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engine parts such as the severe rotational damage to the rotating parts of the engine.
A portion of the throttle box that was also included in the MEC had a portion of the MEC
throttle housing, power lever and shutdown lever still attached to it. The power lever was
found in the open position. It was measured that the power lever was approximately one
third of an inch (about 8 mm) away from the stop. This would be equivalent to
approximately 12 degrees from the full forward stop. However, due to the severe
deformation of this hardware, the accuracy can not be ascertained. It could not be
conclusively determined to which of the two MECs this hardware belonged.
These above findings are consistent with engine operation under power at the time of
impact. No evidence was found of pre-impact engine fire or uncontained engine failure,
[Reference 9].
1.12.3 PCU and Actuator Tear Down and Examination
Most of the actuators and power control units (PCU) were damaged at impact. Forty-nine
of the actuators recovered from the crash site were sent to Boeing Equipment Quality
Analysis (EQA) Laboratories in Seattle for tear down and examination from 28 April
1998 to 8 May 1998. The tear down and examinations were performed under the control
of representatives from AAIC (now NTSC), NTSB, FAA and Singapore MCIT.
1.12.3.1 Tear Down Activities
The activities of tear down and inspection of the actuators/servos are described in
[Reference 10].
Tear down activities were carried out as follows:
• The shipment boxes containing the actuators were in good condition with intact seals
on them.
• Positions of the hydraulic actuators/pistons and other parts were recorded using X-ray
i.e. 200 kV real time X-ray, 300 kV and 425 kV digital radiography and Industrial
Computed Tomography (ICT) modes.
• Measurements of the position of relevant mechanism, sleeves etc., were made directly
on the parts or from X-ray films (using a scale factor for each film).
• Due to deformation, the tear down had to be done by some machining operation. The
cutting operations were performed in such a way to minimize damage.
• The tear down activities were recorded in the form of written reports, video tapes and
still pictures, [Reference 10].
• Samples of hydraulic fluids and deposits found in the actuators and PCUs were taken
and analyzed in the Boeing Analytical Laboratory using Fourier Transform Infrared
15
(FTIR) spectroscopy, X-ray diffraction (XRD) and moisture content determination.
Analysis and characterization were also performed on the water and mud samples
from the river bottom taken on the 24 March 1998. The results of the analysis are
attached in [Reference 11].
1.12.3.2 Spoiler/Flap/Slat/Thrust Reverser Actuators
Flight spoiler actuators – four units per aircraft: (Items 7, 8, 9 & 10 in Appendix E) All
four flight spoiler actuators were recovered and examined. The actuators were found to be
in the fully retracted position (see Figure 18).
Outboard ground spoiler actuators – four units per aircraft: (Items 15, 16 & 17 in
Appendix E) Three out of the four actuators were recovered and examined. The actuators
were found to be in the retracted and locked.
Inboard ground spoiler actuators - four units per aircraft: (Items 18, 19, 20 & 21 in
Appendix E) All four actuators were recovered and examined. The actuators were found
to be in the fully extended and locked position.
Trailing edge flap ballscrews - eight units per aircraft: (Items 24, 25 & 26 in Appendix E)
Three out of the eight ballscrews were found and examined. The ballscrews were found
fully retracted. It is confirmed by the fact that the actuators were interconnected by a
single drive torque tube.
Leading edge flap actuators - four units per aircraft: (Items 40, 41 & 42 in Appendix E)
Three out of the four actuators were recovered and examined. Two actuators were found
to be in the retracted position and one was slightly extended (1.84 inches) with score
marks in the bottom of the bore near the retract position.
Leading edge slat actuators – six units per aircraft: (Items 43, 44, 45, 46, 47 & 48 in
Appendix E) All six actuators were recovered and examined. Five actuators were found to
be fully retracted and one was found to be slightly extended (0.5 inch).
Locking thrust reverser actuators - two units per engine (four units per aircraft): (Items
27, 28 & 29 in Appendix E) –Three out of the four actuators were recovered and
examined. Two actuators were found to be in the stowed and locked position, while the
third actuator was found in the stowed but unlocked position.
Non-locking thrust reverser actuators – four units per engine (eight units per aircraft):
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(13)
