Page:CAB Accident Report, Pan Am Flight 4.pdf/13

- 13 - the flight engineer. A second fuel source was the fuel line on the forward face of the main spar which had a loosened fitting that leaked and supplied fuel for a fire over the strut center spar between the front spar and the nacelle closure rib. A third possible fuel source was the ruptured slat hydraulic line in the inboard gap cover area.

The source of ignition cannot be determined but the possible sources included, the engine exhaust, hot turbine parts, or arcing from exposed electrical leads. The latter is the most probable source because there was an appreciable time lapse between observation of the fuel spray and ignition. These fuel sources wetted much of the upper wing surface before ignition occurred.

The fact that the No. 4 main tank was full of fuel probably prevented more extensive fire damage to that area of the upper wing surface because the fuel acted as a heat sink. The fire in this area reached temperatures ranging from approximately 870°-1165°F., based on the damage incurred to the metal.

The damage to the right outboard wing panel top and bottom skin and ribs could only have been caused by an over-pressure in the reserve tank. This is demonstrated particularly by the manner in which the lower skin separated from the aircraft. The entire panel was forced straight down, taking the attaching flanges of both spars with it. This is plainly the result of a low order explosion. The source of ignition for this explosion could not be determined but could have been autoignition, burn-through, or hot point ignition from a localized hot spot.

The final separation of the wing followed the explosion in the reserve tank. The wing separation is not believed to have been simultaneous with the explosion. The indications of yaw and vertical oscillation on the flight recorder readout and the location of the wreckage on the ground indicate that the wing panel remained on the aircraft approximately 10-11 seconds after the separation of the lower skin panel.

The heat damage to the wing structure was not considered to have been a major factor in the wing failure. Rather, the loss of the lower skin panel, stringer, and spar chord flanges reduced the load carrying capability of the wing below that required to support a 1 "g" condition, thus leading to the failure.

2.2 Conclusions