Instruments

The PPL lesson covered the six basic instruments. Commercial aircraft add air data computers, inertial reference systems, electronic displays, autopilots and warning systems. This lesson explains how they work and how they can mislead you.

1. Airspeeds and Mach number

Speed	Corrected for
IAS	Nothing – as indicated
CAS	Instrument and position (pressure) errors
EAS	Compressibility, which matters above about 200 kt and at high altitude
TAS	Air density – the real speed through the air

The Mach number is TAS divided by the local speed of sound. The speed of sound depends only on temperature: about 661 kt at 15 °C and 573 kt at −56.5 °C. Jets climb at a constant IAS until the "crossover altitude", then at a constant Mach number. Climbing at constant IAS, TAS and Mach increase; climbing at constant Mach above the tropopause, TAS stays constant and IAS decreases.

2. The air data computer

The ADC takes pitot pressure, static pressure and total air temperature and calculates CAS, Mach, TAS, pressure altitude, vertical speed and static air temperature. It feeds the displays, the autopilot, the FMS, the transponder and the warning systems. Airliners have two or three ADCs plus standby instruments so that one faulty source can be identified by comparison.

A blocked pitot or static source in an airliner produces unreliable airspeed: conflicting speeds and overspeed or stall warnings at the same time. The memory actions are to disconnect the automatics and fly a known pitch and thrust setting, then troubleshoot with the checklist.

3. Altimeters

• The pressure altimeter measures height in the standard atmosphere. In air colder than ISA it over-reads; corrections are added to minimum altitudes in very cold weather.
• The radio altimeter measures the real height above the ground by timing a radio signal, typically up to 2,500 ft. It drives the GPWS, autoland and decision height callouts.

4. Gyroscopes and their errors

• Real wander: the gyro axis actually moves, from friction and imperfect balance.
• Apparent wander: the gyro stays fixed in space but the Earth turns under it. For a directional gyro the earth rate drift is 15° per hour × sin(latitude) – about 8° per hour at Amman's latitude.
• Transport wander: moving to a different latitude changes the reference.

Mechanical gyros have largely been replaced by ring laser gyros and fibre-optic gyros, which have no moving parts.

5. Inertial reference systems

An IRS uses gyros and accelerometers to measure every movement of the aircraft, and integrates them into attitude, heading, ground speed, wind and position – with no outside signal.

• Before flight it must be aligned: the aircraft stays still while the system senses the Earth's rotation and gravity to find true north and the latitude. Alignment takes about 10 minutes (longer at high latitudes). The position is entered from the gate coordinates or the GPS.
• Position accuracy slowly drifts (typically up to about 2 NM per hour), so the FMS mixes IRS, GPS and DME/DME positions for the best result.

6. Electronic flight instruments

• The PFD (primary flight display) shows attitude, speed tape (with trend vector and limit speeds), altitude tape, vertical speed, heading and the flight mode annunciator.
• The ND (navigation display) shows the route, the aircraft symbol, wind, waypoints, weather radar and TCAS traffic, in arc, rose or plan modes.
• Engine and system information appears on ECAM (Airbus) or EICAS (Boeing), with warnings in red, cautions in amber.

7. Autopilot and flight director

• The flight director shows the pilot the commands (bars or a flight path symbol); the autopilot moves the controls itself; the autothrottle/autothrust controls the thrust. All follow the modes selected on the FCU/MCP.
• Lateral modes: heading, NAV/LNAV, localiser. Vertical modes: altitude hold, vertical speed, level change/open climb, VNAV, glideslope. Thrust modes: speed, thrust, idle.
• Always check the FMA: armed modes (blue or white) are waiting, engaged modes (green) are active.

8. Warning systems

• GPWS / EGPWS (TAWS): warns of excessive descent rate, terrain closure, sinking after take-off, unsafe terrain clearance and descent below the glideslope. EGPWS adds a terrain database shown on the ND. On "TERRAIN, PULL UP": maximum thrust, pitch up to the stick shaker, wings level.
• TCAS: interrogates other transponders. A traffic advisory (TA, "TRAFFIC, TRAFFIC") means look for the traffic; a resolution advisory (RA, "CLIMB" or "DESCEND") must be followed promptly – even against an ATC instruction – and reported to ATC.
• Stall warning: stick shaker, and on Airbus flight envelope protection.

Flying it at Levant

Before every departure, check the FMA after engaging each mode, set the radio altimeter minimums for the approach and confirm the IRS is aligned before taxi. On IVAO, TCAS works against other pilots' transponders – take every RA seriously.

Check yourself

1. Which speed is corrected for compressibility? EAS.
2. Climbing at constant Mach above the tropopause, what happens to TAS? It stays constant (temperature is constant).
3. What is the earth-rate drift of a directional gyro at 30°N? 15 × 0.5 = 7.5° per hour.
4. TCAS gives "CLIMB, CLIMB" but ATC told you to descend. What do you do? Follow the RA and tell ATC.
5. Up to what height does a typical radio altimeter work? About 2,500 ft.

Simulation training material. Always follow your aircraft's flight manual and checklists.