13.8 Instruments Flashcards
Instrument classification
Flight instruments
Engine instruments
Navigation instruments
Other systems
Flight instruments
Attitude
Altitude
Air speed
Direction
Ratés
Basic T
TL - air speed
TC - attitude indicator
TR - altitude
BC - heading indicator
BL - turn coordinator
BR - VSI
Engine instruments
Power plant status
Amount of power produced
Instrument Ts and Ps
Electrical system health
Nav instruments
Nav info eg VOR ADF DME
GPWS
Weather avoidance
Other systems
Status of systems anti ice etc
Pressurisation systems
Heating and air conditioning
Instrument panels are
Attached so to provide the crew with correct viewing angles
Some are shock mounted to prevent engine and air frame vibrations so they don’t obscure reading and reduce service life
Also painted to reduce glare usually Matte colours
Atmosphere composition
Nitrogen 78%
Oxygen 21%
Other gases 1%
Troposphere
Contains the weather system
Temp drops approx 2c per 1000ft
Tropopause
Between 8-18km pôle to equator
25,000-57,000ft
Temp stops dropping with increasing altitude
Stratosphere
Temperature is considered constant for civil aviation use
Between 8-18km to 50 km
Temperature units
Celsius 1-100c melting point of ice and boiling point of water
Kelvin (absolute 0) -273c - all molecules movement stops
Fahrenheit - 0f - 212f 32f = 0c 212 = 100c
Temp conversions
F= 32 + (9/5 C)
C= 5/9 (F-32)
K= C+273.15
R= F+459
Tropopause temp values
Equator - -80c
45 degree latitude - -56c
Poles -45c
Temp and aircraft performance
At a given pressure an increase in temperature results in a decrease in density
Therefore less lift for increased temps
Pressure
Pressure = force / area = mass x acceleration / area
I’m reassure
In the atmosphere the pressure is caused by the mass air acting under the force of gravity on a given area
Pressure
Force always acts at right angles to the surface that the pressure is exposed to.
Pressure
If the volume is reduced the molecules act on a smaller area thus the force exerted per area unit increases and vice versa
Pressure units
PSI
N/m2
Pascal
Millibar
hpa
1 pascal = 1 N/m2
1mb = 1hpa
Mean sea level pressure
29.92 in hg
1013hpa
Pressure altitude high to low look out below
High pressure to a low pressure without pressure compensation will mean the aircraft’s lower than indicated
Isobars
Are lines on a weather map joining together places of equal atmospheric pressures
Measure the atmospheric pressure in millibars
Eg 1004 indicates a high pressure
976 indicates a low pressure
Density altitude
Is the altitude relative to the standard atmosphere conditions at which the air density would be equal to the indicated air density at the place of observation
Ie density altitude is air density given as a height above mean sea level. Density altitude can also be considered to be the pressure altitude adjusted for non standard temperature
Absolute pressure
Gauge pressure
Differential pressure
Absolute pressure is the measure of the barometric pressure + gauge pressure
Gauge pressure reads the relative pressure above the ambient atmospheric pressure
Pabs = Pg + Pabs
Diff pressure is the difference between internal cabin pressure and external pressure for aircraft
Static pressure
Ps
Is the pressure of a gas when the gas is stationary
(Altitude)
Dynamic pressure
Ram pressure
PD
Is the component of fluid pressure that represents fluid kinetic energy
Pitot pressure
Total pressure
PT
Combines dynamic pressure and static pressure
(Air speed)
Density
Reduces with altitude
Density = mass/volume
Grams or kg per cubic metre
Pounds per cubic feet
The speed of sound
Is dependant on air temperature
Higher the air temp the higher the speed of sound
Standard sea level speed of sound
340m/s
661K
Mach number
Is the ratio of the speed of aircraft to the local sound speed and expressed as a Mach number
Mach number = aircraft speed/sound speed
Mach number
For a given airspeed will depend on altitude.
As altitude increases the Mach number also increases because of the lowering of the local sound speed
Critical Mach number
Used in aerodynamics
Is the lowest mach number at which the airflow over some point of the aircraft reaches the speed of sound but does not exceed its
Mcrit is a fixed value for any given aircraft design and configuration and is always less than 1.
Standard atmosphere SI values
Pressure at MSA - 1013.25HPA
Temperature at MSA - 15c (288K)
Density at MSA - 1.225kg/m3
Temp lapse rate (tropopause) 1.98c per 1000ft
Temperature lower stratosphere -56.5c (216.7k)
Sound speed at MSA - 340m/s (661k)
Gravity 9.81m/s
Variable resistance systems
Parameter controls the resistance when it changes so does the resistance output
Synchros
120 degree separation between phases
Voltage in the stator coils depends on the angles between the rotor coil and each stator coil.
When we turn the rotor the magnetic field in the stator also turns and the voltages in the stator coils change
Voltage reference relates to angular position
LVDTs
Change linear position information into electrical signals
Flight control surface position
Resolvers
Two stators @ 90 degree to each other
Produce a sine and cosine as the rotor is turned
Can give a angular position
Eg throttle lever position
E and I bar
Magnetic unit that is used as a error detector in systems in which the load is not required to move through large angles
RVDTs
Change angular position into electrical signals
Servo loop with DC motor
Anytime there is a difference between the two signals the motor drives the load and feedback until both signals are equal
Polarity of difference signal decides the direction of rotation
Servo loop with AC motor
More torque use AC motor
A chopper circuit makes AC from a DC signal, to drive the AC motor with this signal we need an extra amplifier. System then runs like a DC system
Two phase servo motor
The AC two phase induction motor servo motor may be very small but powerful
Stator has two fields they are represented symbolically by two coils drawn at right angles to each other
Counter clockwise rotation - variable field angle 0 degree 400hz and other field at 90 degrees
Clockwise rotation - variable signal reversed
Braked - disconnect either the variable or fixed field
Absolute pressure instruments
Aneroid capsule
Air pressure increases capsule thickness decreases
Air pressure decreases the capsule expands
Gauge pressure
Is measured from a existing barometric pressure and is the pressure that has been added to a fluid over and above atmospheric pressure
Bourdon tube
Gauge pressure r
Flattens curved Bronze tube sealed at one end and connected to a gear at the other, as the pressure increases the tube straightens which turns the gear and pointer
Used for high pressure systems like oil pressure
Bellows
Lower pressures such as instrument air pressure are measured with a bellows mechanism.
Similar to a aneroid capsule but opposite
Air pressure increase the bellows expands
Air pressure decreases bellows contracts
Sector gear drives a pointer
Differential pressure
Uses a differential bellows
Takes two pressures and indicates the difference
Strain gauges
Resistance changes as force is applied
Piezo resistive
Variable frequency signals
As a parameter increases or des read so does the frequency output
Temp measuring
Bimetallic strip - 2 strips welded together then as heated one expands and moves the pointer
Gas expansion of a bourden tubes
Temp dependent resistors
NTC - resistance decreases with increase in temp
PTC - resistance increases with increase in temp
Temp sensing bulb - resistance increases with increase in temp
Thermocouples
Chromel alumel
Compares reference junction to hot junction
Uses voltage produced from the thermocouple to determine the temp diff between the hot and cold junction
Quantity measurement DC system
Reed switches
Magnetic floats
Variable resistor
Tank unit
Current measure
Ratio meter to minimise error
Quantity measurement capacitance type
Uses a probe which uses the fluid level for the dielectric
More fluid equals higher capacitance
Ultrasonic fuel measurement
Uses ultrasonic pulses to send receive signals using time taken to reflect for measurement,
also needs to know density
Water can effect the system
Works similar to capacitance
Stall warning lift detector
Leading edge of wing detector penetrates are flow, when lift stagnates and stalls it allows the switch to make causing the stall warning alert
Stall warning stick shakers
Alerts the crew to a imminent stall by setting the shakers off and also the alerting annunciation and aurals
Some aircraft also have stick pushers
AOA vane
Alpha vane
Sends Aircraft attitude and AOA data to the stall warning computer
Vane moves a internal synchro for the position signal
Can indicate on the EFIS
Heated
Lightning strike inspection to make sure they are still free moving and not arc welded together
VFR instruments
Air speed
Altitude
Compass
IFR instruments
Airspeed
Altitude (adjustable)
Compass
Attitude indicator
Directional gyro
Rate of turn
Clock
VSI
Pitot static systems
Pressure decreases with altitude
Nonlinear
1Hpa per 28ft
Small aircraft pitot statics
Pitot goes straight to the airspeed indicator
Two flush static ports and sometimes takes internal cockpit pressure on a unpressurised fuselage
Large aircraft pitot statics
More complex system
Uses CAPT, FO and Aux ports
Air data modules used
Pitot tubes
Ram air pressure
Electrically heated
Static port
Uses static air for sensing
Positioned so that the airflow is not disturbed around the port
Position error is compensated by fitting ports to both sides of the aircraft to statically balance
In event of yaw side slip etc
Static air temperature
Temperature of the Undisturbed air around the aircraft.
TAT temp + M number to calculate
Used to calculate true airspeed
TAT
Used for engine power settings
Compressed air temperature
Difference between SAT and TAT is the ram rise
Ram rise is negligible below M.2
Over Mach .2 air speed increase the temp is higher than still air temperature due to kinetic heating and adiabatic heating
Altimeters
Uses the principle of a barometer with gears and pointers attached
Indicates in feet
Aneroid barometer reads altitudes using static air pressure
Some have adjustable Barosettings to adjust for local pressure altitudes
QNH local air pressure at mean sea level
QFE height aviver ground at the airfield
Drum type altimeters
Uses a stack of bellows to drive the pointers
Sensitive altimeters
Uses a minimum of two aneroid capsules to increase accuracy
QFE / standard
Takes mean sea level pressure 29.92” or 1013 mb usually set at 10,000ft for aircraft separation
QFE
Indicates 0 on the altimeter whilst on the ground at the airfield
QNH
corrected pressure for sea level
Altimeters
Large needle 1000ft
Small fat needle 10,000ft
Fitted with a vibrator to reduce lag and sticking
Altitude reporting and altitude encoding
Sends altitude data to the transponder for reporting
Altitude alerting
Approaching selected altitude 900ft to go alt light on the altimeters comes on
Deviating 200ft from selected altitude sets off the alert
RVSM
FL 290-410
Allows 1000ft separation
+/- 25ft at 5000ft
+/- 125ft at 50,000ft
Minimum equipment
Two primary altitude measurement systems
One auto altitude control system
One altitude alerting device
Transponder system
Scale error
Baro set standard and must be within limits
Hysteresis
Is essentially a lagging of the indication caused by the deflection of the metal in the diaphragm not keeping up with the pressure changes
After effect
Error shows up by the altimeter not returning to its original reading after the hysteresis test
Friction
Non servo altimeters test how much friction is needed to keep the instrument reading accurately
Case leak
Performed at 18,000ft pressure to be sure it does not leak more than 100ft in one minute
Barametric scale error
Test determines that the movement of the barometric scale has the proper effect on the pointers
VSI
It is vented to the inside of the case through a diffuser which is a calibrated leak. (Altimeter sealed or evacuated)
Aircraft climbs pressure inside the capsule begins to decrease to a value below the inside of the case and the capsule compresses causing the gears and pointers to indicate
Air speed indicator
Principle of operation formula
PT= 1/2pV2 + PS
ASI
Is a differential pressure gauge that measures the difference between the pitot and static pressure.
In a airtight case in which a thin metal capsule is mounted PT taken into the capsule and internal case uses PS
The capsule expands in proportion to the difference between PT & PS which then drives gears and pointers
Square law compensating
Diff pressure varies with the square of speed
Uses a tuning spring to compensation
1 Knot =
1.15 miles
1.8km
Indicated Airspeed
Is the indicated pitot static airspeed without any compensation or error correction
Calibrated airspeed
Is indicated airspeed corrected for instrument errors position error and installation error
Equivalent air speed
The speed at sea level
TAS
True airspeed differs from equivalent air speed because it hr air speed indicators are calibrated at seal level ISA conditions
Corrected for density
Ground speed GS
Ground speed is the true airspeed corrected with wind speed and represents the speed of an aircraft relative to the ground
Mach number
TAS / local speed of sound
Which changes with altitude
Over speed warning
Uses Vmo and MMO for over speed warning
Air data computer
Used in place of direct reading indicators (EFIS)
Pitot static computers
Gyroscopes
Accelerometers
TAT
Air data modules take in analogue raw data then convert it to arinc and send it to relèvent sources
Gyro scoping instruments
Spins and has two characteristics
Rigidity (position)
Directional/horizon/ position gyros
Provides stable reference for direction and attitude measurement
Precision (rate)
Turn and slip / turn coordinators
Force applied is effected at 90 degrees to the force
The amount of precision is equal to the amount of force applied
Types of Gyros
Vertical gyro 2degrees of freedom
Angular displacement from vertical direction
Artificial horizon / attitude reference / weather radar
Directional gyro 2D of freedom
Sensing angular displacement of horizontal direction azimuth heading
Compass / heading
Rate gyro 1d of freedom
Senses Aircraft angular rate of all 3 axis
Turn & slip / turn coordination
Rate integrating gyro 1d of freedom
Platform stabilisation for INS
Sensing the Integral of aircraft angular rate
Erection of vertical gyros
Air
Electric motors
Spinning balls
Directional gyro slaving
Directional gyro must be set to agree with the magnetic compass and it too must be checked periodically
At least every 15 mins to make sure it hasn’t drifted our agreement with the compass
HSI
Heading indicator combined with VOR/ILS DISPLAY
Usually under the artificial horizon
VOR indicator left/right to from indicators
Gyro wander
Any deviation of the gyro spin axis from its set direction is known as gyro wander (drift)
Real wander
Any physical deviation of the gyro spin axis
Assy metrical bearing friction, unpredictable or able to compensate for
Apparent wander
Gyro spin axis does not physically wander away from its preset direction but to an observer it will appear to change direction
Because the gyro maintains its direction with respect to a fixed point in space
360 d a day
15 degrees an hour
Gyro drift
Directional gyro drift
Earth rate apparent drift 15d an hour x sin latitude
Vertical gyro drift
Earth rate apparent drift 15d an hour x cos latitude
Altitude gyro
Mounted in a double gimbal and has freedom about 2 axis
Rate gyro
Single gimbal freedom about one axis
Attitude and heading reference system
Replaces mechanical gyros
With sensors on the 3 axis and linked to the EFIS
Must be connected to a magnetometer
Can be combined with air data
ADAHRS
Turn and slip indicators
2 minute 3 degree second
4 minute 1.5 degree second
2 min turn standard for light aircraft
4 min turn standard for heavy aircraft
Turn coordinated
Similar to a turn and slip but gimbal axis is tilted 30 degree so it will read when the aircraft rolls and yaws
Gyro instrument pneumatic types
Some aircraft you pneumatic gyro systems
Electrical or Venturi systems
Wet type vacuum pumps
Uses oil in the air system via a metered pump and is discharged with the air
Low altitude
Steel vanes in a steel housing
Dry air pump
Carbon vanes remove the friction of steel on steel
High altitude
Gyro electric motor system
Speed is between 6000 and 20,000 rpm
Advantages of air driven gyros
Cheap
Easy to maintain
Operate without power
Higher rigidity is possible
Operating RPM is more consistant
Performance is not affected by altitude
I formation transmitted to other systems
More freedom about the axis
Instrument case completely sealed
Air gyro disadvantages
Requires being at operating speed for full rigidity
Rotor speed depends on mass flow
Ingested dirt or moisture will create corrosion and bearing wear
Requires a air tight system to operate
Direct reading compasses
To compensate for inclination (tilting) the compass float is weighted on the side nearest the equator
Variation is the difference from a geographical pole to the magnetic poles they are compensated for on aeronautical maps
Deviation caused by the magnetic influences of the compass mounted on the aircraft. A compass swing is performed to minimise deviation and residual deviation is compensated for by use of a deviation card
Soft (earths mag field acting on a material) and hard (permanent) magnetism
Total deviation = A + B sin heading + C cos heading
EASA compass regulations
Deviation card must be near the instrument and show deviation of mag heading no more than 45 d increments
Compass after compensation must not greater than 10 degree deviation
Distance between the compass and any interference shall be so that it doesn’t cause more than 1degres of deviation
Any flight control or undercarriage movement will not cause more than 1degree movement
The effect of the aircraft permanent and induced magnetism as given by coefficients B & C together with any associated soft iron components shall not exceed
A) after correction the greatest deviation on any heading shall be 3D for direct reading compasses and 1D for remote indicating compasses
B) emergency standby compasses and non mandatory compasses need not fully comply with EASA regs but evidence of satisfactory installation is required
Compass swing should be carried out
On aircraft acceptance
New compass fitted
Periodically every 3 months
Major inspection
Following a change in magnetic material of the jet
If aircraft is moved to a new airfield
Following a lightning strike or heavy static
If stood on the same heading for more than 4 weeks
When carrying ferrous freight
Iaw MPD
When compass deviation is suspected
Compensation
Mechanical
Electrical
Remote reading compass
Uses a flux valve in the wing tip or tail which uses earths flux lines to give readings
With a directional gyro
Manual synchronising knob
FDR
Required with MTOW greater than 5700kg and 9 pax
Must retain at least the last 25 hours
10 hours is MTOW is less than 5700kg
Records:
Altitude
Airspeed
Heading
Attitude pitch and roll
Acceleration
Thrust/power settings
Config of lift drag devices
Radio transmission keying
Use of AFCS
AOA
Air temperature
Aircraft MTOW greater than 27,000kg also has to record
Primary flight control positions
Pitch trim
Primary nav info as per EFaiS to crew
Flight deck warnings
Landing gear position
Radio altitude
Must be recording when the aircraft is capable of moving under its own power
Engine start to 5 mins after shutdown
27000kg 32 channels
Less than 5700kg 15 channels
Preflight test switch
Bite
Under water locator beacon 37Khz lasts 30 days
Parameter input formats
Analogue
Digital
Discrete
Positioned in a place that will minimise as much as possible
Damage
Fire
Heat
Shock
Powered by hot battery bus usually
EFIS
PFD
MFD
EIS
On light aircraft sometimes the EIS is combined into the MFD
G1000
Two options
Autopilot built in
Autopilot separate
Two SD slots
Top NDB & software loading
Bottom terrain databases
MEMS technology solid state sensors to provide attitude and heading references
Magnetometer digital compass
Transponder minimum mode C
EICAS
ECAM
Engine indicating crew alerting system
Will give cautions warnings memos status
Electronic centralised aircraft monitoring will give a fault and resolution and serves primary systems
ECAM
Level 1-3 failure
Level 1 - failure to system that degrades it (amber light)
Level 2 - system failure but no direct consequence to flight safety (amber light single chime)
Level 3 - over speed, fire, stall (red warning repetitive chime)
Data displayed in schematic checklist format
Terrain awareness and warning system
Class B - 4 functions
Foward looking terrain avoidance looks forward and down
Premature decent alert
Attention alerts
Class A - 5 functions
Terrain awareness display
GPWS
Mode 1 - excessive decent rate 2450 ra and below
Mode 2 - terrain rising rapidly
Mode 3 - sink rate TOGA
Mode 4 - landing configuration for landing terrain closure
Mode 5 - ILS glideslope
Mode 6 - alerts minimums, DH, bank angle
Mode 7 - (optional) wind shear
EGPWS terrain picture
Solid red - warning terrain approx 30s to collision
Solid yellow - caution terrain approx 60s to impact
50% red dots - more than 2000ft above ref altitude
50% yellow dots - 1-2000ft above ref altitude
25% yellow dots - 500-1000feet above ref altitude
25% green dots - 500-1000ft below ref altitude
12.5% green dots - 1000-2000 feet below ref altitude
Black - no close terrain
Magenta - unknown terrain
Terrain clearance floor
Modifies terrain database for recognised airfields to cancel nuisance warnings
Traffic awareness
Traffic information service - using ADS-B to transmit the traffic position information from a ground facility to the aircraft which displays it on the PFD and MFD
Traffic advisory system - this is an indépendant airborne system utilising directional antennas and a suitable mode S transponder
TCAS
Fitted to all aircraft MTOW greater than 5700kg and carrying more than 19 pax
Directional antennas
Transponder mode s
Standby instruments
ASI
Attitude indicator
Altimeter
Heading indicator
