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necessarily indicate that the mach trim system is a factor contributing to the accident.
Because the Mach Trim system was not implicated as a cause or contributing
factor in the accident, the discussion regarding this system is irrelevant. Therefore,
it is strongly suggested that the discussion in this section be substantially reduced
and that a definitive conclusion be included indicating that there was no evidence of
a Mach Trim system failure that would have been causal or contributing to the
accident.
2.11.2 Emergency Descent due to Fire, Smoke or Depressurization
An emergency descent is necessary when there is a rapid cabin depressurization or when
a fire or smoke occurs in flight. The procedure is to simultaneously retard the thrust
levers, deploy the speed brakes and bank the aircraft to initiate the descent. (Appendix
K). Some forward stabilizer trim is applied to attain a dive which will accelerate the
aircraft towards the maximum speed limit. Once the maximum speed is reached aircraft
is re-trimmed to maintain the speed. This facilitates a limit on maximum rate of descent
to the minimum safe altitude.
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The last pilot radio transmission about two and a half minutes before the descent
sounded normal and there was no mention of any in-flight fire or smoke. Furthermore,
examination of the wreckage showed no evidence of in-flight fire or explosion.
Examination of the recovered oxygen generators showed that they were not activated.
This indicated that there was no rapid depressurization at high altitude.
Based on the above findings, there was no indication of an emergency descent due to fire,
smoke or rapid depressurization.
The first paragraph in this section is not a statement of analysis but of fact.
The information presented refers to procedures (included in the NTSC draft Final
Report as appendix K) to be employed by the flight crew in the event that an
emergency descent is necessary. Moreover, the statement that the emergency
descent procedures call for the pilot to “bank the aircraft to initiate the descent” is
incorrect. The procedures do not specify banking the aircraft as the method to be
used to initiate the descent. Because this information has no relationship to the
accident and implies that a true emergency descent profile is similar to the derived
profiles used in the simulator to match the accident descent profile, this statement
should be removed. However, if it is to remain in the analysis, it must be corrected
by removing the statement “banking the aircraft to initiate descent.”
2.13 Human Factors Aspects of the CVR and ATC Recordings
2.13.1 CVR
(a) The conversations and sounds recorded by the CVR before it stopped were examined.
The CVR transcript (Appendix A) showed that at 09:04:55 the PIC indicated his
intention to go to the passenger cabin " .… go back for a while …. finish your
plate….". At 09:05:00 the PIC offered water to the F/O, and at about the same time,
several metallic snapping sounds were recorded. Thirteen seconds later, at
09:05:13.6 the CVR ceased recording. Analysis of the recording indicated that the
metallic snapping sounds were made by a seatbelt buckle striking the floor. (See
Section 1.16.2)
(b) During the period recorded by the CVR, all door openings or closings were
related to pre-departure activities, in-flight meal service and normal pilot-cabin
crew interaction. In the four minutes following the last meal service, there were no
sounds associated with cockpit door opening or closing. After takeoff from Jakarta,
conversations within the flight deck were between pilot-to-pilot, pilot-to-flight
attendants, and normal pilot-to-ATC radio communications. During the flight, except
for cabin attendants serving meals and drinks to the pilots, there were no indications
of any other person(s) in the cockpit. It is concluded that after the last meal service
and until the stoppage of the CVR, the recording did not reveal any indications that
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person(s) other than the flight crew and cabin attendants attending to their duties were in
the cockpit.
(c) Analysis of the CVR stoppage indicated that the failure of the CVR could not
have been caused by a short circuit or overload. This is because either occurrence
would have resulted in the CVR recording a “pop” sound which was heard on the
test recording but not on the accident recording.
The CVR in-flight tests could not identify the sound of the CVR circuit breaker
being manually pulled as the ambient noise obscured the sound made. The accident
tape did not contain any identifiable sound attributable to manual pulling of the
CVR circuit breaker. It was not possible to determine from the CVR tests if there
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(71)
