Aircraft General Knowledge - Instruments
This domain tests the pitot-static instruments, the gyroscopic instruments and the magnetic compass, including the failure modes that arise from blockages and leaks.
Pitot-static group
- Airspeed indicator: reads dynamic pressure, so it responds to pitot pressure minus static pressure. It shows indicated airspeed; position error and compressibility must be applied to reach calibrated and true airspeed.
- Altimeter: an aneroid capsule responding to static pressure, calibrated to the ISA. It has no inertia, so it lags in a climb or descent.
- Vertical speed indicator: measures the rate of change of static pressure; the lag is about 6 to 9 seconds with an immediate indication of the trend.
Pitot and static failures
- Pitot blocked, static open: the ASI behaves like an altimeter, climbing in the climb and descending in the descent. The altimeter and VSI remain normal.
- Static blocked: the ASI over-reads in the climb and under-reads in the descent; the altimeter freezes; the VSI shows zero.
- Drain and moisture in the pitot can give a false low or zero reading.
Gyroscopic group
- Attitude and heading indicators are usually vacuum or electrically driven; the turn coordinator is electrically driven and can be either for redundancy.
- Vacuum failure is detected on pre-flight run-up limits and in flight by the suction gauge; both gyros then become unreliable and their flags appear.
- The heading indicator processes with the earth's rotation and must be reset to the magnetic compass periodically.
Magnetic compass
- Errors: variation, deviation, turning and acceleration errors, and magnetic dip at high latitude.
Worked example
Climbing at 500 ft per minute with the pitot blocked, the ASI shows a steady increase in speed while the altimeter and VSI read normally. Recognition: pitot blockage. Action: cross-check the other instruments, ignore the ASI, and avoid changing pitch based on the false reading.
Traps
Treating a static blockage as a pitot blockage - the VSI freezing at zero is the giveaway, as is the altimeter freezing.