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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 Section 1.11.1). 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.
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.
The conclusions presented by the NTSC regarding the stoppage of the CVR
are not in full agreement with the evidence. As previously stated in the comments to
the factual portion of the draft Final Report, postaccident examination and testing
proved that there were no mechanical malfunctions or failures of either the CVR or
DFDR that would have caused the recorders to stop recording data.
N-42
Further, it is highly unlikely that the CVR lost power because of a broken
wire, as the NTSC’s analysis suggests, without a related “short circuit” or power
loss to other systems in a related wiring bundle or electrical bus, which likely would
have been reflected on the DFDR. If the short circuit had occurred, the circuit
breaker would have popped, which would have been recorded on the CVR. NTSB
tests established that if an “overloaded and a short circuit” condition had occurred,
the sound of the circuit breaker popping is unique and loud enough to be identified
on the CVR area microphone channel on the ground and in flight. No such sound
was recorded on the CVR from MI 185.
Sufficient evidence has been documented, based on postaccident testing and
examination, to conclude that the failure was not the result of a “fault” or the CVR
“internal power supply and “hold up” capacitor, which appeared to be operating
normally. Additionally, postaccident examination and testing revealed that the
CVR recording exhibited characteristics that would be expected of a normal
electrical power shutdown of the CVR.
NTSC’S COMMENTS:
The evidence indicates that the stoppage of the CVR was not
due to a short circuit or an overload condition. As only a
limited amount of electrical wires was found, the failure of
the CVR as a result of a broken wire cannot be completely
ruled out.
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.
N-43
There were no evidence found that could explain the six-minute time difference between
stoppage of the CVR and FDR.
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(68)
