Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/17

 greatly different altitudes. Flight recorder readouts and crew statements were available for study in a few cases, while in others the crew did not survive and the recorder foil was destroyed or otherwise not available for study. Not all of the involved aircraft were U. S. Registered. The Board does not presume to Judge any investigation that may have been completed or to prejudge any that is still under evaluation. It wishes only to note here that every possible avenue of investigation that could be explored was considered during its lengthy evaluation of this accident. Although in those cases where the crew survived to relate their experiences there were many dissimilarities in the occurrences, there were a few apparent common denominators. Turbulence of varying degrees, small and large, was involved in each case. At various times in the unusual maneuvers involved in each case, the aircraft pitch attitude, airspeed, and altitude varied greatly in both positive—negative or increasing—decreasing directions. The crews indicated that large longitudinal control displacements of both stabilizer and elevator were used and required to maintain control. In some of these cases substantial altitude losses were experienced. Generalizing from a limited number of cases not fully evaluated or clearly understood is usually a technically unsound approach, yet it is still difficult to escape concluding that the phasing relationship between turbulence-induced aircraft motion with control inputs is at least a factor in these occurrences.

Some of the recent preliminary results of the extensive NASA inter-center rough air penetration studies have shed considerable light on the overall turbulence flying problem and have been of great assistance to the Board in its assessment of this accident. This program was Just getting underway at the time of the Board's accident hearing, and in the intervening months since has included, among other things, flight tests, theoretical analysis, and extensive flight simulation tests in a specifically designed simulator. Of particular interest is NASA‘s finding that pilot workload, cockpit acceleration environment, aircraft characteristics, cockpit instrumentation displays, and piloting technique can all be factors in precipitating an upset in some cases. In the work completed to date it has been shown that the simulator, without any pilot control inputs, can fly through the most severe NSSP gust/draft history without excessive G excursions, large airspeed variations or great altitude changes but with, in many cases, large changes in pitch attitude. The inherent or augmented stability of the simulated aircraft will in this type of trial provide the restoring forces required to maintain the trim condition. In most of the trials with a pilot "in the loop," the simulator could be flown successfully through the "storm" and the extent of the G, airspeed, and altitude excursions depended largely on how close the pilot tried to maintain the desired pitch attitude. Some of the trials revealed oscillations in the recorded parameters, sometimes quite large in amplitude, indicating pilot control input out—of—phasing with the simulator motions induced by the imposed gust/draft history. In a few trials the oscillations became divergent and an upset occurred. When the pilot was told to deliberately ignore the pitch attitude display and to rely briefly on controlling airspeed during the simulated penetration, large oscillations of all parameters invariably resulted A wide cross section of pilots, including a number from the airlines, have participated in this simulator program, and NASA is continuing to collect and analyze