WASHINGTON -- Military investigators have been unable to pinpoint a mechanical flaw that caused the main drive shafts of four Marine Corps and Air Force UH-1N helicopters to fail, killing 11 servicemen in the past 17 months.
The two services have issued differing policies on the use of their helicopters in light of the problem. While the Air Force is allowing its UH-1Ns to fly without restrictions, the Marines may fly theirs for no more than 3 hours at a time.
Four of the Marine deaths occurred Oct. 16 despite warnings by the Air Force, which had lost three airmen in a helicopter crash six days earlier. Officials of the two services also had met in July to study the problem.
At least eight other helicopters have had drive shaft-related problems, including as many as six Marine aircraft in the past week.
Two Air Force UH-1Ns made emergency landings in Florida and Oregon this year after crews detected a burning odor in the cockpit, according to a confidential Air Force document and Marine officials, none of whom would speak on the record for this article.
Some investigators believe the odor is the only warning of an impending catastrophe.
The death toll has sent analysts and engineers scrambling to find a cause and cure, but it has also touched off heated arguments among those trying to solve the mystery. As some Marines have accused others of failing to recognize the importance of certain evidence, each service has harbored suspicions about the other's inquiry.
Meanwhile, there are some signs within Marine ranks of declining confidence in the safety of the twin-engine UH-1N Huey, a variant of the Army's single-engine Vietnam workhorse made by Bell Helicopter Textron Inc. of Fort Worth, Texas.
Several Marine generals have had misgivings when told they would be ferried aboard UH-1Ns instead of the larger CH-46 transport helicopter, said a senior Marine officer at Camp Pendleton, Calif., where the Marine units involved in the fatal accidents are based.
Marine UH-1Ns have been flying under new maintenance requirements, including mandatory removal and inspection of the drive shaft every 25 hours.
Military officials said they might be months away from solving the UH-1N problem.
Bell, which has been involved in the investigation, has expressed strong confidence in the UH-1Ns, which were first delivered to the military in 1970.
But Marine and Air Force reports, and test results obtained by The Sun, make it clear that several components of the drive shaft might have defects that cause it to fail.
They show that a synthetic grease common to UH-1N drive shaft assemblies could be prone to losing its ability to lubricate critical moving parts between the helicopter's turbo engines and the transmission, which controls the speed of the main rotors.
The first accident known to the military occurred July 27, 1990, over the Chocolate Mountains in southwest California, when five Marines were flying their UH-1N helicopter to an Arizona bombing range for a night training exercise.
Metal teeth in the main drive shaft assembly suddenly sheared off, investigators said later, sending the helicopter crashing to the ground. Three Marines survived the impact, but the pilot and co-pilot were killed.
The accident was so unlike any previous accidents that Marine officials regarded it as "a statistical anomaly," the investigator said.
An initial probe yielded a variety of possible causes, one or more of which might be blamed for the crash, Marine records show. They were a possible alignment problem, metal failure, improper maintenance, grease loss and the loss of the lubricating ability of the grease inside a metal coupling that connects the drive shaft to the transmission.
Within a month, the Naval Aviation Depot in Pensacola, Fla., suspended use of the lubricant, known as Syn-Tech grease, while the Naval Air Development Center in Warminster, Pa., tested three sample batches at its lubricants lab. The suspension ended after 14 days when tests showed that the grease met Bell Helicopter specifications, lab and Marine officials said.
The grease, a synthetic material designed to withstand high temperatures and resist oxidation, was introduced to the military in 1978 by Syn-Tech Ltd. of Addison, Ill., for use in UH-1N drive shaft couplings, military officials said. It is used for tail rotors of some Army aircraft, but an Army spokesman said that service has not had problems with it.
Syn-Tech did not respond to repeated requests for interviews.
An Air Force report dated Oct. 3, 1991, and sent to all military services to warn of the "high accident potential" of UH-1Ns, disclosed that Navy investigators looking into the July 1990 Marine accident felt that the Syn-Tech grease, though it met Bell standards, "may not release enough oil to provide adequate lubrication."