Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/8

 period of up to 30 seconds and -5G for shorter intervals with no adverse physiological effects. These forces have been duplicated in flight as well as in centrifuge testing. However, they also advised that if one has never been exposed to high negative G forces, the experience could be frightening.

Early in the investigation, Boeing provided the Board with data from two studies which were conducted to determine: (1) the capability of the aircraft to perform the maneuver indicated by the flight recorder readout, (2) what control inputs would be required, and (3) what aircraft response would result from partial or complete loss of the horizontal tail. Initially an analog computer study was conducted. The data derived were then employed in a more sophisticated IBN digital simulation of the flightpath of the aircraft during its final maneuver. Both studies varied longitudinal control inputs to reproduce the vertical acceleration trace of the recorder. The results revealed that while the aircraft was capable of performing the maneuver, full aircraft nosedown deflection of both horizontal stabilizer and elevator was required to achieve the high negative load factors indicated. An intact and operable elevator would also have been required to produce the partial recovery following the initial pushover. In addition, partial or complete loss of the horizontal tail surfaces prior to the partial recovery would have resulted in a much higher rate of change of the pitch attitude and vertical acceleration. In the digital study, pitch attitudes varied from approximately 22 degrees noseup during the steep climb to beyond the vertical in the nosedown direction during the dive. They estimated that negative stall buffet was encountered during a six-second period of the final maneuver.

Following these early studies, and because of the large control inputs indicated, a comparison was made between the known aircraft climb performance capabilities and the actual performance indicated in the flight recorder readout. For this study it was assumed that the power setting throughout the maneuver remained constant at maximum continuous power, which is normal for the climb. It was then possible to compare this normal performance with airspeed-altitude traces indicated on the flight recorder. Any variation from this normal aircraft capability, when comparing a loss in airspeed with a corresponding gain in altitude, represented the influence of an updraft. An opposite variation would be the result of a downdraft. The comparison revealed that drafts of high intensity were acting on the aircraft at the time of the high rate of climb and during the dive. The drafts were not of sufficient magnitude to damage the aircraft structure. However, the possibility that a pilot might be misled by the aircraft response to these drafts was considered. Entry into an updraft produces an initial aircraft response to "weathercock" nosedown into the relative wind. However, it was pointed out that the ultimate effect of the updraft is an altitude and noseup attitude increase. If the pilot attempted to overcome this initial "tuck" with noseup elevator, the rate and amount of change in attitude and altitude ultimately produced by the draft would be exaggerated. The converse would be true for downdrafts.

Boeing also conducted a study by simulating flights of a 720B through various draft histories. The various simulations included one flight with no control input from the pilot, another with sufficient control to maintain a constant horizontal attitude and also a resultant study which included a synthesized draft history. A comparison of the flights indicated that the acceleration forces were less without control inputs than for constant attitude flight. The pitch changes experienced during the stick-fixed flight were fairly large; however, the