Aircraft General Knowledge – Instrumentation
Aircraft General Knowledge – Instrumentation
Instrumentation tests the pitot-static system, gyroscopic instruments, magnetic compass, engine instruments and electronic displays. The CPL candidate must understand how each instrument derives its indication, its errors, and what happens when the source fails.
Core rules
- The pitot-static system supplies airspeed, altimeter and vertical speed. The ASI compares pitot (total) pressure with static pressure; blockage of the pitot leaves the ASI acting like an altimeter, while a blocked static source makes the ASI over-read in climb and under-read in descent.
- The altimeter is set to QNH for altitude above mean sea level, QFE for height above a specific datum, and 1013.25 hPa for flight levels. A 1 hPa error is about 27 ft near sea level.
- Gyroscopes depend on rigidity and precession. A vacuum-driven attitude and heading indicator can fail if the vacuum pump fails; the turn coordinator is often electrically driven.
- The magnetic compass is affected by variation, deviation, acceleration and turning errors. Compass turns must be made with the correct lead/rollout.
- Glass cockpits integrate AHRS, ADC and GNSS; a disagreement between sources requires cross-checking standby instruments.
Worked example
The altimeter subscale is set to 1013 hPa, but the local QNH is 1003 hPa. The subscale is 10 hPa too high, so the altimeter reads high by about 10 × 27 = 270 ft. At an aerodrome elevation of 1,200 ft, the altimeter would indicate about 1,470 ft. The pilot must reset to QNH before using the altitude for terrain clearance.
Traps
- Using QFE as if it were QNH.
- Trusting one instrument after a static blockage.
- Forgetting that attitude and heading indicators may have different power sources.
- Reading a magnetic compass during acceleration as a reliable heading.