Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/15

 of the temperature environment in the balance bay area at the time of the accident. The Board, also, was aware of the significance of the previous incidents and early in the investigation had requested Boeing to provide test data bearing on the possibility of balance bay freezing. The balance bay temperature lapse-rate data collected in late 1963 during a Joint Northwest-Boeing flight test program, clearly demonstrated that the pertinent temperatures were at least as high as the ram air temperature, and for certain components were appreciably higher. Since the ram air temperature determined from the USWB radiosonde data and the flight recorder airspeeds showed the ram air temperature would have been above 40°F for the entire flight, the Board believed it reasonable to conclude that balance bay freezing was not a factor in the accident.

After detailed study of the December, 1964, Northwest-Battelle study report, the Board can find no sound Justification for modifying its earlier conclusion regarding balance bay icing. In developing their thesis that temperatures in the balance bay area were substantially below the freezing level, the report presents no new weather evidence, but rather it presents a different interpretation of the evidence considered by the Board in its analysis. The Board did not find persuasive their "cold—soak" reasoning, their assumption of a 20—degree differential between rain and ambient temperatures, and their method of determining the temperature variation with altitude in the accident area. In the absence of a more conclusive showing that the structural temperatures in the balance bay area were appreciably below the freezing level, the main Northwest-Battelle conclusion that immobilization of the elevators early in the climb precipitated the large longitudinal control displacements is without Substance. However, the Board would be remiss if it did not indicate that much of the material in their report (the flightpath analysis, the significance of the long down elevator period, the human factors influences, etc.) coincides with the Board's views in specific areas.

The Boeing "Performance Analysis" report was most helpful to the Board in achieving a clearer understanding of the complex factors associated with this accident but it, too, was not without its limitations. where it presumes what the pilot might have done at specific times, it is speculative and a derogation of the general soundness of the technical approach used in the analysis. Where it presents a graphic picture of the apparent deviations from "normal" climb performance and the possible significance weather—wise of the deviations, it provides, at least, qualitative information on the severity of the weather encountered, and an appreciation of the problems confronting the crew. Pitch attitudes from this study are in general agreement with the attitudes derived from the earlier flightpath analysis study. The draft velocities from the study are of the same order of magnitude calculated by the USWB. However, the Board is aware that some of the simplifying assumptions, that of necessity had to be used to make the performance analysis (constant engine power throughout, undisturbed air, neglect of the short period dynamic gust response), preclude a literal acceptance of the derived data. Still, the analysis is useful informing an assessment of the events that transpired during the final maneuver.

The NASA wind tunnel tests and their subsequent longitudinal control force analysis provided a very necessary clarification of the elevator and control