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activity in Singapore.
1.16 Tests and Research
1.16.1 CVR Circuit Breaker Actuation Test
Upon the completion of data readout by NTSB, the CVR was taken to AlliedSignal
on 22 January 1998 for further testing. This testing was an attempt to verify if the
termination of the CVR recording was due to loss of power by the pulling of the CVR
circuit breaker or other means. The result was inconclusive. Therefore other tests had to
be performed, see Appendix F.
N-17
There were three tests conducted in a B737-300 aircraft to investigate the CVR
circuit breaker actuation sound signature.
The first test
The first test was carried out on the ground by NTSB and Boeing on 5 February 1998.
The reason for this test was to have quiet ambient condition to provide the best
opportunity for detection of circuit breaker actuation sound signature. The result showed
that the CVR cockpit area microphone did record the CVR circuit breaker actuation.
Actuation of a circuit breaker nearby gave a similar result.
The second test
The test (consisting of on-ground and in-flight tests) was conducted on 14 May 1998 and
15 May 1998 by NTSB.
The purpose of the ground test was to obtain an on-plane, on-ground CVR
recording of the CVR circuit breaker actuation, and the purpose of the flight test was to
obtain an on-plane, in-flight CVR recording of the CVR circuit breaker opening. In both
tests the circuit breaker was actuated manually and through the introduction of faults to
the aircraft’s wiring, i.e. short circuit and overload.
The results of these tests were compared with the accident CVR recording sound
signatures. In the short circuit tests a distinctive 400 Hz tone is recorded on one or more
of the CVR channels. No corresponding signatures could be identified on the accident
recording. The same tests found that the area microphone is able to pick up a distinctive
and identifiable snap sound that the circuit breaker makes when it is violently tripped by
a short circuit. (Note: The CVR continues to run for 250 milliseconds before it runs out of
power from the capacitor. As sounds travel about one foot per millisecond, it would take
only six milliseconds to travel the approximately six feet distance from the circuit breaker
to the area microphone. Hence the CVR is able to record the snap sound of the circuit
breaker.)
The overload tests yielded similar results as the short circuit tests except that
there was a slight time delay for the circuit breaker to trip and the snap sound was
quieter but still identifiable. No corresponding sound signatures could be found in the
accident recording.
The last set of tests was to examine the sound signatures when the CVR circuit
breaker was manually pulled. The snap sound was identifiable on the ground without
engines and air-conditioning operating. However in the flight tests, the addition of the
background cockpit noise present during normal cruise obscures the sounds associated
with the manual in-flight pulling of the cockpit circuit breaker. No corresponding sound
signatures could be found in the accident recording.
N-18
The summaries of the results of the second tests are as follows:
• During an overload and a short circuit, the sound of the circuit breaker popping is
loud enough to be identified on the CVR‘s area microphone channel, both on the
ground and in-flight.
• During an overload and a short circuit, the CVR records unique and identifiable
sound signature on one or more of the channels, both on the ground and in-flight.
• During the manual pull test on the ground, the sound of the circuit breaker is loud
enough to be identified on the CVR recording.
• In cruise flight, normal cockpit background noise obscured the manual circuit
breaker pull sounds. There are no unique electronic identifying sound signature
recorded on the CVR.
The third test
The test was conducted in-flight using a B-737 SilkAir sister aircraft in Singapore on 16
October 1998 and supervised by the Indonesian NTSC, an FAA avionic inspector
(representing NTSB) and Singapore MCIT representatives.
In the third test, several scenarios were performed where the CVR circuit breaker in the
cockpit was manually pulled. The manual pulls were categorized as “soft”, “hard” and
“string” pull. The soft pull was by pulling the circuit breaker with minimum noise. The
hard pull was by pulling the circuit breaker normally. The string pull was by pulling on a
string that was attached to the circuit breaker. This was to simulate a short circuit
causing the circuit breaker to pop out.
All the tests were conducted with an identical AlliedSignal SSCVR 2-hours recorder as
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本文链接地址：
NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(55)
