The Boeing 747, which pushed to the limit the boundaries of commercial aviation technology when introduced in 1966, has become the subject of intensive reexamination in this worst year in civil aviation history, which has seen almost 2,000 deaths.

There are more than 600 copies of the first jumbo jet. Each carries as many as 550 passengers. The four-engine plane has a remarkable record for reliability, but reexamination of some of its basic design concepts has been forced this year by two catastrophic events and two disquieting but not fatal incidents.

The catastrophes were the crashes of the Air India 747 that killed 329 persons and the Japan Air Lines jumbo in which 520 died.

The first troubling but nonfatal incident occurred Dec. 2 when an Air France 747 landing in Rio de Janeiro ran off the runway after one engine ran amok. And on Dec. 15, a British Airways 747 carrying 271 persons dropped a huge wing-flap section on a suburban Boston neighborhood while landing at Logan International Airport.

No injuries were reported in either incident, but happy endings would not be assured if the same things happened at other points in flight or with less skillful pilots. The British Airways crew members used an extraordinary control technique to maintain level flight and land.

"It has been a bad year for that airplane, but there is nothing we are seeing that has tied it together . . . . We haven't found any common thread," said Leroy A. Keith, the first line of defense. He is in charge of aircraft certification for the Federal Aviation Administration's regional office in Seattle, where Boeings are built.

The Air India crash into the Irish Sea June 23 remains a gnawing worry for Keith and others in aviation safety because they do not know exactly what happened.

The crash has all the earmarks of a terrorist bombing, and Sikh separatists have claimed responsibility. But no evidence confirms that account despite an extraordinary international investigation including salvage of many pieces of the aircraft from the ocean floor, 6,000 feet below sea level.

One piece was a section of a baggage-compartment wall and had 13 holes punched in it, from the inside. That was first thought to confirm a bombing but, when the section was examined ashore, the holes were found to have been caused by popping rivets as the wall was torn from supporting stringers while the airplane was coming apart.

"It's a real mystery," Keith said. "I'm really tired of holding my breath on that one. I'd rather find out it was a design problem I could fix than not know what happened." The investigation, under the control of the Indian government, is on hold through the winter.

The Japan Air Lines crash occurred Aug. 12 after the plane lost a tail-fin section and flew uncontrollably for about 40 minutes after takeoff from Tokyo. Much evidence points to collapse of the rear-cabin wall under air pressure as the 747 climbed to cruising altitude. The wall, called the aft pressure bulkhead, had been cracked in 1978 in a landing accident and repaired by Boeing.

Boeing said its "examination of the aft pressure bulkhead at the site of the crash . . . has revealed that a relatively small section of the bulkhead splice approximately 17 percent was not correctly assembled during a repair which Boeing made . . . . "

Although Boeing has not stated that the faulty repair caused the crash, the company has agreed to split with JAL compensation payments to victims' families.

That accident set off a major FAA study of the integrity of pressure bulkheads on all aircraft, not just 747s, and restudy of the integrity of the plane's hydraulic systems. FAA engineers expressed concern about the latter issue during initial reviews of the 747 design 20 years ago.

The 747 is so large that constructing mechanical linkages from the cockpit to all of the plane's controls is impossible. Hydraulic lines do the job instead. When the pilot moves a control lever in the cockpit, that pressure is transmitted through hydraulic fluid lines to actuators at the control site, and the control is supposed to respond.

The FAA's concern at the time of 747 design was that if one hydraulic system were to fail, another could pick up the load. The result is four separate, redundant hydraulic systems.

However, all four systems have connections to controls in the tail, and hoses for all four systems pass through the aft pressure bulkhead. "I have asked our folks to make sure the systems are separated, to revalidate the design," Keith said.

The tricky part of such revalidating is that any change will probably affect something else. "All the changes we have looked at can make other problems worse," Keith said.

As for pressure bulkheads generally, the FAA's study is to make certain that no basic design problem, as opposed to a maintenance-created problem, is lurking to surprise a pilot.

Changes have also been ordered in the 747 tail to ascertain that air escaping explosively through the bulkhead will not expand into the tail section. That is being done by covering a maintenance-access hole in the tail.

The Air France incident in Brazil is simply explained and apparently simple to avoid. A control cable from the throttle to the engine broke just after touchdown. As the tension was released, the engine surged beyond takeoff power, pulling the plane off of the runway as the crew fought for control with the other three engines.

The FAA has ordered a fleetwide inspection of engine-control cables for wear, then regular reinspections while it studies whether design changes are needed.

In Boston, preliminary National Transportation Safety Board analysis indicates that a nut sheared off a bolt holding the wing flap, one of the large control panels that extends from the rear of the wing for takeoffs and landings.

But each flap has eight bolts, and four are supposed to be enough to support it. It is possible that a strong vibration began for some reason and loosened the flap, sources said. The investigation is continuing.

Whatever the 747's problems this year, they have not affected the order book at Boeing, which sold a record $12.44 billion worth of new airplanes this year, including a $3 billion deal with United Airlines and a $2 billion transaction with Northwest Airlines.

Frank Shrontz, Boeing's new president, said in a recent interview with United Press International that "it would be naive to say that an accident like Japan Air Lines had no impact whatsoever on passengers or the investment community or consumer attitudes . . . . I suspect something like that is always going to have some effect. The problem is in trying to measure it."