The Army Safety Center here is the Army's equivalent of the civilian National Transportation Safety Board, but there is one major difference: The safety board's experts investigate in full view of the news media and the public; the Army's do the same thing in secret.

Army witnesses know they will not be identified. Manufacturers know that information volunteered or discovered about their aircraft will not be the source of product-liability suits. And commanders know that mistakes they might have made will not become the subject for disciplinary proceedings. It's the military equivalent of immunity from prosecution.

When Army investigators decide why the accident happened, their findings also are kept secret.

The Air Force and Navy follow similar procedures.

When the safety board investigates civilian accidents, information it develops about the technical performance of aircraft, the actions of pilots and the probable cause of an accident is made public and often forms the basis for civil lawsuits.

Military officers interviewed here agreed there is a theoretical possibility to cover up an embarrassing problem or to delay acting on it. They also said it doesn't happen.

Col. Joseph R. Koehler recently retired as the commander of the safety center. In an interview before his retirement, he said, "It's not costing me anything to go to the chief of staff and say, 'I think we got to ground the fleet right now and go back and take a look at something.' But I have to look at it purely from a safety standpoint. They have to look at it from an engineering standpoint, a cost standpoint . . . . I think people are going to think long and hard before you go to the guy who looks at the scraps of metal that are left after a crash and say, 'Hey, you're whistling Dixie.' I can't see the Army doing that."

The secret investigation conducted by safety center experts is one of two probes the Army mounts after a major accident. The other is a "collateral investigation" conducted by the commander of the unit involved in the accident. It is the basis for any disciplinary action within the unit. The collateral report sometimes is released to the public, but analyses and recommendations usually are withheld.

The Army makes no attempt to cross-check one investigation against the other. "Factual data are usually common to the collateral and safety center investigation," the Army said in an answer to a written question from The Washington Post. "There is, however, no formal cross-checking of the two investigation reports because they evaluate the accidents from different perspectives . . . . "

Expensive modifications to aircraft suggested by accident investigators are slow to happen in civilian as well as military flying. Everybody wants to be sure the change is necessary, that it won't create problems as bad as the ones it solves, that the cost will be worth the benefit and that the time involved to make the change will not seriously damage the mission, whether it is flying passengers for hire or dropping an airborne patrol.

In the case of the mast-bumping problems on the Bell Helicopter Textron teeter-rotor fleet, for example, it took eight years of intermittent studying and worrying before a modification was proposed in 1981 and took another four years and substantial press attention before the Army decided to modify the helicopters.

The mast bumping involved Bell models that have what is technically known as a "teeter-rotor" system, a two-bladed main rotor. An inevitability of the teeter-rotor design is that if one blade goes up, the other goes down, just like a teeter-totter.

The two rotors are attached to the mast of the helicopter through a doughnut-like hub. If the angle of the blades in relationship to the mast of the helicopter becomes too exaggerated, the underside of the hub will bump the mast, the mast will break and the helicopter will crash.

The Army Safety Center, it was discovered, had been cut out of the decision-making process on whether to make the modification.

Since its report, the safety center has been added to the process on whether to approve proposed equipment modifications.

The civilian safety board has a long-established process for following up safety recommendations to make sure that they are implemented or that the Federal Aviation Administration explains why they are not, in which case the safety board has an opportunity to complain publicly. The Army has no centralized process for tracking recommendations and no requirement that anyone tell the safety center what happened later, officials here said.

Employes of columnist Jack Anderson recently lost a suit against the military services to gain access to witness statements, recommendations and conclusions of accident reports. Army General Counsel Susan J. Crawford, in court documents, argued:

"The ways of human nature indicate that those who expect public release of their remarks may well substitute concern for appearances in lieu of candor. In this vein, the limited use of the accident report encourages a frank and open exchange of ideas among investigating officers without fear of recrimination or harassment."

U.S. District Court Judge Louis F. Oberdorfer upheld the Pentagon on the grounds that recommendations and conclusions from investigative boards are not a decision but part of the deliberative process and, thus, exempt from disclosure under the Freedom of Information Act.

C.O. Miller, an aviation-safety expert who has worked for the military as a consultant and who once was chief accident investigator for the safety board, said, "As an investigator, I can develop more facts more quickly than I can in the environment of a safety board investigation."

When Miller was with the safety board, the Government in the Sunshine Act had not yet forced the board's deliberations and much of its investigations into public view. The fact they are now public is a point of controversy among some senior investigators at the board.

However, Donald W. Madole, a Washington attorney specializing in aviation litigation, said, "Not one person has ever claimed that the Sunshine Act has resulted in the failure of the aerospace industry to reveal the kind of information they revealed before the Sunshine Act. If there is a real question of national security, I can see that, but I cannot see the government adopting a system that hides its workings."