Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/14

 climb, the noseover, and during at least most of the dive. The angle-of-attack,pitch attitude, equivalent elevator angle, and stick force time histories resulting from the digital computer study, coupled with the derived flightpath in space (see attachment D) provided a graphic picture of the final maneuver and a clearer understanding of the problems confronting the crew. Perhaps the most significant and initially puzzling finding was that the maneuver required (a) full nosedown stabilizer trim and full down elevator, (bl full down elevator for about eight seconds and (c) a return to the full up elevator position about nine seconds later. This one finding was perhaps the most convincing of all the evidence indicating an essentially intact aircraft down to a lower altitude, even when the inherent limitations of the overall digital study were taken into account. The Board is cognizant of the fact that this study was prepared on the basis of operation in still air, and that the study results would be somewhat modified if it had been possible to incorporate into the study the effects of the gust or draft history through which the flight was undoubtedly flying. However, it is clear that for gusts to be considered as the major contributory generating source for the initial negative G portion of the maneuver, their velocities would have to have been inconceivably high because of the large gust gradient (rate of gust onset) required and the relatively long time interval (about ten seconds) over which the negative G built up to its maximum value. Gust velocities inconceivably greater than the most severe gusts measured during the NSSP would be required. The results of the simulated gust computer studies provided still an- other indication that gusts and/or drafts alone, even of the type and magnitude believed to have been imposed on Flight 705, would not generate a G trace of the type shown on the flight recorder record.

The picture of the final maneuver, then,that emerged from initial consideration of the evidence was that of an intact aircraft describing a path in space as a result of unusual longitudinal control displacements. It was inconceivable to consider the captain imposing such 1arge control displacements unless prompted to do so by the most exceptional circumstances, and it was equally difficult to conceive of any control difficulty that could account for the elevator-stabilizer time history required for the maneuver. None of time possible control malfunctions, such as a runaway stabilizer trim drive or an autopilot hardover, would be consistent with the developed evidence, nor would they be expected to produce such drastic results. The two most likely possibilities were those outlined in the Northwest-Battelle studies and in the Boeing studies. Each of these possibilities received thorough consideration by the Board in its final assessment of the available evidence.

Two of the three broad conclusions outlined in the summary of the Northwest-Battelle study are in essential agreement with the Board's assessment of the evidence as presented in preceding paragraphs. They also conclude that the wreckage examination disclosed no physical evidence of a failure which causedthe accident, and that ". . analysis of flight recorder data has produced strong evidence that positioning of the elevator and horizontal stabilizer were directly responsible for the final maneuver from which the airplane did not recover." The manner in which they arrive at these two conclusions is much the same as the Board's, and their report contains an excellent, detailed exposition of the reasoning associated with these conclusions. In arriving at their third.broad conclusion that immobilization of the elevators due to freezing precipitated the captain‘s control inputs, they chiefly relied on the previously reported incidents of balance bay freezing, and on their own