Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/20

 16,000 feet and the airspeed was building up rapidly. At this time the flight recorder G trace changes toward positive G, indicating a recovery attempt was initiated. However, the recorder flight—path analysis indicates the elevator was returned initially to neutral, remained there for a few seconds, and then moved to the full up position. By this time the airspeed was at or beyond 470 knots, the altitude was nearing 10,000 feet, and the vertical acceleration was again moving in a negative direction, indicating that the excessive airspeed and air loads were precluding a successful recovery at this time. During the dive the pilot undoubtedly attempted to retrim the stabilizer in the aircraft noseup direction, but these attempts were unsuccessful because the high down elevator loads had by that time stalled the stabilizer electric drive motor, preventing system operation

by the pilot control column trim.switches. Although the Boeing recovery calculations indicate that a successful recovery could be made from about 14,000 feet and an airspeed at or below 320 knots, it would be unreasonable to fault the crew for not being able to do so in view of the cockpit conditions existing at the time and the extremely high control forces required throughout such a recovery. Besides, it appears that the rapid upward elevator displacement required by the Boeing recovery calculation might only have precipitated an earlier elevator and horizontal tail failure.

Clearly, many factors, which individually would not be considered as extreme hazards, were involved in producing this accident In many ways this accident is a classic illustration of the man—machine—environment causal triangle concept. Weather was a factor in this accident but the evidence is clear in indicating, that it was not greatly different from weather which might be encountered during routine airline operation. It is indeed unfortunate that the airborne radar did not guide the crew through "softer areas" during their climbout.

The Board is also convinced that the aircraft characteristics played

an important part in this accident. The cockpit acceleration environment induced by fuselage bending response in heavy turbulence, together with the acceleration amplification at the pilot's head as a result of pilot—seat belt—cushion response, probably caused blurring of the instruments and was annoying—to-alarming to the crew. In its extreme, this characteristic can have a significant effect on a pilot's actions and reactions during rough air penetrations. This unfavorable characteristic is present in all large, swept wing transports. The lightening of elevator control forces at high down elevator angles in pitching maneuvers is another undesirable characteristic which undoubtedly compounded the pilot's problem in this instance. If, as it appears, a force lightening to near zero or a mild force reversal did occur in this instance, then the pilot would be faced with a hazardous problem While it can be argued that the developed evidence does not absolutely prove that force lightening to near Zero or that mild reversals did occur, the Board believes that these arguments leave moot the question of whether the total evidence refutes such a possibility. In the Board's view, therefore, extensive control force lightening to at least within the system friction band provides the only reasonable explanation for the approximate eight seconds of down elevator input and, accordingly, was an important contributing factor in this accident. The powerful effect of the movable horizontal stabilizer is another aircraft characteristic involved in the final maneuver However, the movable stabilizer feature is essential to the aircraft design and other methods can be utilized to preclude serious out—of—trim conditions,