Aircraft General Knowledge - Instrumentation
Instrumentation tests pitot-static and gyroscopic instruments, air-data computers, electronic displays and warning systems, with emphasis on interpreting failures and setting the correct pressure reference.
Core instruments
- The altimeter measures static pressure; QNH gives elevation above mean sea level, QFE gives height above a reference, and 1013.25 hPa gives pressure altitude.
- The airspeed indicator uses pitot pressure and static pressure; blocking the static source affects altitude, VSI and airspeed indications.
- The VSI is a rate instrument with lag; it shows trend, not instantaneous vertical speed.
- Gyroscopic instruments depend on rigidity and precession; AHRS/IRS provide attitude and heading without traditional gyros.
Worked example
A field has elevation 500 ft and QNH 1000 hPa. With the altimeter set to 1000 hPa it reads 500 ft. If the crew sets 1013 hPa, the indication increases by about 30 ft per hPa: 13 × 30 = 390 ft, so it reads about 890 ft. The aircraft has not climbed; the pressure reference changed.
Traps
- Increasing the altimeter setting increases indicated altitude.
- QFE reads zero at the reference point, not at sea level.
- A blocked static port causes the altimeter to freeze and the ASI to over-read in climb.
- Do not treat VSI lag as a failure without cross-checking altitude and attitude.