- 7 -
The center tank[1] bottom skin was separated in three pieces. Stiffeners and access doors were bowed outward. The largest section was bowed chordwise in an outward direction. There were no signs of fire damage.
There was considerable ground-fire damage to the right inner wing.
The left outboard flap and spoiler skin were heavily sooted underneath caked mud. There was also soot under the mud on the inboard flap gear box area. The left inboard flaps showed evidence of fire damage. Measurement of the jackscrews indicated the left wing flaps were up. The right outboard flap showed heavy fire damage and the bottom skin was burned away. These flaps were extended 112 to 212 degrees.
The ailerons and spoilers on both sides, except the outer half of the left outboard aileron, exhibited evidence of in-flight fire. The aileron and rudder trim were neutral and the stabilizer was trimmed for 0.8 degrees noseup.
The recovered wing and tail components were laid out in their respective positions for study of fire damage and the left wing tip and outboard sections were examined for evidence of lightning damage.
Electrical resistance measurements were taken across 21 fuel tank access plates. Readings obtained varied from 0.000 ohms to 250,000 ohms. The higher readings were obtained where bondings had been broken.
A magnetic strength survey[2] was made of the steel components of the left and right wings, center fuel tank, and the horizontal and vertical tail sections. The readings ranged from light to moderate with occasional areas of heavy magnetism.
Similar surveys were conducted on a USAF B-707-100 and a Pan American B-707-139 with 3,454 and 10,540 hours flight time, respectively. The results were generally similar except that the readings on the aft end screws on the fuel tank access plate of the left wing were higher on N709PA than on the two tested aircraft this work was an attempt to delineate the path of the lightning-induced current through the wing structure. Despite the one variation in results of the three magnetic surveys, no information significant to the investigation was obtained.
The left outer wing and other parts were examined and analyzed by the National Bureau of Standards (NDS) in an effort to detect ignition points and confirm lightning damage. Special attention was given to the left wing tip and parts of the No. 1 reserve tank; the left fuel vent surge tank, the HF antenna probe cover, a piece of top skin from the right side of the center fuel tank with float valve attached, and the float valve from the right wing reserve tank.
Lightning discharges can be hazardous to aircraft fuel systems by possibly igniting the fuel vapor within the tanks. Direct strokes may penetrate the wall of
- ↑ See attachment No. 1.
- ↑ Magnetic strength surveys were made using a comparative Gaussmeter with a probe containing ferrous core coils, current amplifier, microammeter, and sensitivity adjustment. Movement of the probe across parts checked induced voltage into probe coils and results were shown in micro amperes.
