_ 24 _
than 120 percent of the design dive Speed of the aircraft If, however, the stiffness is reduced, forced oscillations become more likely depending on amount of stiffness reduction and on equivalent airSpeed. More speCificall , the data show that if the stiffness is reduoed to some value less than 8 x 10 inch pounds per radian, whirl mode could become a driVing force on the wing in the cruising speed range. The tests further showed that whirl mode of catastrOphic proportions could develop, reduce its frequency, and couple with the wing in a period of from 20 to 40 seconds.
Certain causal possibilities can be eliminated from further discussion be- cause of a complete lack of eVidence or ev1dence to the contrary
l. ColliSion with another aricraft
2. Structural failure due to turbulence during this flight
3. Structural failure from fatigue
4. Structural failure as a result of boost and/or autopilot malfunction 5. Sabotage
The shattered upper planking of the left inboard wing suggested a strong possibility of failure due to excessive positive loading. The horizontal tall or rear fuselage showed no such eVidence; however, Lockheed testified that at 275 KIAS (last known airspeed) the wing and tail were about equally critical under peeitive loading. There was further testimony that above 275 knots the wing becomes the more critical of the two.
This leads to the premise that high—load wing failure (if it existed) occurred at an airspeed in the order of 275 knots (cruise) or higher. Such an overload failure, with boost, autopilot, and turbulence out of the picture, would have to develop from a pullup maneuver brought on by colliSion aVOidance or following loss of control. Since there was no known conflicting traffic, there is mithing to substantiate a theory of colliSion aVOidance.
Loss of control has occurred in other instances becausecfi‘a.pilot's 1natten- tion to duty resulting in a dive or diVing spiral. An analySis of a plot of the witness Sightings,however, places the ball of fire at or above 15,000 feet. If, then, the ball of fire (wing—tank fuel ignition} was at or above 15,000 feet it would require a climb, intentional or not, prior to any loss of control of a type which would create exceSSive airspeed (Note. It is extremely difficult to con— ceive of a recovery from an "unusual position“ causing structural failure without first having exceSSive speed, particularly at the gross weight of this aircraft at the time of the accident.) This hypotheses cannot be maintained for it first presupposes a climb for which there would be no known purpose. If it be argued that the climb 15 unintentional, it becomes necessary to assume an extremely lengthy inattention. It must also be remembered that a scant four minutes prior to impact, or about three minutes prior to the witnessed neise, the flight reported 15,000 feet.
All this leads to a conclusion that, even with indications of high positive loading, there is a causal factor far more inSidious than exceSSive air loads.
