F-4B/N Air Data Computer

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The air data computer receives inputs of static pressure, pitot pressure, total temperature and angle of attack. These inputs are corrected in the ADC to compensate for errors in the sensing equipment.

The ADC can be toggles using the three-position switch found inboard of the throttle levers. The three positions are:

  • OFF: ADC is turned off and no error corrections are made
  • NORM: Normal ADC operation, providing corrected input to the altimeter and airspeed instruments
  • RESET: Spring loaded position that resets the ADC. It is not simulated and functions the same as the OFF position when held there.

The ADC should normally be set to NORM.

Instrument errors

Airspeed indicator

The airspeed indicator shows aircraft airspeed and mach. When the ADC is operating, the indications are Calibrated Airspeed and True Mach. When the ADC is OFF, it outputs Indicated Airspeed and Indicated Mach.

When the ADC is inoperative and the aircraft is transitioning from subsonic to supersonic speeds, an erroneous airspeed signal is generated, as shown in the following charts.

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Altimeter indicator

Barometric altimeter indications will differ from the real value based on two factors:

  • Altimeter lag
  • Altimeter error

ADC operation will fully compensate for altimeter error and significantly reduce altimeter lag.

Altimeter lag

When the aircraft is in a dive, indicated altitude is higher than the actual one, because the needle "lags" behind the actual altitude. This error increases with dive angle and speed and is illustrated below.

Altimeter error

Altimeter error appears when:

  • transitioning from transonic to supersonic speeds and remains until the aircraft decelerates to subsonic speeds.
  • in landing configuration with half or full flaps. This differs between early F-4B aircraft and later F-4B/F-4N with drooped ailerons

==== Aircraft without drooped ailerons (early F-4B)

==== Aircraft with drooped ailerons (late F-4B and F-4N)

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