Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/13

 and because portions of this engine's mounting structure had been repaired as a result, of the Fort Lauderdale accident involving N724US a year earlier. However, meticulous study of the aircraft wreckage mockup not only eliminated this causal area, but also disclosed no evidence to support the theories of in-flight explosion, fatigue failure, or control system malfunction.

NASA's review for the Board of the methods and techniques used by Boeing in demonstrating substantiation for gust loads and flutter showed that these were in accordance with established procedures and in agreement with current design practices. Moreover, NASA found the results of Boeing's analyses to be reasonable Fluster protection was provided in the design to speeds in excess of 120 percent of V$D$ (the dive speed), and no unusual dynamic response characteristics were found for either positive or negative gusts within the design limits. The analysis of the gust intensities an the accident area at the tune, prepared by the USWB, effectively demonstrated that the weather was severe but not unusual. Thus barring the statistically remote chance of an extreme gust encounter, the maximum gusts the flight might have encountered were within the design limits. These findings are persuasive in concluding that the single event possibilities of excessive gust loading or flutter were not the cursor. cause of the final accident maneuver. Accordingly, the Board concludes that no single catastrophic event was the cause of the final maneuver and that a rationale for the maneuver hes elsewhere. Corroboration of this view is provided by the results of the wreckage tragectory studies and the flight recorder readout analyses.

While the Board recognizes the limitations inherent in any wreckage trajectory study, it nevertheless is convinced that such studies can be useful in providing at least a gross picture of the breakup altitude and sequence. Here, the trajectory study was helpful an establishing that the aircraft structure was essentially intact throughout most of the final maneuver and that the initial separations did not occur until the aircraft had descended below 10,000 feet. Had structural failure started earlier an the maneuver, wreckage pieces would have been found outside of the ground scatter pattern, a pattern which was consistent with breakup below 10,000 feet. The short breakup time interval involved in the inflight disintegration generally masked the actual breakup sequence, although there were some indications that light pieces of empennage and wing structure separated early in line sequence.

The structural strength data review also tended to support a breakup at a lower altitude, Although the design regulations required that strength he provided for only a -1G limit load, the aircraft design incorporated strength in the negative direction considerably in excess of that value. The horizontal tall could withstand the high loads associated with maneuvering to -3.2G in the early part of the noseover, and would not be expected to fall under the condition unless the elevator was deflected upward suddenly at an extremely high rate, well in excess of the rate indicated by the recorder readout analysis. However, the manner an which elevator and stabilizer did fall suggested that this type of loading did occur later in the dive. The forward fuselage could also withstand the initial high negative G loading and would not fail until the horizontal tail separated. The wing could be expected to exceed its design strength at either of the high negative G loadings, but would have been wore critical at the lower altitude loading.

The early analog and digital recorder readout studies by Boeing were most helpful in demonstrating that the aircraft was intact during the initial steep