Instruments
Instruments (CAA NZ AME)
This domain tests the AME's understanding of how aircraft instruments sense, transmit and display pressure, gyroscopic and engine parameters, and how maintenance and pre-flight checks detect errors. Expect pitot-static systems, gyros, magnetic compasses, engine indications and electronic displays.
Core rules
- Pitot pressure is total pressure; the static port supplies ambient pressure. The ASI displays dynamic pressure (pitot minus static).
- A blocked pitot with an open static port causes the ASI to overread in a climb and underread in a descent: trapped pitot pressure stays fixed while static pressure falls or rises.
- A blocked static port makes the ASI behave like an altimeter: it underreads in a climb and overreads in a descent. The altimeter freezes at the altitude where the static port blocked.
- Altimeter errors include position, instrument, temperature and pressure-setting error. Cold air makes the altimeter overread; the practical rule is "high to low, look out below."
- Gyroscopic instruments rely on rigidity in space and precession. Attitude and heading indicators are normally vacuum/air-driven; turn coordinators are often electric.
- The magnetic compass is the only north-seeking instrument; deviation is aircraft-specific and recorded on a compass card. In the Southern Hemisphere, turning and acceleration errors are opposite to the Northern Hemisphere rules.
Worked example
An aircraft is at an indicated altitude of 10,000 ft with an OAT 20°C colder than ISA. Estimate the true altitude. Rule of thumb: error is about 4 ft per 1,000 ft per °C of ISA deviation. Error = 20 × 4 × 10 = 800 ft. The altimeter overreads in cold air, so true altitude is about 9,200 ft. Reason: the altimeter assumes the ISA pressure lapse; cold, dense air has a faster pressure lapse, so the aircraft is lower than indicated.
Traps
- Reversing blocked-pitot and blocked-static symptoms.
- Applying Northern Hemisphere compass rules in New Zealand.
- Treating QFE as QNH: QFE reads height above a specific aerodrome, not altitude above mean sea level.
- Forgetting that a heading indicator must be reset to the magnetic compass; it is not north-seeking.