Page:CAB Accident Report, Braniff Airways Flight 542.pdf/25

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It thue becomes necessary to consider "whirl mode" which has been described,

'1 phenomenon shown by wind tunnel tests and analySis to be a potential destructor. Sons evidence of oscillatory motion was found in the left wing and No. l QEC/hacelle while this is not positive evidence of whirl mode, it is certainly compatible with the motions shown by tests to eXist during the latter stage of excitation.

Another factor which is compatible with, but not proof of, whirl mode is the intense neise attested to by groundwitnesses. Analyses by Lockheed and Board technical personnel have shown that during whirl mode the propeller tips approach sonic velocity without increase in r.p.m. or airspeed, and probably produce‘a noise in the order of 120 decibels. The witnesses heard such a noise at a time which would place the nOlSB about 33 seconds prior to the fuel ignition. Analysis has shown that whirl mode, from inception to destruction, would last about 20 to 40 seconds. No avenue of investigation has revealed any other reason for the sound.described and later identified by the witnesses.

As mentioned earlier, the left wing showed indiCations of high positive load. This is in complete contrast to the right wing failure at Cannelton. There is no way to establish with any degree of certainty this difference in wing failure patterns, but it is possible to rationalize a possibility. The first impulse of a pilot, when subgected to either severe Vibration, a runaway propeller noise, or both, is to slow the aircraft down. Normal action wouhdbe to reduce power and to climb. 0f the two, climbing is the more immediately effective, particularly in the Electra, which takes several minutes to reduce Speed from 275 to 200 knots by power reduction. There is, then, the possibility that in the excitement and in his desire to slow down quickly, the pilot exerted back pressure sufficient to fail the wing earlier than if failure had resulted from oscillation alone. fhis is not to imply that the pilot applied a stick force capable of failing a egructurally sound wing, but rather that his action dictated direction and time 0 failure.

There remains one point, the element of "prior damage," which cannot be satisfactorily explained. According to Lockheed, the stiffness factor of the QEG must be substantially reduced to produce an undamped whirl mode, or propeller precesSion This suggests damaged or failed structure, engine mounts, or engine structural components. No such evidence was found. The No. l QEC and powerplant were examined minutely for fatigue, with negative results. No other type of failure was discovered which could be definitely conSidered damage prior to whirl EGder QEC failure, and impact. There is serious doubt whether such a determina- tion could be made with any degree of accuracy. For example, there were several pure tenSion and compression failures in the QEC structure which could have occurred prior to whirl mode or early in the preceSSion. Furthermore, there is nothing in the aircraft's recent history, such as hard landings or turbulence, to indicate the posSibility of prior damage, nor was there on the final flight, as far as can be determined, any incident leading to structural damage prior to the accident.

ConcluSion

There was in this investigation no positive indication of the cause. For this reason, an attempt has been made in this report to elindnate certain possi~ bilities by application of the available evidence to each of them. Once these .ossibilities have been disposed of, the only remaining causal factor for which