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At the end of a flight, the pilot may assess the accuracy of an IRU:

  • A

    By comparing the position after landing with the one on the IRS.

  • B

    By checking the amber NAV END light.

  • C
    By checking the red END light.
  • D
    By checking the air speed.

Inertial reference systems (IRS) are usually composed of up to three individual inertial reference units (IRU), each of which has its own laser gyros and accelerometers supplying data to separate computers. Every IRS output is available in up to three independent copies, which are likely to differ slightly. Greater discrepancies indicate that at least one unit is drifting. To assess the performance of individual IRUs after a flight, the pilot may:

  • Review the residual indicated ground speed when the aircraft has parked;
  • Review the drift given as NM/h.
This can be done by comparing a known position, such as the coordinates of the aircraft parking stand with the calculated positions of the inertial systems.

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