Truss System Makes Flight 800 Airliner Reassembly Possible
The official word on the cause of the tragic crash of TWA Flight 800 in July 1996 hasn't been released. Even so, the reconstruction of a 94 ft. long section of the Boeing 747-131 jetliner -- performed in a former U.S. Navy hanger on Long Island during the winter and spring of 1997 -- went a long way toward providing answers.
Designing a way to put the pieces of the shattered aircraft back together, was a challenge
handed to Wiss, Janney, Elstner Associates Inc., Chicago, Ill. Michael J. Koob, PE, a WJE senior consultant on the project, presented a series of slides during the February breakfast meeting of the Great Lakes Fabricators & Erectors Association. His firm not only came up with a solution but implemented it as well, putting the many fragments of the plane back together again.
Koob said Flight 800's departure from New York City's John F. Kennedy Airport was about one hour late. Curiously, the plane had been delayed at the gate while its cargo was being searched. Someone had checked a bag on it but didn't board. After checking into that issue and finding nothing suspicious, the flight lifted off at 8:02 p.m., bound for Europe.
At 8:31 p.m., the aircraft had crashed in the ocean, approximately eight miles south of East Moreches, Long Island.
There were a number of theories floated about as to the cause of the crash -- including a surface-to-air missile strike caused by a test firing that went wrong. Investigators focused on what appeared to be obvious. There had been an explosion in the aircraft's center fuel tank. The plane's six tanks, three on each wing, contained sufficient fuel for the trip overseas. However, the center fuel tank, was nearly empty.
Koob said the 747's center fuel tank is about the size of a two car garage and can contain approximately 13,000 gallons of fuel. On Flight 800, it's estimated the tank had anywhere from 50-100 gallons of fuel within it. Because of the heat of the day, and heat that may have been rising from air conditioning equipment mounted beneath the center tank, some of that fuel had probably vaporized. This would create a potentially explosive condition. Vaporized jet fuel, mixed with the right combination of air, can be set off by a spark, possibly generated by deteriorated equipment or wiring.
The aircraft used for Flight 800 certainly was a veteran, Koob said. Built in 1971, it had logged 90,000 flight hours and an estimated 30,000 takeoffs and landings. "ThatÌs quite a few flight hours," he observed. On its last flight it was carrying 230 passengers and crew. While still climbing, it had reached an altitude of 13,600 ft. and an airspeed of 343 mph when the explosion happened.
The blast took place where the wing meets the aircraft's fuselage, Koob said. It was strong enough to separate the forward and nose sections of the plane, which fell first into the sea. Still under power, the remainder of the jetliner continued to fly for a while until it became unstable and dived.
The wreckage was scattered on a relatively flat, sandy ocean bottom about 110-120 feet beneath the surface. It was gathered and brought to three hangers in the old base, which had been used in jet fighter construction for the Navy during the 1980s. It was sold to the nearby city of Riverhead, N.Y., about a year before the crash.
WJE was contacted by the National Transportation Safety Board to submit a proposal for the aircraft's partial reconstruction in September 1996. The approximately $500,000 contract was finally awarded on Dec. 18. The work was 90% complete by Mar. 21, 1997, was examined for "punch list" items, and reached the 100% mark on Apr. 18 of that year.
What the NTSB wanted was a reconstruction of the 747 that ran about 45 ft. from either side of the fuel tank explosion area, or from Station 500 to Station 1600 on the aircraftÌs plans. WJEÌs design parameters focused on:
- Minimizing obstructions to viewing.
- Minimizing alterations to the pieces of the aircraft.
- Providing a capability to reinstall the aircraftÌs seating.
- Providing a means for moving the reassembled aircraft section.
Koob said he was aware of two aircraft crash reconstructions prior to Flight 800, with one using pipe scaffolding and the other attaching aircraft segments to a truss system. His firm took the latter approach, designing a 60,000 box truss system measuring 110 ft. 2 in. in length with a width of 9 ft. 2 in., to support the cross section of the 747, which towers about 28 ft.
"The box truss acted as a spine for the aircraft," he said.
The truss is supported on either end by columns bearing on the hanger floor slab. The legs of the truss were designed to go down the floor aisles. Transverse floor beams were connected to the truss. Holes were drilled on 4 in. centers on all members, straight and bent, so that they could be moved if needed.
The truss design took about two weeks. It was fabricated by Delhi Steel and delivered to the reconstruction hanger on Jan. 30, 1997. Syracuse Rigging erected the system, finishing it on Feb. 7. Actual mounting of the fragmented parts of the airplane started on Feb. 10.
Koob was one of the WJE workers who actually reassembled the plane. He worked 12 days at a time, took a weekend off, then returned for another 12 day work cycle, until the reassembly was complete. A typical WJE crew consisted for two structural engineers and two technicians. Besides the challenge of bolting the aircraft fragments to the assembly, the WJE crew had to hang the remains of the center fuel tank from the floor beams, reinstall landing gear boxes and mechanical systems, and rebuild the 747's air conditioning units.
"We rigged every piece and brought it into our system," Koob said, simply, in describing the jigsaw puzzle nature of his work.
The center fuel tank, for example, was hung from the floor beams using 1/2 inch diameter coil rods. Ribs were custom bent to match the general configuration of the aircraft. Small pieces of the plane had to be tie-wired to welded wire fabric before they were assembled into place.
Photographs of the completed aircraft section clearly show smoke patterns and related damage that were generated by the center fuel tank explosion and resulting fire. What wasn't found was indisputable evidence of a missile attack.
Recently WJE returned to the project to assist in moving the reassembled aircraft to another hanger, about three-quarters of a mile away. Steerable dollies were put under the truss columns and a tractor trailer towed the assemblage to its new home, making the move in approximately two hours. None of the pieces attached to the framing system were disturbed by the move.
