Air Data Computers and Pitot-Static Systems
What the ADC does
The air data computer combines pitot pressure, static pressure, total air temperature and angle-of-attack inputs, applies position and instrument corrections held in the air data module, and outputs barometric altitude, computed airspeed, Mach, TAS, static air temperature, vertical speed, over-speed warnings and the encoded altitude sent to the transponder. Modern aircraft carry two or three ADCs with cross-comparison and automatic fault flagging.
Blockage diagnosis
- Pitot blocked, static clear: the ASI loses its dynamic input and behaves like an altimeter - over-reading in a climb, under-reading in a descent.
- Static blocked: the altimeter freezes at the trapped pressure, the VSI reads zero, and the ASI over-reads in a descent (pitot pressure rises while static stays fixed) and under-reads in a climb.
- Pitot heat is the first defence; alternate static sources are used when the static ports are blocked.
Monitoring points
Disagreement between ADCs usually appears as an amber ADC or altitude-comparator flag. RVSM airspace requires the altitude-keeping and comparator systems to be serviceable, and standby instruments must be checked against the primary displays.
Worked example
An aircraft with a blocked static port descends from FL300 to FL200. The altimeter stays at 30,000 ft, the VSI indicates zero, and the ASI over-reads because pitot pressure has risen while the trapped static pressure remains at the FL300 value. The crew should fly attitude and power, select alternate static if available, and advise ATC.
Traps
Confusing the two blockage patterns; forgetting pitot heat; accepting a single ADC after a disagreement; and assuming the altimeter will follow a QNH change when the static system is blocked.