A U.S. Department of Energy helicopter lost over Chesapeake Bay in March was part of a fleet of planes used to combat nuclear extortion, a government contractor who operated the aircraft said yesterday.

The WAMO-1 helicopter carried no special equipment, however, on what began as a routine flight March 24 from Andrews Air Force Base to Middletown, Del. It was scheduled to pick up a navigation device for another craft, according to Roy Lounsbury, manager of the aerial measurements system of EG & G Inc., a firm that specializes in nuclear-related problems.

"The crash is still a mystery," Lounsbury said, dismissing any suggestions of sabatage or hijacking in the plane's disappearance. A month after the plane failed to reach its destination, the bodies of the pilot and passenger washed ashore above the Chesapeake Bay Bridge. Parts of the plane itself appeared in the Chester River.

The plane itself has not turned up. "We were hoping this summer with all the activity on the bay would turn up something of the plane, but nothing did," Lounsbury said.

Lounsbury's company operates and maintains another government-owned helicopter another government-owned helicopter and one other airplane based at Andrews, as part of NEST, the government's Nuclear Emergency Search Team program to respond to nuclear terrorism and accidents.

EG & G is one of four contractors participating in the NEST program, created in 1974 to help the FBI find and defuse nuclear weapons planted by terrorists. NEST has responded to at least five threats against U.S. cities, all of them hoaxes.

For the past three decades, Lounsbury said, his firm has also assisted with the government's nuclear-testing program in Nevada, where it operates two helicopters and three airplanes for the government.

The missing helicopter, Lounsbury said, had seen service at Three Mile Island, ferrying scientists with geiger counters to the site of the nuclear reactor crisis. It had also flown to Canada in 1978 to assist in the recovery of nuclear material dropped by an exploding Russian satellite.

On another occasion, the helicopter had taken EG & G employes to Wilmington, N.C., to assist the FBI in cracking a case of industrial nuclear blackmail.

The pilot on the final flight was Richard Eicher of Huntingtown, Md. His passenger was Gerald Milton of Bowie, a Federal Aviation Administration employe in the control tower at Andrews, who went along to familiarize himself with the aircraft.