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installed in the accident aircraft.
All four channels of the CVR recordings of the above three tests were analyzed using the
same NTSB signal processing software that was used to analyze the accident CVR
recording.
Several tests were done to document the sound that were recorded on the CVR during a
soft, hard and string pull of the CVR circuit breaker. The test closely matched the data
obtained from the second test (NTSB in-flight test above).
The NTSC draft Final Report’s discussion about the CVR (and DFDR later
in the report) requires a correction of terms so as to avoid confusion when
referencing the electrical power that is being supplied to both recorders. The
“power source” is where the CVR and DFDR receive their respective electrical
power from within the aircraft whereas the “power supply” is a component that is
integral to the CVR and DFDR units. The use of “power supply” when describing a
power interruption from the aircraft power source is incorrect because it implies
that an internal failure occurred or may have occurred within a respective recorder.
N-19
To eliminate any potential confusion regarding the electrical power being
supplied to the CVR and DFDR, the draft Final Report should be reviewed and the
term “electrical power source” inserted where the term “power supply” is currently
used.
NTSC’S COMMENTS:
The term “electrical power source” is meant to include the
external as well as the internal power supply to the
individual units.
In addition, examination and testing requested by the NTSC revealed no
evidence to suggest that a mechanical malfunction or failure of either the CVR or
DFDR caused either recorder to stop recording data.
The discussion of the postaccident CVR testing does not address the
distinctive 400 Hz tone (or “hum”) that was recorded on the CVR tape. For
purposes of clarity, the following information (an excerpt summary from the
February 20, 1998, Boeing test report) should be added to this section to discuss the
basis for the 400 Hz tone:
Power line hum components, located at multiples of 400 HZ were evident
in the CVR area microphone signal. The level of some of these hum
components increased when the overload was applied until the circuit
breaker popped. These levels were even more dramatic during the
ground fault conditions….It should be noted that some of these hum
components persisted in the signal, after the breaker has opened, to the
very end of the recorded data. Tones are good candidates for detection
amongst broadband boundary layer excitation. Tracking the amplitude
of a particular power-line hum component may indicate circuit overload
or faults.
NTSC’S COMMENTS:
It has been adequately covered in the report that failure of
the CVR is not due to an overload or a short circuit
condition.
1.18.3 PIC’s Background and Training
1.18.3.1 Professional Background in RSAF
The PIC joined the Republic of Singapore Air Force (RSAF) as a pilot trainee on 14 July
1975. He obtained his ‘wings’ (fully operational) on 25 March 1977. During his RSAF
career, the PIC flew many different types of fighter and training aircraft. He held senior
flying and instructing positions. In 1970s, the PIC was selected to joint the RSAF’s Black
Knights aerobatic team. He reached the rank of Captain in 1980 and was promoted to
N-20
Major in 1989. In 1991, the PIC applied to leave the RSAF under a voluntary early
release scheme. The PIC met the eligibility requirements for the early release scheme as
he was 35 years old and had at least six years in his immediate preceding rank. His
application was accepted.
The PIC’s reason to leave RSAF and join SilkAir was to keep flying and to spend more
time with his family.
The PIC obtained a US Federal Aviation Administration (FAA) Commercial Pilot
Licence on 19 November 1991 and an Air Transport Pilot Licence on 26 November 1991
in Benton Kansas. He left full-time employment in the RSAF on 29 February 1992. He
had approximately 4,100 hours flying experience at that time. The PIC served as a
squadron pilot in the RSAF on a part-time basis from 1 March 1992 to 30 April 1993. He
subsequently served in the military reserve, as a Major, in a non-flying capacity. In
January 1997, the PIC was promoted to Deputy Director Air Liaison Officer in his
reserve unit.
The NTSC draft Final Report’s description of the captain’s “Professional
Background in RSAF” should be expanded to include information developed by the
HPG regarding his military service or significant events that occurred during his
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(56)
