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The results suggested that a separation had taken place at a low altitude near or below
12,000 ft., while the aircraft was flying at a high Mach number.
2.3.2 Results of Flutter Studies
Flutter studies undertaken suggested that the applied static loads and aerodynamic flutter
margins developed during the cruise and descent above 20,000 feet are below the design
and certification requirements or specifications and thus do not support the separation of
the empennage structure above that altitude.
Below 18,000 ft altitude, the estimated descent speed exceeded the 1.2 Vd and an onset of
an empennage 22 Hz anti-symmetrical flutter mode was calculated to have had occurred
at approximately 5,000 ft altitude and 570 KEAS.
At approximately 3,000 ft and 600 KEAS a lower frequency 12 Hz anti-symmetrical
flutter mode was calculated to have had occurred.
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2.3.3 Explanation to the Break Up of the Empennage
Close examination of the wreckage (Section 1.12) 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.
2.4 Power Control Units and Actuators
2.4.1 Main Rudder PCU
As stated in Section 1.12.3.3.a, the position of the main rudder PCU actuator piston was
measured to be in a position equivalent to a 3° left rudder deflection. The main rudder
actuator piston was found bent at the forward and aft ends. It could not be determined
whether its position reflected the last position before impact.
The yaw damper or modulating piston showed an equivalent of 0.5° left rudder
deflection. However, as the modulating piston is a hydraulic piston, the position found
may not necessarily indicate that the piston was at that position before impact.
Examination of the servo valve of the main rudder PCU showed no scratch marks on the
surface of the slides, as well as in the internal bores of the valve. Corrosion deposits were
found in the cavities of the PCU and other actuators examined. The presence of corrosion
deposits could be due to immersion in the river water, or during the period of storage after
recovery from the riverbed.
During the inspection, a force of 260 pounds (about 130 kg) was required to remove the
secondary slide from the servo valve. This gripping force was due to a deformation of the
valve housing - which was constructed from a stack of wafers. The deformation was
caused by the impact forces on the PCU actuator rod and the servo valve body.
It is noted that some issues arose during investigation of the Colorado Spring (in 1991)
and Pittsburgh (1994) accidents involving B737 aircraft. One of the issues was a
possibility of the servo valve jamming due to thermal shock resulting in rudder reversal.
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.
The rudder PCU fitted on the aircraft complies with the applicable Airworthiness
Directives (AD) as described in Appendix J. The modifications to the PCU included one
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that enabled circulating of hydraulic fluid to warm up the valve to prevent the thermal
shock problem.
Another possible uncommanded rudder movement is due to a corrosion caused by waste
water to the yaw damper system components in the electrical and electronic (E&E)
compartment. Such an incident occurred to a B737 in Bournemouth, UK (in 1995). An
inspection of sister B737-300 aircraft of SilkAir was performed in March 1998. This
aircraft was selected because it was fitted with similar internal configuration and forward
galley modifications, as was the accident aircraft. The inspection showed that the E&E
compartment of the sister aircraft was clean and free from any contamination. It could not
be determined if the E&E compartment of accident aircraft was in similar condition.
However, assuming that the accident aircraft was in similar condition, there would not
have been corrosion caused by toilet water leaking into the E&E compartment.
2.4.2 Standby Rudder PCU
The standby rudder PCU was found at a position of 1° right rudder deflection, which was
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(23)
