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system and not due to an “uncommanded” or “runaway” trim condition.
NTSC’S COMMENTS:
There is no evidence to conclude that pilot input was
responsible for the final position of the stabilizer.
The effect of a system run-away of the horizontal stabilizer trim was simulated in the
Garuda Indonesia Training Simulator as well as Boeing M-Cab Simulator, see Appendix
G. A trim change from 4.5 to 2.5 units changed the aircraft attitude from a nose-up to a
nose-down attitude. The simulator results showed that, with such a trim change, it took 1
minute and 23 seconds to descend from 35,000 feet to 19,500 feet. However, the last five
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.
The phrase in the preceding paragraph, “if the simulation was correct”
should be removed unless there are specific doubts regarding the accuracy of the
simulation. If there is evidence to support the accuracy or inaccuracy of the
simulation, this information should be discussed in detail in the factual and the
analysis. Further, Boeing does not have any additional qualifiers about the
simulation accuracy other than the verification by flight test to .89 Mach and
extrapolation to .99 Mach.
NTSC’S COMMENTS:
This qualifying phrase recognises that simulation tests
cannot fully replicate actual flight conditions especially
flight conditions beyond the normal flight envelope.
N-40
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.
There was sufficient evidence to indicate that the actuators had performed as
intended. Thus, this section should be modified to include a conclusion that there
was no evidence of a mechanical malfunction or failure of any flight control PCU or
actuator that either caused or contributed to MI 185’s departure from cruise flight
or the resulting accident.
NTSC’S COMMENTS:
As only 73% of the highly fragmented wreckage was recovered,
airplane-related mechanical malfunctions or failures can not
be totally ruled out.
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
conversations in the cockpit was consistent with normal flight operation.
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
N-41
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 16) 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
the CVR stopped, the FDR was still recording TCAS and DME-1 parameters. This
indicates that the CVR stoppage was not due to power loss at Elex Bus 1.
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(67)
