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agreement with the analysis on the break up of the empennage as discussed in Section
2.3.
2.11 High Speed Descent Issues
2.11.1 Mach Trim System and its Function
The aircraft was equipped with a Mach Trim system to provide stability at the higher
operating speeds, i.e. higher Mach numbers. Mach trim is automatically accomplished
above Mach 0.615. When the Mach Trim system is operative it will normally compensate
for trim changes by adjusting the elevator with respect to the stabilizer, as the speed
increases. With the Mach Trim inoperative, the aircraft could exhibit a nose down
tendency ("Mach Tuck") as speed increases. However, the expected control forces to
overcome the “Mach Tuck” are light. Additionally when the speed exceeds the maximum
limit, audible overspeed warnings are activated.
Since the aircraft was cruising at subsonic speed (Mach 0.74) and trimmed for level
flight, the aircraft will eventually return to the trimmed condition after a minor speed
disturbance.
For the aircraft to dive, a significant disturbance resulting in an increasing speed must
have taken place. Such a disturbance could be initiated by changing aircraft elevator or
stabilizer trim. Should the airspeed increase to the point where it becomes transonic, and
as the lift resultant moves aft and local supersonic flow develops, the nose-down pitching
moment could be sufficiently large that the aircraft becomes speed unstable, i.e.
continuing speed increase of the aircraft. Once the aircraft is in a transonic dive, a
recovery from the dive becomes more difficult because of an increase in control column
forces, due to the aircraft’s increasing nose down pitching moment, as well as a large
reduction of elevator effectiveness due to the formation of shock induced air flow
separation in front of the elevator.
It is possible to recover from a transonic dive by timely action of the pilot, by reducing
thrust and deploying the speed brakes. Should the pilot not initiate a prompt recovery
action, the recovery becomes more difficult.
During the tear down examination, the mach trim was found in the fully retracted
position. The fact that this actuator was found in the retracted position may not
necessarily indicate that the mach trim system is a factor contributing to the accident.
2.11.2 Emergency Descent due to Fire, Smoke or Depressurization
An emergency descent is necessary when there is a rapid cabin depressurization or when
a fire or smoke occurs in flight. The procedure is to simultaneously retard the thrust
40
levers, deploy the speed brakes and initiate the descent12 (Appendix K). Some forward
stabilizer trim is applied to attain a dive that will accelerate the aircraft towards the
maximum speed limit. Once the maximum speed is reached aircraft is re-trimmed to
maintain the speed. This facilitates a limit on maximum rate of descent to the minimum
safe altitude.
The last pilot radio transmission about two and a half minutes before the descent sounded
normal and there was no mention of any in-flight fire or smoke. Furthermore,
examination of the wreckage showed no evidence of in-flight fire or explosion.
Examination of the recovered oxygen generators showed that they were not activated.
This indicated that there was no rapid depressurization at high altitude.
Based on the above findings, there was no indication of an emergency descent due to fire,
smoke or rapid depressurization.
2.12 General Human Performance Issues
This section analyses the general human performance issues such as medical, professional
qualification, training, fatigue, impairment, improper in-flight management, etc.
• The flight-crew's medical background and recent activities were examined. All
medical files reviewed showed no significant medical history. Medical personnel,
family, and friends reported both pilots to be in good health. Neither pilot flew in the
two days before the crash. Family and friends reported routine activities and rest
during that time period. Evidence from the CVR and the conduct of the flight (based
on FDR and ATC communications) indicated that neither pilot experienced any
medical difficulties in flight. Further, there was no evidence found of incapacitation or
impairment. Therefore, the investigation concludes that until the stoppage of the flight
recorders there was no evidence found to indicate that the performance of either pilot
was adversely affected by any medical, psychological or physiological condition.
• Both pilots held valid airman licenses and medical certification. They had received all
required training, including unusual attitude recovery training. At the time of the crash
both pilots were within duty-time limits. Peers, instructors, and supervisors described
　
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本文鏈接地址：NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(28)