10 Instruments & Autoflight

What is the practical relationship between IAS, CAS, TAS and Mach?

IAS is the cockpit indication; CAS corrects IAS for instrument/position error; TAS corrects for air density/compressibility and represents speed through the airmass; Mach is TAS divided by local speed of sound.

What pressures does the airspeed system compare?

Total (pitot) pressure and static pressure; their difference is dynamic pressure.

What happens if the pitot inlet and drain are both blocked while the static source remains normal?

The airspeed indication behaves approximately like an altimeter: it increases in a climb and decreases in a descent regardless of actual constant IAS.

What happens if the pitot inlet is blocked but the drain remains open?

Pitot pressure equalises toward static pressure and the airspeed indication falls toward zero.

What happens if the static source becomes blocked?

Altimeter freezes, VSI trends to zero, and airspeed underreads in a climb and overreads in a descent.

Why are unreliable airspeed events potentially difficult to diagnose?

Different pitot/static failures can create plausible but mutually inconsistent indications, so crews must compare independent sources and aircraft attitude/thrust behaviour.

What is an Air Data Computer primarily doing?

Converting pressure and temperature inputs into parameters such as altitude, vertical speed, airspeed, Mach and temperature-related data.

What is the main difference between barometric altitude and radio altitude?

Barometric altitude is referenced to atmospheric pressure; radio altitude measures actual height above the terrain directly beneath the aircraft.

Why can radio altitude change over terrain even with constant barometric altitude?

It follows the changing distance between the aircraft and the surface below.

Why is radio altitude particularly important in low-visibility operations?

It provides an independent height-above-ground reference and is used by many automatic landing and warning functions.

What is the fundamental difference between an inertial reference and an air-data reference?

Inertial systems derive attitude, heading, acceleration and position from gyros/accelerometers; air-data systems derive atmospheric parameters from pressure and temperature sensors.

Why does an unaided inertial position gradually become less accurate?

Small sensor errors integrate over time and create position drift.

Why are independent attitude/air-data sources important on transport aircraft?

Disagreement between independent sources allows erroneous information to be detected and isolated.

What is the primary purpose of standby flight instruments?

To retain essential attitude, airspeed and altitude information after failure of the normal integrated displays or their sources.

What is the difference between a Flight Director and an Autopilot?

The Flight Director displays guidance commands for the pilot; the Autopilot automatically moves the flight controls to follow guidance.

Can the Flight Director operate with the Autopilot disengaged?

Yes.

Can automatic thrust normally operate independently of the Autopilot?

Yes, depending on aircraft design; disconnecting the Autopilot does not necessarily disconnect automatic thrust.

What is the Flight Mode Annunciator (FMA) used for?

It shows the autoflight modes that are active and armed; it is the primary reference for understanding what the automation is actually doing.

What is the difference between an active and an armed autoflight mode?

Active mode is currently controlling the aircraft; armed mode is waiting for its capture/engagement conditions.

Why should every significant automation selection be followed by an FMA check?

The selected control input does not guarantee the expected mode became active; the FMA confirms the actual system state.

What is mode reversion?

An automatic change to another autoflight mode when the selected mode can no longer operate or its conditions are no longer satisfied.

Why is mode reversion a major automation threat?

The aircraft may remain under automatic control but follow a different pitch, roll or thrust objective than the crew expects.

What is automation surprise?

A mismatch between what the crew expects the automation to do and what it actually does.

What is the best defence against automation surprise?

Continuous mode awareness: select, verify on the FMA, monitor aircraft response and intervene early if it differs from the intended flight path.

What is mode confusion?

Failure to correctly understand which autoflight mode is active, armed or controlling a particular axis.

Why should pilots monitor aircraft response rather than only mode annunciations?

A correctly annunciated mode can still produce an undesired flight path if the target, data or energy state is inappropriate.

What is the fundamental principle for choosing automation level?

Use the level of automation that best supports situational awareness and workload; revert to simpler modes or manual flight when they are more appropriate.

Does engaging the Autopilot transfer responsibility for flight-path monitoring to the automation?

No.

What is the relationship between FMS guidance and the Autopilot/Flight Director?

The FMS computes navigation and trajectory guidance; the autoflight system follows that guidance when the appropriate modes are selected.

Why should FMS predictions not be treated as guarantees?

They depend on assumptions about winds, temperature, aircraft performance, route and restrictions that may change.

Why can a managed/VNAV descent become high or low on profile despite correct programming?

Wind changes, ATC constraints, speed changes, configuration and differences between predicted and actual aircraft performance alter the energy state.

What is the basic energy-management problem during descent?

Potential energy and kinetic energy must be managed so altitude and speed targets are achieved simultaneously.

Why can “being high” and “being fast” become mutually reinforcing?

Descending more steeply can increase speed, while reducing speed often requires drag or a shallower descent, making altitude loss harder.

Why is configuration an important energy-management tool?

Flaps, gear and speedbrakes change drag and therefore the aircraft’s ability to decelerate or descend.

What is the basic capture concept in autoflight?

A guidance mode initially reduces the error toward a target/path, then transitions into a mode that maintains the captured target/path.

Why is intercept geometry important for LOC or glide-path capture?

Excessive intercept angle, rate or inappropriate position may prevent stable capture or create overshoot.

Why is glide-path capture from below normally preferred?

It reduces the risk of false glide-slope capture and promotes a predictable stabilised intercept.

What does an autoland system fundamentally control?

The approach, flare and landing, and depending on system capability, rollout.

What is the difference between fail-passive and fail-operational automatic landing capability?

After a failure, fail-passive leaves the aircraft in a condition requiring pilot takeover without significant flight-path deviation; fail-operational retains sufficient automatic capability to complete the intended landing/rollout as certified.

Why does very-low-DH CAT III operation require greater system redundancy?

There may be insufficient height or visual reference for safe manual takeover after a system failure.

What is the fundamental difference between GPWS and EGPWS/TAWS?

Conventional GPWS primarily detects hazardous terrain closure from aircraft sensor data; EGPWS/TAWS additionally uses aircraft position and a terrain/obstacle database to provide predictive terrain awareness.

What is a key limitation of predictive terrain systems?

Their protection depends on valid position, terrain/database information and system configuration; they do not replace flight-path or chart monitoring.

What is ACAS/TCAS designed to protect against?

Mid-air collision risk by independently assessing nearby transponder-equipped traffic and, when required, issuing vertical resolution guidance.

What is the difference between a TCAS Traffic Advisory and Resolution Advisory?

A TA alerts the crew to potentially conflicting traffic; an RA provides vertical manoeuvre guidance to resolve a predicted collision threat.

Should a pilot manoeuvre solely on the basis of a TA?

Normally no; visually acquire/monitor the traffic and prepare for a possible RA unless another safety reason requires manoeuvring.

What parameter fundamentally drives TCAS threat assessment?

Primarily predicted time to closest point of approach, not distance alone.

Are TCAS RAs horizontal or vertical manoeuvre commands?

Vertical.

What takes priority if an RA conflicts with an ATC vertical clearance?

The RA is followed promptly; ATC is advised as soon as workload permits.

Why must both ACAS-equipped aircraft respond correctly to coordinated RAs?

The systems coordinate complementary vertical manoeuvres; failure of one aircraft to respond reduces protection for both.

What does airborne weather radar primarily detect?

Energy reflected from precipitation particles, especially liquid water; stronger reflectivity generally produces stronger returns.

Why can a strong weather-radar return hide hazardous weather behind it?

Heavy precipitation can attenuate the radar beam, creating a misleading weak or blank area beyond the strong echo.

What is a radar shadow?

An area behind strong attenuation where little or no radar energy returns; it should not be interpreted as clear weather.

Why is antenna tilt management important?

Incorrect tilt may scan the ground or above/below relevant weather and create a misleading picture of the flight path ahead.

Can conventional airborne weather radar reliably detect clear-air turbulence?

No. Without sufficient precipitation or detectable moisture, hazardous turbulence may produce little or no radar return.

Why should radar gain generally be left in the manufacturer-recommended automatic/calibrated setting for normal weather avoidance?

Displayed intensity then has a predictable relationship with return strength; inappropriate manual gain can hide or exaggerate echoes.

Why is the shape and trend of a weather-radar return as important as its colour?

Rapid growth, gradients, hooks/fingers and changing structure can indicate significant convective development even before maximum displayed intensity.