- 7 -
No action was taken by dispatch to furnish the flash advisory or radar information to Flight 75. Dispatch did not know if Captain Paddack had received the advisory prior to departure. It was stated by dispatch personnel that they believed the flash advisory indicated improved conditions over those previously forecast and that all of the weather data indicated the thunderstorms were scattered, thus circumnavigable.
With respect to providing an en route flight weather information, one dispatcher said, "If the trip is operating in a scattered thunderstorm condition, the likelihood of any extended period in solid cloud is almost negligible. If he is operating where you would expect solid IFR conditions, it is a condition that requires all of the information he can get." It was stated that because many Capital aircraft are not radar equipped there was no distinction in the information furnished the en route flight based on this equipment.
Following the accident a study of the weather conditions prevailing in the accident are at the time of the accident was made by a U. S. Weather Bureau research meteorologist. The results of this study showed there were large rapidly developing thunderstorms in the vicinity of Martin Airport, located about 2-1/2 miles southwest of the accident area. Utilizing several techniques it was also determined that extreme turbulence[1] most probably existed at 14,000 feet in the thunderstorm cells and area around them. It was also shown that extreme turbulence may exist not only in the thunderstorm cell but up to five miles around it.
The pilots of another Viscount observed Flight 75 deviate at the Westchester omni. At that time these pilots, using radar, noted no indications of severe thunderstorm cells on airway V-3. Consequently, they continued on the airway and experienced no difficulty. An executive pilot also stated that from a location north of the accident scene he observed a severe thunderstorm cell on airborne weather radar near the accident scene. He described it as twice as strong as any other in the vicinity. He immediately altered course to avoid it. Still another pilot operating according to visual flight rules reversed course in the accident area to avoid instrument conditions of a thunderstorm.
Analysis
On the basis of all the available evidence it is the Board's analysis that the inflight disintegration occurred as the result of loads imposed on the aircraft which exceeded its design strength. It is the Board's opinion that the forces were from a high indicated airspeed in turbulence. The Board believes that this airspeed was generated during an involuntary descent from 14,000 feet which followed loss of control of the aircraft in extreme turbulence. The Board is convinced that no preexisting weakness or condition contributed to the breakup and that no malfunction or failure of the aircraft, its systems, or its components led to the circumstances under which the disintegration occurred.
From examination of the major fractures, breakup patterns, and from design considerations it is believed that the initial failure in the destruction sequence was the nearly simultaneous downward failure and separation of the horizontal stabilizers at the No. 2 hinge points. This is confirmed by the fact that the symmetrical stabilizer failures could only occur with both wings intact. Also, under ultimate loadings on the aircraft the stabilizers would be expected to fail.
- ↑ Extreme turbulence is defined by NASA as a rarely encountered turbulent condition in which the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage.
