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first. Furthermore, the breakup sequence and the nature of the mass of fractures are entirely consistent with this as the initial occurrence.
Following separation of the right and left stabilizers the aircraft pitched down violently so that all four nacelles broke upward from combined inertia and gyroscopic loads. Immediately thereafter both wings were subjected to extreme downloads under which the right separated and the structural integrity of the left wing was destroyed. With the nacelles, right wing, and stabilizers gone, drag induced by the left wing yawed the fuselage violently to the left. Forces to the left tore off the vertical fin with portions of the fuselage attached, the latter already weakened when the left stabilizer stub tore away. During the subsequent gyrations the left wing broke up, its fuel cells were opened, and the flash fire occurred. At the same time the remaining fuselage disintegrated. The Board believes that the major disintegration sequence took less than one second and that during the latter part of the sequence occupants of the plane were exposed in a random manner to the flash fire and attendant high concentration of carbon monoxide.
The high indicated airspeed which the Board believes existed at breakup is suggested by several singular factors which, in their cumulative value and with the overall patterns of evidence, make the existence of excessive speed nearly irrefutable.
An important consideration is that unless an airspeed in excess of cruising was present the strength of the Viscount is such that forces causing the horizontal stabilizer failures which occurred cannot be developed. Below cruising speed the horizontal tailplanes will stall at loadings less than those necessary to cause failure.
The high indicated airspeed is also suggested by the structural damage to the passenger seats, propeller reduction gearing assemblies, the engine mount "W" struts, and possibly by the ante-mortem injuries to two or three passengers. The damage and the injuries resulted from pullup loads which were in the opposite direction to the loads imposed on these subjects by the breakup forces. This damage had to be made prior to the breakup and is compatible with a descent in which high speed was attained, followed by a recovery in rough air in which positive "g" forces had to have occurred.
A further indication of an excessive airspeed and one more definitive of the amount was the blade angle of the No. 3 propeller, 52 degrees. It is believed the indications of blade angle were made during breakup, therefore, airspeed calculated from the blade angle would be valid at that time. From technical data relating to airspeed and propeller blade angles it was shown that with the 52-degree angle there is no throttle position at which true airspeed could be less than 295 knots. Because this airspeed is excessive it is entirely logical to assume the throttle would have been closed to slow the aircraft. With the throttle closed a 52-degree blade angle reflects a true airspeed of 335 knots, which is 15 percent in excess of the Viscount never-exceed speed or about 5 percent in excess of D, the maximum speed demonstrated in certification. Loads at such an airspeed, combined with gust and/or maneuvering load, could easily exceed the strength of the aircraft.
From the evidence of a high airspeed, combined with pullup forces already discussed, it is the Board's opinion that an involuntary descent occurred before the inflight disintegration. The foregoing conclusion is supported by the fact that the breakup occurred at about 5,000 feet and it is not reasonable, under the
