Federal Aviation Administrator Langhorne Bond indefinitely grounded all U.S.-operated DC10 jumbo jets yesterday because, he said, there is evidence of a "possible design problem" in the engine mounting assembly.

Bond's extraordinary action came the early morning yesterday, just hours after the FAA had fought a court order that would have done the same thing. The turnabout in FAA attitudes came after inspectors confirmed that two airplanes previously checked and cleared apparently had developed cracks in their engine mounting assemblies in less than 100 flying hours.

The decision disrupted the travel plans yesterday of about 70,000 passengers, airline industry spokesmen estimated. The long-range disruption could be much worse, because aviation experts agreed, it looks like the troubled DC10 wiil be grounded for weeks, at minimum.

This grounding is the fourth, and by far most serious, formal action the FAA has taken against the DC10 since the May 25 crash of American Airlines Flight 191 in Chicago that killed 275 persons.

McDonnell Douglas, the manufacturer of the plane, called the FAA action "extreme and unwarranted. We are exploring every legal avenue, including the possibility of going to court" to have the order overturned, McDonnell Douglas spokesman Frank Tomlinson said yesterday.

The normal appeals route for McDonnell Douglas would be through an administrative law judge of the National Transportation Safety Board, then to the board, then to a U.S. Court of Appeals. Even on an expedited basis, that route could take days.

Meanwhile, both the FAA and the safety board were extending their investigations beyond the Chicago crash into the entire process by which the DC10 first was declared fit to fly.

The FAA said that formal orders of investigation against both McDonnell Douglas and American Airlines will be issued. Those orders permit the FAA to subpoena records and witnesses and to take formal testimony.

"Our decision is open-ended and indefinite," Bond said. "It will remain in force until we have found the problem, analyzed it and cured it." He said the exclusive area of concern was with the "pylon design aspects of it." The pylon is a support structure that holds engines on the wings of the DC10. In the Chicago crash, the left engine and pylon fell off the wing just as the airplane was lifting off the runway. In subsequent inspections, 68 aircraft were found to have some problem with their engine mounting assemblies.

Despite those discoveries, however, Bond had been satisfied that a rigorous inspection program would solve the problem. DC10s were ordered out of the sky three times, but permitted to fly again after inspections or repairs, if necessary.

When asked what changed his mind, he said, "My certitude has switched from the position of high likelihood of no risk to sufficient likelihood of risk, and that's enough to put the planes on the ground.

"Our investigation has proceeded now for a week and a half, and each something about this incident, and this has clinched it for me. It can't be solved, in my judgment, by an airworthiness directive. It has to have complete grounding of the fleet and a review."

Bond's three previous actions have been "airworthiness directives," formal orders to airline requiring certain inspections. The action taken this time is the suspension of the "type certificate" for the DC10 granted to McDonnell Douglas. Without that certificate in force, operation of the airplane is illegal.

U.S. airlines operate 138 DC 10s, and another 137 are flown by foreign carriers. Most foreign carriers yesterday were following the FAA's lead and grounding their DC10s. Furthermore, Bond last night issued a regulation banning the operation of foreign DC10s in U.S. airspace.

Asked why American Airlines alone among U.S. operators was singled out for a formal "order of investigation," Bond said, "Maintenance procedures at American Airlines are ones we have focused on most heavily."

Safety board investigators have determined that some of the cracks found on DC 10 engine mounting assemblies could have been caused when the engine and its support pylon were reinstalled in the wing after maintenance.

McDonnell Douglas, in its formal statement, said, "Such cracks have been found only on aircraft operated by airlines whose past procedures for removal and reinstallation . . . have been contrary to McDonnell Douglas recommended procedures."

Although American Airlines was not named in the McDonnell Douglas statement, American responded shortly thereafter saying that it was disturbed and astonished" by the McDonnell Douglas statement.

"American changed its first DC10 pylon on April 17, 1977, at Los Angeles," the American statement said. "Two McDonnell Douglas representatives . . . observed the entire process and actively participated in the procedure."

Nonetheless, the safety board said Monday that American Airlines had used a different procedure to dismount the engine and pylon from the wing than the one recommended by McDonnell Douglas. The safety board recommended against the different procedure, and American announced Monday that it had changed to the recommended method.

Bond told of the decision to ground the DC10 by transatlantic telephone. He was in London Tuesday, en route to the Paris Air Show, one of aviations's great events. He returned to Washington yesterday morning.

He seemed confused yesterday about exactly when he received information that led to the grounding of the fleet. At one time he said he got the first report of possible design problems at 8 p.m. At 8 p.m. Tuesday, FAA attorneys were scurrying around town trying to get a stay against a court order to ground the aircraft. U.S. District Court Judge Aubrey E. Robinson Jr. finally stayed his own order about 9:30 p.m.

Senior FAA officials intimately involved in the discussions in Washington Tuesday night, however, said that there was no indication of new problems until about 11 p.m.

"I honestly felt at 6 p.m. Tuesday that we had it firmly under control for the first time," said Charles R. Foster, associate administrator for aviation standards. Foster and FAA attorneys worked with the Justice Department to try to convince Robinson of that fact.

About 11 p.m., John R. Harrison, Foster and others were on the phone to Bond.

"Foster was saying ground 'em," said Jerome Doolittle, Bond's official spokesman, who attended many of the FAA meetings Tuesday. "Foster was saying, 'Jesus Christ, we can't go into court now and say we have this under control. It would be perjury.'"

Bond was convinced.

The new cracks found on the American planes were in a pylon flange next to the aft bulkhead, one of the three points at which the pylon attaches to the wing. One of the American planes had previously been inspected once under the recent Bond order; the other twice. No cracks had been found.

The new cracks were spotted at a reinspection Bond ordered every 100 hours or every 10 days after the first inspection. The cracks were confirmed through the use of a "dye penetrant," a substance that makes cracks more visible to the naked eye.

The clincher for Bond was that other cracks have been discovered in airplanes that have not had their pylons removed. Age also does not appear to be a factor, Bond said. That leads inevitably to the possibility of a "design problem."

McDonnell Douglas said in its statement that all of the eight planes that have been found with cracks in the aft bulkhtad are DC10-10 series. These are also DC10-30s and DC10-40s, and "no such cracks have been found" on them, McDonnell Douglas said.

Bond said, "we believe there is sufficient design commonality" to justify grounding the entire DC10 series. If the FAA is erring, Bond said, it will be "erring on the side of safety."

The Air Transportation Association of America, the airline lobby, estimated a week ago that the DC10 represent about 12 percent of the available seats on domestic carriers. With the total fleet grounded, the loss in revenue to U.S. carriers would average about $6 million a day, the ATAsaid.

Bond had to consider that in his order, because he is charged by the law with the worrying about the economic health of the industry as well as the safety of flight. Critics of the FAA have often pointed to that law as creating an irreconcilable conflict of interest.

"The consequences of his decision are very grave," Bond said. "It will affect every carrier flying DC10s, the manufacturer and the traveling public. It was a very weighty, costly decision. I did not make it lightly, but my concern is safety, and I don't think I could've any other course."

McDonnell Douglas stock dropped 2 1/2 points yesterday on the New York Exchange, to 20 7/8. Since May 24, the day before the Chicage crash, McDonnell Douglas has dropped 7 5/8.

The national Transportation Safety Board investigation, meanwhile, has extended to three cities, Tulsa, Dallas and Long Beach, in what turned into a highly technical search for the answers to why flight 191 crashed.

Specialiests in Tulsa are tearing down the remnants of the three General Electric CF6 engines on the crashed airplanes to find out if there were any problems there. Early indications are that the engines functioned properly.

In Dallas, a flight simulator is being programmed to see if the actions of flight 191 can be reproduced. After the engine fell off the wing, the plane rose to about 600 feet, rolled to the left, then fell to ground. The leading theory is that the plane's hydraulic systems were disabled when the engine came off, rendering the plane uncontrollable.

In Long Beach, safety board experts are poring through the documents that led to the certification of the DC10 by the FAA in the first place, in an attempt to determine what areas received special attention.

Most of the work at the crash site in Chicago has been completed, and anlysis of that work is now under way here, board experts said.

In the U.S. District Court yesterday, the Airlines Passengers Association sought an extension of Judge Robinson's injunction ordering the FAA to ground the plane. Robinson denied the request, and let his injunction stand until Monday, when a full hearing is scheduled.

Because of Tuesday night's events, U.S. Attorney Royce Lamberth said, there was no basis for the federal government to seek a stay of the injunction.

"It would be rather difficult to obtain," said Robinson. CAPTION: Picture, Charles R. Foster: "I honestly felt at 6 p.m. Tuesday that we had it firmly under control for the first time." By Joel Richardson - The Washington Post