Fundamentals of Flight Instruments: Pressure, Gyroscopes and Errors
Pressure sensing
The pitot-static instruments depend on two pressures: total (impact) pressure at the pitot head and static pressure at the static ports. The airspeed indicator compares the two, the altimeter and vertical speed indicator (VSI) use static pressure alone. Understand that indicated values are derived from a calibration assuming standard atmosphere (ISA): sea-level pressure 1013.25 hPa, temperature 15 °C, lapse rate 1.98 °C per 1000 ft. Any departure from ISA produces a systematic error, which is why true altitude and true airspeed differ from indicated values.
Gyroscopic principles
Two properties govern attitude and heading instruments: rigidity in space (a spinning rotor maintains its plane of rotation) and precession (a force applied to the spin axis produces movement 90° later in the direction of rotation). Rate instruments (turn coordinator, turn needle) exploit precession to indicate rate of turn; attitude and heading indicators exploit rigidity. Vacuum- or pressure-driven gyros have erection and realignment limitations; electrically driven gyros have different failure signatures.
Errors to recognise
- Position error: static port location causes errors at high angles of attack and high speed.
- Density error: airspeed indicator under-reads in low density (high altitude, high temperature).
- Temperature error: altimeter over-reads in cold air (dangerous - terrain clearance reduced).
- Lag and hysteresis: VSI lags, altimeter has mechanical lag and hysteresis.
- Manoeuvre and turning errors: compass and gyro instruments.
Worked rule of thumb
For altimeter temperature error, a useful approximation is 4 ft per 1000 ft per °C of ISA deviation. At 10 000 ft with the temperature 20 °C below ISA, the altimeter over-reads by about 4 × 10 × 20 = 800 ft: the aircraft is 800 ft lower than indicated. Always apply cold-temperature corrections when flying approaches in very cold conditions.
Question traps
Distinguish "over-reads" from "under-reads" carefully; remember that the altimeter senses static pressure only, and that the ASI cannot distinguish density from pressure error without a TAS computation.