Instruments
Instruments tests how cockpit indications are produced, what they actually represent, and how they fail. The questions link pitot-static theory to altimetry, airspeed and Mach indications, and cover gyroscopic and electronic systems such as IRS, AHRS and EFIS.
Pitot-static and altimetry
- The altimeter is a pressure instrument calibrated to the ISA. Set QNH below the transition altitude and QNE (1013.25 hPa) above the transition level; the transition layer separates them.
- Near the surface, 1 hPa ≈ 27 ft. Flying from high pressure to low pressure without resetting makes the altimeter overread: from high to low, look out below.
- Temperature error: at a constant indicated altitude in air warmer than ISA, true altitude is higher; in colder air it is lower. A useful approximation is 4 ft per 1,000 ft per °C of ISA deviation.
Airspeed, Mach and gyros
- IAS is affected by position and instrument error (CAS), compressibility (EAS) and density (TAS). Mach is TAS divided by the local speed of sound, so for a given Mach number TAS falls as temperature falls.
- IRS uses ring laser gyros and accelerometers, aligns before flight and drifts slowly. AHRS is a lower-accuracy attitude and heading source.
Worked example
At an indicated altitude of 10,000 ft the ISA deviation is +20°C. True altitude ≈ 10,000 + (4 × 10 × 20) = 10,800 ft. The aircraft is about 800 ft higher than the altimeter suggests because the warm column expands and the pressure levels are lifted.
Traps
- Resetting QNH while climbing through the transition altitude rather than at the transition level.
- Confusing IAS, CAS and TAS when the question gives Mach number and temperature.
- Assuming an IRS position stays exact; position error grows with time.