Page:CAB Aircraft Accident Report, Northwest Airlines Flight 705.pdf/12

 a discontinuance of the turn then in progress to level the wings, a good technique in rough air penetrations. However, at one point in the second encounter at about 1343, the heading trace breaks sharply, the altitude drops, and the acceleration is at about +1G level, indicating some form of lateral upset.

The recorder indicates that the flight passed out of the heavy turbulence area at about 1346 while climbing through 15,000 feet. From this point to the beginning of the final maneuver at about 1347.25, the recorder traces show a mild oscillating motion of the aircraft as it climbed from 15,000 feet to 17,250 feet. The acceleration excursions are no greater than +0.2G and the altitude variations are small, but discernible during the oscillations. The half cycle time varies from about 16 seconds to 25 seconds.

It is evident from the flight recorder traces that the accident maneuver started some 12 minutes after lift-off at Miami and ended about 45 seconds later when disintegration of the airframe occurred in flight. In this brief time interval the aircraft climbed steeply, reaching a climb rate about three and one half times its normal rate, pitched nosedown, and dove toward the ground at high airspeed. At the start of the maneuver the aircraft was in a level turn at 17,250 feet, and had been so for about 12 seconds. The airspeed had increased approximately ten knots over the leveloff airspeed of 260 knots, the heading was still changing toward the 360-degree clearance heading, and the vertical acceleration had returned to +1G after the slight decrease during leveloff. About one minute earlier, while climbing through 15,000 feet, the aircraft had passed out of a heavy turbulence area into a light turbulence area through which it was still flying at about the start of the final maneuver. Several radio contacts with departure control were made by the flight in this one minute interval before the maneuver started, and two contacts were made with ARTCC in an approximate ten second interval following the initiation of the final maneuver. None of these transmissions indicated concern or alarm, and none referred to an aircraft mechanical difficulty.

In analyzing the final maneuver, assessing the various possibilities, and ascribing the probable cause, the Board has used not only the data developed during the initial investigation and public hearing, but also valuable research, test, analyses and study data from many sources developed subsequent to the hearing. In fact, the lengthy time interval between the accident and the release of this report has been due to the necessity for awaiting the outcome and evaluation of significant efforts such as the NASA wind tunnel tests and analysis and the Northwest-Battelle studies. Further, all of the jet transport accidents and incidents that occurred before and after this accident were carefully for clues that might assist in a greater understanding of the events that transpired in the last 45 seconds of flight.

Early in the investigation, before the results of the flight recorder analyses and other pertinent studies were available, the extensive in-flight structural breakup was suggestive of a single catastrophic even such as (1) an in-flight explosion, (2) a fatigue failure of a main component, (3) a control system failure or major malfunction, (4) an excessive gust loading, (5) flutter, or (6) a "static-type" failure of a major component resulting from prior damage due to traversing the heavy turbulence area, an earlier incident or a combination of these prior damage possibilities. This last possibility received early consideration because of the distinctive manner in which the No. 3 engine separated