Instruments, Warning Systems and Performance Limitations
What this domain tests
This domain covers pitot-static and gyroscopic instruments, warning systems, and the performance limitations that constrain dispatch.
Core rules
- The pitot-static system feeds the airspeed indicator, altimeter, and vertical speed indicator. The ASI compares pitot pressure with static pressure; the altimeter measures static pressure against a set subscale; the VSI measures rate of change of static pressure.
- Errors include position error, density error, compressibility error, and lag. The altimeter has lag and a barometric error if the subscale is wrong.
- Gyroscopic instruments use rigidity in space (attitude and heading) and precession (turn coordinator). AHRS replaces spinning gyros with solid-state sensors.
- Warning systems include stall warning, master caution and warning, fire, GPWS, and TCAS.
- Performance limitations include MTOW, MLW, MZFW, CG limits, V speeds, runway length, and WAT limits.
Worked example
You are at an airfield with QNH 1003 hPa. The altimeter subscale is set to 1013 hPa. The difference is 10 hPa. Near sea level, 1 hPa ≈ 27 ft, so the altimeter reads about 270 ft too high. On approach this is dangerous: the aircraft is lower than indicated. Set the local QNH.
Traps
Using QFE when QNH is required; forgetting that 1 hPa is about 27 ft, not 30 ft; reversing the sign of the subscale error; treating the VSI as instantaneous; confusing MTOW with MLW.