Helicopter Crash Still a Mystery : Tragedy: Air Force report fails to provide clear cause for 1991 accident in which three were killed. A separate document on the incident remains confidential.

A newly released Air Force report on the October, 1991, helicopter crash that killed three servicemen at Edwards Air Force Base gives no clear cause for the accident.

The document, called an accident investigation report, contains the only findings on the crash that the Air Force intends to release. There is a separate safety mishap investigation report, which typically states a cause, but its findings are confidential.

Air Force officials said this week they have nothing more to say on the subject.

“The document is designed to be stand-alone. It says everything we can say on that particular incident,” said Capt. John Boyle, a spokesman for the Air Force Materiel Command at Wright-Patterson Air Force Base in Ohio.

Air Force officials would not say whether they had found a cause for the crash. But the report, nearly 550 pages long, does detail the final flight of the 20-year-old twin-engine UH-1N helicopter. The craft, on a routine parachute training mission, began bucking and swerving, then turned virtually upside-down, lost its rotor blades, caught fire, and fell 8,000 feet to the desert. Two crew members survived the crash.

The UH-1N model involved, built by Bell Helicopter Textron Inc. of Texas, has had various reported problems in recent years. A spokesman Friday confirmed that the company has been inspecting and modifying those craft used by the Air Force, but refused to detail the problems or the modification.

Another UH-1N crashed at Edwards on Jan. 14, 1991, killing two crew members and injuring two. A Marine Corps UH-1N crashed July 27, 1990, in Imperial County, killing two crew members and injuring three. Four Marines died Oct. 16, 1991, in a UH-1N crash near the Salton Sea.

The Air Force had blamed the January, 1991, crash at Edwards on failure of the helicopter’s main drive shaft coupling, likely because of overheating caused by misalignment. But they had ruled out that cause for the October, 1991, Edwards crash. No cause was available on the Marine crashes.

Air Force officials said they always keep confidential the safety report, the one that typically contains a cause, because its findings are used to help prevent future crashes. Servicemen testifying in that process are promised that nothing they say can be used against them or become public.

The document released on the October, 1991, crash found no drug impairment by crew members, no sign of maintenance or mechanical problems likely to have contributed to the crash, and no external conditions--such as bad weather--that might have caused the mishap.

Killed in the crash were the pilot, Capt. Jay D. Burdett, 30, of Albion, N.Y.; an aerial photographer, Staff Sgt. John R. Anderson, 26, of Columbia, Pa., and survival instructor Staff Sgt. Kurt H. Ellington, 30, of Charlotte, N.C. Two crew members were thrown free of the craft and parachuted to safety. One was injured.

According to the report, as the troubled craft began veering to the right and flying nearly upside-down, the main rotor blades sliced off its tail, then came loose. In the process, the rotor blades also hit the cabin four or five times, and apparently punctured a fuel tank.

The report does not indicate what caused the craft to begin bucking and veering, nor does it state what led to the problems with the craft’s rotor blades. The two surviving crew members reported hearing only some metallic-type noise at the outset.