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ATC radar points showed a much faster descent of the accident aircraft, i.e. 32 seconds
from 35,000 feet to 19,500 feet. Therefore, if the simulation was correct, the change of
horizontal stabilizer trim position alone would not have resulted in the fast descent after
leaving FL350.
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During autopilot operations, the trim operates automatically to correct changes in the
center of gravity position due to fuel use or movements of people along the aisle. Had a
run-away occurred during autopilot operations, the autopilot would disengage. The trim
wheel would also turn continuously. The warning from the autopilot disengagement and
the turning of the trim wheels would alert the pilots to this situation. Both pilots were
trained to recognize such a condition and to take appropriate corrective actions.
The manual trim wheel is available as a back-up in case there is a malfunction in the main
electrical trim system. It is possible to change the trim position using the trim wheel, but
the pilot would have to crank the wheel in a prolonged manner to effect a large trim
change. No evidence was found that the main electrical trim system had failed.
During normal operations, the main electrical trim switches on the control wheel are used
to neutralize the force on the control columns. In this mode of operation, pushing the
main electrical trim switches forward continuously would move the horizontal stabilizer
to a limit of 2.5 units when the flaps are retracted. The horizontal stabilizer trim position
was found in a 2.5 units position during the tear down examination. This matches the
forward limit of the main electrical trim.
2.4.6 Other Actuators
During the tear down examination, the following components were found to be in the
stowed or retracted position:
• Flight spoiler actuators
• Outboard ground spoiler actuators
• Inboard ground spoiler actuators
• Trailing edge flap ballscrews
• Leading edge flap actuators
• Leading edge slat actuators
• Mach trim actuator
• Thrust reverser actuators
The fact that these actuators were found in the stowed or retracted position does not
necessarily suggest that their respective systems were not activated during the descent. If
the respective systems remained in the stowed or retracted positions, they would not have
been factors contributing to the accident.
2.5 Powerplant
In the examination of the recovered engine parts, main control units and throttle box, the
following were found:
• There was no evidence found of melting or soot on recovered engine hardware,
indicating that there was no pre or post impact engine fire.
• All major rotating engine hardware, i.e. fan, compressor, high pressure and low
pressure turbine disks were recovered with no indications of high energy uncontained
engine failures.
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• Indications were found that both engines were operating at high engine rotation speed
at the point of impact.
Therefore, the engines were considered to be not a factor contributing to the accident.
2.6 Stoppage of the CVR and FDR
2.6.1 CVR Stoppage
The CVR recording ended while the aircraft was still cruising at an altitude of 35,000
feet, about seven minutes before the last radar return. Up to the CVR stoppage, the
conversation in the cockpit was consistent with normal flight operations.
The CVR stoppage could have occurred due to a malfunction of the unit itself or a loss of
power to the unit. The loss of power to the unit could be due to power interruption to the
Electronics Bus 1 that supplies power to the CVR, short circuit or overload, CVR circuit
breaker pulling or break in the wiring.
The entire two-hour recording was found normal. There were no observed anomalies
when power was transferred on the ground in Jakarta. It appeared that the recorder’s
internal energy storage capacitor was operating normally by providing continuous
recorder operation in spite of momentary aircraft electrical power interruptions,
[Reference 4].
The examination of the CVR unit performed by the manufacturer (Appendix F)
confirmed that the CVR was functioning properly. The recording had characteristics that
would be expected of a normal electrical power shutdown of the CVR. Therefore, the
stoppage of the CVR could be a result of the loss of power to the unit.
According to the aircraft wiring diagram 24-58-11 (Figure 17) the power to the CVR was
from the Electronics Bus 1 (Elex Bus 1). The Elex Bus 1 also supplies power to other
systems, such as the FDR, DME-1, TCAS, ATC-1 etc. Parameters of DME-1 and TCAS
were recorded in the FDR. Analysis of the FDR recording showed that six minutes after
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(25)
