Air Data Computer (ADC): Inputs, Outputs, and TAS Computation Logic

022-02-08Pablo Asensio Martínez2026-03-271 min

The Air Data Computer (ADC) is a digital computer that processes inputs from various sensors to calculate accurate flight parameters for flight instruments and systems (EFIS, FMS, AFCS, Transponder, etc.).

Inputs

The ADC receives raw data from:

  1. Total Pressure ($P_t$): From Pitot tubes.
  2. Static Pressure ($P_s$): From Static ports.
  3. Total Air Temperature (TAT): From temperature probes (e.g., Rosemount).
  4. Angle of Attack (AoA): From alpha vanes/probes.

Outputs

Based on these inputs, the ADC computes:

  • Altitude: Pressure Altitude, Baro-corrected Altitude.
  • Speed: IAS, CAS, TAS, Mach Number.
    • Note: The ADC is essential for calculating TAS, as it compensates for position, compressibility, and density errors using the SAT derived from TAT input. ($TAS = f(CAS, P_s, SAT)$).
  • Temperature: SAT (Static Air Temperature) is calculated from TAT and Mach number.
  • Warnings: Overspeed (VMO/MMO), Stall warnings.

Redundancy

Modern aircraft typically possess two independent ADCs (ADC 1 and ADC 2) fed by independent sensors.

  • Failure: Switching facilities allow instruments to be fed from the alternate ADC if one fails.
  • Standby: Standby instruments (ASI, Altimeter) usually usually have their own direct pitot/static sources, separate from the ADC system.

Key Calculation

The ADC calculates Static Air Temperature (SAT), which cannot be measured directly in flight. It uses the measured TAT and the Mach number to derive SAT, which is then used to compute True Airspeed (TAS).