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
GPWS
- Alerts and warnings by mode
Autopilot typical flight path (diagram)
