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According to CS-25, in case of an EFIS failure, standby instruments must normally be…
  • A
    available without additional flight crew action.
  • B
    switched on manually by the flight crew.
  • C
    switched on manually. or in case of a combination of failures, switched on automatically
  • D
    available upon activation by the Commander or Co-pilot.

Refer to figure.
Many modern aeroplanes employ integrated flight systems, which include indicators that can display not only pitch and roll attitude data from a remotely located vertical axis gyroscope, but also associated guidance data from radio navigation systems. In these systems there is no longer a need for a separate artificial horizon to be fitted, but one is fitted in order to satisfy the airworthiness requirements as a standby attitude indicator. This provides the necessary indication should the circuits controlling the aeroplane attitude display fail.

Additionally, COMMISSION REGULATION (EC) No 859/2008 states, for IFR or night operations — Flight and navigational instruments and associated equipment (OPS 1.652)

(l) Those aeroplanes with a maximum certificated take-off mass in excess of 5 700 kg or having a maximum approved passenger seating configuration of more than nine seats must be equipped with an additional, standby, attitude indi- cator (artificial horizon), capable of being used from either pilot’s station, that:

1. is powered continuously during normal operation and, after a total failure of the normal electrical generating system is powered from a source independent of the normal electrical generating system;
2. provides reliable operation for a minimum of 30 minutes after total failure of the normal electrical generating system, taking into account other loads on the emergency power supply and operational procedures;
3. operates independently of any other attitude indicating system;
4. is operative automatically after total failure of the normal electrical generating system; and
5. is appropriately illuminated during all phases of operation, except for aeroplanes with a maximum certificated take-off mass of 5 700 kg or less, already registered in a Member State on 1 April 1995, equipped with a standby attitude indicator in the left-hand instrument panel.

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