A Japan Air Lines jumbo jet that crashed Monday night with the apparent loss of 520 lives seemed to be flying normally until a loud and sudden "bang" occurred above the rear of the passenger cabin, according to an off-duty flight attendant who survived the crash.

The cabin's interior simultaneously "turned white," probably due to mist caused by rapid loss of air pressure. The plane began weaving through the air and descending rapidly. Passengers put on life jackets and assumed crash positions in their seats. Two or three strong shocks occured at impact.

The account from Yumi Ochiai, 26, was provided today by Japan Air Lines as searchers continued to find more pieces from the plane's tail section floating in sea water 90 miles south of the mountainside that the jet hit at about 7 p.m. Monday.

On Tuesday, a 15-foot section of the plane's vertical stabilizer was found in the water. Today, part of its rudder and an air duct connected with an auxiliary power unit in the plane's rear were picked up. Investigators surmise that the plane also lost other parts that have not been found.

The discoveries proved that the tail section was heavily damaged in midair. That left little doubt as to why the crew was unable to control the jet, a Boeing 747, but provided no immediate answers as to how the damage occurred.

Before the crash, the jet's crew radioed a cryptic message about the right rear door being "broken." But today, Ochiai, who was sitting across the cabin and several rows forward from the door, said she had not been aware of any problem with it.

Transport Ministry officials said this evening that the door had been found at the crash site, hanging open on its hinges with its lock mechanism intact. They suggest that it could have been opened by the force of the impact. That cast doubt on suggestions that it had opened accidentally and seemed to eliminate theories that it had come free and smashed into the tail.

Speculation on the cause of the tail damage has produced several possibilities, including metal fatigue, accidental explosion, collision with another object and sabotage with a bomb. Most analysts here, however, are downplaying the bomb idea because of a lack of direct evidence.

Any bomb would presumably have been in the cargo area below the cabin or in the rear of the cabin itself, where Ochiai was sitting. Her account, regarded as the observation of an aviation professional, does not point to a bomb going off in either place.

Some Japanese press agencies reported today that Ochiai, now recovering from broken pelvic and hand bones, had said she could see "blue sky" through a hole punched in the cabin ceiling before the plane went down. But an airline spokesman said he was unable to confirm this.

Japan Air Lines today confirmed that the 9-year-old jet had sustained two minor accidents previously. In the first it dragged its tail more than 400 yards in 1978 and in a second dragged its far right engine during a landing in 1982. However, the plane was later inspected and judged airworthy.

Meanwhile, investigators combing the crash site, located about 60 miles west-northwest of Tokyo, recovered the plane's two flight recorders and wrapped them in blankets for removal to laboratories where they will be analyzed.

They should provide valuable clues about the cause. One is designed to record conversations and noises in the cockpit. The other should yield a running account of the final flight with such technical readings as altitude, air speed and manipulation of the jet's control systems.

Ochiai and three other survivors were found in the wreckage on Tuesday morning. No more survivors were found today and expectations mounted that they were the only ones among the 524 people aboard the plane to have lived through the crash.

One hundred twenty-one bodies or fragments of bodies were flown by helicopter today from the site to the nearby town of Fujioka. Fifty-one of them were reported to have been identified by this evening.

Meanwhile, a technical team from the Boeing Corp. arrived here today to assist in the investigation. Experts from the U.S. National Transportation Safety Board are also scheduled to take part.

Japan Air Lines president Yasumoto Takagi announced tonight he intends to resign "as soon as the situation has settled down." He told reporters: "I want to take responsibility." Such resignations are common in the Japanese business and political world, where leaders are held to have ultimate responsibility for all acts of their subordinates.

Takagi today called on Prime Minister Yasuhiro Nakasone to inform him of his decision. Nakasone reportedly admonished Takagi for the accident saying that discipline at the airline had become lax.

Japanese newspapers reported that Nakasone pointed out that his own departure on a trip to Western Europe last month was delayed after maintenance problems were discovered in the Japan Air Lines jet that he was to take.

The jet that crashed took off from Tokyo's Haneda Airport at 6:12 Monday evening on a 50-minute domestic flight to Osaka. It crashed about one hour later. It appears to be the worst single-plane disaster in aviation history.

Ochiai told Japan Air Lines officials that she was seated in row 56, four rows from the end of the plane, and reading a magazine when she suddenly heard a loud "bang" from above her at about 6:25 p.m.

With that, several things happened that aviation specialists say indicate a rapid loss of pressure in the cabin. Oxygen masks dropped from overhead compartments. A white haze, probably mist caused by moisture in the air turning into vapor, formed inside and Ochiai's ears hurt.

Ochiai also noticed that an air vent under one of the cabin crew's seats had opened. The vents function to equalize pressure in different compartments of a plane in the event of depressurization in one.

The jet began what Ochiai called a "Dutch roll," rolling from wingtip to wingtip and from nose to tail.

After a while, she saw Mt. Fuji, located about 60 miles southwest of Tokyo, out of a left-hand window and concluded that the jet was going back to Haneda Airport. "Oxygen ran out after 10 minutes but there was no trouble breathing," she said.

A flight attendant announced the plane was experiencing an emergency. Ochiai helped a women attendant on duty show passengers how to get into life vests and take up crash positions, in which passengers are buckled into their seats and lean forward, placing their heads on their knees.

Ochiai then put on her own belt and assumed the crash position. The plane began descending sharply. When it hit the ground, "there were two or three strong shocks. Seats and cushions flew all around me."

When the motion stopped she realized that a seat was on top of her. "I felt like my stomach had been torn apart. Pain, pain. With a lot of effort, I finally succeeded to get rid of my belt but my body was between two chairs. I couldn't move." She saw helicopters overhead and waved her hand, but they did not seem to notice. Gradually, she fell asleep.

Her place in the wreckage was not threatened by fire. Later, she heard men's voices and realized it was morning. The voices were those of rescuers, who pulled her out and flew her to a hospital.