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Sound Barrier (1952)

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  • #16
    This was not a deep stall incident Bonekicker!
    I think it might have been on a Douglas DC8 - they did suffer from problems with the Pitch Trim Compensator in early days.
    Here is an extract from an Airline Safety website which describes various control related airline accidents.

    An Eastern Airlines DC-8 crashed into Lake Pontchartrain about 5 minutes after taking off from the New Orleans Moisant Airport. All 58 on board perished. The water was only 20 ft. deep, yet only 60 % of the wreckage was recovered, because the breakup was so extensive. The FDR tape was too damaged to help the analysis. Instead, they used the maintenance records of that plane, and of other DC-8s, to conclude that the pilots had trimmed the stabilizer to the full nosedown position to counter the excessive noseup attitude that, in turn, was caused by a malfunctioning PTC (Pitch Trim Compensator) that had extended too far. Then, when the upset occurred, they could not trim the stabilizer back to the noseup position because the severe forces, generated by their pulling back on the yoke, stalled those jack screws.

    They tried putting the engines into reverse (inboard engine reversing in-flight was allowed on the DC-8), and that nearly worked; the plane was almost level when it hit the lake.

    Accident investigators found other instances of misrigging of the PTC, during the course of that investigation. A bushing was installed upside down on the Eastern plane, which would have caused the PTC to extend even further.

    As a result of this accident, modifications were made to the DC-8 stabilizer trim system:

    · A warning light was installed to alert the pilots when a PTC malfunctions and begins extending too far.

    · The nose down travel limits of the horizontal stabilizer was reduced.

    · The PTC actuator bell-crank arm was redesigned.

    · Changes in flight crew and maintenance training in how to deal with PTC unwanted extension or complete malfunctions.

    After these accidents, changes were made in pilot training. It became a “No No” to use horizontal stabilizer trim to ride out turbulence of any kind. The new procedure was to disconnect the autopilot and seek to maintain a “reasonable” – not “perfect” -- attitude, using only the elevators. Maintaining speed and/or altitude were to become secondary considerations; maintaining attitude was primary in moderate to severe turbulence, to prevent jet upset.

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    • #17
      I didn't know about the DC8 having such problems, so I'll have to read up on it. Its in many ways a classic example of a chain of events which by themselves might have been OK, but together became a disaster.

      There is of course the story in Fate is the Hunter of the DC6 which survives only because of the accidentally perfect combination of angle of attack and throttle setting, which was actually discussed the other day on DKos http://www.dailykos.com/story/2017/0...e-Hell-You-Say I suspect you'll like the 'Kossack Air Force' threads - Major Kong and others has done some great diaries.

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      • #18
        I will have a read of that thanks BK
        Many of the airliner control incidents have happened whilst flying on autopilot,I remember many years ago there was a 707? cruising at high altitude (41,000' ?) on autopilot,one of the outboard engines wound down very subtly and gradually without causing any flight deck captions/warnings - the autopilot just kept feeding in gradual rudder and aileron inputs to keep the a/c straight and level until of course eventually the control settings were outside the autopilot parameters and the autopilot disconnected LOL - all the controls snapped back to neutral and the Assymetric thrust caused by the other (still producing cruise thrust) outboard engine yawed the aircraft severely enough to invert the aircraft - the crew eventually managed to regain control at much lower altitude and there were of course injuries on board,one reason why it is always a good idea to stay strapped in whilst flying.

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