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impact area. Multiple lightning-strike marks were found on the left wing tips. There was an area of extensive damage on the top surface of the tip along the end rib, in and adjacent to the joint where the wing tip cap and the top wing skin were attached to the end rib. The damaged area extended from the trailing edge of the wing to a point about three feet eight inches from the leading edge, measured along the end rib. Within this area there were numerous spots where the metal surface and rivet head showed indications of melting, and associated dendritic patterns were were visible on the wing surfaces. The largest single indication of lightning damage was an irregular shaped hole about 112 inches in diameter. There was evidence of higher heat in this area and fused metal was found around the hole. A smaller hole was burned adjacent to the outboard edge of the access cover over the fluxgate compass transmitter. Four rivet heads and small areas of metal adjacent to the rivets were burned off the top surface near the trailing edge of the wing tip. Numerous small, lustrous craters were found in the wing tip cap ranging in diameter from 1⁄16 to 1⁄8 inch. The depth of the craters ranged from 1⁄32 inch to complete perforation of the 1⁄16 inch thick wing tip skin. Evidence of fusion was found at the edges of two drilled holes in the wing tip cap and one of these holes had been enlarged by melting around the periphery. A darkened area was found on the end rib directly opposite one hole. The paint had been discolored by heat and there was localized charring of the paint. A few small specks of fused material were found on the paint. There was no evidence of pitting in the metal surface of the rib. The lightning damage nearest the fuel tank vent outlet. Metallographic examination of several areas of lightning damage showed characteristic deposits of porous fused metal on the damaged surfaces and a distinct boundary between the affected and unaffected metal.
The surge tank box[1] was intact except for a 2.2 inch opening along the top extending from spar to spar. This opening encompassed the surge tank end of the vent passages from the fuel tanks. The ends of the fracture curled up and were moderately sooted. The bottom surge tank skin was curled downward at a fracture line along the tank and closure rib. Very slight outward bulging of the tank end rib was noted. The interior of the surge tank was heavily sooted on all sides and the sealant was burned inside.
The exterior of the wing fuel tank ram vent was moderately sooted inboard of the recess in the bottom skin. The heaviest concentration of soot was below and aft of the tank vent screen. There was evidence of heat in the inlet of the ram air vent scoop.
The wire bundle for the fluxgate compass transmitter showed signs of heat damage and charring of the insulation of one wire. This wire bundle was located in the surge (illegible text) ducting area. Disassembly of the surge (illegible text) revealed light sooting on the interior wall and heavier sooting on the outside surface of the duct.
A 27-inch section of the forward spar was bulged outward and the bottom spar cap was bulged downward. There was slight sooting or discoloration on the reserve tank side of the spar and slight charring of the sealant at the juncture of the spar and the closure rib.
- ↑ See attachment No. 1, a schematic of the B-707 fuel system.
