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would have been in the best position to manually pull the CVR circuit breaker at the time
that it stopped. (It should be noted that the captain had pulled a CVR circuit breaker on a
previous occasion.4)
The DFDR stopped recording approximately 6 minutes after the CVR stopped
recording. There was no evidence of any malfunction of the DFDR until the moment it
stopped recording. Examination of other aircraft systems and the review of the air traffic
control radar tapes revealed that the DFDR is powered through the same electrical bus
(Electronics Bus 1) as ATC-1 (one of the airplane’s two radar transponders) and the Mach
trim actuator. The radar transponder (which was likely ATC-1 during the accident flight)
continued to operate and return data for a short time after the DFDR stopped. In
3 The DFDR gave no indication of any other electrical problems associated with the cessation of the CVR or
electrical problems preceding the subsequent cessation of the DFDR.
4 The NTSC draft Final Report acknowledges this incident in section 2.14.3, which describes the incident as
follows: “for non-technical reasons the PIC infringed a standard operating procedure, i.e., with the intention
to preserve a conversation between the PIC and his copilot, the PIC pulled out the CVR circuit breaker, but
the PIC reset the circuit breaker in its original position before the flight.”
N-9
addition, the Mach trim actuator was found at its high speed (not cruise speed) setting,
indicating that it was powered and operational during the airplane’s high-speed dive. It
can be concluded that the absence of a malfunction of the DFDR up to the point at which
it stopped, combined with the fact that the transponder continued to transmit and the
Mach trim actuator continued to operate after the DFDR had stopped, indicates that the
stoppage was not due to a loss of power to Electronics Bus 1. However, the stoppage
could be explained by someone manually pulling the circuit breaker.
NTSC’S COMMENTS:
As mentioned above, although it may be possible that the CVR
was intentionally disconnected, there is no evidence to
suggest as such.
The NTSC draft Final Report suggests that the cessation of the CVR and DFDR
could in each case be explained by a broken wire. Although this is technically correct, the
probability of two such unrelated wire breaks occurring several minutes apart and
affecting only the CVR and DFDR is so highly improbable that it cannot be considered a
realistic possibility.
NTSC’S COMMENTS:
• The draft final report stated that “A break in the wire
supplying power to the CVR could also lead to CVR
stoppage without any sound being recorded. However, from
the limited quantity of wiring recovered, it could not be
determined if a break in the wiring had caused the CVR to
stop.” The report did not conclude or rule out that the
stoppage of the CVR was due to a broken wire.
• The draft final report did not make any statement linking
FDR stoppage with wire breakage as implied by NTSB.
N-10
c) Recovery of the airplane was possible but not attempted.
The NTSC draft Final Report contains a recommendation that flight crews be
trained in “recovery from high speed flight upsets beyond the normal flight envelope…to
enhance pilot awareness on the possibility of unexpected hazardous flight situations.”
This recommendation implies that the NTSC has concluded that the accident may have
been caused by an unexpected unusual flight upset and that the flight crew was not
properly trained to recover from such an upset. However, such a conclusion is not
supported by the evidence.
NTSC’S COMMENTS:
The evidence showed that the airplane had exceeded its
flight envelope during its high-speed transonic descent.
The recommendation does not in any way imply that NTSC has
concluded that the accident may have been caused by an
unexpected unusual flight upset. This recommendation is to
generate crew awareness of the narrow margin between the
normal high-speed flight regime and the limits of the
flight envelope, and the hazards of exceeding the normal
flight envelope.
Regardless of the reason for the airplane’s departure from cruise flight, it could
have been easily recovered using conventional techniques that both pilots had received
training for and that were within the capabilities of both pilots. Further, there was ample
time for the pilots to take such corrective action to return the airplane to a straight and
level attitude and flight. Both pilots had training in unusual attitudes, and the captain was
an accomplished fighter pilot adept at aerobatic maneuvers as evidenced by his
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(51)
