Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/19

 In the preceding paragraphs of this analysis, a discussion of the more significant evidence has been presented and the Board's views and reasoning with regard to the evidence has been noted in some detail. While some of the evidence purports to show second by second the actions of the pilot throughout the final maneuver, the Board finds it difficult to agree in every detail with the suggested sequence in either the Boeing or Northwest-Battelle studies because of their speculative nature and, in some instances, their erroneous assumptions. Moreover, it is neither necessary nor possible to be so precise in setting out the events of N724US's last 45 seconds of flight. The Board does believe, however, that it is possible to delineate a generalized picture of these events from the evidence that is available and that this picture is sufficient for determining a definitive probable cause and for providing a clear understanding of the general problem.

It seems evident that shortly after 1347 the aircraft once more entered an area of severe turbulence. The climb that started at about this time could have been initiated by the air drafts or by the pilot but most probably was due to a combination of these The rapidly decreasing airspeed, increasing rate of climb, and the high nose attitude that soon developed would provide the necessary cues for any pilot to take drastic action to prevent what would appear to be an impending stall. Acting on this concern and, quite probably, while being subjected to severe vibrating accelerations from the turbulence, the pilot used full down elevator and aircraft nosedown stabilizer trim to change the aircraft's flightpath. Although the flightpath analysis study indicates the stabilizer trim was applied before the elevator, the Board finds it difficult to believe that a pilot would use trim before using elevator in a situation of this type and is more inclined to believe that they were used in combination.

Although these large control displacements would have the effect of arresting the speed decrease and high climb rate and would return the nose high pitch attitude to a near level attitude, they would also develop extremely high negative G forces on the aircraft. The Board is convinced that these high negative G forces when considered along with the elevator control characteristics, help to explain why a successful recovery was not made. The negative G forces shown on the flight recorder would result in a chaotic situation in the cockpit of any airliner with a crew totally unaccustomed to forces of this type and magnitude. Besides the distraction of warning lights and ringing bells which were probably actuated under the negative G conditions, loose items such as briefcases, charts, logbooks, etc., would be tossed around. The crew members, themselves, would be forced upward against their belts and the average airline pilot would probably have difficulty keeping his feet on the rudder pedals and his hands on the control wheel It is for this reason that the Board finds it inconceivable to believe that the pilot continued to apply full down elevator during the initial high negative G period. It is much more reasonable to believe that the elevator control forces lightened in the manner revealed by NASA's analysis of the wind tunnel results, but to a greater extent than was established in that analysis. Control force lightening to within the system friction band range or actual force reversal very likely did occur. No other plausible reason is evident. With the control forces reduced to zero or reversed and the pilot's hands off the control wheel as a result of the high negative G effects, the control column would remain in full forward or nosedown position.

It appears that when the pilot managed to place his hands on the control wheel some eight seconds later, the aircraft was in a vertical dive at about