Engine Device Blamed for Fatal Copter Crash : Accident: A report says warnings about drive shafts overheating were issued before the fatal plunge.

The Jan. 14, 1991, crash of a UH-1N helicopter at Edwards Air Force Base that killed two crew members and injured two others was caused by the overheating and failure of a coupling on the craft’s main engine drive shaft, according to an Air Force report released Monday.

Bell Helicopter-Textron, the maker of the craft, issued advisories in 1976 and 1981 warning of drive shaft overheating and recommending a modification to test for the problem, according to the crash report. Also included was a subsequent 1981 Air Force memo finding “insufficient justification� to take that action.

The coupling failure, which also crippled the helicopter’s tail rotor, caused the craft to lose power and crash on the northern section of the base. The report did not give a cause for the overheating, although it said the crew earlier had reported a burning odor.

The narrative of the publicly released report did not mention the prior warnings on overheating or say what steps, if any, the Air Force has taken since 1981 to deal with the problem. Drive-shaft problems are suspected of causing at least three other crashes around the nation that have killed nine servicemen.

Those crashes include one at Edwards on Oct. 10 that killed three people, an Oct. 16 crash near the Salton Sea that killed four Marines from Camp Pendleton on July 27 and a crash in the Chocolate Mountains that killed two other Marines.

Some military officials have focused on possible inadequacies of a synthetic lubricant known as Syn-Tech that is used on the helicopters. However, the report found “no evidence� of defective materials, and an attached memo said the 1987 batch of grease used on the craft met Bell’s specifications.

However, a report in the Baltimore Sun said the military nonetheless has decided to begin replacing the grease with another product in 1996. The same report, citing a Navy document, alleged that Bell had lowered its specifications for Syn-Tech in 1984. Military officials could not be reached for comment.