Flight instruments and instrument systems
This domain tests how pressure and gyroscopic instruments behave in IFR flight, how altimetry is set and interpreted in New Zealand airspace, and how to recognise instrument failure before it corrupts a procedure.
Pressure instruments
- The pitot-static group is ASI, altimeter and VSI. A blocked pitot leaves the ASI behaving like an altimeter; a blocked static source makes the ASI over-read in a climb and under-read in a descent, and freezes the VSI.
- Altimeters are set to QNH, QFE or standard. New Zealand's transition altitude is 13,000 ft AMSL; above it, standard 1013.2 hPa and flight levels apply, with the transition level set by QNH.
- QFE is common at New Zealand aerodromes for take-off and landing clearances; approach minima are published in AMSL and flown on QNH.
Gyroscopic and glass systems
- Attitude and heading references are gyroscopic or AHRS-derived; turn coordination comes from the turn coordinator or yaw damper data. Glass cockpits combine an ADC and AHRS, so a single blocked static port can affect several displays at once.
Worked example
Aerodrome elevation 1,000 ft, QNH 1000 hPa. Using 30 ft per hPa, the pressure difference is about 33 hPa, so QFE is roughly 967 hPa. On the ground with QFE set the altimeter reads 0 ft; with QNH set it reads 1,000 ft. On approach, minima published as 1,520 ft AMSL are flown with QNH, not QFE.
Traps
- Cold air: when temperature is below ISA the altimeter over-reads, so true altitude is lower than indicated - significant in New Zealand terrain.
- Flying from high pressure to low pressure without resetting leaves the altimeter over-reading: "high to low, look out below".
- Confusing QFE with QNH on a charted minimum costs marks and, in real flight, terrain clearance.