U.S. airlines moved rapidly today to beat a 3 a.m. (EDT) Tuesday deadline to inspect wing engine mounting assemblies on their DC10 jumbo jets or ground them.

The deadline order came in Washington from Federal Aviation Administrator Langhorne Bond as a result of the National Transportation Safety Board's inquiry into the crash of an American Airlines DC10 here last Friday that killed 273 people in the nation's worst aviation disaster.

NTSB investigators here stressed that their concerns about the integrity of the engine mounting assembly extend beyond the broken bolt that was found to have snapped in the DC10 that crashed Friday. "The whole technical area" of engine attachment to the wing is under suspicion, said Douglas Dreifus, chief of the safety board's technical team.

Dreifus said parts of wreckage recovered by the team showed there had been a failure in the aft pylon bulkhead, one of the three points where the engine and supporting pylon were fixed to the wing.

The famous broken bolt, recovered after two days of searching the grass along Runway 32 Right at O'Hare International Airport, "is just a part of the whole," Dreifus said.

Bond's airworthiness directive stressed that in highly technical language when it required U.S. airlines to "visually inspect the inside forward flange of each wing engine pylon aft bulkhead for cracks" in addition to inspecting or replacing the two bolts in the center support point. There are three points where the pylon attaches to the wing.

It was unknown just how much disruption in air travel would be caused by Bond's order, but most airlines said there would be little or none. United Airlines, for example, has 37 of the domestic fleet's 134 DC10s, and had completed inspection today on 13 of them as mechanics returned to work after a lengthy strike.

United expects to have 24 of its 37 DC10s properly inspected and flying Tuesday. Replacement bolts were being flown in to some United bases from the McDonnell Douglas factory in Long Beach, Calif. Simple replacement of the bolts plus inspection of the aft pylon bulkhead was one option provided in Bond's order.

American and Northwest Orient Airlines, with 30 and 22 DC10s, respectively, reported that they would complete their inspection or replacement programs before the deadline. Other airlines, including Continental, National and Western, reported that there might be minimal disruption in service. Western Airlines, for example, canceled one DC10 flight today.

American reported that it had found no problems with the engine mounting assemblies on any of its other DC10s as a result of inspections completed so far. Elwood Driver, vice chairman of the safety board, said during his regular press briefing tonight that no discrepancies on other DC10s inspected today had been reported, but that the inspection program was not completed by all airlines and all reports were not yet in.

Bond's order was mandatory only for domestic airlines, but many foreign governments were ordering their carriers to follow the same procedure. Swissair immediately grounded all nine of its DC10s until an inspection is completed. Lufthansa at first declined to grounds its 11 DC10s, then reversed itself. Dutch, Canadian, Scandinavian and British airlines, including Laker, also immediately required inspection procedures or grounding of their DC10s.

Although Bond used the verb "ground," it was the consensus of aviation officials that his action would result in little grounding, since he permitted DC10 flights to continue through the Memorial Day holiday.

Asked why at his Washington press conference, Bond responded that "the risk seems sufficiently small." He also was asked if he would fly in a DC10 before it had been inspected and he said "I will and I have," calling it a "very fine aircraft."

In Los Angeles, McDonnell Douglas, manufacturer of the DC10, issued a statement late today that said ". . . the probable cause of the tragic accident at O'Hare has not been determined.We do not yet know what relationship the bolt found near the runway may bear to the loss of the aircraft . . .

"However, because it is the prudent thing to do, McDonnell Douglas on Sunday, May 27, issued an alert service bulletin to all DC10 operators advising them to conduct an inspection of their aircraft in the area where thepylon supporting the engine attaches to the wing."

The company's alert service bulletin recommended that the inspection take place within seven days or 50 flight hours, but the statement did not note this.

McDonnell Douglas said that there are 274 DC10s now in service, "averaging more than 750 flights and more than 135,000 passengers daily."

Consumer advocate Ralph Nader, who had called Saturday for an immediately grounding of DC10s, said Bond's order "should go further than just examining the type of bolts that failed . . ."

The safety-board's Dreifus said that the exact sequence of events of the breaking apart of the engine assembly in the DC10s left wing may not be known for weeks. Metallurgists are studying the problem as more and more fragments from the plane are recovered from the runway and the crash site, about half a mile away.

"There is an indication of [movement] in the area," Dreifus said. He said it was uncertain whether the bolt or the bulkhead had failed first. In any cae, none of the three support points was designed to carry the load if one of the others failed, NTSB investigators have said.

Dreifus, Bond and others also stressed the investigation is still concentrating on the question of why the plane could not fly with one of its three engines missing.

DC10s are supposed to fly with one engine "out," in other words,turned off or shut down. It is another matter to fly with one engine ripped off the wing.

Bond said that "there are no procedures, no training, no nothing . . . that a crew would go through or we could simulate" to deal with an engine falling off.

However, the safety board's Driver thought the point important enough to make a special statement at his press briefing tonight. He said that a McDonnell Douglas engineer agreed with the board's position that "the DC10 will fly and climb on two engines if the third engine, nacelle and pylon are missing . . ."

The flight data recorder, one of the black boxes recovered at the scene, Driver said, showed that the plane continued climbing to about 600 feet above the ground and that there was only a minor (2-degree) change in course after the engine fell off.

The recorder also showed that the other two engines continued to develop full takeoff power for a full 33 seconds after the engine fell off. By the time, however, the plane had rolled 107 degrees to the left - almost on the side - despite indications on the flight data recorder that the crew was doing "the best it could to correct for the unusual attitude of the aircraft," Driver said.

Thirty-three seconds after the engine fell off, the flight data recorder stopped working, apparently for lack of electrical power. Therefore the board does not have a complete technical record of what happened once the plane was flying on its side. In a matter of seconds, it fell to the ground, nose first, exploded and disintegrated.

Dreifus said that preliminary research indicates that the American Airlines jet lost two of its three hydraulic systems at some point during its 38-second flight. Some hydraulic lines were apparently severed when the engine broke loose.

The loss of hydraulics could deprive the crew of its ability to control the plane. There is redundancy in the hydraulic systems so that if one fails another will supply control, but Dreifus said "we gotta sit down with the Douglas manuals and work this through."

It is not known yet what systems or controls were rendered inoperative when the engine assembly separated and rotated up and over the wing. The engine possibly even "flew" over the entire aircraft because it came to rest on the right side of the runway.

There has been some speculation that the engine might have struck the tail, but NTSB investigators are inclined to discount that without ruling it out completely until they have further examined the strike marks on the engine and determined their cause.

"This is one where I think we're going to get all the answer," Dreifus said. "We're working with hardware, which is a lot easier."

Bond's directive today is the first "grounding" in the Federal Aviation Administration's 27-year history. The predecessor Civil Aeronautics Administration once grounded the old Martin 202 for almost a year and permitted both the Douglas DC6 and the Lockheed Constellation to voluntarily resign their airworthiness certificates until they could fix problems found to cause crashes.

"It's defeat every time an accident of this kind happens," Bond said. "It's a defeat for the FAA, for the designer and for the air carrier." Later, he said, "Somewhere, somehow along the way something was not done that should have been done. It's a personal loss for all of us." CAPTION: Picture 1, American Airlines technicans in Los Angeles performing required inspection. AP; Picture 2, In Washington, the FAA's Langhorne Bond said, "It's a defeat every time an accident of this kind happens," By Larry Morris - The Washington Post; Picture 3, while in Chicago, federal investigators were picking up pieces of DC10 wreckage from gouge in earth caused by jet's left wing. AP; Illustration; Investigators think forward link bolt (above center) sheared, shifting excess weights to front and aft attachments (upper right), and that the rear attachment then fractured, causing engine to fall off plane. By Richard Furno - The Washington Post