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(Items 32, 33, 34, 35, 36, 37 & 38 in Appendix E) (center and lower) Seven of the
actuators were recovered and examined. The actuators were found to be in the stowed
position.
Thrust reverser synchronization locks - two units per engine (four units per aircraft):
(Items 30 & 31 in Appendix E) - Two out of the four locks were recovered and examined.
The locks were found in locked positions.
1.12.3.3 Actuators Found in Non-neutral Position
During the tear down examination, the following components were found to be not in the
neutral position:
a) Main rudder power control unit (Item 1 in Appendix E) - The main rudder actuator
piston was found bent at the forward and aft ends. X-rays were taken to determine the
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internal state of the PCU. The rudder PCU was found in a position equivalent to 3
degree left rudder deflection. The measurement was made directly from the X-ray
film.
In view of the condition of the actuator piston and the fact that the actuator is of the
piston type, its position could have been changed on impact. Its position may not be
indicative of the last position at impact.
b) The servo valve of the main rudder PCU (Item 1.a. in Appendix E) was X-rayed using
computerized tomography prior to removal. The primary slide of the main rudder
PCU servo valve was found to be in the half-rate position while the secondary slide
was in the neutral position. The primary slide was able to move freely in the
secondary slide. The secondary slide had to be removed with a force of 260 pounds.
This could have been due to deformation of the servo valve housing during impact.
No corrosion and scratch marks were observed on the primary and secondary slides.
A mapping by means of a mini Boroscope was carried out on the internal surface of
the secondary slide and the servo valve housing and this was recorded on video. No
corrosion and scratch marks were observed on the internal surfaces of the secondary
slide and the servo valve, [Reference 10].
c) Inspection showed that the yaw damper or modulating piston inside the main rudder
PCU (Item 1 in Appendix E) was in a position equivalent to a half degree of left
rudder deflection.
d) The rudder trim actuator (Item 14 in Appendix E) was found to be close to neutral.
e) The cam of the feel and centering unit (Item 22 in Appendix E) had impact marks
indicating a left rudder deflection, (Figure 12).
f) Standby rudder power control unit (Item 2 in Appendix E)- The standby rudder PCU
was found in a position equivalent to one degree right rudder deflection. The
measurement was taken directly from the X-ray film. As the actuator is of the piston
type, the position may not be indicative of the last position at impact.
g) Aileron power control units and autopilot servos (Item 3 and 4 in Appendix E)– There
are two ailerons PCUs, which are mechanically linked to a control quadrant. The
piston of aileron PCU (3529) was found in a position equivalent to 3.5 degrees left
aileron down, while the piston of aileron PCU (3509) was found in a position
equivalent to 1.2 degrees left aileron up. There are two aileron autopilot servos (Item
11.a. & 11.b. in Appendix E). Both were found to be in the disengaged positions.
Disengagement of detent pistons prevents modulating piston inputs to the PCUs.
h) Elevator power control units and autopilot servos (Items 5 and 6 in Appendix E)-
There are two elevators PCUs which are mechanically linked to one output torque
tube connected to the left and right elevators. One PCU piston (3559) was found to be
in an equivalent elevator position of 1.5 degree trailing edge up. The elevator PCU
(3560) was found with the piston missing and its position could not be determined.
There are two elevator autopilot servos (Item 12.a. & 12.b. in Appendix E). Both were
found to be in the disengaged position. Disengagement of detent pistons prevents
modulating pistons inputs to the PCUs.
i) Horizontal stabilizer jackscrew (Item 23 in Appendix E) – The pictures of the broken
jackscrew are shown in Figure 13. The jackscrew ball-nut was found in a horizontal
stabilizer trim position of 2.5 units, equivalent to 0.5 degree horizontal stabilizer
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leading edge up4. During the tear down examination, the ball-nut could originally only
rotate approximately 1/8 inch around the circumference. Only when an additional
force is applied was it able to rotate an additional 3/8 inch. (Note: During cruise, as
shown by the FDR data, [Reference 2], the horizontal stabilizer was at about 4.5 units,
equivalent to 1.5 degree horizontal stabilizer leading edge down.)
j) Throttle box (Item 39 in Appendix E) - The throttle box had a power lever and a fuel
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(14)
