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yaw damper modulating piston, the rudder trim actuator, the rudder trim and feel
centering unit, the standby rudder PCU, the aileron PCUs, the elevator PCUs, and
the horizontal stabilizer jack-screw components, revealed no indications or evidence
of pre-impact malfunctions.
Based on the evidence and postaccident testing, a definitive conclusion can be
made regarding the flight control systems. Therefore, it is suggested that the
NTSC’s draft Final Report be modified to include the following:
There was no evidence of a mechanical failure of any of the flight
control systems or related components that would have been causal or
contributing to the accident.
Also, it is suggested that the following conclusion be added for completeness:
Separation of the empennage components/parts were not the cause of
the departure from cruise flight or the resulting impact with terrain
but, rather, were the result of an overspeed condition that occurred
after the airplane departed cruise flight.
N-53
• Examination of the 370 kg of recovered electrical wires, connectors and circuit
boards showed no indication or evidence of corrosion, shorting, burning or arcing in
these wires or parts.
• The CVR stopped recording at 09:05:15.6 and the FDR stopped recording at
09:11:33.7. The examination of the CVR and FDR showed no malfunction of the
units. The stoppages could be attributed to a loss of power supply to the units.
However, there were no indications or evidence found to conclude on the reason for
the stoppages due to the loss of power. The cause of the CVR and FDR stoppages
and the reason for the time difference between the stoppages could not be concluded.
The NTSC draft Final Report suggests that the cessation of the CVR and
DFDR could in each case be explained by a broken wire. Although this is
technically correct, the probability of two such unrelated wire breaks occurring
several minutes apart and affecting only the CVR and DFDR is so highly
improbable that it cannot be considered a realistic possibility.
• The inspection of the aircraft maintenance records did not reveal any defects or
anomalies that could have affected the airworthiness of the aircraft or that may have
been a factor contributing to the accident.
• The horizontal stabilizer trim was found to be in the 2.5 units position which
matched the forward nose-down limit of the manual electrical trim.
This conclusion should be expanded to include a definitive statement that the
2.5 units of nose-down trim was the result of a sustained manual input and not
attributed to a malfunctioning system resulting in a “runaway.”
Flight Operations
• Weather and Air Traffic Control were not factors contributing to the accident.
• Audio spectral analyses on Air Traffic Control communications and the accident
CVR indicate that the last communication from the MI 185 at 09:10:26, occurring at
a position approximately abeam Palembang was performed by the F/O.
• The examination of the flight deck noise and sounds concludes that the metallic snap
recorded on the CVR was made by a seatbelt buckle hitting against a metal surface.
• Based on flight simulations, it was observed that the simulated descent trajectory
resulting from any single failure of flight control or autopilot system would not
match the radar data.
• Based on the same flight simulations, it was also observed that the trajectory shown
by the radar data could have been, among other possibilities, the result of the
N-54
combination of lateral and longitudinal inputs together with the horizontal stabilizer
trim input to its forward manual electrical trim limit of 2.5 units.
To clarify the conclusion at bullet 5, it is suggested that the following
sentence be added to the end:
Despite the stabilizer trim being at the 2.5 unit nose-down setting (its
forward limit), the aircraft would have remained controllable and
appropriate flight control input would return the airplane a normal
flight attitude.
Additionally, the following should be added to the draft Final Report:
No single mechanical failure of the airplane structure or flight control
systems was found that would have resulted in movement of the
airplane that matched the recorded radar data points. Further, there
was no evidence of any combination of systems failures. Thus, no
known or postulated mechanical failure was found that resulted in a
flight profile that matched the accident radar data. However,
changing the flight control input manually in multiple axes did
provide a flight profile that matched the last recorded ATC radar
data points. Therefore, it is probable that the airplane was likely
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(74)
