ATPL 022 Instrumentation cheat sheet

Resolution is the smallest change a sensor can display or resolve; accuracy is closeness to the true value; precision is repeatability of the same reading.

Random error scatters around the mean and is reduced by averaging; systematic (bias) error shifts every reading and cannot be averaged out, only calibrated out.

Servo (force-balance) sensors null the deflection with a restoring torque, giving low hysteresis and high linearity compared with direct-deflection sensors.

Bridge circuits: a single active strain-gauge arm gives low output and thermal error; four active arms give maximum sensitivity and inherent temperature compensation.

Piezoresistive pressure sensors change resistance with applied stress; capacitance sensors change gap and therefore capacitance, giving very small moving mass and good dynamic response.

A synchro transmits an angular position as three-wire AC voltages; a resolver gives two quadrature outputs and can resolve 360 degrees unambiguously.

Sensor output is typically ratiometric to its supply, so a supply-voltage error appears as a scale error in the indicated parameter.

Total temperature exceeds static temperature by the recovery factor times the dynamic temperature rise; probe recovery factor is below 1, so TAT probes indicate slightly less than true total temperature.

Sample rate must exceed twice the highest signal frequency (Nyquist) to avoid aliasing; an anti-alias filter is applied before digitising.

Pitot-static system: pitot pressure is total (stagnation) pressure; static ports supply static pressure; airspeed is derived from the difference (impact pressure).

ASI indication uses the compressible-flow relationship, so a given impact pressure corresponds to a lower CAS at high altitude than at sea level.

Altimeter is a pressure capsule referenced to static pressure, calibrated to the ISA; subscale setting aligns the instrument to the local pressure datum.

Altimeter errors: position (static source), instrument, barometric (wrong subscale), temperature (cold air gives true altitude lower than indicated) and lag/blockage.

Altimeter check: with the subscale at 1013.25 hPa at aerodrome elevation, the indication must be within tolerance of field elevation.

VSI is a differential-pressure (choked capillary) instrument; it indicates rate of change of static pressure, so it lags and reads zero in a steady climb after settling.

Machmeter computes M from impact pressure and static pressure, so it needs both pitot and static inputs; an ASI needs the same inputs but different computation.

Blocked static: ASI behaves as an altimeter, altimeter freezes, VSI reads zero; blocked pitot with drain open gives ASI falling toward zero in climb.

Air data computer outputs computed airspeed, Mach, TAT/SAT, pressure altitude, and baro-corrected altitude to displays and other systems.

TAT probe measures total (recovery) temperature; SAT is computed by removing the ram rise using Mach number from the air data computer.

Ram rise = TAT - SAT, approximately proportional to M squared; static air temperature is always lower than total air temperature in flight.

Resistance temperature detectors use a platinum element with a positive, near-linear resistance/temperature characteristic; thermocouples produce a small voltage proportional to the temperature difference between junctions.

Thermocouple cold-junction compensation is required because the output depends on the reference junction temperature, not the hot junction alone.

Ice detectors are typically vibrating-probe or magnetostrictive types that change frequency when ice accretes, then heat to shed and reset.

Ice detector activation normally switches on airframe and engine anti-ice, and may inhibit or modify autothrust and stall-warning schedules.

Pitot and static heaters are separate from airframe anti-ice; pitot heat failure can give erroneous airspeed while other parameters remain plausible.

Total air temperature probes are heated to prevent icing; heater failure can cause a false low TAT and consequent thrust-limit errors.

Rigidity in space: a spinning rotor maintains its spin axis in inertial space; precession is the response to an applied torque, at 90 degrees in the direction of rotation.

Attitude indicator uses a vertical gyro (or laser/air-data aided platform); it is erect and gives pitch and roll relative to the local vertical.

Rate gyro (rate-of-turn indicator) measures rate of turn about the vertical axis and is calibrated in degrees per minute; it is not a heading indicator.

Turn coordinator uses a canted gyro so it senses both rate of turn and rate of roll, allowing it to indicate skid or slip in a turn.

Directional gyro/heading indicator is a horizontal-axis gyro with a drift error from Earth's rotation and transport wander; it needs periodic synchronisation to magnetic heading.

Slaving: a flux valve/flux gate senses the horizontal component of Earth's magnetic field and drives the gyro to magnetic heading via a slaving amplifier.

Flux valve errors include turning error (dip-related) and acceleration error; a 'slave free' mode allows manual heading setting when the flux valve is unreliable.

Remote indicating compass combines a gyro-stabilised heading with a magnetic reference, providing a stable, slaved magnetic heading to displays.

Gyro drift is corrected by erection (attitude) and slaving (heading); uncorrected drift causes slow, monotonic indication errors.

Direct-reading magnetic compass errors: deviation (aircraft magnetic fields), variation (geographic), and turning/dip and acceleration errors from magnetic dip.

In the Northern Hemisphere, turning through north the compass under-reads (lags), and turning through south it over-reads (leads); the effect is reversed in the southern hemisphere.

Acceleration error: on east/west headings, acceleration causes a northerly turn indication in the northern hemisphere; deceleration causes a southerly turn indication.

Deviation is minimised by adjusting compensating magnets and recorded on a deviation card; residual deviation is applied by the pilot.

Variation is the angle between true and magnetic north, taken from charts; magnetic heading = true heading minus easterly variation (add westerly).

Magnetic dip is the vertical component of Earth's field; it is zero at the magnetic equator and maximum at the magnetic poles.

The flux valve must be mounted where the field is least disturbed; it senses the horizontal component, so it is unreliable near the poles where dip is high.

Heading references: true heading is used for high-latitude operations and for some area-navigation displays; grid heading is used in polar regions.

INS uses accelerometers plus gyros; acceleration is integrated once for velocity and twice for position, so bias errors grow with time (position error grows roughly with the square of time for bias).

Schuler tuning: the platform is tuned to a period of about 84 minutes so that horizontal acceleration does not cause a false tilt, keeping the vertical reference accurate.

Ring-laser gyros measure rotation by the Sagnac effect; they have no moving parts, no gimbal lock, and require dither to avoid lock-in at low rates.

Aligning an INS requires a known position and level attitude; gyrocompassing determines true north from Earth-rate sensing.

GNSS position uses pseudoranges to at least four satellites; a fourth satellite resolves the receiver clock offset.

RAIM detects (and with extra satellites, excludes) a faulty satellite; RAIM availability depends on satellite geometry and is weaker at high latitudes and during certain geometries.

SBAS (e.g. EGNOS/WAAS) provides integrity and correction data; GBAS provides local differential corrections for precision approach.

GNSS/INS integration: GNSS bounds inertial drift while the INS provides continuity during signal loss and aids reacquisition.

Dilution of precision (DOP) quantifies geometric quality; high DOP means poor geometry and larger position error for the same ranging error.

Flight director computes steering commands and displays them as pitch and roll bars; the autopilot follows the same commands to move the control surfaces.

Autopilot modes: lateral (heading, VOR, localiser, LNAV) and vertical (altitude hold, vertical speed, flight level change, glide slope, VNAV).

Autopilot disconnect must be accompanied by an aural and visual warning; the stabiliser trim and pitch trim are typically inhibited or limited in autopilot.

Autothrust controls thrust to hold a target (speed, Mach, or thrust rating); autothrottle servos move the thrust levers, and engagement requires valid air data.

Autothrust modes include speed, Mach, thrust (takeoff/go-around), and retard; a thrust-limit mode prevents over-boost.

Takeoff/go-around mode arms autothrust to set takeoff thrust; a fault in TAT or air data can cause incorrect thrust limits.

Autopilot authority is limited by control laws (e.g. bank angle, pitch limits); excessive deviation causes the autopilot to disengage.

Autoland requires at least two autopilots, radio-altimeter data, and a fail-passive or fail-operational architecture depending on the category.

EFIS displays: primary flight display shows attitude, airspeed, altitude, heading and flight director; navigation display shows route, heading, track and weather.

Display symbology colours: green normal/engaged, amber caution, red warning, white/cyan information, magenta active route.

EICAS/ECAM present engine and system parameters, with caution and warning messages; the master caution and master warning lights are separate.

Alert levels: warning (immediate action), caution (awareness, may need action), advisory (information); aural warnings accompany the highest levels.

Comparator/monitor logic compares redundant sources (e.g. two ADCs, two IRUs) and flags a disagreement to the crew.

Electronic displays use redundant symbol generators; a single display-unit failure can often be compensated by reversion or switching.

Aural warnings include stall (stick shaker plus aural), overspeed (clacker), altitude alert, and configuration warnings; each has a distinct sound.

Standby instruments (attitude, airspeed, altimeter) provide a backup if the primary displays or air data fail.

Engine parameters: N1/N2 (spool speeds), EGT/ITT, fuel flow, oil pressure and temperature, and thrust (EPR or N1).

EGT is a thermocouple-based indication; a rapid rise or exceedance indicates a hot start or engine distress and requires immediate action.

N1 and N2 are measured by tachometers (magnetic or optical) and displayed as percentages of a reference speed.

Fuel flow is measured by a turbine or volumetric flowmeter; total fuel used is integrated from flow and compared with tank quantity.

Engine vibration monitoring uses accelerometers on the engine; rising vibration can indicate imbalance, bearing wear, or FOD.

FADEC controls fuel metering and thrust limits, and provides fault reporting; it uses air data for thrust setting and limit protection.

System monitoring includes hydraulic pressure/quantity, electrical bus voltages, and pressurisation; each has caution/warning thresholds.

Trend monitoring compares parameter trends over time to detect deterioration before a limit is reached.

ARINC 429 is a unidirectional, point-to-point bus using 32-bit words and differential signalling; it is widely used for avionics line replaceable units.

ARINC 629 and MIL-STD-1553 are bidirectional/multiplex buses; 1553 uses a command/response protocol with a bus controller.

AFDX/Ethernet uses virtual links and redundancy management for high-integrity data transfer.

Bus data integrity is protected by parity, checksum, or cyclic redundancy check; a failed word is discarded and may be replaced by the last valid value or flagged invalid.

Line replaceable units are connected to computers that perform voting or comparison; dual-channel architectures compare outputs and flag disagreement.

Electrical power: AC generators (constant frequency or variable frequency) and DC via transformer-rectifier units; essential and emergency buses supply critical loads.

A bus failure can degrade multiple systems simultaneously; the affected systems revert to alternate sources or declare invalid data.

Battery and standby power supply essential instruments and displays after generator loss; the standby horizon and standby air data are on a separate source.

Pitot blockage: ASI behaves as an altimeter in climb/descent; with drain open, ASI falls toward zero in climb and rises in descent.

Static blockage: ASI behaves as an altimeter, altimeter freezes, VSI reads zero; altitude and speed indications become unreliable together.

Total air data failure: airspeed, altitude and Mach disagree with other sources; the crew must use standby instruments and cross-check attitude.

Attitude failure: a failed attitude indicator may show a slow drift or a fixed erroneous attitude; cross-check with standby horizon and other instruments.

Heading failure: a failed flux valve or slaving gives a slowly drifting heading; select 'slave free' and use the standby compass or another source.

Display failure: a blank or frozen display is detected by comparing with the other display and the standby instruments; reversion switches may restore the display.

Autopilot/autothrust failure: disconnect warnings and mode reversions; the crew must fly manually and cross-check thrust and speed.

Warning hierarchy: red warning requires immediate action; amber caution requires awareness; advisory is information only.

Failure management relies on cross-checking independent sources (standby instruments, other ADC/IRU) to identify the faulty source.

Pitot heat failure can cause a false low airspeed while altitude remains correct; the crew should suspect a pitot problem if airspeed is inconsistent with attitude and thrust.

Static port blockage on one side causes a position error; the aircraft may need to use the alternate static source.

A failed air data computer may cause multiple systems (autopilot, autothrust, displays) to degrade simultaneously; identify the source and revert.

Inertial reference failure: attitude and heading may be lost; the standby instruments and GNSS/other sources provide backup.

GNSS loss: navigation degrades to inertial or radio aids; RAIM warnings require reversion to another navigation source.

Display and alerting degradation: a failed symbol generator or display unit causes loss of primary information; reversion or standby instruments are used.

Engine parameter failure: a failed sensor may show a fixed or zero indication; cross-check with other parameters and trend data.

Electrical bus failure: essential and emergency buses maintain critical instruments; non-essential loads are shed.

Crew response to warnings: identify, verify with independent sources, and apply the appropriate checklist; do not act on a single unverified indication.