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the investigator-in-charge shall immediately initiate action to ensure that the
aviation security authorities of the State(s) concerned are so informed.” In the case
of MI 185, this notification was necessary because the technical examination of the
aircraft wreckage revealed there was no evidence of a mechanical malfunction of the
aircraft structure, systems or powerplants that would have caused the aircraft to
depart cruise flight. Further, the HPG developed sufficient personal background
data pertaining to the captain to warrant the “relevant aviation security authorities”
to conduct a further investigation of the captain.
NTSC’S COMMENTS:
The notification to the relevant aviation security
authorities was not made because there was no evidence of a
mechanical malfunction of the aircraft structure, systems or
powerplants. The findings of the HPG at that time (July 99)
was taken into consideration in the notification.
2.3.3 Explanation to the Break Up of the Empennage
Close examination of the wreckage (Section 1.22) supports the results of the flutter
analysis (Section 2.3.2) and the trajectory analysis (Section 2.3.1).
The above results suggest that the separation of the empennage parts could have had
occurred at an altitude near or below 12,000 ft, due to an unstable flutter as the aircraft
exceeded 1.2 Vd.
N-38
These two sentences may be misleading. To provide clarification, the NTSC
draft Final Report should be revised to state that evidence indicates that the
separation of the empennage components/parts was not the cause of the
departure from cruise flight or the accident but was the result of an
overspeed condition that occurred after the airplane departed cruise flight.
NTSC’S COMMENTS:
The two sentences are satisfactory.
2.4 Power Control Units and Actuators
2.4.1 Main Rudder PCU
In the controlled laboratory test condition [Reference 16], it was found that problems
due to thermal shock can arise. This can happen if the warm hydraulic fluid (at +77°C)
rushes into a cold-soaked servo valve (at -40°C) causing the slides to expand against the
valve housing. In such a temperature difference, a valve jamming could occur causing the
rudder to move uncommanded or in a direction opposite to the rudder pedal command
(rudder reversal). In real flight, the hydraulic temperature would not reach that high
(+77°C) a level.
An introductory paragraph should be included in section 2.4.1 that explains
why the rudder PCU was examined and described in greater detail than the other
actuators/PCUs. This introduction would provide the reader with a brief
background about the known rudder PCU anomalies identified in previous
accidents and the reason for the additional examination.
Further, for clarification, the last two sentences in this section should be
revised to read, “In such a temperature difference, if the valve jams and the pilot
commands additional rudder input, the result could be an unintended rudder
movement in a direction opposite to commanded input (rudder reversal). However,
the temperature of the hydraulic fluid rising to +77°C is not likely in normal inflight
operations.
Finally, this section should contain a conclusion statement that indicates that
the investigation determined that the rudder PCU was not a cause or contributing
factor in the accident.
NTSC’S COMMENTS:
In view of the FAA 737 Flight Controls Engineering Test and
Evaluation Board Report and the FAA AD --- which recommends
that -- , the NTSC is of the view that this section should
not contain a conclusion statement on whether the rudder PCU
was or was not a cause or contributing factor in the
accident.
N-39
2.4.5 Horizontal Stabilizer Jackscrew
A malfunction affecting both trim switches on a control wheel could also cause a runaway.
It was not possible to ascertain if such an occurrence took place. However, had a
run-away occurred due to a malfunction of the main electrical trim system, it would take
about 10 seconds to change from 4.5 to 2.5 units (at a rate of trim change of 0.2 unit/sec
at flaps retracted position). The trim wheel would turn continuously. The movement of the
trim wheels and the sound produced would have been noticed by the pilots. Both pilots
were trained to recognize such a condition and to take appropriate corrective actions.
This paragraph should be modified to include a conclusion that based on the
evidence derived from the last recorded FDR position, the NTSB simulation and the
physical evidence found during the wreckage examination, the stabilizer trim was
moved to the full nose-down limit through pilot input to the main electric trim
　
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本文鏈接地址：NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(66)