Aviation Investigation Report A98H0003

1.6.8  Standby Flight Instruments (STI1-7)

Two standby flight instruments (one that displays the aircraft's attitude, and one that displays the aircraft's altitude and airspeed) are located in the centre of the lower instrument panel for use by the captain or first officer (see Figure 11). There was no provision for a self-contained, independent electrical power supply for standby communication and electronic navigation capability, nor was this required by regulation.

The standby attitude indicator (SAI), sometimes referred to as a gyro horizon, provides a vertical, stabilized reference that makes it possible to visually monitor the aircraft's attitude, in pitch and roll, with respect to the horizontal plane. The SR 111 SAI was self-contained and electrical power was being supplied by the aircraft's battery bus. A warning flag appears on the face of the instrument if electrical power to the unit is lost or removed, or if the gyro speed decays to a predetermined speed below which the gyro has insufficient rotational speed to provide reliable information.

The standby altimeter and airspeed indicator are combined in one instrument. They are connected to the auxiliary pitot and alternate static systems, and do not require electrical power to perform their intended function; electrical power is required for the vibrator that prevents the pointers from sticking.

Primary power for the two standby instruments' integral lights (STI1-8) was being supplied by the 115 V AC Bus 1 (phase B) circuit breaker (CB) B-523 (labelled MAIN & PED INSTR PNL LTG) located on the lower main CB panel at position A-13. The wiring for the primary electrical power circuit integral lights runs below the cockpit floor and not through any area where heat damage[28] was observed; therefore, there is no reason to suspect that these lights ceased to function. Back-up electrical power for the integral lights was supplied by the left emergency AC bus.

A direct-reading, standby magnetic compass (see Figure 11) is installed in the cockpit forward of the overhead panel on the windshield centre post. The instrument does not require electrical power to operate. Electrical power for lighting of the compass (STI1-9) was supplied by the 28 V direct current (DC) Bus 1. The switch for the compass light is on the overhead switch panel, near the compass. The standby compass is normally kept in a stowed position with the light off. As is the case with all direct-reading magnetic compasses, the accuracy of the instrument in the MD-11 is degraded when the aircraft is accelerating or decelerating, and when the aircraft is not in straight and level flight.

[28]     Heat damage is defined as damage caused by exposure to significantly elevated temperatures. This includes charring, melting, shrinkage, and discolouration of materials due to heat.

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