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Which altimeter is used in case of Rad alt failure in the SAR modes of the AFCS?

  • A
    Doppler.
  • B
    Barometric.
  • C
    GPS.
  • D
    Inertial.

Refer to figures.
Beginning with the Doppler radar or Doppler velocity sensor, is an instrument that uses the Doppler effect to measure the velocity of the helicopter relative to the ground. It works by transmitting a radar signal that bounces off the ground and returns to the sensor, and the frequency shift in the returning signal is used to calculate the helicopter's speed and direction.

In addition, the Global Positioning System (GPS) provides highly accurate location information for a vertical fix.

Moreover, the inertial navigation system is a self contained system that can provide continues information on the helicopter’s:

  • position;
  • track;
  • heading and
  • groundspeed, without any external assistance.

Therefore, the altimeter used as a backup for the Radio Altimeter (Rad Alt) in Search and Rescue (SAR) modes of the Automatic Flight Control System (AFCS) is typically a Barometric Altimeter. It measures altitude based on atmospheric pressure rather than radio waves, and can provide an estimate of altitude in case the Rad Alt fails or encounters interference.

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