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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.
The CVR is equipped with an energy storage capacitor. The function of this capacitor is
to provide power for 250 milliseconds after electrical power is removed from the unit
such as when the aircraft power is switched from ground power to APU generators or the
engine generators. Another function of this capacitor is to enable continued recording for
another 250 milliseconds after power loss to the unit.
Had there been an overload or short circuit, the resultant popping of the CVR circuit
breaker in the cockpit would have been recorded as a unique and identifiable sound
signature by the CVR (see Sections 1.11.3 and 1.16.1). Based on the examination of the
results of the circuit breaker pull tests, there was no such sound signature in the MI 185
CVR recording found. This indicates that there were no short circuit or overload to cause
the CVR circuit breaker to pop out.
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The results of the CB pull tests showed that the sound signature associated with manual
pulling of the circuit breaker is obscured by the cockpit ambient noise. Hence, no
conclusion can be drawn whether the circuit breaker had been pulled manually.
A break in the wire supplying power to the CVR could also lead to CVR stoppage
without any sound being recorded on the CVR. However, from the limited quantity of
wiring recovered it could also not be determined if a break in the wiring had caused the
CVR to stop.
Thus, the cause of the CVR stoppage could not be concluded.
2.6.2 FDR Stoppage
The FDR stopped recording at 09:11:33.7, or 6 minutes and 18.1 seconds after the CVR
stoppage, and approximately 35.5 seconds before the aircraft started its descent, see
Section 1.11.1 and Figure 2. Data recorded by the FDR indicates that the flight was
normal until the FDR stoppage time. It was concluded that until the stoppage of the FDR,
there were no indications of unusual disturbance (e.g. atmospheric turbulence, clear air
turbulence, or jet stream upsets, etc.) or other events affecting the flight.
The FDR stoppage could have occurred due to a loss of power supply to the FDR, or the
malfunction of the unit itself.
The recording of the ATC radar plots during the descent of the aircraft until 19,500 ft
indicated that the aircraft ATC transponder continued operating after the FDR had
stopped recording. SilkAir stated that generally flight crews use ATC-1 flying outbound
from Singapore, and ATC-2 inbound. ATC-1 is on the same bus as the FDR, while ATC-
2 is powered from Elex Bus 2, i.e. a different power source. No conclusion could be
drawn as to the reasons for the CVR and FDR stoppage at different times.
The FDR was determined to be functioning normally until it stopped. The stoppage of the
FDR could not be determined from the available data.
There were no evidence found that could explain the six-minute time difference between
stoppage of the CVR and FDR.
2.7 Radio Transmission Voice Recognition
The last radio transmission on VHF from MI 185 was at 09:10:26 when it acknowledged
ATC’s call that MI 185 was abeam Palembang, to maintain FL350, and to contact
Singapore Control at PARDI, by responding “SilkAir one-eight-five roger, 134.4 before
PARDI”. Voice spectrum analysis identified that the F/O made this last radio
transmission, see Section 1.16.3. This information reveals that the F/O was in the cockpit
about 1.5 minutes prior to the descent.
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2.8 Maintenance Aspects
2.8.1 Aircraft Maintenance
The aircraft and the engines were found to be properly maintained by SIA Engineering
Company (SIAEC) in accordance with the Maintenance Schedule ref. MI/B737-300 Issue
No.1 and approved by the Civil Aviation Authority of Singapore (CAAS). The
maintenance task cards and inspection reports were reviewed. The required checks were
carried out within the stipulated period. There were no adverse findings on any
matter/defect that would contribute to the accident.
A review was carried out of the aircraft technical logbooks, deferred defect logbooks and
in-flight log defects. It did not reveal any defects that could have affected the
airworthiness of the aircraft.
Therefore, the inspection of the maintenance records did not reveal any defects that may
have contributed to the occurrence.
2.8.2 Patch Repair
On 24 April 1997 the aircraft was damaged in a ground incident, resulting in a dent to the
fuselage skin. The patch repair, designed and performed according to Boeing Structure
Repair Manual (SRM), was carried out by SIA Engineering Company on 28 April 1997.
The patch repair was recovered from the riverbed. A damage to the upper forward corner
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(26)
