The brother of one of the 301 people who died Aug. 19 on a burning Saudia Airlines Lockheed L1011 jumbo jet, just after it landed in Riyadh, yesterday sued the airline, Lockheed and Trans World Airlines, which trained the crew and provides technical assistance to Saudia.

The suit -- apparently the first filed in the United States concerning the second-worst single airplane disaster in history -- comes as reliable information is beginning to trickle out of Saudi Arabia. -

Both a recent Saudi News Agency report on the official preliminary investigation and U.S. aviation sources say investigators have determined that:

The fire began in a cargo compartment -- not the passenger cabin as was first reported.Early speculation suggested that a small butane gas stove carried by a Moslem pilgrim was the source of the fire. The real source of the fire in the cargo compartment is still unknown.

The cockpit crew noticed a fire warning light a full five minutes before deciding to return to the Riyadh airport, where the flight had originated. "It was incumbent upon the pilot to take an immediate decision to return to the airport . . . but . . . it appears that the aircrew . . . wished first to confirm the warning," the Saudi News Agency report said. An old problem in aviation is that warning lights malfunction often enough themselves that crews ignore the warning until they have confirmed the problem by other means.

Ten minutes later, the plane landed at Riyadh. Instead of halting the aircraft as quickly as possible and immediately evacuating it, the plane taxied to the end of a lengthy runway, then turned to the ramp that leads to the normal loading and unloading area, "thereby wasting valuable time which might have been used for evacuating passengers," the news agency report said.

"The experts think," the news agency said, that the spread of poisonous fumes throughout the inside of the Lockheed Tristar was so rapid that it exhausted the oxygen supply "so that nobody could open the exits or the doors from the inside."

Rescue crews were delayed in attempts to open the doors from the outside because the three engines continued running for some time after the plane came to a halt.

There has been substantial speculation within the U.S. aviation community that the doors could not be opened because the inside of the aircraft remained pressurized for high altitude flight after the plane landed. The doors on an L1011 are plug type, and inside pressure would increase the difficulty of opening them.

A Lockheed spokesman said yesterday that depressurization of the cabin would have to have been initiated by the cockpit crew. The implication of that statement -- and the U.S. speculation -- is that the crew failed to throw a switch that would have made it possible for the doors to be opened easily.

The lawsuit, which seeks millions of dollars in damages, was filed in federal district court here and a California state court in Los Angeles. It alleges essentially that Saudia was negligent in its maintenance and operation of the airplane, that Lockheed was negligent in manufacturing the airplane with materials that would give off poisonous fumes and that TWA was negligent in training the crew and maintaining the plane.

Lockheed and TWA had no comment on the suit. A TWA spokesman said that TWA trains the Saudia crews and provides technical assistance on other matters but that it does not perform the maintenance on the aircraft. The flight engineer -- the third-ranking member of the cockpit crew -- was an American employe of Saudia who had been hired by TWA for Saudia.

The plaintiff in the suit is Walter Muller, brother of Jack A. Muller, who died in the accident along with his wife. Walter Muller himself has many years of experience in the aerospace industry and is familiar with the problem of toxic substances in airline cabins. "The problem is not new," Muller said in a press release, "but all that anyone has been willing to do is write reports about it."

Federal Aviation Administrator Langhorne M. Bond has told reporters and testified in Congress that flammable interiors present a tough challenge to the FAA and the manufacturers. "You have three problems," Bond said in a recent interview. "The materials will either flame, give off dense smoke or give off toxic fumes. So far, manufacturers have found ways to suppress any two of the three problems, but not all three."