Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/18

 simulator data. It would be presumptuous of the Board to state what the conclusions of this excellent NASA program are or will be. However, the preliminary results from the program have persuaded the Board to conclude that, under certain conditions and circumstances, the unfavorable coupling of pilot control inputs and turbulence—induced aircraft motions can create a hazardous in—flight situation.

Many individuals and organizations have devoted considerable effort to the human factors, design, and operational aspects associated with rough air penetrations since the occurrence of this accident. Notable among the individuals is Paul Soderlind, Manager, Flight Operations Research and Development Division, Northwest Airlines- One of his technical papers prepared in late 1963 received wide distribution throughout the airlines, and his personal presentations to many groups of airline pilots and other industry personnel served to highlight and reemphasize the precautions that should be taken in making rough air penetrations, especially at higher altitudes. Another of his papers, presented in mid—l964 discussed potential pilot "miscues" from primary cockpit flight instruments and some pilot sensory cues which can be misleading under certain weather conditions. The importance of using the attitude indicator as the chief a reference instrument in turbulence, and the need for still further improvements in attitude instrument design are other significant conclusions reached by Captain Soderlind in this paper. All of these points were of extreme interest to the Board and were helpful in the overall evaluation of the accident evidence.

As a follow—on to the work performed in connection with this accident investigation, additional comprehensive rough air penetration computer simulation studies were conducted by Boeing to provide more information on the general problems associated with rough air penetrations. Specific study goals included validating recommended turbulence penetration speeds and piloting techniques, evaluating pitch attitude excursions in severe turbulence, and determining if simple modifications to the autopilot could be incorporated to assist the pilot during rough air encounters. Severe turbulence history profiles from the NSSP data and from actual transport encounters were used in the simulations. The preliminary results of this study are particularly interesting and add to the information provided by Boeing's earlier studies and by NASA's simulator studies. Providing the entry speed is not appreciably lower than the recommended values, the aircraft will do a pretty good Job of flying itself through the "storm." Little is gained by trying to maintain rigid attitude control since this can produce excessive aircraft loadings without appreciably affecting the altitude and airspeed excursions that occur during severe encounters. Large pitch attitude of 40 degrees nose up can occur in severe turbulence but moderate counteracting elevator inputs will prevent excessive speed reductions that could result in a stall The use of the autopilot on Manual Mode offers some advantages but considerable stabilizer trim activity can occur in some types of turbulence and could present a serious danger if the autopilot was disengaged either deliberately or inadvertently at a time when the trim varied appreciably from the in—trim setting. Simulations of rough air penetrations with an autopilot "modified" so as to deactivate the stabilizer trim showed that this type of autopilot configuration would do a very satisfactory job of flying through the rough air. A final preliminary study result, perhaps the most significant, was that the principal cues available during instrument flying in rough air can be confusing and contradictory and that the attitude indicator is the most consistently reliable reference instrument for rough air penetrations.