A limited range of electromechanical instruments are normally provided as back-up; alternatively, electronic standby flight instrument displays may be provided. Standby systems are fed from independent power supplies and sensors to ensure continuity of operation in the event of main system failure.
Standby Attitude Indicator
A self-contained, two coloured moving drum indicating aircraft attitude.
Standby Turn and Slip Indicator
A self-contained Turn and Slip, gyro driven rate of turn/slip indicator, with power warning flag
A conventional wet compass suspended in damping fluid used as a backup magnetic heading reference. Normally mounted in the forward window, away from metal and sources of electro-magnetic interference. Commonly accurate to within 10 degrees of magnetic north.
A combined mechanical Clock/Stopwatch with hands for hours, minutes, seconds, and time elapsed.
Standby Airspeed Indicator and Altimeter
Caution - Whilst standby airspeed indicators and altimeters provide vital information in the event of failures to digital air data systems, they will not provide reliable information if the fault is within the pitot static system to which they are connected. In the event of a power failure, the internal vibrator in the standby altimeter may cease to function and the instrument may be subject to lag in the descent. It should, therefore, be tapped regularly to ensure an accurate reading.
EU-OPS 1.652 "IFR or night operations - Flight and navigational instruments and associated equipment" contains the requirement [sub paragraph (l)] that:
"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 indicator (artificial horizon), capable of being used from either pilot’s station, that:
- 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;
- 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;
- operates independently of any other attitude indicating system;
- is operative automatically after total failure of the normal electrical generating system; and
- 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."