From probes to an air-data computer

Why does the temperature probe read warmer in fast flight?

Decelerating air at the probe converts part of its kinetic energy into heat.

Static temperature and recovered temperature

Static air temperature describes the undisturbed air. A total-air-temperature probe slows the flow and measures a warmer recovered temperature. For ideal air, complete recovery gives TAT = SAT × (1 + 0.2M²), with temperatures in Kelvin. A recovery factor r modifies this to SAT × (1 + 0.2rM²). The factor belongs to a defined probe installation, not a universal correction selected by the pilot.

Ram rise is therefore a temperature difference caused by the measurement process. It is not the same as a warm air mass. The air-data computer can combine measured temperature and Mach to infer SAT. Probe heating prevents ice but its effect must be compensated by the design. In the model increase Mach at fixed SAT, then change recovery factor and explain why the recovered temperature changes.

Angle of attack is another physical input

An angle-of-attack vane aligns with local airflow; a null-seeking probe detects a pressure difference and rotates until it is balanced. The sensed local angle must be calibrated to the aircraft reference. Angle of attack is the angle between a chosen aircraft reference and the relative airflow, not pitch attitude, climb angle or flight-path angle. A high pitch attitude can coexist with different angles of attack as the flight path changes.

AoA inputs can serve stall warning, air-data corrections and flight-envelope protection. A frozen vane may continue to provide a plausible constant value, making a wrong but valid-looking signal more difficult than a clear loss flag. Sensor heating, channel comparison and fault monitoring are safeguards, but their actions depend on the aircraft. Cross-check disagreement using the approved procedure; a single coloured AoA cue is not a complete measure of aircraft energy or control margin.

Why can one air-data fault affect several systems?

A computer distributes processed measurements; it does not make the inputs independent.

Inputs become a consistent calculated set

An air-data computer receives total and static pressure and temperature, with configuration, AoA and stored calibration data where required. From them it computes quantities such as altitude, IAS, Mach, SAT, TAS, vertical speed and speed limits. It can correct known position, instrument, compressibility and density effects. The calculated outputs share relationships, so a wrong input can influence several apparently different displays together.

In a distributed system, air-data modules near probes can convert pressure into electrical data before transmission to a processor. This avoids some long pneumatic lines but introduces electrical power, data-bus and validity dependencies. A display failure can remove only one presentation, whereas a source or processor failure can remove multiple functions. Identify which part of the signal chain is unavailable rather than calling every disagreement an instrument failure.

Selection changes what the aircraft trusts

Air-data outputs may feed flight instruments, autoflight, thrust control, transponders, pressurisation and warning systems. Switching a display to another source can restore a presentation while leaving other consumers on the original source. Conversely, selecting a common source may make screens agree without removing the originating fault. A block diagram is useful because it distinguishes physical redundancy from the visual appearance of two screens.

An ADIRU combines air-data and inertial functions within an architecture that may have separate controls and consequences. Selecting an inertial channel off can have alignment implications that differ from changing an air-data display source. Follow the exact aircraft failure procedure, confirm the affected unit, and understand the downstream effects before an irreversible selection. The laboratory fault selector shows which outputs share a source, while its calculated numbers remain synthetic examples rather than a reconfiguration checklist.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.