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in conflict with the position of the main rudder PCU, which was at 3° left rudder. The
standby rudder PCU is normally unpressurized. (The standby rudder PCU is activated by
selecting a switch located on the forward overhead panel to the “Standby” position when
there is failure of normal system).
As the standby PCU was a piston type and furthermore it was unpressurized, its position
could change on impact. Its position may not be indicative of the rudder position at
impact.
2.4.3 Aileron PCU
Examination of the Aileron PCU 3529 indicated a 3.5° left aileron down position, while
the Aileron PCU 3509 indicated 1.2° left aileron up position. Both actuators were piston
type and were mechanically linked, the evidence found was contradictory, and therefore
the position of the ailerons at impact could not be determined.
2.4.4 Elevator PCU
The Elevator PCU 3559 was found to be in an elevator position of 1.5° trailing edge up,
while the Elevator PCU 3560 was found with the piston missing and therefore the
elevator position could not be determined. Both PCU actuators were mechanically linked
to an output torque tube connected to the left and right elevators.
As the elevator PCU actuators were piston type, their position could change on impact.
Therefore the position of the elevator at impact could not be determined.
2.4.5 Horizontal Stabilizer Jackscrew
During the tear-down examination, the horizontal stabilizer jackscrew was found in a
position equivalent to a horizontal stabilizer trim position of 2.5 units. It was also
observed that the ball nut could not be moved freely due to impact deformation. The
upper end counter-bore (aft portion) was ovalized by the screw-shaft being forcibly
33
extracted from the ball nut. The ball nut could be rotated 1/8 inch around the
circumference, and only when an additional force is applied was it able to rotate an
additional 3/8 inch, [Reference 10].
FDR data showed that the horizontal stabilizer position was at an average of 4.5 units
while the aircraft was cruising at 35,000 ft. At this trim setting, the horizontal stabilizer
leading edge is in a down position and corresponds with an aircraft nose-up attitude,
[Reference 17].
The last data on the FDR showed the horizontal stabilizer trim was at 4.58 units, while the
horizontal stabilizer jackscrew was found in a position equivalent to a horizontal
stabilizer trim position of 2.5 units. The difference in the trim positions indicated a
change of about 2.0 units (degrees) of the horizontal stabilizer position.
The horizontal stabilizer trim normally can be operated by three ways:
• by the autopilot trim (e.g. to automatically correct any out-of-trim conditions);
• by manipulating the main electrical trim switches on the pilots’ control wheels (e.g.
during manual flying); or
• by cranking the stabilizer trim wheels (e.g. when the main electrical trim fails).
Uncontrolled movement of the horizontal stabilizers could occur (e.g. a stabilizer runaway).
If such a run-away occurs, in a normal situation, it can be overcome by moving
the control column in the opposite direction.
To operate the horizontal stabilizer using the main electric trim system, the two switches
on each of the control wheels have to be operated simultaneously, one to power the
electric motor and two to release the brake and connect the clutch of the jackscrew. With
such a switch arrangement, operation of one switch or a malfunction in one circuitry
would not change the jackscrew position.
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. (But, at least one
Service Bulletin reported a malfunction during an approach maneuver, see Appendix L.)
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.
The effect of a system run-away of the horizontal stabilizer trim was simulated in the
Garuda Indonesia Training Simulator as well as Boeing M-Cab Simulator, see Appendix
G. A trim change from 4.5 to 2.5 units changed the aircraft attitude from a nose-up to a
nose-down attitude. The simulator results showed that, with such a trim change, it took 1
minute and 23 seconds to descend from 35,000 feet to 19,500 feet. However, the last five
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(24)
