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control forces were relaxed. This would result in a descent at a speed exceeding 1.2
Vd, in agreement with the analysis on the break up of the empennage as discussed in
Section 2.3.
Bullet 3 should be modified as follows for correctness and clarity:
Although manipulation of the primary flight controls without
horizontal stabilizer trim would result in a descent time history
similar to that of the last ATC radar points, this would and large
control require control column forces greater than 50 pounds column
inputs. However, the simulations indicated that if the control column
input forces had been relaxed, the aircraft would have initiated a
return to a nose-up attitude due to its inherent stability.
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Bullet 4 should be modified as follows for correctness and clarity:
Among other possibilities, a combination of either control column
inputs and/or changing the stabilizer trim from about 4.5 to 2.5 units,
in combination with aileron inputs could result in a descent time
history similar to that of the last ATC radar points. This simulated
descent trajectory would result in the aircraft entering an accelerating
spiral and being subjected to a loading of less than 2 G. Furthermore,
the aircraft would continue in the spiral even when the control forces
were relaxed. This would result in a descent at a speed exceeding 1.2
Vd, which is in agreement with the analysis of the breakup of the
empennage as discussed in section 2.3.
Based on the data derived from the simulations, the following conclusion can
be made regarding the maneuvers necessary for the airplane to fly a profile similar
to that of MI 185:
No single mechanical failure of the airplane structure or flight
control systems was found that would have resulted in movement of
the airplane that matched the recorded radar data points. Further,
there was no evidence of any combination of systems failures. Thus,
no known or postulated mechanical failure was found that resulted in
a flight profile that matched the radar data. However, changing the
flight control input manually in multiple axes did provide a flight
profile that matched the last recorded ATC radar data points.
Therefore, it is probable that the airplane was likely responding to
sustained flight control inputs from the cockpit.
NTSC’S COMMENTS:
As there was no FDR data available from before the
commencement of descent up to time of impact, it is not
possible to conclude that the airplane was responding to
sustained flight control inputs from the cockpit.
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
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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,
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
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NTSC Aircraft Accident Report SILKAIR FLIGHT MI 185 BOEING B(70)
