An air data computer, or ADC, is an avionics unit that turns pressure and temperature measurements into the flight numbers pilots use every second. It receives total pressure from the pitot tube, static pressure from static ports, and outside air temperature from a temperature probe. From these inputs, it calculates indicated airspeed, altitude, vertical speed, true airspeed, and Mach number.
This matters because accurate air data helps the pilot control the aircraft safely and helps autopilot and flight management systems make correct decisions.
The ADC compares pitot total pressure with static pressure to find dynamic pressure, which is linked to airspeed. It uses static pressure to estimate altitude because atmospheric pressure decreases with height. Temperature correction is needed because air density and speed of sound depend on temperature, so true airspeed and Mach cannot be found from pressure alone.
Modern ADCs condition sensor signals, convert them to digital values, apply calibration and correction algorithms, then send clean data to cockpit displays and other aircraft systems.
Key Facts
- Dynamic pressure is found from q = Pt - Ps, where Pt is pitot total pressure and Ps is static pressure.
- For low-speed incompressible flow, dynamic pressure relates to speed by q = 1/2 rho v^2.
- Indicated airspeed is mainly based on the pressure difference between pitot and static pressure.
- Altitude is computed from static pressure using a standard atmosphere model.
- Mach number is M = v / a, where v is true airspeed and a is the local speed of sound.
- Speed of sound depends on temperature: a = sqrt(gamma R T).
Vocabulary
- Air Data Computer
- An avionics computer that converts pressure and temperature sensor inputs into airspeed, altitude, Mach number, and related flight data.
- Pitot Pressure
- The total pressure measured by a pitot tube, combining static pressure and pressure caused by the aircraft moving through the air.
- Static Pressure
- The pressure of the surrounding air measured by static ports, used to determine altitude and as a reference for airspeed.
- Dynamic Pressure
- The pressure difference caused by motion through the air, calculated as total pressure minus static pressure.
- Mach Number
- The ratio of an aircraft's true airspeed to the local speed of sound.
Common Mistakes to Avoid
- Using pitot pressure alone for airspeed is wrong because airspeed depends on the difference between total pressure and static pressure.
- Treating indicated airspeed and true airspeed as the same is wrong because true airspeed changes with air density and temperature.
- Ignoring temperature when finding Mach number is wrong because the speed of sound depends on absolute temperature.
- Assuming static pressure only affects altitude is wrong because static pressure is also needed as the reference pressure for airspeed and Mach calculations.
Practice Questions
- 1 An ADC measures pitot total pressure Pt = 86.0 kPa and static pressure Ps = 80.0 kPa. What is the dynamic pressure q?
- 2 At a certain altitude, the air density is 0.90 kg/m^3 and the dynamic pressure is 5000 Pa. Using q = 1/2 rho v^2, find the airspeed v in m/s.
- 3 A pilot sees the airspeed display drop while altitude appears normal. Explain whether a blocked pitot tube or blocked static port is more likely, and justify your answer using how the ADC uses pressure inputs.