Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/21

 From all the evidence available to the Board, it is abundantly clear that flight on instruments in heavy turbulence can present a difficult problem to any pilot who departs too far from the recommended practice of using the attitude indicator as the main reference instrument for maintaining control. If the pilot places undue emphasis on any other flight instrument during his normal scan routine, a serious miscue with drastic consequences can occur. Similarly, attempts to maintain "perfect" attitude control can be equally hazardous, because of the high loadings induced, the danger of overcontrolling by the use of large control displacements, and the possibility of inducing an undesirable oscillatory motion of the aircraft. "Loose" attitude control,or moderate counteracting control inputs, appears to be the best method of counteracting the effects of heavy turbulence.

The HZ-4 attitude indicator installed in N724US, was one of the newer types then available, and provided an adequate, although by no means optimized, attitude reference display for normal or near normal pitch attitudes. However, during high pitch angles, interpretation of the attitude is extremely difficult because the horizon reference line on the indicator recedes from the face of the instrument. This results from the sphere within the instrument rotating, and the line moving deeper into the instrument housing, away from the face. While this display peculiarity may not have been a factor in the initial climb portion of the maneuver, it almost certainly would have been a complicating factor during the noseover and recovery attempt.

The Board's discussion of the factors involved in the final maneuver would not be complete without some reference to the control technique used by the pilot as indicated by the recorder flightpath analysis. As mentioned earlier, the Board believes that the pilot operated the controls to obtain the full down elevator and full aircraft nosedown stabilizer trim. Some of the more important factors having a bearing on the pilot's control actions have already been covered in preceding paragraphs. Other factors, such as limited experience in this type of aircraft, his recent return from an extended leave, and cockpit workload, occasioned in part at least in this instance,by the large number of communications to and from ATC, also may have had some influence on his flying technique, but their effect, if any, is more subtle and difficult to correlate with the developed evidence. The pilot, believed to be flying the aircraft, had wide airline experience, with over 17,000 hours to his credit, in many types of aircraft and most assuredly in all types of weather. By present standards he was qualified, and possessed average or better flying abilities. However, the Board is convinced that a clearer understanding of the "limits" of an "average" airline pilot must be found in order to insure a safe matching of the man to the machine and the environment. Perhaps statistical methods will have to be applied in prescribing a realistic capability range for the "average" pilot in order to provide the aircraft designer with more meaningful data to use in achieving a safe design that provides for full consideration of all associated human factor elements.

In the course of its lengthy study of the huge mass of evidence, the Board deliberated long on the form and context of a probable cause for this accident An initial reaction to the complex interrelationships of the many involved factors was that it would not be possible to ascribe a definitive probable cause, that no one single factor caused the accident. Still, the preponderance of evidence pointed toward a general causal area, and the Board,