ATPL 022 Instrumentation flashcards

36 cards. Read the term and the definition here. The deck at the top of this page is optional.

Resolution versus accuracy

Resolution is the smallest change a sensor can resolve or display, while accuracy is how close the indicated value is to the true value. A sensor can be precise but inaccurate if it has a systematic bias.

Random versus systematic error

Random error scatters readings around a mean and is reduced by averaging, while systematic (bias) error shifts every reading in the same direction and cannot be averaged out, only calibrated out.

Servo force-balance sensor

A force-balance sensor nulls the deflection with a restoring torque, so the moving element barely moves. This gives low hysteresis, high linearity and better accuracy than a direct-deflection instrument.

Synchro versus resolver

A synchro transmits angular position as three-wire AC voltages, while a resolver gives two quadrature outputs and can resolve a full 360 degrees unambiguously. Both are used for remote position indication.

Pitot-static system basics

Pitot pressure is total (stagnation) pressure and static ports supply static pressure. Airspeed is derived from the difference, while altitude and vertical speed are derived from static pressure alone.

Altimeter temperature error

The altimeter is calibrated to the ISA, so in colder-than-ISA air the true altitude is lower than indicated. In warmer air the true altitude is higher than indicated, which matters for terrain clearance.

Blocked static port effects

With a blocked static port, the ASI behaves as an altimeter, the altimeter freezes at the trapped pressure, and the VSI reads zero. Airspeed and altitude become unreliable together.

Blocked pitot with drain open

If the pitot is blocked but the drain is open, the trapped pressure leaks to static. In a climb the ASI falls toward zero, and in a descent it rises, while altitude remains correct.

TAT versus SAT

The TAT probe measures total (recovery) temperature including ram rise. SAT is computed by removing the ram rise, which is approximately proportional to Mach number squared, so SAT is always lower in flight.

Thermocouple cold junction

A thermocouple output depends on the temperature difference between its hot and reference junctions. Cold-junction compensation is required so the indicated temperature reflects the hot junction alone.

Ice detector operation

Vibrating-probe ice detectors change frequency when ice accretes, then heat to shed the ice and reset. Detection normally activates airframe and engine anti-ice and may modify autothrust and stall-warning schedules.

Rigidity and precession

A spinning gyro maintains its spin axis in inertial space (rigidity), and an applied torque causes precession at 90 degrees in the direction of rotation. These two properties underpin all gyroscopic instruments.

Rate gyro versus turn coordinator

A rate gyro measures rate of turn about the vertical axis in degrees per minute. A turn coordinator uses a canted gyro so it also senses rate of roll, allowing it to show skid or slip.

Flux valve slaving

A flux valve senses the horizontal component of Earth's magnetic field and drives the directional gyro to magnetic heading through a slaving amplifier. Slave-free mode allows manual heading when the valve is unreliable.

Gyro drift correction

Attitude gyros are corrected by erection to the local vertical, and heading gyros by slaving to the magnetic reference. Uncorrected drift causes slow, monotonic errors in the displayed indication.

Compass turning error

In the Northern Hemisphere the direct-reading compass under-reads (lags) when turning through north and over-reads (leads) through south. The effect reverses in the Southern Hemisphere and is largest at high dip.

Compass acceleration error

On east or west headings in the Northern Hemisphere, acceleration causes a northerly turn indication and deceleration a southerly turn indication. The error reverses in the Southern Hemisphere.

Variation versus deviation

Variation is the angle between true and magnetic north and comes from charts, while deviation is the error caused by the aircraft's own magnetic fields and is recorded on a deviation card.

Magnetic dip

Magnetic dip is the vertical component of Earth's field, zero at the magnetic equator and maximum at the magnetic poles. High dip makes the flux valve unreliable because it senses the horizontal component.

INS error growth

Inertial navigation integrates acceleration once for velocity and twice for position, so accelerometer bias errors grow with time. Position error grows roughly with the square of time for a constant bias.

Schuler tuning

The inertial platform is tuned to a period of about 84 minutes so that horizontal acceleration does not cause a false tilt. This keeps the vertical reference accurate during manoeuvres.

Ring-laser gyro

A ring-laser gyro measures rotation by the Sagnac effect using counter-rotating light beams. It has no moving parts and no gimbal lock, and requires dither to avoid lock-in at low rates.

RAIM and SBAS

RAIM detects and can exclude a faulty satellite using redundant ranging, but availability depends on geometry. SBAS provides wide-area corrections and integrity, while GBAS provides local differential corrections for precision approach.

Flight director versus autopilot

The flight director computes steering commands and displays them as pitch and roll bars. The autopilot follows the same commands to move the control surfaces, so it can be engaged with or without the director.

Autothrust modes

Autothrust controls thrust to hold a target speed, Mach or thrust rating. Modes include speed, Mach, takeoff/go-around thrust and retard, with a thrust-limit mode preventing over-boost.

Autoland requirements

Autoland requires at least two autopilots, radio-altimeter data and a fail-passive or fail-operational architecture depending on category. The autopilots must be able to disconnect safely on failure.

EFIS display colours

EFIS symbology uses green for normal or engaged, amber for caution, red for warning, white or cyan for information, and magenta for the active route. The PFD shows attitude and air data; the ND shows route and weather.

Alert levels

Warnings require immediate action and are red with an aural warning, cautions require awareness and may need action and are amber, and advisories are information only. Master caution and master warning lights are separate.

Engine parameters

Engine monitoring includes N1 and N2 spool speeds, EGT or ITT, fuel flow, oil pressure and temperature, and thrust as EPR or N1. A rapid EGT rise indicates a hot start or engine distress.

FADEC function

FADEC controls fuel metering and thrust limits and provides fault reporting. It uses air data for thrust setting and limit protection, so a faulty air data source can cause incorrect thrust limits.

ARINC 429 characteristics

ARINC 429 is a unidirectional, point-to-point bus using 32-bit words and differential signalling. It is widely used between avionics line replaceable units and is simple but requires many separate connections.

Data bus integrity

Bus data 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, which can degrade dependent systems.

Pitot blockage with drain open

If the pitot is blocked but the drain is open, trapped pressure leaks to static, so in a climb the ASI falls toward zero and in a descent it rises. Altitude indications remain correct.

Static blockage effects

With a blocked static port, the ASI behaves as an altimeter, the altimeter freezes and the VSI reads zero. Airspeed and altitude become unreliable together, so standby instruments are needed.

Failure cross-check

Failure management relies on cross-checking independent sources such as standby instruments and other ADC or IRU outputs. A single unverified indication should not be acted on until the faulty source is identified.

Display failure response

A blank or frozen display is detected by comparing it with the other display and the standby instruments. Reversion switches may restore the display, otherwise standby instruments provide the primary information.