Instrumentation Flashcards
Ergonomics
The science of relationships between people and machines
CS-25 colour standardisation for instruments
White: Present status
Blue: Temporary status
Green: Normal operating range
Yellow/Amber: Cautionary range
Red: Warning or unsafe operating range
2 pitot tube methods
- Air flow brought to rest against a “stagnation wall” which senses the pressure and transmits readings to instruments.
- Air flow directed via tubes to the instruments where it can be read directly.
Position error
- What impacts it during flight?
Aka pressure error.
Error in reading of pitot & static pressure due to position of the instruments, affected by attitude & AIRSPEED.
Static vent vs static head
Static head is on pitot tube, 90 degree directed holes. It is affected by turbulence from the pitot head and has high position error.
Static vents are in a more neutral position and less affected by suction, sideslipping & yawing. Static vents can be duplicated on each side to reduce sideslipping/yawing effects.
High speed probe
Advanced version of pitot tube which can cope with shock waves.
Manoeuvre induced error
- main axis causing issues
- time of effect
Errors in pressure sensing instruments during manoeuvres, especially changes in pitch attitude (entering climb, descent). Can last up to 3 seconds after control movement ceases at low altitude, 10 seconds at high altitude. Unpredictable in strength and direction.
Duplicate pitot system setup
Typically feed left pitot data to left pilot, right pitot data to right pilot, maybe a warning in case they differ by too much. Do not generally cross feed the information in processing.
[Backup ASI fed from third pitot source]
Duplicate static vent setup
Unlike the pitot system, dual static systems will be coupled so that error due to side-slipping or yawing can be resolved.
Direct measuring thermometer
- descriptions
- 2 materials
Bi-metal strip of Invar (low coefficient of expansion) and brass (high coefficient of expansion). When heated the brass expands more causing the fused strip (which is set up as a helix) to bend and move an indicating arrow.
Resistive sensor
Aka Resistive Temperature Detector (RTD) or Wheatstone Bridge
Uses electrical resistors of different materials (NICKEL) where resistance varies with temperature, including one variable resistor and a galvanometer (sensitive voltmeter) in the middle. Adjusting the variable resistor to balance resistance indicates temperature.
Wider temperature range than bimetallic sensor, but needs power.
Thermocouple
Used to detect very high temperatures (e.g. jet engine exhaust). Circuit loop connecting sensed area (Tsense) with cold area (Tref) of known temperature, with one side in chromel and one in alumel. Potential difference will be created as temperature at Tsense changes which can be measured with galvanometer.
Low accuracy.
Formula E = K x T(H)
K = constant, E = volts, T(H) = temperature at hot sensor (Tsense)
Radiation pyrometers
- what aspect of radiation is measured?
For measuring highest temperature areas, measures frequency of radiation being emitted.
Used for turbine blade temperatures, just behind turbine inlet, hottest part of jet engine.
Total Air Temperature Probe
- description
- materials used
Remote measuring device (as opposed to direct measuring). Air intake outside of boundary layer, perpendicular to airflow takes in air, bends it 90 degrees to separate water and pure platinum or nickel (high thermal conductivity) wire used as sensor.
Ice in total air temperature probe
The probe includes a heater, which is self-compensating as heater resistance increases with heat, reducing heater current.
Can impact temperature readings but by less than 1C.
Measuring temperature on ground
Air to air ejector (aspirator) used to achieve airflow over a sensor. Engine bleed air from an APU or running engine creates suction effect near the temperature sensor which draws external air in to be measured.
Air temperature gauge errors (3)
Instrument error
Environmental error (solar heating of probe, ice accretion)
Heating error: Adiabatic (compression) and kinetic (friction) heating
Heating error is the biggest problem
Kinetic heating in temperature sensing
Primarily affects direct reading thermometers. Caused by molecules of air impacting the flat temperature sensor and friction effects creating heat.
Adiabatic heating in temperature sensing
Primarily affects remote reading (total head or rosemount) thermometers. Caused by air travelling at high speed being brought to rest and releasing energy as heat.
Total Air Temperature (TAT)
TAT is the theoretical measured air temperature based on static air temperature (SAT) and airspeed, which contributes a theoretic Total Ram Rise on top of the SAT.
Why is Total Air Temperature (TAT) displayed to pilots?
Relevant for assessing icing conditions. TAT below 10C in cloud or precipitation indicates icing risk.
Ram Air Temperature
This is what we detect in the instrument. Crucially this detects only part of the theoretical total ram rise (the measured ram rise) so will be slightly lower than TAT.
Ram air temperature = SAT + measured ram rise
Recovery factor
(in temperature sensing)
The percentage of the Total Ram Rise that is detected by a sensor, designated K(r).
K(r) = Measured ram rise/Total ram rise
Alternative terms for:
- SAT
- TAT
SAT = COAT (Corrected outside air temperature) or just OAT
TAT = IOAT (Indicated outside air temperature)
Formula for SAT
SAT = TAT / (1 + 0.2 x K(r) x M^2)
where K(r) = recovery factor
M = mach number
TEMPERATURES IN KELVIN!
ISA temperature variances at different levels (C per ft)
MSL to 36,090ft (11km): 1.98C per 1000ft
36,090ft to 65,617ft (11km to 20km): Isothermal -56.5C
65,617ft to 104,987ft (20km to 32km): 0.3C per 1000ft
Jet standard atmosphere
Simplified model of the atmosphere used by jet manufacturers.
Assumes temperature reduces from 15C at MSL by 2C per 1000ft with no upper limit.
Accelerometer types (3)
Can have a simple weight on a spring driving an indicator, or suspended on a thin metal blade.
More sophisticated use an E and I bar system detecting more subtle movements through potential difference in E-bar coils.
Or MEMS (micro electric mechanical system). An IRS (inertial reference system) needs 3 of these chips at right angles to sense acceleration in 3d.
Dynamic pressure formula
Dynamic pressure = 1/2 x rho x V^2
ASI errors
Instrument errors (small errors can be assessed in lab and datum card produced)
Position error (aka pressure error, mainly from false static pressure sensing, a joint datum card with instrument error can be produced)
Manoeuvre induced error (chiefly changes in AoA giving transient errors)
Mnemonic for blocked pitot or static error on ASI
PUDSOD
Pitot blocked => Underread in descent
Static blocked => Overread in descent
Calibrated Airspeed (CAS)
Aka Rectified Air Speed (RAS)
This is IAS, corrected for instrument and position errors (perhaps with a datum card).
Equivalent Airspeed (EAS)
- description
- significance
This is CAS corrected for compressibility error only (not density error). As such it is the most accurate measure of dynamic pressure and is used as the basis for all limit speeds. Limit speeds are translated back with errors to find markings for the IAS.
True Airspeed (TAS)
This is EAS corrected for density error (the fact that air density <> 1225g/m^3).
Calculated by reference to pressure and temperature information.
Mnemonic for IAS, CAS, EAS, TAS and corrections
ICE T & Pussy Cat Dolls
I: IAS - correct by P: Position error to get
C: CAS - correct by C: Compressibility…
E: EAS - correct by D: Density…
T: TAS
V(YSE)
Base rate of climb speed with one failed engine.
Marked with a blue line on ASI
ASI accuracy tolerance
From CS-25, greater of
+/- 3%
5kt
Pressure altitude
- Actual definition
- What I remember
Defined as the altitude in standard air corresponding to actual static pressure.
Remember: Altitude displayed on altimeter with 1013 set in pressure setting.
Aneroid & pressure capsule
Aneroid is partially evacuated, sealed capsule used to measure pressure. A pressure capsule is similar but is fed pressure from a source, so can be used to measure differential pressure (e.g. compare pitot pressure to atmospheric pressure.
Bellows
Series of aneroid or pressure capsules on top of each other (maybe with a spring) to increase range of pressures that can be sensed.
Bourdon tube
- Description
- Uses
Curved tube with oval cross section, thinner material on inside of tube than outside. Increase in pressure will thus straighten the tube.
Used for oil & hydraulics.
Pressure transducer
Strain gauge converts pressure to electrical input. Wheatstone bridge arrangement used to detect small voltage change (requires current but NOT dependent on level of the current received) and transmit pressure readings remotely.
Calibration of altimeter vs ASI
Altimeter is calibrated in accordance with ISA over its entire operating range (-5,000ft to +80,000ft).
ASI is calibrated only in accordance with ISA @ MSL.
[This is why ASI diverges from TAS at altitude, whilst altimeter displays useful height information at all levels]
Temperature compensation in altimeter
Bimetal compensator in linking mechanism compensates for temperature effect.
Sensitive altimeter
Advancement of the single pointer altimeter, contains a bank of 2 or 3 capsules with geared linkages to drive the 1000ft per revolution, 10000ft per revolution and 100000ft per revolution indicators.
Improvements available for altimeters
Jewelled bearings reduce friction and associated lag.
Knocking/vibrating devices can overcome initial inertia of the gear train when transmitting movement from the capsules to the pointers [NOT called lock-in]
Digitiser in altimeter
Reads pressure data and transmits to transponder on 1013 pressure setting basis.
Counter-pointer altimeter
Digital counter to show current altitude clearly (3 pointer version can be hard to read) and a pointer with 1000ft rotations to give a sense of rate of change.
Servo-assisted altimeter
Higher accuracy, especially at high altitude where pressure changes are smaller.
Capsules pivot a straight “I” bar next to a separate “E” shaped bar. The centre leg of the E is excited to create 180 out of phase current in the other 2 legs (which are wired together and detected).
If “I” bar moves it will pivot relative to the 2 legs and generate error current between them, which can be detected and amplified to be displayed.
Altimeter accuracy requirements
20m (60ft) for altimeters with test range up to 30,000ft
25m (80ft) for altimeters with test range up to 50,000ft
Altimeter errors
Position/pressure error
Instrument error
Manoeuvre induced error
Barometric error (pressure setting not accurate)
Time lag
Temperature error
Altimeter temperature error correction formulaic
0.4% per 1C away from ISA
Density altitude
The altitude in the standard atmosphere at which the prevailing density would occur.
Location for pre-flight altimeter checks
Apron (apron elevation information should be displayed in aerodrome flight clearance office)
VSI time lag for metering unit
4 to 6 seconds
VSI metering unit construction
More complex than a simple hole as this would have a detection of hectopascals per minute, not feet per minute. A combination of different holes (called ‘capillary’ and ‘orifice’) achieve required feet per minute detection.
Metering unit feeds the case (capsule gets fed directly).
VSI Errors
Instrument error
Position (pressure) error
Manoeuvre induced error
Time lag error
Instantaneous Vertical Speed Indicator (IVSI)
This uses a small vertical acceleration pump (or dashpot) which provides boosts static pressure change into the body providing quick reaction to pressure change. By the time the boost disperses the differential pressure will be set up.
[MAY REFER TO ACCELEROMETER]
IVSI errors (2)
Will be highly sensitive during turbulence, fluctuations should be ignored.
Piston will sink towards the bottom of the cylinder in a steep turn, giving false indication of a climb.
VSI accuracy check on the ground
Needs to be within +/- 200 ft of zero when on the ground at temperatures from -20 to +50.
Outside those temperatures, +/- 300ft is ok.
Effect of temperature variance from ISA on VSI
This is compensated for via the capillary and orifice.
Vane type alpha sensor
[Alpha = AoA]
Aerofoil on a pivot in the airflow which has a transducer (or synchro-transmitter) to detect the angle which is transmitted to cockpit. Contains a heater to prevent icing.
Conical alpha sensor
Conical sensor sticking out into air flow, free to rotate, with slots at 90 degrees. It will rotate to equalise pressure through the slots. Potentiometers (WIPER?) will measure the angle it is at and transmit data to cockpit.
Integral heater to prevent icing.
Effects of shock waves
- More drag
- Less lift
- Mach tuck
- Buffeting
- Reduction in control effectiveness
Which direction do shockwaves move in?
Rearwards as speed increases
Formula for local speed of sound
LSS = 38.95 x sqrt(T)
where LSS in kt
T is absolute temperature in K
Mach number
TAS / LSS
Relationship between mach number, CAS, TAS and temperature
Mach number and CAS are NOT affected by temperature.
TAS IS affected by temperature.
Thus if MN or CAS kept constant while temperature is changing (altitude constant), TAS will change (LSS is changing).
What is measured as a proxy to establish mach number
D / S
[Dynamic pressure / static pressure]
Machmeter construction
Has two capsules, an airspeed capsule fed by pitot pressure (body is fed by static pressure so that it the airspeed capsule gives dynamic pressure) and altitude capsule.
Both capsules drive a ratio (2 directional) arm which is linked to a ranging arm that determines the ratio between the two inputs, feeding a needle.
Machmeter errors
Instrument error
Position error (designed to always over-read for safety)
Manoeuvre induced error
Note: Temperature, density and compressibility errors cancel out
Types of mach number based on corrections
MMR: Machmeter reading (uncorrected)
IMN: Indicated mach number (MMR corrected for instrument error)
TMN: True mach number (IMN corrected for position error)
ECTM diagram
Shows relationship between EAS, CAS, TAS and MN as you climb (assuming one remains constant). Only for standard conditions (i.e. troposhere)
Impact on ECTM diagram of isothermal layer or inversion
Affects the relationship between TAS and MN. In isothermal layer they will move together, in inversion they will swap direction.
M(MO) and V(MO)
- description
- What factor causes V(MO) to vary in flight?
MO: Maximum operating, alternative to NE
V is for IAS
M is in terms of mach number
V(MO) starts to reduce with altitude once mach number becomes the limiting speed.
Relation of V(NE) and V(MO)
V(MO) always less than V(NE).
Large aircraft won’t have a V(NE) in reality, operate with the V(MO) that changes and is indicated by the barber pole.
Air Data System (ADS)
- Components (5)
Consists of the following:
Air Data Computer (ADC)
Sensors: pitot, static, temperature
Displays
Power pack
Weight on wheels switch
Peripheral component of ADC
AoA vane
AoA data is passed to other computers
Purpose of weight on wheels switch for ADS
To disable stall warning on the ground
Analogue vs digital ADC
Analogue system uses mechanical methods and voltage measurement/transfer to establish and share readings.
Digital version converts analogue data to digital at the input level to be processed by a digital ADC.
Built in Test Equipment (BIT/BITE) test types for ADC
Power up BITE: Applied on start-up or after a break, tests microprocessor, memory store, air data functions
Continuous BITE: Automatic testing of inputs and outputs once per second
Maintenance BITE: Allows maintenance crew on ground to carry out tests with a test setting
Colour convention for poles of a magnet
North: Red
South: Blue
[Note: North means the pole that points to Earth’s north pole]
Direction of lines of flux in magnet
Flow from North to South (outside of the magnet, can visualise a line of flux internal from S to N completing the loop)
Place in a magnet where magnetism is concentrated
At the poles
Methods of Magnetisation
(and direction of resulting magnetisation)
- Stroke an iron bar in same direction with same end of a magnet. The last end touched will be opposite pole to the “stroking” pole.
- Align iron bar in line with a magnetic field and subject to hammering/vibrating. Will be opposite poles to the magnet creating the flux.
- Iron bar can simply be placed in line with a magnetic field (opposite alignment to the magnet creating the field).
- Placing the iron bar in a solenoid.
Methods of Demagnetisation
Shock: Place at right angles to the earths magnetic field and hammer it.
Heat: At about 900C magnetism is lost and doesn’t return on cooling.
Electric current: Place inside solenoid with AC current that gradually reduces to zero (AC means magnetism is constantly reversed) [called degausing]
Magnetic materials
Ferrous metals such as iron and steel.
Examples of hard and soft iron
Hard iron is hard to magnetise but retains magnetism well.
Hard: Cobalt, chromium & tungsten steel
Soft: Silicon iron, pure iron
Directive force
The horizontal component of the earth’s magnetic field at a given point, designated H.
Total force is T and vertical component is Z.
Dip
This is the angle between horizontal and the force of earth’s magnetism at a given point.
Z/H = tan(dip)
Vertical card compass
- description
- aka
Aka b-type, e-type.
This is the one most commonly in use, with a compass card attached to the magnet assembly, suspended in liquid in the bowl. A vertical lubber line on the glass window allows heading to be read off.
Requirements of compass magnet system
- Horizontal
- Sensitive
- Aperiodic
Horizontality in vertical card compass
Compass pendulously suspended so that its weight creates a moment force that opposes the moment created by dip.
Resulting angle is around 2 degrees in mid latitudes of NH.
Sensitivity in vertical card compass
Increase magnet moment by joining several short magnets together (a long one is undesirable). Circular magnet also possible.
Reduce friction with lubrication, reduction in magnet weight and iridium-tipped pivots in jewelled cup.
Aperiodicity in vertical card compass
Ideally want “dead beat” compass, settles down on heading immediately after displacement by turbulence of manoeuvres.
Achieved by:
Several short magnets instead of one long one reduces moment of inertia.
Damping liquid (plus damping wires in grid ring compass).
Compass liquid issues
Expansion with temperature dealt with via an expansion chamber or “Sylphon tube” (although low coefficient of expansion in liquid is desirable).
“Liquid swirl” effect reduce by having a liquid with low viscosity.
Compass accuracy requirement
Part 25: +/- 10 degrees
Correction on magnetic compass when turning through N/S in NH
- Mnemonic
- Calculation
UNOS
UnderSHOOT turning through North
OverSHOOT turning through South
(i.e. when turning through north, stop before you reach the target heading, through south, go past target heading)
Amount to x-shoot: (Lat + bank angle) / 2
Correction on magnetic compass when accelerating in W/E direction in NH
ANDS
Accelerating - shows turn to north
Decelerating - shows turn to south
[Accelerate shows turn to nearer pole]
Compass turning error factors
Compass turning error is a function of bank angle, so greater angle of bank will give a greater error.
Steep turns and compass turning error
If bank angle and dip sum up to 90 degrees compass can become completely unreliable. A turn away from dip angle can cause compass horizontal to go past perpendicular to flux lines and indicate 180 degrees in wrong direction.
Liquid swirl effect
- effect on turning error
Body of the compass drags the liquid in direction of the turn, which drags the magnet in that direction.
In NH when turning through N the effect INCREASES magnitude of turning error (opposite through S and in SH).
Compass manoeuvre errors at magnetic equator
No vertical component of magnetic force at magnetic equator, so only get the liquid swirl effect.
Tend to under-read on turns as liquid swirl acts like friction on the magnet turning.
Typical gyroscope rpm
4,000 to 55,000 rpm
Definition of horizontal vs vertical gyro
Defined based on the spin axis, so horizontal gyro has a horizontal spin axis with the rotor spinning vertically.
Gyro: Rigidity
The property of a gyro that causes it to maintain its axis in a fixed direction in space unless subjected to an external force.
Gimbal
Pivoting frame within which a gyro sits, each gimbal gives one degree of freedom to movement of the gyro (i.e. an axis around which movement can be measured - spin axis doesn’t count).
Gyro: Precession
A torque applied to the gyro will be precessed through 90 degrees in the direction of rotation of the gyro.
Strength of rigidity and precession
Rigidity is increased by increased rpm and increased moment of inertia (combining mass and radius).
Precession however is reduced by an increase in rpm or moment of inertia.
Figuring out which orientation gyro is for pitch, roll, yaw with 1 gimbal
(picture of a plane with 3 gyros dotted around it)
1 gimbal indicates a rate gyro, rigidity gyros need 2 gimbals.
So look for the direction in which the gyro cannot turn. Force in that direction is precessed to the free direction and measured.
Gyro wander categories (2)
Wander is any movement (real or apparent) of gyro over time.
Drift: Horizontal movement
Topple: Vertical movement
[Note: Horizontal gyros can drift or topple, vertical can only topple]
Real wander
Aka random wander.
This is when a gyro wanders in relation to inertial space.
Caused by manufacturing imperfections.
Apparent wander
- Description
Changes in apparent orientation of the gryo as a result of changes in the observers frame of reference.
e.g. rotation of the earth and movement of gyro around earth (transport wander).
[Transport wander may or may not be considered part of apparent, more likely to refer to rotation of earth]
Apparent wander
- Calculating amount and direction
sin(latitude) x 15 deg per hour
RIGHT or NEGATIVE in NH
LEFT or POSITIVE in SH
Apparent drift & topple at equator & poles
Topple of vertical gyro is maximum at the equator, drift of horizontal gyro is maximum at the poles.
[Horizontal drift is measured as SIN, vertical as COSINE]
Transport wander
- description
- calculation
More accurately Transport Drift.
Horizontal gyro whose spin axis is aligned with meridian. If it is moved around the earth its angle relative to new meridian (i.e. heading to true north) will change by:
Change in long x sin(mean latitude).
Calculations on amount of drift of directional gyro
(Transport, latitude nut, time)
Northern Hemisphere ONLY!
Time advancing and moving EAST:
RIGHT drift, NEGATIVE heading
Lat nut and moving WEST:
LEFT drift, POSTIVE heading
Displacement vs Rate Gyros
- description
- # gimbals and DoF
Displacement gyros measure angles and have 2 gimbals and 2 degrees of freedom.
e.g. Directional indicator, Artificial Horizon
Rate gyros measure angular rate and have 1 gimbal and 1 DoF, with springs to assess rate rather than distance.
e.g. Rate of Turn indicator, yaw dampers
Space vs Tied tyros
Space gyros have gyroscopic inertia with reference to a point in space. They are free to wander with no source of correction, so need to be highly accurate and are expensive.
Tied gyros get restored back to orientation periodically, e.g. directional indicators.
Earth Gyros
An earth gyro is a special case of tied gyro, which has its vertical or horizontal maintained in respect of local gravity (e.g. artificial horizon).
Ring Laser Generator (RLG)
- description
- advantages
Ring lasers use 2 light paths travelling around a glass prism in a triangle in different directions, distance travelled by each assessed through their frequency, which is affected as the prism moves.
Advantages: Zero spin up time (run up time), low sensitivity to g-forces.
Ring Laser Gyro Issues
Frequency lock is when input rates are very low and the two lasers shift frequency and lock together. Resolved with “dither” which rotates the triangular block forwards and backwards around the axis to escape the lock.
Real wander due to (for example) thermal expansion is offset through processing.
Rate integrating gyro
- How they work
- Where they are used
- Degrees of freedom
Measures rate of displacement to a high degree of accuracy. Gyro is mounted inside a cylinder (which is the ONE degree of freedom), which is held in viscous fluid within another cylinder.
The rotation of the cylinder is used to measure angular displacement in the third axis direction.
A pickoff and torque motor can be used on the cylinder to reset it (otherwise the axis of detection moves over time).
Used in inertial units only.
Gimbal lock
- description
- solutions
Occurs when a gimbal is turned to its fullest extent (e.g. 90 degree bank) losing one degree of freedom. Results in toppling of the mechanism, unless prevention method exists.
Aerobatic craft could have 4 dimensional gimbal or gimbal flip mechanism, where a motor flips the outer gimbal by 180 degrees.
Directional gyro - orientation
Directional gyros are horizontal gyros, i.e. spin axis is horizontal, the rotor spins in the vertical plane.
The rotor axis lies in the yawing plane.
Directional gyro
- Degrees of freedom (and which planes)
2 degrees of freedom (2 gimbals)
Free in pitch and roll (but not yaw, which is what we are measuring)
Direction gyro
- What is it “tied” to?
Aircraft horizontal
NOT Earth horizontal
Suction Directional Gyro
- Control System
Control system is the means by which orientation is maintained through manoeuvres.
Air jets applied in rotation direction from outer gimbal. If aircraft banks the gyro is free and moves out of yawing plane, needs a force to move rotor axis back into yawing plane.
Suction Directional Gyro
- Control System components (2)
1) Aircraft banking means air jets have sideways force on rotor, which is precessed by gyro to correct back to yawing plane
2) “Exhaust” flow off rotor hits symmetrical rotor plates on outer gimbal, which provides force if rotor is off centre from gimbal to move rotor back to yawing plane.
Effect (1) is for big corrections, (2) for fine tuning.
Caging device
Allows the inner and outer gimbals to be locked to allow DGI to be synchronised and prevent toppling during manoeuvres.
DGI limits before toppling
Air driven: 55 degrees (pitch/roll)
Electronic: 85 degrees
[Although depending on rotor axis, 360 degrees of either roll or looping may not cause a topple (fore/aft rotor axis allows 360 degree roll). This depends on aircraft.]
Directional gyro
- Gimballing error
Errors in DGI during banking, which will during a 360 degree turn follow a double sine curve. Recovers once S&L flight resumed.
Due to the two gimbals not being at 90 degrees to each other during a turn.
Latitude Nut correction
Latitude nut can be wound in or out from neutral position on its thread on inner gimbal. In neutral position it balances a weight on the other side.
Moving it creates a net weight and force on inner gimbal which is precessed by the gyro to have a steady change in heading over time.
This is used to correct wander of the gyro due to earth rate and position will depend on latitude of the gyro.
Directional Gyro
- On which gimbal are the latitude nut and air stream located
Air on outer gimbal.
Latitude nut on inner gimbal.
DGI rotor speed error
Rotor not spinning at correct rate will result in errors, including incorrect amount of precession causing latitude nut to have incorrect amount of force.
Artificial horizon gyro
- Orientation (Horizontal or vertical)
- Type
- Rotation direction
Vertical gyro (spin axis is vertical, rotor spins in horizontal plane).
Earth Gyro type (sub-type of tied gyro).
Rotates anti-clockwise when viewed from above (electric are clockwise)
Artificial horizon limitations
Air driven: 60 degrees pitch and 110 degrees roll
Electric: 85 degrees pitch, complete freedom in roll
Beyond this the gyro topples, taking 10 to 15 minutes to re-erect (due to high rotor speeds - high rigidity, limited precession)
Artificial Horizon control system (air driven)
Pendulous unit under the rotor has four slots in four directions where air flow exits, each half covered by a pivoting vane. When the gyro moves relative to gravity, the vanes (pulled by gravity) close or fully open slots, creating unbalanced force of exiting air. This force is precessed to make the gyro line up with gravity.
Acceleration error in air driven artificial horizon
False indication of pitch up and roll to right under acceleration:
Pitch up: Acceleration affects left and right control system vanes, force to port is precessed to a pitch up force.
Roll right: Weighted rotor base lags in acceleration, which force is precessed as a turn to starboard (right wing down).
Turning errors in air driven artificial horizon
As with acceleration, impact of turning on the vanes and weight of pendulum will cause incorrect banking reading (order of 2 degrees) varying over a 360 degree turn.
False pitch up: Peaks at 180 degrees of turn
Bank: Under read after 90 degrees, over read after 270 degrees
Artificial Horizon (Electronic)
- Control System
The electronic artificial horizon uses mercury levelling switches (one in pitch, one in roll) to detect changes vs gravity and activate torque motors to correct.
Pitch torque motor on outer gimbal “rolls” the gyro, which is precessed to correct pitch. The roll torque motor on the inner gimbal pitches the gyro, which force is precessed to a roll change.
Artificial horizon
- Turn error compensation
Both air driven and electric type artificial horizons have a compensation built into the horizon line and rotor axis to compensate for turning error.
Managing acceleration & turning errors in electric artificial horizon
Rotor speed is higher (higher rigidity) and rotor is less bottom heavy, reducing acceleration error in general.
There are also pitch and roll cut out switches which stop the torque motors for over 10 degrees of bank, but acceleration errors left as acceleration is short lived.
Electric artificial horizon
- Fast erection system
Push a button to increase voltage to the torque motors, increasing speed of the control system to correct orientation. Need to be in S&L flight with no acceleration when pressing the button.
Corrects at 120 degrees per minute instead of 4 degrees per minute.
Artificial horizon
- Adjustable aeroplane datums
Only really appear in USA. Highly recommended for light aircraft to set the datum on the ground then leave it alone. EASA demand they are removed or rendered inoperative for craft over 6000lb (2727kg).
Vertical gyro unit
Has the same idea as the artificial horizon but used remotely to generate data for a flight director (or combined flight director and attitude indicator display) or Attitude Director Indicator (ADI).
ADI
Attitude Director Indicator
Can be electronic or mechanical.
Shows pitch & roll attitude plus flight directors and potentially ILS/glidescope info.
AHRS
Attitude & Heading Reference System
Vertical gyros combined with electric compass feed to an AHRS system. Likely use 2 single degree gyros for roll & pitch (heading from compass) rather than one 2 degree gyro.
Most up to date use a 3-axis accelerometer, a 3-axis gyro and a 3-axis magnetometer.
Note: ATTITUDE AND HEADING, NOT POSITION
Rate of Turn Indicator construction
Horizontal axis gyro has one degree of freedom so that if the aircraft banks it remains horizontal. If the aircraft turns (yaws) however, the force of the gyro being turned is precessed to a “banking” force, which is opposed by springs. The force of the spring creates a secondary precession opposing the rate of turn of the aircraft.
The secondary precession is measured to assess rate of turn.
Rate of Turn indicator measures yaw rate or change of heading rate?
Rate of turn is the change of heading rate.
However the turn indicator actually shows us the angular velocity around the yaw axis of the aircraft, which isn’t exactly the same thing.
Rate of Turn Gyro rotation speed
Slower than artificial horizon and DGI gyros as it principally uses precession, not rigidity to work.
Rotation is away from pilot (as if rolling forward away from the pilot).
Toppling of rate of turn gyro
As there is only one gimbal the gyro will not topple when it hits the stops, but it does have a stop at around 20 degrees per second turn speed.
Effect of varying rotor speed
Precession is increased at reduced rotor speed.
What matters is the torque produced by secondary precession in opposition to the rate of turn. With higher precession, less torque is needed to create the secondary precession force, so a lower than true rate of turn (under-read) will occur.
Impact of aircraft speed on rate of turn indicator
(and which speed is relevant)
Rate of turn indicator is calibrated at a given speed (spring tension) and will be inaccurate at different TAS. However the amount of error is small even for large departures from design TAS.
Rate of turn errors in looping plane
This is an issue for steep turns, which effectively have forces in the looping plane.
If the gimbal is tilted (due to yaw) before movement in looping plane commences, the steep turn will increase the gimbal tilt leading to over-read.
Calculation for bank angle for rate 1 turn
Bank angle = 10% x TAS + 7 degrees
Skidding vs slipping
Skidding - oversteer
Slipping - understeer
Turn-coordinator differences to rate of turn indicator
Turn coordinator gyroscope is mounted with axis at 30 degrees to the aircraft longitudinal axis making it sensitive to banking as well as turning.
The coordinator responds to bank first to give a correct initial turn direction indication, but balances out correctly with rate of turn due to springs.
Weakness of turn coordinator
Sensitivity to yaw & roll (even pitch if TAS is wrong) makes it sensitive to TAS. Only rate 1 turn at designed TAS will be accurate (maybe within about 5% of design TAS).
3 aims of compass swing
1) Observe deviation on a series of headings
2) Remove as much deviation as possible
3) Record the residual deviation
Hard iron vs soft iron magnetism
(considering deviation)
Hard iron magnetism of the aircraft is permanent and not related to heading.
Soft iron magnetism is induced by the earths magnetic field. We focus primarily on the magnetism induced by the vertical component of earths magnetism (Vertical Soft Iron, VSI) only. Obviously this is nil at magnetic equator.
3 coefficients of deviation
A) Mechanical issue of displaced lubber line, corrected using compass bolts.
B) Correction required due to magnetic deviating forces acting on compass, measured on Easterly or Westerly heading.
C) Same as (B) but Northerly or Southerly heading.
When compass swing is required
- CHANGE to components (direct replacement not a problem)
- Significant modification/repair involving magnetic material
- Doubt about compass accuracy
- Per maintenance schedule
- Carrying unusual ferromagnetic loads
- Compass subject to shock
- Aircraft hit by lightning
- Significant change in magnetic latitude
- After long term storage on one heading
Cause of deviation changes by heading
Note that deviation changes on different headings due to alignment of the effective poles of the aircraft magnetism relative to the compass and magnetic north.
HOWEVER, aircraft magnetism is not itself affected by heading.
Remote Reading (Gyromagnetic) Compass
Uses a flux detector to correct a directional gyro to maintain synchronisation with magnetic north.
Digital interpretation can also be passed to other systems.
Remote Reading (Gyromagnetic) Compass
- Flux Detector
Aka Flux gate or Flux valve
Used by remote indicating compass, located in wing tip or tail (far from systems to reduce deviation).
3 legs made of soft iron detect Earth’s magnetic field, suspended on a Hooke’s joint so it can move up to +/- 25 degrees in pitch/roll.
Will still detect Z component in balanced turns.
May be held in liquid to dampen oscillations.
Remote Reading (Gyromagnetic) Compass
- Flux Detector diagram
Central core (not visible) connects the two sections
Remote Reading (Gyromagnetic) Compass
- How earths magnetic field is detected by flux detector
Static magnetic field can’t be detected, so an AC exciter field is passed around the core of the flux detector. This saturates the magnetism at peak current.
The fields generated in the 2 sides of the C are opposing waves therefore current in coils wrapping both sides sums to zero.
But the amount of earths field flowing through each horn disrupts this, causing a non-zero resultant field through each section.
[Note: Horns not super important, just increase sensitivity]
Remote Reading (Gyromagnetic) Compass diagram
Remote Reading (Gyromagnetic) Compass
- How flux detector interacts with gyro
Coils around flux detector legs are linked to coils in an error detector. A self-synchronising (SELSYN) rotor sits in the middle of it which has zero current only if aligned at 90 degrees to magnetic north.
If out of alignment a current is generated, amplified and DC rectified by PRECESSION amplifier and energises coils around a permanent magnet.
This rotates the gyro, the bevel gears, the drive shaft, heading card and error detector - until current is zero.
Remote Reading (Gyromagnetic) Compass
- Mnemonic for gyro correction process
FEAT
- Flux valve
- Error detector
- Amplifier (precessor)
- Torque motor
Erection of Remote Indicating Compass gyro
If gyro spin axis moves out of line with aircraft horizontal, pickups to a torque motor will move off insulated sections and generate a torque to re-erect.
RMI
Radio Magnetic Indicator
Combination directional indicator (driven by remote compass) and VOR/radio indicator.
Card rotates so that heading is at the top, a bug can be set and a green arrow will sit on top pointing to the radio bearing.
Annunciator
Can appear on RMI.
This indicates if the error detector is moving the gyro alignment in one particular direction, which would indicate it is a long way out of alignment.
Flickering between one and the other indicates correction of minor differences, a good sign!
Initial alignment of remote indicating compass
- time
- method
Remote magnetic unit only corrects at 2 degrees a minute, so initially use the knob to cage and adjust the gyro manually.
Remote Reading Compass
- DG/COMP switch
Allows selection between directional gyro mode and COMP or MAG modes (i.e. corrected by flux detector).
Note: No latitude nut so all compensation is manual in DG mode.
Horizontal Situation Indicator
RMI with additional VOR deviation markings across the middle
Remote Indicating Compass accuracy requirement
Within 5 degrees
[If accuracy within 1 degree, some authorities say compass correction card not required]
Difference between remote indicating and direct reading compass
Remote version “senses” rather than “seeking” magnetic north, therefore more sensitive
How is deviation controlled for remote indicating compasses
Using permanent magnets placed around the flux detector
Acceleration and turning errors in Remote Indicating Compass
Steady pitch (up to 25 degrees) not a problem as the flux detector hangs on hooke joint. But will move off horizontal and pick up Z component of earths field in balanced turns, acceleration etc.
Sensors may be in place to switch off magnetic correction when such manoeuvres are detected.
Inertial Navigation System (INS) vs Inertial Reference System (IRS)
INS was the first iteration, standalone system used only for navigation using gyros.
IRS is a modern system which uses strapdown method and ring laser gyros.
IRU and ADIRU
Inertial Reference Unit (a single inertial platform, may be several in an IRS).
Air Data and Inertial Reference Unit
Labelling of axes for inertial navigation purposes
X axis: North - South
Y axis: East - West
Z axis: Vertical
3 solutions to maintaining axis direction in INS
Stable platform: Platform kept level (with local gravity) and oriented with north
Wander angle: Platform level but not kept aligned with north
Strapdown: System fixed to the plane but mis-alignment is measured and monitored
Stable platform INS
- How platform is stabilised
Platform is suspended in 2 gimbals.
2 torque motors (pitch and roll) keep platform level as aircraft manoeuvres.
Azimuth torque motor aligns north.
Stable platform INS
- Measurements on the platform
2 accelerometers for axes X and Y
3 rate integrating gyros sense rotational displacement
Stable platform INS
- Alignment process (5)
- Time taken
1) Battery check
2) Align vertical (use torque motors until zero gravity acceleration sensed)
3) Align with true north (azimuth torque motor until East gyro senses zero rotation)
4) Rotation at North gyro indicates latitude
5) Pilot inputs lat and long for initial position (lat checked against internal)
NOTE: Takes 10-15 mins
Stable platform INS
- NAV mode
Rate integrating gyros now responsible for keeping platform level. Accelerometers for measuring acceleration in X and Y axes.
Acceleration integrated twice to give distance moved.
Stable platform INS
- Calculation of lat and long
Distance moved in X axis converted to degrees gives latitude.
Y axis distance needs to be fed into “secant gear” (1/cos) along with latitude result, to determine longitude in degrees.
Stable platform INS
- Extra info required for windspeed info and magnetic headings
INS only knows about location, i.e. ground track and ground speed.
To give out magnetic headings and windspeed need TAS (from ADC) and variation (data stored internally).
Otherwise only track, true heading and drift can be calculated.
Stable platform INS
- ATT mode
Disabled NAV function of INS and provides ATTITUDE AND HEADING info only to relevant systems (not navigation, i.e. current position)
NOTE: NAV mode cannot be re-selected after switching to ATT
INS display choices
- TK/GS
- HDG DA
- XTK/TKE
- POS
- WAY PT
- DIS/TIME
- WIND
- DSRTK/STS
Has two numerical outputs, use a dial to select which numbers you see:
TK/GS: True track, Ground Speed
HDG DA: True heading, drift angle
XTK/TKE: Distance from track, track error
POS: Lat and long of current position
WAY PT: Lat and long of waypoint
DIS/TIME: Distance and time to waypoint
WIND: Wind direction and speed
DSRTK/STS: Desired track (i.e. heading) and system status
Stable platform INS
- Waypoint navigation (manual and auto)
2 digit display indicates “from” waypoint number and “to” waypoint number.
MAN selection makes you overfly waypoint with a warning but no action
AUTO will switch to next waypoint and turn just before it to “cut the corner”
Stable platform INS
- Waypoint navigation (intercept and go direct)
Changing waypoint selection to (e.g.) “23” when not inbetween waypoints 2 and 3 will lead to an interception of the path between 2 and 3, before continuing to 3.
Changing selection to “03” will establish current position as waypoint zero and go direct to waypoint 3.
Trihedron
This is a set of 3 axes covering 3d space.
Inertial system is designed to measure acceleration across the 3 axes of a stable trihedron, regardless of the aircraft trihedron (pitch, roll, yaw)
Major disadvantage of stable platform INS
Can’t maintain alignment over the poles, needs to switch to dead reckoning system until emerging on other side of the pole.
Wander Angle INS description (differences from stable platform)
Wander Angle INS doesn’t maintain north alignment, which gets around the Stable Platform problem over the poles.
Direction of north is established initially with pilots latitude input along with east and north rate integrating gyros in alignment phase.
Change in alignment then detected by azimuth gyro.
Calculations need to be adjusted using the alignment info.
Inertial system errors
- classification
Bounded: Fixed error (in angle or distance) that stays constant or oscillates around a mean
Unbounded: Error that increases over time
Note that a track angle error may be fixed (so bounded), but will lead to a XTK error that increases with time (so unbounded)
Main error of concern in INS systems
Drift - an unbounded error due to drift in the gyros
Corrections required in an inertial system (4)
Earth Rotation (15 degrees per hour)
Transport Wander
Coriolis effect
Centripetal acceleration (following great circle path)
Real wander due to gyro imperfections can’t be corrected
Schuler error
Bounded error based on the idea that the INS, always pointing to the centre of the earth, behaves like a pendulum with oscillatory period of 84.4 minutes.
Schuler tuned INS accounts for the 84.4 minute periodicity error.
INS & IRS system accuracy
- expected values
- how to measure it (2)
No minimum, but expected to be within 1.5NM per hour for INS
IRS expected around 0.5NM per hour
Measured by comparing final position to IRS position, OR looking at residual ground speed when stationary.
Altitude in inertial systems
IRS have a concept of altitude, INS ignore it. Thus IRS require 3 accelerometers instead of 2.
HOWEVER - Inertial altitude or IRS is NOT accurate and is not an input into other systems, needs correction from barometric data
Inertial Reference System (strapdown)
- Description
- 3 types of parameter measured
Uses 3 ring laser gyros, orthogonally mounted to aircraft structure, to detect measure ANGULAR ACCELERATION and ANGULAR RATE.
MEMS accelerometers measure LINEAR ACCELERATION in earth X, Y and Z.
6 sensors collectively called the computing trihedron.
IRS Initialisation
- Process
- Ring laser gyro sync time
Not a true initialisation, no alignment, just detection of starting info.
Accelerometers determine direction of gravity.
Ring laser gyros will detect alignment change which must be earth rotation, thus giving direction of north.
Latitude calculation inaccurate so lat and long input by pilot.
Takes 10 mins usually (5 mins @ equator).
IRS fast realign
Switch from NAV to ALIGN for a quick 30 second re-alignment if limited time.
Generally better to switch off and start a full alignment process.
IRS Corrections
Same corrections required as INS (earth rotation, transport wander, coriolis and centripetal acceleration).
Schuler errors are also a problem.
All are corrected via calculation.
NOTE: Altitude not Schuler tuned, requires barometric data to correct or will drift exponentially.
Usage of IRS data
Unlike INS, IRS carries out no navigation of its own. Data is passed to the FMS (Flight management system) which feeds from other systems to drive outputs and control the aircraft.
FMS, EFIS & CDU
FMS - Flight Management System (the software)
EFIS - Electronic Flight Information System (display unit of FMS info)
CDU - Control Display Unit (panel to manage FMS, 1 per pilot)
Update frequency of FMS navigation data
- What to do if database will expire during flight
Every 28 days (data uploaded a week or so in advance), in accordance with AIRAC cycle defined by ICAO.
If database is to expire during flight, stick to the old one.
What happens to input data (e.g. waypoints) if nav database is changed?
What happens to it at the end of the flight?
If the database is changed it all gets deleted!
However doesn’t get deleted at end of the flight.
Essentially creates a storage space within each nav database version.
Data set in FMS nav database (7)
- Airport data (4 letter ICAO designator)
- Runway data
- STAR (STandard ARrival procedures)
- SID (Standard Instrument Departure)
- VOR/DME data (3 letter designator)
- NDB data (2 or 3 letters)
- Waypoint data (5 letters)
NOT obstacles or terrain, NOT ATC frequencies
Update frequency of FMS performance database
As required, based on information for the individual aircraft, to reflect changes as aircraft ages (for example).
OR after an FMS software update.
Data in FMS performance database (6 groups)
- Aircraft flight envelope
- Thrust and Drag characteristics
- Speeds: V1, V2, VR, Vx, Vy, cruise, endurance, holding
- Masses: ZFM, TOM, LM
- Fuel flow (standard configuration only!, don’t trust FMS for non-standard!)
- Altitudes: Maximum, optimal
LNAV and VNAV
- Which controls speed (horizontal)?
Lateral navigation (horizontal sense, navigation between geographical locations)
Vertical navigation (climb and descent profiles)
VNAV controls speed (vertical and horizontal)
Amendment of FMS route info
Route data (and other nav data) in FMS nav database is read only.
Pilots can make amendments to routes they pull up, which do not affect the underlying route data in the FMS database.
Use of FMS VNAV
- phases of flight
Likely to use VNAV to control climb and descent (including speed), to keep to desired profile.
However it is rarely used in cruise as flight level is more likely to be chosen based on ATC instructions, rather than for performance considerations made by the FMS.
Requirement for AP VNAV to carry out a climb or descent
Pilot has to input an altitude in the control panel which sets the limit for how far the autopilot can climb/descend
FMS position determination
- Sources
- Algorithm
FMS inputs: DME, INS/IRS, GPS, VOR, LOC
[ILS = LOC, no NDB]
Kalman filter is an algorithm to determine likely location based on conflicting data (e.g. imperfect GPS and navaid info)
FMS position fixing source priority
DME/DME cross cut
DME/VOR
VOR/VOR
GPS (could be highest priority if available)
[Note: IRS can be unreliable in the short term, e.g. due manoeuvres]
NDB, VORAN etc. unlikely to be used
RAIM
- Description
- What facility do they enable?
Receiver Autonomous Integrity Monitoring
Part of FMS, uses GPS to monitor performance and exclude erroneous GPS satellites.
Required for RNP approaches, THUS NO RNAV/RNV APPROACHES WITHOUT GPS.
Cost management in FMS
Cost Index used to tell FMS how to prioritise fuel burn vs other per hour costs. Low index means high fuel cost, minimise fuel burn. High index means minimise flight time.
Cost Index =
Flying cost per hour (dry) in $ per hour
/ Fuel cost in cents per lb
Why is GPS altitude inaccurate?
As the mathematical model of the world to which GPS 3d coordinates are compared is an estimate only.
FMS Inputs (9)
- Clock time
- EFIS control panels
- ADC (Air Data Computers)
- IRS (Inertial Reference System)
- Navaid data (VOR, DME, ILS)
- APFDS (Autopilot Flight Director Sys)
- Air/ground switches
- Fuel flow & contents
- Flap position
FMS Outputs
Various displays (including EFIS panel, CDU panel, flight displays and navigation info)
Flight control computers, autothrottle, AFCS (automatic flight control system)
CDU Scratchpad
Input area at bottom of CDU screen where you can type. Once you’re happy with the input, click a left side button to apply it to a chosen field (kind of copy and paste).
Can also grab data from fields down to scratch pad by pressing the right side button next to the field.
RNP in FMS
Required Navigational Performance
An input from pilot on required accuracy of positional determination for a given route. Compared to Actual Navigation Performance (ANS) by the FMS.
Defined as radius within which FMS is 95% confident you are located.
FMS setup process steps
1) IDENT
2) POS INIT
3) ROUTE (initialisation then programming)
4) PERF INIT
FMS setup process
- IDENT and POS
IDENT - Confirm aircraft type and active nav database are correct
POS INIT - Displays last position, you input airport code and gate ref to establish initial position
POS REF - Shows position based on FMS and the two IRS plus their indicated ground speeds
POS SHIFT - Shows navaid info
FMS setup process
- ROUTE and PERF
ROUTE - Select nav database route and input runway (Standard Instrument Departure designator for runway will be displayed) and ACTIVATE the route
PERF INIT - Input performance info such as
- wind
- temp @ altitude
- ZFW
- reserves required
- transition altitude
(or “request” data from ACARS) and TAKEOFF
FMS setup process
- TAKEOFF
On TAKEOFF screen, OAT is displayed (can be overwritten, often to higher temp to reduce thrust and $ - called FLEX takeoff).
QRH (quick reference handbook) speeds (V1, V2, VR) displayed, which can be replaced with ACARS data if required.
FMS setup
- What if fuel amount entered is wrong (e.g. more fuel added)?
FMS takes feeds from fuel data so will update calculations based on the actual fuel amounts. May display an error around difference of true fuel from the amount entered in FMS.
Map shift
When the map jumps on the navigation screen due to an update in position fixing data in the FMS
Windspeed used by FMS
Default is to use current windspeed for the current nav leg and the input cruise windspeed for remaining legs (for ETA purposes).
Can input forecast windspeeds for future legs to improve ETA.
FMS - Overfly
Default is to turn before reaching waypoints, but can set a waypoint to overfly in which case turn only commences overhead the waypoint.
NOTE: Activated on a waypoint by waypoint basis, not for the whole route!
FMS - Lateral offset
Ability of some FMS to allow a path to be flown a set number of miles to the left or right of the set route
FMS - Cruise speed options
ECON CRUISE - Best range speed, modified by the cost index (default)
Long Range Cruise (LRC) - 4% faster than still air best range speed, but with 99% of the range - used when no cost index available (e.g. diversion)
Required Time of Arrival (RTA) - set RTA to a given waypoint
FMS
- What configuration changes can be accounted for in fuel planning?
Normal configuration or
SINGLE ENGINE
Nothing else, so fuel calculations will be wrong for landing gear stuck down etc.
TIME planning not affected (based on ground speed etc.)
FMS - Consideration on approach
Raw data from navaids should be checked on arrival as FMS’s blended position may not be optimal by this point in the flight. Recommended to check it against verifiable data such as ILS.
FMS - APPROACH page
Approach page gives details of target threshold speed (Vref) for different flap settings, along with flap setting recommended for selected approach and reference for that approach.
Single FMC Dual MCDU
Can have a single Flight Management Computer with 2 Multifunction Control Display Units, instead of 2 entirely separate FMS. May need to take care to only enter info into one MCDU at a time.
Dual FMC modes
Dual mode (aka Master/Slave) (default) - One is designated to take input and shares data with the other. They communicate and compare data, each controlling its own autothrottle (etc.) using CROSS-TALK BUS.
Independent - Allow different functions to be used on the different units at the same time. No cross checking.
Single - One FMC has failed and the other feeds to both pilots.
Dual FMC system if one of two CDU fails
Single remaining CDU can be used to input into either of the two FMCs
EFIS components
Displays
- Electronic Attitude Director Indicator (EADI) [or Primary Flight Display (PFD)]
- Electronic Horizontal Situation Indicator (EHSI) [or Navigation Display (ND)]
EFIS Control Panel
Symbol Generator
Remote Light Sensor
EFIS control panel items
- Pressure setting
- Navigation Display mode selector
- Navigation Display range
- Flight director/LS display selector
- ADF/VOR select switches
- Other data display buttons
Symbol Generator
Generates graphical outputs for displays, so sits between the displays and all other inputs and computers/systems.
Have one for each pilot. If one fails, can use a third backup one or drive both pilots displays from a single symbol generator (but both will see the exact same thing).
Primary Flight Display (PFD)
Or EADI (Electronic Attitude Director Indictor) brings together ALL instruments from the T layout (plus slip ball!).
VSI on the right with a white arrow to current VS.
DI on the bottom with heading bug and track.
Useful limits (such as speed targets, pitch limits, glidescope deviation) will be overlaid.
PFD
- Decision height & radio height
Below 2,500ft radio altimeter data available. Will be displayed as well as the decision height. Different output for boeing and airbus but both have magenta flashing DH as it is approached (along with audio warnings).
PFD
- pitch limits
Yellow “eyebrows” at high part of attitude indicator display which show pitch limit, when you’ll get light stick shake. This is the pitch that should be targeted on wind shear go-around or GPWS response.
Flight Mode Annunciator (FMA)
Summary at top of PFD indicating the flight mode active (e.g. FMS autopiloting VNAV? LNAV?)
When are Flight Mode Annunciator (FMA) changes accompanied by aural warning?
Only when the change could lead to an undesirable aircraft state, otherwise just the FMA change and perhaps visual warning somewhere else.
Rising runway
A green runway symbol on the attitude indicator of PFD, representing the last 200ft of radio altimeter height.
Requires radio altitude <2,500ft with valid radio altitude data provided and a valid ILS localiser frequency selected.
Navigation Display (ND)
- Magnetic or True North orientation?
Default is magnetic between around 60 or 65 S to 73N, true outside that band.
However it can be manually overridden in general.
Navigation Display - description
Mode selectable FLIGHT PROGRESS display
ND
- speed info displayed
GS
TAS
Wind direction/Speed
In other words - the dead reckoning triangle!
ND
- Full or expanded modes
Full mode (rose in airbus) shows the entire compass circle like a conventional HSI, relevant for certain displays such as navaids.
Expanded mode (arc in airbus) zooms in on the top half of the rose to provide more info about the area ahead.
ND
- Deviation from target per dot in VOR and ILS modes
2 dots either side of target
VOR: 5 degrees each dot (20 degrees covered in total)
ILS: 1.25 degrees each dot (5 degrees covered in total)
ND
- Nav mode
Displays heading info along with information about the desired track (i.e. planned route)
ND
- Display of weather radar info in modes
Boeing: Only expanded, not full modes.
Airbus: Both rose and arc modes.
NEITHER include it in plan mode.
ND
- Map mode
Map type view with aircraft position and route as magenta line connecting waypoints.
Aircraft either at bottom of screen (expanded mode) or CENTRE MAP mode (full mode).
Also shows weather radar and navaids (blue or green if tuned).
ND
- Plan mode
An expanded compass arc appears at the top to show heading but this is NOT connected to the data below, which is a TRUE north oriented plan of the route. Simple plan of the route, presumably for browsing route before takeoff, where map mode is more like looking at an interactive map during flight.
ND
- Weather radar colouring
In mm/hour
Black: 0-1
Green: 1-4
Yellow: 4-12
Red: 12-50
Magenta: 50+ (Or doppler turbulence in TURB mode)
ND
- wind display
Arrow which will be oriented with the heading (plus speed indication), so showing relative wind direction.
EFIS amber items
Cautions or abnormal sources
Items of concern appear where they normally would but coloured yellow/amber to advise of abnormal source or problem.
Boeing special EFIS failure warning
Altimeter on failed side is blanked and replaced with a fail flag until the display is switched to working info
Airbus ND screens
- ROSE LS (Landing system for ILS, GLS, MLS)
- ROSE VOR
- ROSE NAV
- ARC
- PLAN
Airbus ND colour differences
Route and waypoints coloured in green instead of magenta
EICAS and ECAM
Boeing: Engine Indicating & Crew Alerting System (EICAS)
Airbus: Electronic Centralised Aircraft Monitoring (ECAM) [E NOT = ENGINE!]
EICAS/ECAM description
Both consist of two screens.
Top one is likely to be higher importance.
Top: Engine/warning display
Bottom: System/status display [including advisories to crew, information about status problems]
EICAS modes
Operational: Normal flight crew mode
Status: Displays data around dispatch readiness on the lower screen, flight crew mode.
Maintenance
What does following ECAM/EICAS procedures mean?
Following an electronic checklist. Need to be aware that actions can be irreversible and cause damage to aircraft. Don’t just blindly follow the list without thinking about consequences (e.g. shutting down engine).
ACARS
AirCraft Addressing and Reporting System
Early datalink system covering items like:
- OOOI times (Out of gate, Off ground, On ground, Into gate)
- Load sheet & passenger routing data
- METARs, TAFs
- D-ATIS
- Maintenance Reports
- Text messages
Controller Pilot Data Link Communication (CPDLC)
Uses ACARS infrastructure (or other methods) to provide data communication between pilot and ATC.
ATC instructions can be issued via data and voice communication cut out entirely.
How CPDLC data is passed
ATC or airline communicate with a network like ACARS or more recently the Aeronautical Telecommunications Network (ATN) which finds the best data route to aircraft via VHF (cheap but LOS only), HF (available in polar regions but ionosphere issues) or SATCOM (FASTEST, expensive, not in polar regions)
CPDLC aircraft equipment
Collectively called Air Traffic Service Unit (ATSU), not to be confused!
Communications Management Unit (CMU) deals with messages in and out, displayed on the MCDU (Boeing) or dedicated Data Communications Display Unit (DCDU, Airbus), visual & aural warnings for messages and a printer.
Controls HMI (Human Machine Interface)
CPDLC in use
Connect to 2 units at a time (only one active). Connect by sending a CONTRACT MESSAGE with 4 letter ATU code.
Messages can then be sent which can be uploaded to FMS (e.g. clearance changes).
Note: Will NOT be used for takeoff or landing clearances
ADS (B and C)
Automatic Dependent Surveillance
ADS-B (Broadcast) is transponder system where aircraft broadcasts Mode S data that can replace radar control more cheaply.
ADS-C (Contract) uses ACARS for ATC to interrogate aircraft FMS.
ADS-C contents
- default (4)
- on request
Default is lat/long, altitude, timestamp and figure of merit (FOM - accuracy).
On-request data can include weather, predicted future positions, intended route, FMS projected levels.
Datalink mayday
Sending CPDLC mayday message puts it and ADS-C into emergency mode (high reporting rate).
Or can just directly put ADS-C into emergency mode, but less likely.
Satellite ADS-B
Generally ADS-B broadcasts on 1080MHz but has LOS limitations. There is now a satellite network available to replace this however.
ADS-C Contract types (4)
- Periodic
- On demand
- On event
- Emergency
Airbus vs boeing automation
Airbus has a hard “envelope”, automation will prevent aircraft going outside acceptable parameters (e.g. speed down to stall speed, pulling >2.5g).
Boeing may have “soft” envelope, but pilot has more discretion to take more drastic action in emergency situation.
[Note: Referred to as different “last operator”, with pilot being last operator for boeing, aircraft for airbus]
Effect of temperature (internal and external) on cabin humidity
Very cold air outside aircraft is the cause of low humidity.
Warmer air inside the aircraft would decrease relative humidity however.
AFCS & APFDS
Autopilot Flight Control System
AutoPilot Flight Direction System
AFCS axis control types
Single: Roll only
2 axis: Roll & pitch
3 axis: Roll, pitch & yaw
Power requires additional subsytems
3 axis AFCS modes
- Heading, altitude & vertical speed - capture and hold
- IAS or mach hold
- Coupling to VOR track or ILS localiser & glidepath
- Coupling to FMS horizontal & vertical profiles
- Autoland (optional)
Autopilot elements (6)
- Mode display
- Computers
- Sensors
- Comparators
- Amplifiers
- Servo actuators
Autopilot regulatory requirements
No requirement for fitting, but certain operations can’t be carried out without one.
e.g. Reduced Vertical Separation Minima (RVSM), ETOPS, CAT II, mode approaches.
Only mandatory requirements is no single pilot IFR without a 2-axis autopilot with altitude and heading hold.
AFCS stated functions (2)
1) Stability - control of the aircraft around its CoG
2) Guidance of CoG along a determined flight path
Auto-pilot
- Inner vs outer loops
Inner feedback loop is a simple feedback look from the servo motor back to the controller, thus ensuring control surfaces are moved correctly. Used for stability control.
Outer feedback senses the aircraft behaviour as a result of the control surface movements and feeds this back to the controller. Used for guidance.
Auto-pilot
- Gain variation
Auto-pilot model knows that lower speed (for example) will require bigger control deviations to achieve the same result.
Relates to control/stability balance. High level of gain gives high level of control but instability (oscillation).
Auto-pilot
- Gain adaptation
A computer reference model understands how the aircraft should behave. The model is adjusted based on feedback, the actual response of the aircraft to control surface changes.
Auto-pilot
- Mixed mode
- Mode to look out for
Mix between manual and automatic control for special phases of light.
For example pilot controlling pitch and autothrottle speed (thus vertical nav in mixed control).
Could also refer in exam to takeoff mode to avoid TAILSTRIKE.
Auto-pilot phases (3)
Initial: Attitude changed to obtain new trajectory
Capture: Aircraft follows “pre-determined rate of change of trajectory” to achieve the parameter
Tracking/hold: Parameter maintained
Flight envelope protection items
- Basic
- Fly by wire
Basic: Overspeed and stall protection
Fly by wire:
- Pitch attitude
- Bank angle
- Excessive roll or pitch rate
- Excessive G force
- Flap or undercarriage speed limits
Flight Envelope Protection main inputs (3)
- Airspeed
- AoA
- Aircraft Configuration
Fly by wire aircraft with sidesticks - how does it respond to impending stall
TOGA thrust
NOT stick shaker
Overriding overspeed protection
May be necessary for flight safety.
In Airbus pushing constant nose down pitch will allow speed higher than threshold.
Alternatively there could be an override button or switch.
Auto-trim
- Description
- What happens if it fails?
Operates during auto-pilot control to prevent “snatch” when disengaged and reduce servo motor load. Consists of a screw-jack in the horizontal stabiliser.
If it fails in flight auto-pilot will disconnect immediately.
Flare auto-trim
Auto-trim in autopilot mode is slow and behind pitch movement in the flare during autoland. So pitch trim is rolled aft before the flare and stick force held against it in anticipation, to prevent excessive back pitch required during the flare.
Mach trim
Active whether auto-pilot is on or not. A separate actuator initiates elevator movements to oppose mack tuck. Or it could be part of auto-trim.
Older craft used a mach strut which lengthened control runs in the trans-sonic range.
Pitch trim indicator green area
Indicates take-off setting range
Trim in fly by wire aircraft (normal operation, not autopilot)
Pilot inputs move the elevator, fly by wire system adjusts entire horizontal stabiliser to eliminate control force.
So no manual trim from pilots.
Overspeed protection - what can the aircraft do?
Initiate pitch up.
NOT reduce thrust.
Yaw damping
- Description
- inputs
- For manual flight or autopilot?
- Any feedback to pedals?
Activates when “yawing rate is NOT constant” to prevent oscillation or dutch roll, using a RATE Gyro and/or IRS.
Used in manual flight and autopilot.
NO feedback to rudder pedals
Dutch roll filter
Designed to detect the difference between a command input and a yaw due to dutch roll or disturbance
Self-induced oscillation
- which axes
Unwanted pitch and roll introduced by autopilot.
NOT related to yaw or dutch roll.
AFCS protection systems
- interlock
- torque limiters
- synchronisation
Interlocks: Electrical switches in series preventing activation if (e.g.) electric issues, roll control knob not centralised, fault in altitude reference unit
Torque limiters: Limit torque the servo motors are capable of applying (use spring-loaded coupling & friction clutch)
Synchronisation: Autopilot follow through on manual movements to prevent snatching on engagement (not auto-trim which prevents snatch on disengagement), autopilot won’t turn on if this is broken!
Disengage autopilot
- warnings
If disengaging with button (control column or central bar/button) get a warning tone, push again to cancel.
If disengaging through other means get a more insistent warning of unintended disengagement.
Disengage autopilot
- methods
- Takeover button on sidestick
- A/P button on FCU/glare shield
- Push (hard) on sidestick
- Move trim wheel (too much)
- Engage another AP (except APP mode)
- Engagement condition lost
- Failure in IRS
- Pressing TOGA switch
- Hydraulic failure
[NOT moving thrust lever, autothrottle is separate, NOT dropping landing gear or overshooting localiser]
FCU
MCP
FMGC
Flight Control Unit
Mode Control Panel
Flight Management Guidance Computer
FCU/MCP located on glare shield, allow control of the FMGC (auto-pilot computer)
Having more than 1 autopilot engaged at a time
Can only have 1 autopilot engaged at a time (except approach mode - 2 or 3 are ENGAGED).
Autopilot control laws priorities
Aim to achieve the flight path or stability desired, but above all to ensure flight limitations are not breached (speed, load factor, pitch & bank limits).
Autopilot
- V/S mode
V/S has a roll wheel to control vertical speed (direction of the roll wheel consistent with trim wheel for pitching nose). Can have it with SPEED mode so autothrottle controls thrust to achieve IAS, or manually controlling IAS.
Airbus also has a flight path angle mode (FPA) in degrees.
Autopilot
- ALTITUDE mode
- Modes to control vertical position and speed
Usually set an altitude target in combination with a V/S mode setting. ALT HOLD will be engaged once altitude is reached.
Alternatively use VNAV or LVL CHG to change altitude, which would both also control thrust and speed.
Autopilot
- ALT NTV and ALT HOLD
ALT NTV (altitude intervention) cancels VNAV profile and continues climb/descent to the altitude target input.
Selectin ALT HOLD will hold the current altitude (autopilot 1 takes from captains altimeter, 2 from FO). If QNH is set marks an altitude, if 1013 follows FL. Subsequent altimeter setting changes doesn’t impact autopilot.
Autopilot
- Which modes does maximum bank affect?
HDG SEL (set heading) and VOR modes will be controlled by maximum bank, but not LNAV.
Autopilot
- VOR tracking
Captain or FO set a VOR radial (mode on PFD in white, to show armed) and use HDG SEL or LNAV to get to it. VOR mode engaged when it is reached and autopilot follows it (mode on PFD goes green).
Overhead VOR will disengage and maintain heading until it can pick up “from” bearing on the other side.
Autopilot
- Control wheel steering
May be referred to as “hold”, e.g. “hold wings level” (or pitch angle, attitude).
Rarely used function where autopilot engaged with no modes, basically manual flying but servo motors hold control positions if you let go of control stick.
Defined as ability of pilot to make INPUTS TO AUTOPILOT via the control wheel.
Autopilot
- Touch Control Steering
(airbus name)
Optional mode.
With the TCS button held the controls are manual, servo motors disengaged. When the button is released the autopilot re-engages on previous mode.
[Airbus: PRIORITY button]
Autopilot
- Targets in takeoff mode
Pitch: for a given attitude, usually for a reference speed such as V2
Roll: to maintain runway centreline, perhaps with localiser signal
Autothrottle modes
- Take-off (/ground roll)
- Climb
- Climb with v/s
- Cruise (/alt hold)
- Approach (i.e. glide)
[Thrust settings possible]
Take-off: N1 hold
Climb: N1 hold (AP controls speed with pitch)
Climb with v/s: IAS hold (AP controls v/s with pitch)
Cruise: IAS/Mach hold
Approach: IAS hold
[Thrust full (N1 hold) or idle]
Autothrottle flight envelope protection parameters
[Name of mode]
Minimum flight speed
Maximum flight speed
NOT Takeoff thrust or any other thrust limit
[Called reversion mode]
Autothrottle hazards
- can engine parameters be exceeded?
Autothrottle takes feeds from engine systems and cannot exceed engine limitations.
Main hazard is not being able to achieve the speed set due to being outside performance envelope.
Boeing Autothrottle features (2)
1) Autothrottle must be selected before takeoff, takes over on pressing TO/GA (take-off/go around).
2) If Autothrottle is armed it will engage at high alpha for stall protection
Autoland
- Intercept localiser (lateral path) or glidescope first?
Always intercept localiser first, otherwise you’ll commence descending on wrong heading and obstacle clearance isn’t guaranteed.
Risk of starting ILS approach too high
“False lobes” are false glidescopes reflected above the true one (e.g. at 6 deg, 9 deg,… for a 3 deg glidescope) so a false glidescope could be followed if it is selected too high.
Autoland
- ILS Category altitudes
Cat I: 200ft
Cat II: 100ft
Cat III: Touchdown onwards
[NOTE: These are ILS categories, NOT just for autoland - can carry out manual ILS landings on these categories]
Autopilot
- Lateral guidance
This refers to control modes to do with heading/roll/horizontal plane.
So localiser/VOR intercept/track, track hold, FMS LNAV
but NOT speed hold.
Autoland
- Fail operational vs fail passive
Fail operational (AKA FAIL SAFE) means the plane can continue autoland in the event of a single failure below alert level.
Fail passive means crew must take over in the case of a single failure.
Above alert level crew go-around or crew takeover should be carried out for BOTH types.
Fail passive
- Expected trim and flight path situation on failure
There will NOT be a significant out of trim condition, or deviation of flight path or attitude.
Only effect is auto-land will not be completed.
Autoland
- How fail operational is achieved
Either with 3 autopilots working together, which can vote out one which is in error, or 2 autopilots and:
External monitoring system which can identify and disconnect a failed autopilot or
If a primary fail passive has a secondary independent guidance system such as a HUD (called hybrid system).
Autoland
- Alert height
Often around 200ft, above this height an alert gives pilots time to react and (for example) modify decision height.
After this point only a go around is possible, and may momentarily touch down. Only critical failures would lead to an alert [AURAL OR VISUAL!], lesser failures would allow fail passive landing to be completed.
Action in case of ILS signal loss warning below alert height
If visual approach is possible, cancel the warning and land manually. Otherwise go-around.
Note: Critical warnings such as ILS signal loss are not disabled below alert height (unless backed up by a working ILS signal) as they require a go around (unless visual).
Autoland
- ILS frequencies
VHF (metric) for localiser
UHF (decimetric) for glidescope
Just need to tune VHF, UHF channel is paired.
Autoland
- Requirements for ILS categories
Runway equipment (e.g. ability to ensure runway is clear) and crew training necessary for category.
For aircraft, CAT I can be done with automated aileron and elevator.
CAT II requires rudder actuators to line up with runway due to crosswind
CAT III requires ability to flare and rollout capability (nose wheel steering on landing).
Autoland
- Process (initial stages)
Tune an ILS frequency to enable APP mode to be armed (& tune 2nd nav for a dual channel approach).
Arm APP mode and engage 2nd autopilot to intercept (HDG SEL or LNAV).
When localiser intercepted VOR/LOC mode engages (goes green) and autopilot turns onto localiser, G/S is armed.
G/S activated when glidescope intercepted (from below).
2nd autopilot armed at 1500ft RA (must engage by 800ft or failure), FLARE mode armed.
Autoland
- Process (final stages)
400ft, 300, 200, 50, 25, 5, 0
400ft RA: Auto-trim
300ft RA: sensitivity to glidescope deviations muted to prevent pitch OSCILLATION
200ft RA: Aircraft lined up with runway centreline with aileron/RUDDER to control drift
50ft RA: FLARE mode takes over pitch, 2ft per second descent
25ft RA: THROTTLE close
5ft RA: GA inhibited. Touchdown mode to decrease pitch attitude.
0ft: rollout
Autoland
- Autothrottle after landing
Pilot manually selecting reverse thrust cancels autothrottle.
After 2 seconds with no reverse thrust, autothrottle disengages automatically.
Autoland
- GA mode
(what does AP control, what does pilot control)
Go around mode is armed at 1500ft in autoland. Pushing TO/GA button causes +15 degree pitch attitude and full thrust.
At 2000fpm climb thrust is dropped to maintain speed.
Pilot needs to retract gear and flap.
Autoland
- Back course
Allows localiser component of autoland to be utilised in the wrong direction of an ILS approach runway if it is only set up in one direction
Autoland
- 3 channel display
Boeing 3 channel display has a 2 output autoland display which says “LAND 2” if 2nd autoland ok, “NO LAND 3” if 3rd autoland failed, “NO AUTOLAND” if autoland not available.
Autoland
- Can it be used if random piece of equipment is non-functional?
Even if an unrelated piece of equipment is non-functional, need to check autoland is still approved according to airline’s manuals and MEL.
e.g. co-pilot heated windscreen - can’t assume ok to autoland even though it isn’t related
Command vs demand
Command is issued by autopilot, telling the control surfaces what to do.
Demand is issued by flight director, advising the pilot what he needs to do.
Colours of flight director markings
Yellow on analogue ADI instruments.
Magenta on EFIS systems (i.e. PFD)
Which flight parameters does flight director cover?
Only pitch and roll. EFIS system may have other “magenta” target parameters (speed, deflection from ILS/VOR, glidescope departure) but these are other systems (e.g. autothrottle), not part of flight director.
What does it mean if flight director is centred?
It means the current bank angle is as demanded by the flight director.
In other words, moving towards an off-centre command bar will move the command bar gradually towards the centre, NOT move the plane indicator over to the side!
Typical dual flight director operation
As with autopilot, default mode is master/slave. Independent operation is rarely used.
Note: It is possible for both flight director and autopilot to be active (autopilot will be following the flight director, very closely!)
Indication of whether flight director or autopilot is active
Green text at top of PFD (below the main annunciator section) saying CMD for autopilot or FD for flight director.
NOTE: Both can be active, in which case flight director is “giving guidance as a backup”, autopilot will be matching flight director exactly
What to do in case of doubt around flight director instructions
Switch it off if there is doubt about its accuracy.
e.g. if it contradicts with ILS, trust your understanding of ILS
Boeing & Airbus alerting systems
Boeing: Master Caution
Airbus: Flight Warning System
3 levels of alerting under CS-25
Warnings: Require immediate recognition and corrective action IS required. Red colour code [or MAY be required IMMEDIATELY?]. 2 senses (visual, aural, tactile).
Cautions: Require immediate AWARENESS, correction action WILL be required. Amber colour. (2 senses)
Advisories: Require awareness and corrective action MAY be required. Any colour except red or green (yellow or white popular). No aural warning required.
Suppression of warnings & cautions
Often inhibited at start up and critical stages of flight (e.g. from passing 80kt on take off up to 400ft height).
Stall warner operation
- when it must activate
Speed at which stall warner is activated (V(SW)) must be at least 5kt or 5% of CAS above stall speed. Must continue until angle of attack is below where it was at V(SW) point.
Inputs to stall warning module
Alpha indicator
CAS (air data computer)
Flaps/slats
A/G relay
Thrust settings
[NOTE: These are ALL considered REQUIRED, not optional]
Outputs of stall warning module
Stick shaker, audible warnings, visual warnings. If there is a stick pusher (especially likely if aircraft is vulnerable to deep stall) it will be initiated by the stall warning module.
Stick pusher inhibition
Can be manually over-ridden in case of error.
Will also be automatically inhibited in phases of flight (e.g. flare) where it would be dangerous.
Fly-by-wire stall protection levels
Alpha V(LS): 1.23 x stall speed, maximum approach alpha
Alpha prot: Autopilot won’t increase alpha beyond this point even if descending, pilot can do so manually
Alpha floor: TO/GA thrust applied
Alpha max: Maximum performance alpha (just below C(Lmax)), used with TO/GA thrust for terrain avoidance
What commands can stall protection carry out?
Inhibiting nose up
Increasing thrust
Fly-by-wire modes (4)
Normal
Alternate (limited protection)
Direct (pilot commands passed directly)
Mechanical backup
Autothrottle stall protection level
Speed not allowed to fall below 1.3 x V(S)
What is “stall protection”?
i.e. stick pusher, this is automation preventing the stall.
Not the same as stall warning, which is letting the pilot know of an impending stall.
Altitude alerting system
Required for aircraft with > 9 passenger seats or > 5,700kg.
Visual and auditory warning when approaching a set altitude, or deviating from it.
Radio altimeter functionality
- description
- frequency (type & numbers)
[Only below 2,500 ft]
Transmitter and receiver of radio beam in 30 degree cone. Signal varies frequency between 4200 and 4400MHz continuously and gap between frequency detected back is measured.
Frequencies used varied at different heights for accuracy.
[SHF range - centimetric]
Radio altimeter position
2 antennas, near landing gear, so needs to be calibrated to show 0ft when main wheels on the ground - accounting for cable length and residual height.
Radio altimeter accuracy
+/- 2ft in first 500ft
+/- 1.5% at higher
Features that are dependent on radio altimeter
- GPWS
- Auto-throttle (as part of autoland)
- Autoland
- Cat II or III landings
What happens if radio altimeter fails during approach (in general)?
Height indication is removed.
[Not necessarily any warnings as it isn’t necessary for visual landing]
GPWS, EGPWS & TAWS
Ground Proximity Warning System (mandatory for public transport aircraft) is vertical only.
EGPWS (Enhanced) and TAWS (Terrain Avoidance Warning System) include forward looking terrain avoidance based on a 3d database.
GPWS modes
Mode 1: Sink rate
Mode 2: Ground proximity
Mode 3: Altitude loss after TO or GA
Mode 4: Incorrect landing configuration
Mode 5: Below glidescope deviation
GPWS Alert vs Warning
Warning: More urgent, requires immediate climb
Alert: Less urgent, requires corrective response
[Only mode 5 (glidescope deviation) likely to be an alert]
GPWS
- Mode 1 vs Mode 2
- Altitude data source
Both use radio altimeter, so only work with terrain in 30 degree cone (vertical cliff not sensed).
Mode 1 (sink rate) detects excessive BAROMETRIC RoD below 2500ft (RA operating range). Caution @ 40-60 secs to impact, warning @ 20-30 secs.
Mode 2 (ground proximity) detects approaching terrain (even if barometric altitude not falling).
[Mode 2 only uses radio altitude, mode 1 uses ACD alt for RoD, radio altitude only for activation]
GPWS
- Mode 3, 4(A/B), 5 arming/disarming
Mode 3 (TO/GA) is only armed when gear is up and flaps aren’t in landing config, to prevent going off during approach.
Mode 4 (landing config) has inhibitions for gear (4A) or flaps (4B) in case of planned gear up or flapless landing.
Mode 5 (glidescope) arms when ILS is tuned and below a certain altitude, so can be inhibited in case of visual landing.
GPWS
- Alerts and warnings by mode
GPWS
- Modes 6 & 7
Non-regulatory modes
Mode 6: Additional altitude alerts (e.g. calling out heights, “RETARD” alert)
Mode 7: Windshear alerts and warnings based on various available inputs INCLUDING AoA (warning means mandatory go-around, UNLESS in VMC, clear of cloud and immediately obvious to captain there is no risk)
EGPWS/TAWS
- How it works
- 3 tools it provides
Uses position fix (needs to be accurate) and a database of terrain (more detailed near airports).
Provides:
1) Terrain clearance floor (based on 3 degree approach around NEAREST airport)
2) Terrain look ahead alerting
3) Terrain Alerting and Display (similar display to weather radar)
EGPWS/TAWS
- colour coding of terrain
Red: >2,000ft above
Yellow: 500ft below to 2,000ft above
Green: 2,000ft to 500ft below
Black (no colour): Over 2,000ft below
Blue: Water
EGPWS database expiry
- Overall database
- Obstacle database
Overall database doesn’t expire (but does get updated regularly)
Obstacle database expires every 28 days
GPWS
- Nuisance vs false warnings
- pilot reaction
Nuisance warning is genuinely triggered but not relevant. In VMC it can be ignored, but in IMC a go-around is required.
False warning is an error.
Flight warning system priority
- GPWS
- TCAS
- Stall
- Windshear
1) Stall
2) Windshear
3) GPWS
4) TCAS
ACAS/TCAS I and II
Airborne Collision Avoidance System & Traffic CAS
Effectively interchangeable
ACAS I provided traffic info based on SSR transponder info, ACAS II provides manoeuvre advice in pitching plane and is now mandatory for >5700kg craft.
TCAS information sources
“Squitters” packets of info sent out from mode S transponder equipment (ADS-B equipment sends out even more info).
Return times from mode A, C or S transponders to compute bearing and distance.
Also altitude from mode C transponders.
If both craft have mode S and TCAS II, TCAS is coordinated between them.
Equipment used by TCAS
TCAS uses the aircrafts own transponder to interrogate other transponders, so a transponder failure means TICS won’t operate normally.
Uses 4 antenna (top and bottom, mode S and directional antenna)
TCAS
- When is a warning given?
Traffic advisory: Potential collision threat
Resolution advisory: Imminent (or serious?) collision threat
TCAS
- Tau area
A small, variable volume of airspace around the aircraft where collision could occur in a set amount of time
- Traffic advisory: 48 to 35 secs
- Resolution advisory: 35 to 15 secs
[True time boundaries could differ]
TCAS
- Traffic and resolution advisory
Traffic advisory for larger Tau area simply advises of traffic: “TRAFFIC TRAFFIC”
Resolution advisory gives avoidance instructions: e.g. “CLIMB CLIMB”
[Note: Resolution advisory linked to autopilot which would carry out the required manoeuvre itself]
TCAS
- Limitations based on equipment of aircraft detected
Mode A transponders can only be identified in 2d plane, no vertical info so resolution advisory not possible.
Mode C is sufficient for resolution advisory.
TCAS
- Resolution verbal instructions
“CLIMB”, “DESCEND” or “LEVEL OFF”
“CROSSING” climb or descend if crossing altitude of the other aircraft
“ADJUST” or “INCREASE” climb or descend (adjust means decrease)
“MAINTAIN VERTICAL SPEED” or “MONITOR VERTICAL SPEED”
Changing from climb to descend adds “NOW” to the end
“CLEAR OF CONFLICT” means all clear
TCAS
- Range
Outside terminal areas: 30nm +/- 2700ft
Reduced in high traffic areas to minimum of 6nm
TCAS Inhibition
- Aural warnings
- 1450ft
- 1100ft
- 1000ft
- 500ft
- 360ft
- All aural warnings inhibited by priority warnings (stall, windshear, GPWS)
- No increase in RoD below 1450ft
- No descent below 1100ft
- No resolution advisory below 1000ft (all aircraft amber circles)
- No aural commands below 500ft
- Returns discarded below 360ft due to aircraft on the ground and below 50ft declares itself on ground to other aircraft
TCAS
- Display unit
- Which scale is used (e.g. heading scale, altitude, vertical speed, speed)?
- Visual output types (2)
- Which displays used
Commands displayed as a VSI, with coloured areas (green for target, red for danger).
2d “radar” type view shows icons for aircraft.
Either displayed on dedicated display (maybe called variometer) including both, or on EFIS - which would put VSI info on PFD and “radar” view on ND.
TCAS
- Colouring of aircraft on TCAS display
Red box is RA (resolution advisory) aircraft
Amber/yellow circle is TA (traffic advisory)
Cyan/white lozenge is proximate (no alert) traffic within 6nm and 1200ft
Cyan/White hollow lozenge is “other” outside 6nm and 1200ft
TCAS
- No-bearing advisories
If TCAS can’t get a bearing but identifies a RA or TA craft, will be displayed in text in appropriate colour at bottom of the TCAS/VSI display
TCAS
- Reaction to alerts
TA are for information only, follow ATC
For RA you should follow the guidance and ignore ATC, but inform ATC and return to guidance after “CLEAR OF CONFLICT” given
TCAS
- Deconfliction aim of TCAS
- Pilot reaction time expected
- G value expected on response
- RoC/RoD expected
300-500ft deconfliction targetted
5 second reaction time (+ 3 seconds to achieve climb/descent)
CLIMB/DESCEND RA expects 0.25g and 1500ft/min
INCREASE CLIMB/DESCEND expects 0.35g and 2500ft/min within 2.5 seconds
Does TCAS take into consideration stall speed?
NO
Ignore anything about 1.2 x or 1.3x stall speed!
Flight data recording
- CVR, DFDR, CVDR
Cockpit Voice Recorder (CVR)
Digital Flight Data Recorder (DFDR)
Cockpit Voice and Data Recorder (CVDR)
[Note: Over 5700kg need entirely independent CVR and DFDR]
CVR - what is recorded?
All audio, including:
- Aural environment in flight deck
- All audio in headsets
- Voice comms from or to the flight deck
- PA audio (from flight deck only)
- Aircraft interphone audio
- Navaid audio
CVR retention requirement
Current rules are 2 hours for >5700kg, 30 mins for less.
However some questions refer to 30 mins (previous requirement)
Permission required for voice recordings to be used other than for accident investigations
All crew must agree
DFDR - what is recorded
Standard:
Time, attitude, airspeed, pressure altitude, heading, acceleration, thrust, flap/slat configuration, spoiler/brake selection.
Aircraft over 27000kg:
Primary flight control positions, pitch trim, radio altitude, nav info displayed to pilots, cockpit warnings, landing gear position, any unique design parameters.
DFDR retention requirements
25 hours (10 hours only for <5700kg)
When DFDR records
Starts on engine start up.
Event buttons allow pilots to mark an event.
Number of consecutive flights possible with defective DFDR
8
How are the number and type of FDR parameters to be recorded by data retention devices established?
Based on:
- Recording capacity and
- Applicable operational requirements
NOT externally demanded (unlike CVR)
ACMS/AIDS
Aircraft Condition Monitoring System (Boeing)
Aircraft Integrated Data System (Airbus)
System that records advanced detail of flights for maintenance and safety useage.
e.g. fuel, pneumatic systems, APU, engines, landing gear.
Data sent to a processing unit which can record it, pass on to DFDR and be sent via ACARS.
Tachometer - what does it measure?
Measures speed of rotation - RPM
Mechanical or magnetic tachometer
Cable (up to 2m) connected directly to engine spins a magnet within an aluminium drag cup. Spinning magnet creates a turning force in the aluminium cup, opposed by a hairspring, which is proportional to rotation speed and can be measured directly on an indicator.
Electrical tachometer
- 3 types
DC: DC generator at engine sends DC voltage to be measured at instrument. But voltage loss causes errors and commutator & brushes cause sparks affecting radio.
Single phase AC: Solves spark problem, but voltage rectified to DC has the same voltage loss issue.
3 phase AC: Uses squirrel cage [DRAG CUP], synchronous motor and magnetic tachometer instead of voltmeter, measures frequency instead of voltage so voltage drop no problem.
Electronic tachometer
Turbines too fast and no gearbox so use inductive probe (requires power) to detect passage of teeth on a notched “phonic” wheel. The AC signal generated is rectified to a digital pulse which can be input to computers. Frequency measured, not voltage, so no voltage drop issues.
N1, N2, N3
- description of n1
Refer to the three spools in a single turbine engine which each can have speeds monitored.
N1 is low speed or low pressure shaft.
N1 indicator
- metric displayed
Tachometer for N1 engine spool. Calibrated in terms of % of manufacturer defined speed.
Indicator will go over 100% (likely to be a yellow or red zone).
Synchroscope
- description
- how it functions
To synchronise multi-engine rpms (usually prop but potentially turbine). One engine is the master, every other engine has a propeller symbol on the synchroscope which is stationary if synched to master, spins one way or the other if fast or slow.
Note: Works by comparing frequency difference between the engine driven alternators (i.e. 3 phase tachometers).
What is the purpose of the synchroscope?
Synchronising RPMs primarily reduces vibrations and noise.
NOT related to fuel consumption.
Vibration sensor
- high or low frequency biggest issue?
- 2 types
Used for turbines where vibration indicates a big problem. Vibration causes acceleration so high frequency is the main concern. Either piezoelectric or magnet/coil sensors send signals, which are averaged. Display shows a number with no units to indicate level of vibration.
Can vibration between N1 and N2 engine parts be discerned and displayed?
Yes
What does vibration in turbojet engine mean?
Rotor imbalance
Hydraulic torquemeter
Measured in the gearbox. Helical gears are used which create a force on the oil in the gearbox, directed towards a hydraulic piston, which measures the level of force and thus torque being generated.
Electronic torquemeter
An outer shaft is placed around the shaft connecting engine to gearbox. Only connected to engine. Thus the twisting of the main inner shaft can be measured.
Measured as a phase (not frequency which is identical!) difference between positions on the two shafts which can be measured.
Gas Turbine Thrust Indicator
We calculate Engine Pressure Ratio:
EPR = TURBINE outlet pressure / COMPRESSOR inlet pressure
[ OUT / IN]
Some engines use a series of pressure sensors, called an integrated EPR (IEPR).
Check on EPR gauge
Ice can block the compressor inlet valve so check result against N1 tachometer reading (less accurate for thrust but not affected by icing).
Fuel information monitored
- Fuel contents (volume sometimes a proxy for mass)
- Fuel flow
- Fuel temperature (avoid waxy turbine fuel)
- Filter status
Resistive fuel volume indicator
- Description
- 2 types of measurement device
Use a float connected to a variable resistor.
Measure position with ratiometer or galvanometer (wheatstone bridge). Ratiometer preferred as it isn’t impacted by voltage.
Capacitive fuel mass indicator
Measures capacitance with 3 tubes (outer is earth, inner is low potential, middle is high potential), with capacitance of tubes affected by dielectric being fuel or air. Far less affected by movement compared to float.
A reference unit in the unusable fuel (always submerged) allows changes in fuel density to be compensated for so it INDICATES MASS NOT VOLUME.
What you see on error of capacitive fuel mass indicator
Reduces to zero
Drip stick & magnetic stick
Use sticks out of underside of wings going into fuel tanks.
Drip stick has a hole in the top (inside tank) and tap at bottom. Pull down until fuel starts dripping from the tap, measure off tube position.
Magnetic stick similar but sealed, pull down until the magnetic float supports the stick.
Venturi volume flow indicator
Fuel flow passed through a narrowing (venturi) and pressure difference between the venturi and wider tube indicates flow.
Volume, not mass.
Variable orifice volume flow indicator
- Description
- Measuring density
- Safety valve
Fuel goes through a semi-circular piece with a hinged flap blocking flow. Flow pushes the flap around and increases the gap, so angle of the flap indicates flow (measured electrically).
Temperature sensitive resistors can adjust to show density instead of volume.
A safety valve will bypass the device in-case of blockage (increased pressure on inlet forces the sprung safety valve open).
Turbine volume flow indicator
Turbine immersed in fuel flow can have rpm measured in same way as turbine electronic tachometer (phonic wheel).
Described as counting a “tally of the impulses”.
Doesn’t cope with large flow rate or temp changes (measures volume, not mass) so not common now.
Mechanical mass flow indicators
Swirl the fuel in pipes and measure angular momentum.
1) Swirl with an impeller continuously, measure rotation at a reaction turbine anchored to a spring (i.e. a stator).
2) Impeller connected to a rotor by a sprung shaft, so will twist and have phase difference to the rotor, which is measured.
Electronic mass flow indicators
Fuel initially turns a hydraulic driver which rotates the shaft.
It is then straightened by a stationary straightener, passes through a drum and an impeller. The deflection between magnets on the drum and impeller record fuel flow.
Calculating fuel used
Answer is always integration of fuel flow over time, never starting fuel minus current fuel.
System Bus
- Description
- Serial vs parallel
- 3 bus types for memory
Link that allows computer systems to talk to each other and CPU, memory and input/outputs to connect.
Could be described as SERIAL or PARALLEL bus depending on whether data is transferred bit at a time, or simultaneously
Memory uses: Address bus, data bus, control bus
Software certification
- Nature
- Classification
Software needs to be certified, which is done in conjunction with hardware (not as a standalone item).
Classified by impact of failure:
A - Catastrophic - loss of ability to continue safe flight and landing, e.g. fly-by-wire system
B - Hazardous
C - Major
D - Minor
E - No effect (e.g. entertainment system)
[B/C/D increase workload but don’t prevent safe flight, e.g. FMS failure]
Software Certification Regime
EUROCAE
Multi-processing vs multi-tasking
Multiple CPUs in a single computer is multi-processing
Single CPU running multiple programs is multi-tasking
Electronic Flight Bag (EFB)
- 2 types
Collective name for electronic tools used in the cockpit, classified as either portable or installed, which can be removed without using tools.
If installed type it must be part of a minimum equipment list.
Electronic Flight Bag (EFB) software certification types
Type A: No safety effect [“A ok”] (such as manuals)
Type B: Minor safety effect (including charts, performance calculations)
Impact of data entered into EFB
Can be highly integrated with the FMS so errors in the input data (weather, weight etc) can impact take off speeds or other performance calculations directly.
Disadvantages of portable EFB (5)
Requires certification
Can’t integrate with EFIS
Needs separate wires for charging
Can’t be used in all phases of flight (would need a certified mount or secure stowage device)
May not be visible in all light conditions
2 types of Electronic Checklist (ECL)
Sensed: Uses inputs to detect which items have been done and amends checklist accordingly.
Unsensed: Just an electronic version of a paper checklist
HUD system components
- Projector
- Stowable screen/combiner lens
- HUD computer
- Declutter button (on primary controls, allows pilot to cycle through levels of info displayed)
- Brightness setting (optional)
Declutter modes
- Display declutter
- Crosswind declutter
Display declutter - gets rid of artificial runway first, then other items
Crosswind declutter - removes airspeed and altitude tapes to open up lateral view
HUD focus point
Focused at infinity, so can be viewed at same time as outside
Information on HUD
- TAWS?
- TCAS?
Replicates the PFD, NOT ND
- Attitude (3d)
- Speed (+ speed trend)
- Heading
- Flight path vector (track & vertical path)
- FMA
- Crew alerting system (CAS)
- TAWS
- Windshear commands
NOT ACAS/TCAS
What is flight path vector (FPV)?
This is your current flight path (i.e. combined vertical and lateral track). Can be displayed as a point on the PFD to show where you are headed, e.g. if in a glide, attitude is upwards bank but you are heading downwards.
Synthetic Vision System (SVS)
- description
- where is it displayed?
Synthetic cockpit view based on terrain and ground features database and augmented reality type overlays.
Shown on HUD or PFD.
Enhanced Vision System (EVS)
Displays view from infra-red cameras which shows much clearer view of the world than you can see in low visibility conditions.
Works best when temperature differences high (e.g. tarmac vs grass at different times of day) but there is a cross-over point when these will have the same temperature and EVS view will be of little help.
Do HUD, EVS or SVS decrease landing minima?
HUD might as it aids awareness and reaction speed.
EVS improves view of real world significantly so can reduce landing minima.
SVS is dependent on accuracy of the information however so doesn’t increase landing minima.
Manifold pressure guage
Used in single engine piston aircraft to measure pressure in INTAKE system near the INLET valve.
ISA pressure @ MSL in psi and Hg
14.5 psi (half of tyres 2 bar = 30psi)
29.92 Hg
Primary source of attitude data in large aircraft
INS/IRS
NOT old school gyros
RNP -X (Radio Navigation Performance) requirements
+/- Xnm lateral and longitudinal 95% of the time
Where are details of warnings displayed if no EFIS?
Central Warning System - Annunciator Panel
or Central Warning Panel
What is a take off warning?
May be used instead of take off configuration warning. Should be an audible alert at start of take off roll if configuration is incorrect.
[Note: Relates to CONFIGURATION items only (e.g. brakes, flaps, spoilers) not controls like elevators]
How to correct balance ball (with and without using rudder!)
With rudder - step on the ball.
With bank - bank in the opposite direction (e.g. ball to the right => too much left rudder => bank more to left to balance)
In a balanced turn are forces balanced through normal axis of plane or real world vertical?
Through normal axis of the plane
Process on master warning or master caution alert going off
1) Acknowledge the warning/caution
2) Silence the alarm
3) Follow SOPs
4) Inform ATC
Is TAS displayed on the PFD?
Is pressure setting display on PFD?
No, can appear on ND though, where it is more relevant
Pressure setting is shown underneath altimeter ribbon. Could be abbreviation like “STD” for 1013.
Torque definition
Force x lever arm
Questions on consequence of equipment failure
Look out for failure of “the” vs “a”. For example, failure of a radio altimeter isn’t a problem, failure of BOTH radio altimeters is!
FANS
- Initiated by whom?
- Primarily used where?
- Which communications method?
Allows more safe sharing of airspace.
Initiated by ICAO, taken up by manufacturers.
Primarily used in oceanic areas currently.
Uses satcom.
What does autobrake max setting result in?
Maximum brake force, WITH MODULATION!
ADI
Attitude DIRECTOR Indicator
Combines attitude indicator and flight director (and/or VOR/ILS radials)
NOT directional gyro
What does “unreliable airspeed” mean?
This is an incorrect airspeed indication which is NOT picked up by systems and displayed as a warning or removed from displays. Something to be concerned about and look out for.
Performance factor
A positive or negative adjustment factor, part of performance data for an aircraft in FMS. Applies adjustments to fuel flow, perhaps due to aircraft age.
Requirements for mounting of standby instruments
NO REQUIREMENTS!
Likely to be small and not easy to use in an emergency!
Non-precision approach accuracy requirement
0.3NM (RNP - 0.3)
Materials
- Temperature wire (TAT)
- Bimetal thermometer
- Thermocouple
- Hard iron
- Soft iron
Thermometer wire - Nickel (or platinum)
Bimetal thermometer - Invar & Brass
Thermocouple - Alumel & Chromel
Hard iron - Cobalt, Chromium, Tungsten
Soft iron - pure iron, silicon iron
Does TAWS need connection to 4d FMS system or 3d position sensor?
3d position sensor
Autopilot typical flight path (diagram)
