Aviation Investigation Report A98H0003

1.12.6  Examination of Standby Instruments

The SAI was recovered and examined to determine whether it was electrically powered and functional prior to impact, and to determine whether the indicator could help determine the attitude of the aircraft at the time of impact. The indicator's casing was extensively deformed from impact damage. This deformation resulted in capturing the pitch and roll indicators at approximately 110 degrees right bank and 20 degrees pitch down. The attitude scale showed a distinct red imprint that matched the shape and size of the red warning flag. The flag comes fully into view in less than one second with the loss of electrical power to the unit, or if the rotational speed of the gyro falls below 18 000 rpm. The location of the imprint showed that the warning flag had been fully displayed at the time of impact.

The shaft of the gyro rotor mass was fractured in torsional overload; there were superimposed rotational rub markings. Both the rotor mass, and the housing, exhibited surface rub. There was a transfer of machine tool markings between the two. This is indicative of there having been high rotational speed when the rotor mass contacted the enclosure, which would have occurred when the support shaft fractured. Therefore, it is concluded that at the time of impact, the gyro was rotating with high rotational energy.

If electrical power to the SAI is lost, the rate of spool-down of the gyro will allow the indicator to continue to provide reliable attitude information for five to six minutes. Electrical power to the unit is provided from the aircraft's battery bus, through CB C-01 located on the overhead CB panel.

The only part of the standby altimeter/airspeed indicator that was identifiable was the airspeed dial drum. The dial drum was extensively deformed; however, it exhibited two distinct indentations at the 80 knot and 120 knot graduations. These two indentations appear to have been made at the time of impact by contact with two internal supports that were located at the rear of the indicator, but in close proximity to the drum as it rotates. Using a serviceable airspeed indicator, the indentations on the airspeed dial drum were lined up with the internal supports. The comparison placed the 300 knot graduation at the airspeed index mark, indicating that the aircraft was travelling at about 300 knots at the time of impact.

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Date modified :
2012-07-27