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A modern transport jet is flying at its cruising altitude of 37 000 feet in normal cruise flight. Suddenly and without warning, the pilots realize that the autothrottle system moved the thrust levers to idle, accompanied with the FMA 'RETARD' message. In order to control the situation, the pilot flying increases the thrust back to its initial position and takes over manual control of the thrust levers. What could be the probable cause of this issue?

  • A
    The autothrust/autothrottle has automatically disengaged itself causing the engine thrust target to be idle.
  • B
    The autothrust/autothrottle computer has failed and as a consequence the default thrust setting of idle is applied automatically.
  • C
    A total failure of the radio altimeter, I.e. NO output, which can cause unexpected behaviour WITHOUT warnings.
  • D
    Spurious (Incorrect) radio altimeter indications, which can cause unexpected behaviour WITHOUT warnings.

If the Flight Mode Annunciator (FMA) displays "RETARD" during cruise flight, this would be an abnormal situation as the "RETARD" mode is normally only displayed during the approach and landing phase of flight.

The "RETARD" mode indicates that the autothrottle moves thrust levers to the aft stop. RETARD mode is followed by ARM mode which means that no autothrottle mode engaged. The pilot must manually adjust the thrust to maintain the desired speed or altitude.

If the radio altimeter were to provide intermittent or unreliable height information, the autothrottle system may become confused or unable to maintain a stable flight profile.

In the event of the FMA displaying "RETARD" during cruise flight, the pilot should immediately take action to manually adjust the engine thrust to maintain a safe and stable flight. The pilot should also follow the appropriate procedures to troubleshoot and correct the issue with the autothrottle system or its inputs, which may involve switching to an alternate system or using manual control of the engines for the remainder of the flight.

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