A 15-foot-long section from the tail of the Japan Air Lines jumbo jet that crashed Monday was found floating in water yesterday about 90 miles from the crash site, indicating that the aircraft, with 524 persons aboard, was seriously disabled before it went down.

About 30 minutes before the crash, the crew of the plane radioed to ground control that its right rear door was "broken." Aviation specialists here have speculated that the door blew out and damaged the plane's tail section.

The floating section was found about 10 miles west-southwest of the Miura Peninsula south of Tokyo, an area over which the jet passed.

This morning a coast guard vessel picked up a glass-fiber tube bearing the mark "747" in the same vicinity in which the stabilizer section was found, the Japanese Maritime Safety Agency reported. The tube's function was not known.

Early yesterday rescue teams flown by helicopter to the heavily wooded mountainside 60 miles west-northwest of Tokyo where the jet crashed found at least 52 bodies, the Japan Self-Defense Agency announced. By 6:30 p.m., when the search was called off for the night, only four survivors had been located and hopes dimmed that others had lived through the crash, which scattered wreckage over a mile-long area.

Initial reports that seven persons had survived were incorrect, authorities said yesterday.

The four survivors, including two children, were found in the shattered rear section of the plane, which had not been destroyed by fire. Live television broadcasts from circling helicopters showing the wreckage in mountainous terrain seemed initially to offer little hope of survivors.

The crash ranks as the worst single-plane disaster in the history of commercial aviation. It occurred about one hour after the jet took off from Tokyo's Haneda Airport on a domestic flight to Osaka.

During the night, rescue workers hacked out a helicopter pad near the crash site to be used to ferry the dead out. By late this morning, 49 bodies or fragments of bodies were reported to have been taken by helicopter to Fujioka. Four of the dead have been identified.

Aviation specialists here are looking closely at characteristics of the plane's rear door. JAL spokesman Shinji Waterai said yesterday that the door was one of many possible causes of the crash.

He said that the door in question was not touched or opened during the approximately one hour that the Boeing 747 spent on the ground in Tokyo after flying in Monday afternoon from Fukuoka city on Japan's Kyushu Island as Flight 366.

A Boeing spokesman in the United States has discounted the door theory, saying that there are too many precautions built into its design to allow it to blow out.

A Boeing spokesman, responding to reports that the component found floating today was part of the plane's vertical stabilizer, said later that it was "as important to the plane as a wing," but would not speculate on how it broke off or whether its loss caused the jet to go down, The Associated Press reported.

Following their report about the "broken" door, the crew radioed that it was unable to control the plane. Loss of a large section of the tail could be expected to cause loss of control.

JAL, meanwhile, confirmed last night that the aircraft had "bounced" on an Osaka runway in 1978 because of pilot error, damaging outside panels and its mainframe when it dragged its tail section for about 400 yards. Officials here were reported to be studying a possible link between this and Monday's crash.

Airline officials said that the plane was well maintained. They also said there was no report of bad weather in the area of the crash.

Authorities here have dismissed as a probable hoax anonymously telephoned claims that a radical group destroyed the jet. One call went Monday to a JAL reservations center, while a second reportedly was received at the Japanese government's National Security Council.

The caller to the airline, apparently a Japanese, said: "We are the Kakumaru, and it was our power that blew up the plane." Kakumaru is the Revolutionary Marxist Faction, a small but long-established radical leftist group.

Police are skeptical that the group would make an indiscriminate attack on civilians. In addition, investigators do not appear to have found any direct evidence of sabotage.

Investigators should be able to locate the two flight data recorders carried by the plane. One recorded cockpit conversations and the other technical data on the plane's final flight, such as air speed and manipulation of controls.

The four survivors, moreover, also should be able to give clues about the crash.

The crash scene was brought live into Japanese homes at sunrise yesterday as television cameras in helicopters panned over a picture of total desolation.

Ten hours after the crash, parts of the jet's wreckage still were emitting gray smoke. From the air, there was no sign of any of the 524 persons who had been aboard.

It seemed nothing short of a miracle when the first rescue teams reached the site later and pulled four persons alive from the wreckage -- a mother and her 8-year-old daughter, a flight attendant and a 12-year-old girl.

Television cameras caught Keiko Kawakami, the 12-year-old, impassively staring to the sky as she was hefted toward safety on a stretcher across a carpet of broken baggage, metal fragments and other debris.

Keiko dutifully answered questions from reporters as she lay awaiting treatment. Do you remember the accident? No. Where do you feel pain? I don't have any pain. Until what point do you remember? I don't know. Later, in the hospital, she inquired about her missing parents.

Her grandmother, Kimie Kawakami, was one of the viewers. "I touched the screen and stroked her head," she said later.

Hiroko Yoshizaki, 35, survived with her daughter Mikiko, 8. Three other members of their family did not. They all were returning from a visit to Tokyo. JAL flight attendant Yumi Ochiai, 26, the fourth survivor, was on a private trip.

All four are expected to recover from their injuries.

About 1,500 persons have gathered at Fujioka, near the crash site, to await news on their loved ones. Yesterday, authorities began laying plastic on the floor of a gymnasium in preparation for turning it into a morgue. Coffins arrived by truck.

Only 22 of the passengers on Flight 123 were foreigners. They included six Americans. Japan Air Lines identified them yesterday as:

* Christopher, Okja and Scott Kim, no address available.

* Edward Anderson of 7642 South Oneida Ct., Englewood, Colo.

* Michael Hanson of 4129 F South Evanston Circle, Aurora, Colo.

* Ward Wallach, an English teacher employed by the airline, of Los Alamitos, Calif.