Aircraft General Knowledge - Instruments
What is tested
Instrument questions focus on how pitot-static and gyroscopic instruments fail, how EFIS, ADC and IRU systems present data, and what the pilot must do after a disagreement. Know the standby instruments and the meaning of red and amber flags.
Pitot-static
- The ASI compares pitot total pressure with static pressure.
- Blocked pitot while static is clear: the ASI behaves like an altimeter, overreading in climb and underreading in descent.
- Blocked static with pitot clear: the ASI overreads in descent and underreads in climb; the altimeter freezes at the blockage altitude; the VSI reads zero.
- A leak changes indications slowly. Position error, compressibility error and density error affect high-speed and high-altitude readings.
Gyroscopic
Rigidity in space gives attitude and heading reference; precession gives rate indications. A vacuum or pneumatic failure usually affects attitude and heading indicators, while the turn coordinator is often electric. AHRS and IRS provide attitude and heading without a conventional gyro but need alignment and can drift. Compare standby instruments and cross-check.
Worked example
During descent, the static port blocks. The altimeter stops changing, the VSI goes to zero, and the ASI overreads because the trapped static pressure is lower than the rising ambient static pressure, so the pressure difference across the ASI is too large. Treat the ASI as unreliable, level off, use the standby altimeter and static alternate source, and advise ATC.
Traps
The ASI is not a true speedometer; it indicates dynamic pressure. Do not confuse gyroscopic rigidity with precession. Do not chase a single instrument; use the power-attitude-performance cross-check.