The crash of a small private jet plane into a McLean subdivision April 28 appears to have started when the left wing ruptured because of excessive stress, the chief U.S. crash investigator said yesterday.

However, investigator Richard G. Rodriguez said, it will be weeks before the reason for that rupture, and the deaths of all four people on board the plane, is known.

"There is no indication of weakness in the left wing or of any structural failure," Rodriguez said. The wing, part of a twin-jet Hawker-Siddeley 125, ruptured because its design load was exceeded," he said.

Because the wing also was a giant fuel tank - filled with 450 gallons of jet propellent - the rupture spilled fuel into the sky. The fuel ultimately exploded and produced a fireball that was seen for miles despite the rain and clouds of that dark April evening.

Airplanes parts ranging in size from small rivets to huge sections of tail fell out of the sky and onto a McLean subdivision just north west of Tysons Corner. One home was destroyed and another damaged, but there were no serious injuries on the ground.

Rodriguez and his team of experts from the National Transportation Safety Board's Washington field office at Dulles Airport are trying to reconstruct the last moments of the plane's flight. A model of the HS-125 - including the N40PC tall number of the ill-fated plane - sits on Rodriguez' desk.

Rodriguez refused to speculate yesterday on what could have caused the rupture in the wing. "There could be 25 reasons," he said "and if I began to list them for you, I would leave out the most important one."

Investigators have learned some things. The fuel explosion occurred outside the airplane, and although its blast may have contributed to a breakup of the craft, it was not primary. There is no evidence of a bomb or other form of sabotage. The engines both appeared to be functioning normally.

Other aviation experts suggested yesterday that the kinds of stresses that would make a wing rupture could include, obviously, an unusual sudden maneuver by the pilot for some as yet unexplained reason, extraordinary turbulance, or some kind of failure in the control system of the aircraft that would force an unusual maneuver. But they cautioned that an early attempt to pin the blame on one factor would be premature.

The parts of the plane that have been recovered are laid out - from nose to tail and wingtip to wingtip - on a concrete apron next to a workshed at Dulles. Investigators were still poking through the rubble yesterday.

A major effort in the weeks to come will be to determine the sequence of the plane's breakup - what fell off first, what separated second, etc.

"These planes are not designed to fly with cracks in the wing," Rodriguez said as he pointed to the left wing tank.

A chart of where every part fell will be made and each part will be weighed and checked for its aerodyamic properties - how fast if would fall. "Then we figuratively try the old movie trick of running the film backwards to see if we can put it together again in the sky." Rodriguez said.

It is an imprecise science at best, but such detail is the only method the safety board's investigators have of figuring out what happened.

Two tools that are helpful in solving the mysteries of commercial airliner crashes are missing in this case - the so-called "black boxes," or cockpit voice recorder and flight data recorder. Those instruments, required in commercial aircraft, are not required in private planes.

Safety Board Chairman Webster B. Todd Jr. told reporters after another private crash some months ago that "with these corporate jets now costing $2 million to $3 million, we're beginning to think that maybe they should spend a few more thousand dollars for these recorders."

The cockpit voice recorder is nothing but a microphone in the cockpit connected to a tape recorder. Crew conversations, the sound of engines and switches and radio transmissions that the microphone captures often tell investigators what happened. The only voice recording in this case - the air traffic control tape - shows no panic or even the declaration by the crew members of a problem.

The flight data recorder is a sophisticated device that preserves many operational characteristics of a flight - power settings, altitude, etc. Both "black boxes" are built to be crash resistant.