A hatch door on the nation's only flyable B1 bomber blew off at 16,000 feet while the aircraft was over the Occoquan Reservoir in Northern Virginia yesterday.

The bomber, which was carrying a four-man crew, including Gen. Charles A. Gabriel, Air Force chief of staff, returned safely to Andrews Air Force base 10 minutes after it had taken off for a flight to Offutt Air Force Base, Neb.

Lt. Col. Tom Cooper, an Air Force spokesman, said there was no damage to the aircraft, one of four prototypes of the politically controversial bomber. The plane was carrying no armaments.

"We do not have any idea why the hatch separated," Cooper said. He added that the incident is being investigated by the Air Force and Rockwell International, the builder of the B1.

Air Force authorities said they notified local police in Virginia immediately after the door separated. Two helicopters from the 1st Helicopter Squadron at Andrews searched for the door in the vicinity of the reservoir for more than three hours before giving up at 2:30 p.m.

"I would think if it had hit somebody we would have been notified by now," said Capt. Clemmer Montague, Andrews Air Force base spokesman.

About the size of a large car door, the hatch is one of four on the B1. They lie directly over each of the four crew positions and are designed to be opened in event the crew has to use the ejector seats. It weighs 150 pounds and is painted the same beige and brown "desert camouflage" colors as the plane.

B1 Aircraft No. 4 was "a showbird," said Capt. Montague. It had been in storage at Edwards Air Force Base in California until last June when it was dusted off and tuned up for the Farnborough Air Show in London. The B1 program, which was canceled by President Carter in 1977, has been revived by President Reagan, and building the bomber is one of the top priorities of the Air Force.

The plane had flown to London, refueling in mid-air, without incident. On its return trip, the Air Force decided to include a stopover in the Washington area. The bomber had been on display at Andrews since Monday and had been visited by Air Force brass, two senators and 18 congressmen during the week.

"There is nothing to indicate that anything done on the tours had anything to do with the hatch separating," said Coooper.

Cooper said the bomber was given a thorough preflight examination before it took off at 7:57 a.m., piloted by Lt. Col. LeRoy Schroeder. Gen. Gabriel, a fully qualified command pilot who was taking an "initial orientation flight," was sitting in the copilot's seat. Lt. Col. Tom Alexander and Rockwell test pilot Doug Benefield occupied the two seats to the rear.

The bomber was climbing between 12,000 and 16,000 feet at 300 knots when, after 10 minutes, a light indicated the left rear hatch door was not secure. Benefield went to examine it.

"He went forward to tell the pilot he saw nothing wrong with it and about that time the hatch separated," Cooper said. "It was pulled off by the airstream. They could hear it come off and some papers on the navigator's table were blown out, but there was no great rush of wind."

The pilot immediately returned to Andrews and made a smooth landing. Cooper said there had never been any problems with hatch doors on the B1. Air Force officials plan to replace the missing hatch and try a second time to fly No. 4 home to Edwards on Sunday.