Flight Instruments and Avionics
Flight instruments and avionics tests pitot-static instruments, gyros, the magnetic compass and the avionics fitted to typical CPL aircraft.
Pitot-static
- The ASI measures dynamic pressure. Errors include position, density (IAS versus TAS) and compressibility.
- The altimeter is set to QNH for altitude above mean sea level, QFE for height above the aerodrome, and 1013 hPa for flight levels. Near sea level, 1 hPa is about 30 ft.
- The VSI shows rate of climb or descent and lags behind the actual change.
- Pitot blocked with drain open: the ASI behaves like an altimeter. Static blocked: ASI over-reads in a climb and under-reads in a descent, the altimeter freezes, and the VSI reads zero after its lag.
Gyros and compass
- Attitude indicator and directional gyro rely on rigidity and precession; vacuum or electric. The attitude indicator must be erect before use and can tumble.
- The turn coordinator shows rate of turn and roll, electrically driven.
- Compass errors: deviation from the card, variation from the chart, turning and acceleration errors. The Northern Hemisphere rules (UNOS, ANDS) reverse in New Zealand: turning through north overshoots, and acceleration indicates a turn to the south.
Worked example
The altimeter reads 8,500 ft with 1013 hPa set, but the local QNH is 1003 hPa. The pressure difference is 10 hPa, about 300 ft. True altitude ≈ 8,500 − 300 = 8,200 ft. Flying from high to low pressure without resetting the subscale puts the aircraft lower than indicated.
Traps
- Setting QFE where QNH is required.
- Expecting a normal ASI with a blocked pitot.
- Using an attitude indicator that has tumbled.
- Applying Northern Hemisphere compass rules in New Zealand.