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GCAA Ghana ATPL Theory study materials
GCAA Ghana ATPL Theory study resources

GCAA Ghana ATPL Theory: Aircraft General Knowledge: Instrumentation

GCAA Ghana ATPL Theory · Work through your exam, one topic at a time.

All guide sections

Aircraft General Knowledge: Instrumentation

Instrumentation questions test the sensing principle behind each display and the error you must apply in flight. Ghana ATPL papers lean on pitot-static errors, gyroscopic properties, and the gap between what is displayed and what is true.

Pitot-static group

  • The ASI measures dynamic pressure. Indicated airspeed carries position, instrument and compressibility error, plus density error when converted to TAS. Colour arcs: white flap range, green normal, yellow caution up to VNE, red radial line.
  • The altimeter is an aneroid capsule referenced to the subscale. QNH gives altitude above mean sea level; QFE gives height above the reference datum; QNE (1013.25 hPa) gives flight levels.
  • The VSI responds through a calibrated leak: it lags, and only reads steadily in a steady climb or descent.
  • The Machmeter combines pitot and static pressure to show M = TAS / local speed of sound.

Gyroscopics and heading

  • Rigidity in space and precession are the two properties used. Attitude and heading indicators are earth-referenced gyros; the turn coordinator uses rate precession.
  • The direct-reading magnetic compass suffers deviation, turning error (acceleration, latitude) and dip error; it is reliable only in steady level flight.

Modern flight deck

  • Air data computers feed the EFIS; IRS/AHRS supply attitude and heading with no moving parts and must be aligned before flight; EICAS/ECAM present engine and system warnings.

Worked example

You cruise at an indicated altitude of 5 000 ft with OAT -20 °C. ISA temperature at 5 000 ft is 15 - (2 × 5) = 5 °C, so the air is 25 °C colder than ISA. The rule of thumb is 4 ft per 1 °C per 1 000 ft: 25 × 4 × 5 = 500 ft. The altimeter over-reads and true altitude is about 4 500 ft - 'high to low, look out below'.

Traps

Mixing up QFE and QNH when the stem says 'height'; applying temperature error in the wrong direction (in cold air the aircraft is lower than indicated); expecting the VSI to respond instantly; and forgetting that standby instruments may use separate static sources.