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The NTSC draft Final Report’s discussion regarding the stoppage of the FDR
needs to be revised to indicate that in addition to the possibilities mentioned, the
DFDR’s stoppage can also be explained by someone manually pulling the circuit
breaker. This discussion should also be revised to reflect that the DFDR is powered
through the same electrical bus (Electronics Bus 1) as ATC-1 (one of the airplane’s
two radar transponders) and the Mach trim actuator. The radar transponder
(which was likely ATC-1 during the accident flight) continued to operate and return
data for a short time after the DFDR stopped. In addition, the Mach trim actuator
was found at its high speed (not cruise speed) setting, indicating that it was powered
and operational during the airplane’s high-speed dive. It can be concluded that the
absence of a malfunction of the DFDR up to the point at which it stopped, combined
with the fact that the transponder continued to transmit and the Mach trim actuator
continued to operate after the DFDR had stopped, indicates that the stoppage was
not due to a loss of power to Electronics Bus 1. However, the stoppage could be
explained by someone manually pulling the circuit breaker.
The NTSC draft Final Report’s discussion of the SilkAir practice of flight
crews using ATC-1 when flying outbound from Singapore and ATC-2 when
returning to Singapore is not documented in either the AAIC Operations or HPG
reports. Further, this statement is contrary to information provided to the HPG
group that the transponder in use during a flight typically corresponds to the pilot
flying. Therefore, in the case of the accident flight, ATC-1 would have been
selected. It is suggested that this statement be corrected.
Finally, the NTSC draft Final Report should include a discussion of human
actions as a possible cause of the CVR and DFDR stoppage.
NTSC’S COMMENTS:
As the data pertaining to the use of the ATC-1 and ATC-2 is
not captured by the FDR, it cannot be confirmed for the
accident flight, whether ATC-1 or ATC-2 had been selected.
The NTSC draft Final Report contains a discussion of the
human aspects of stoppage of the CVR recording. Discussion
of the human aspects of stoppage of the FDR recordings was
not meaningful as there was no available CVR recording to
assist in the discussion.
2.10 Simulated Descent Profile
The last five ATC radar points recorded represent the flight trajectory of the aircraft
from the cruise altitude 35,000 feet to approximately 19,500 feet. Each point consisted of
data relating to time, altitude and geographical coordinates.
N-44
Simulator tests and computer simulation fly-out studies were done to determine failures
or combination of failures of the flight control and autopilot systems that could result in
the extreme descent trajectory. Aircraft flight data were not available for the time period
after the stoppage of the FDR. The initial condition for these tests and studies was cruise
configuration at 35,000 feet based on the last known FDR data. The altitude range for
the simulations was from 35,000 feet to approximately 19,500 feet.
The results of these simulation studies (Appendix G) are summarized as follows:
• Any single failure of the primary flight controls such as hard-over or jamming of
aileron, rudder or elevator did not result in a descent time history similar to that of
the last ATC radar points. In simulations of these flight control failure conditions the
aircraft could be recovered to normal flight manually.
• Any single failure of the secondary flight controls such as hard over or jamming of
yaw damper, or runaway of the stabilizer trim would not result in a descent time
history similar to that of the last ATC radar points. In simulations of these flight
control failure conditions the aircraft could be recovered to normal flight manually.
• Manipulation of the primary flight controls without horizontal stabilizer trim would
result in a descent time history similar to that of the last ATC radar points. But this
required large control column input forces and the aircraft was subjected to a
loading exceeding 2 G. However, if the control column input forces were relaxed, in
the simulations the aircraft would recover from the steep descent due to its inherent
stability.
• Among other possibilities, a combination of changing the stabilizer trim from about
4.5 to 2.5 units and an aileron input could result in a descent time history similar to
that of the last ATC radar points. This simulated descent trajectory would result in
the aircraft entering an accelerating spiral and being subjected to a loading of less
than 2 G. Furthermore, the aircraft would continue in the spiral even when the
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(69)
