Federal Aviation Administrator Langborne Bond grounded all 134 domestic McDonnell Douglas DC10 jumbo jets yesterday because inspections he had ordered "are turning up grave and potentially dangerous deficiencies" in the engine mounting assemblies.

Bond's unconditional arder immediately threw a crimp into the operations of the eight U.S. airlines that own the three engine wide-engine wide-bodied jets, and inconvenienced an undetermined number of their passengers across the country.

Some flights were canceled, and others were made smaller, substitute aircraft. It could be three days before all planes are reinspected and those with no problems are permitted to return to service. Bond estimated.

The order stemmed from the continuing investigation by the National Transportation Safety Board into the crash last Friday of an American Airlines DC10 that killed at least 273 persons in Chicago.

Furthermore, according to sources close to that investigation, new structural problems in the wing supports for the engine assemblies have been discovered by the board's investigators.

Additionally, there is growing evidence that a massive hydraulic system failure, possibly initiated when the engine fell from the left wing, ultimately made the plane uncontrollable.

"We've found some new areas of stress that we're going to check," a board expert said. "We're looking at the entire area where the pylon [which supports the engine] attaches to the wing. There are cracked areas on the wing side that could be metal fatigue."

Those cracks were discovered yesterday as investigators continued to examine the wreckage. All previous reported findings of metal fatigue or cracks in important supports had involved three points where the pylon attaches to the wing - not the wing itself.

A bolt in one of those attaching points clearly broke on the DC10 sometime during a chain of events that included an engine falling off the left wing and the crash.

When that broken bolt and other cracks in the support structure were discovered, Bond ordered DC10s grounded until the bolts were replaced and the aft bulkhead, another fractured support point, was inspected.

Inspectors turned up other problems on seven airplanes, according to FAA officials. John Harrison, director of safety for the FAA, said that "there were cracks, corrosion and sheared bolts. Problems were found in the assembly and in the hub assembly. Loose bolts were found in aft attach fittings. Cracks were found in thrust link assemblies." All of the parts he listed are housed in the pylon or pylon attachment.

On one United Airlines plane inspected in Chicago, according to two highly placed FAA officials, and engine was so loosely fastened to its mountings that it was possible to move it laterally by hand.

United denied the claim through a spokesman, and also said that problems had been found with only one of its planes, not four, as FAA spokesmen said. In any event, United grounded its own fleet of 37 DC10s an hour before Bond acted.

"My concern is that we're finding all kinds of things, not just one thing," a senior FAA official said.

The FAA, with McDonnell Douglas, was working out a detailed inspection plan to cover all of the possible sources of concern, according to FAA spokesmen. Once that inspection is completed, the DC10s will be returned to service, assuming they found free of problems.

That could be as early as Thursday or Friday. Thereafter, the engine support assemblies would have to be inspected every 100 hours or every 10 days, whichever comes first.

The inspections will be performed by FAA-licensed mechanics, who are employes of the individual airlines, under FAA supervision. Though such an arrangement is standard practice in U.S. avaiation, it has occasionally been criticized by safety experts who charge that airline employes are subject to commercial pressures and might put safety second.

The FAA response is that its supervisory techniques are sound and that any other procedure would require the addition of hundreds of federal employes.

At a news conference yesterday, Bond said that, in addition to all domestic DC10s, he was also grounding the seven Airbus Industrie A300 widebodies that Eastern Airlines uses. The A300, he said, has the same engine and pylon combination as the DC10.

Later in the day, however, the FAA reversed itself and told Eastern Airlines that it could go ahead and operate the A300, because it was satisfied that the plane's configuration differed sufficiently from the DC10's that there was no fear of a similar problem. Eastern continued to fly the A300s all day because, according to spokesman Jim Ashlock, "We never got official notification not to."

Bond also said that he has directed a review of "procedures for inspection of all engine mountings of all wide-bodied aircraft." In addition to the DC10 and the A300, the other wide-bodies are the Lockhead £1011 and the Boeing 747.

Bond's grounding order is not binding on the foreign operators of 140 DC10s, but many of them followed suit yesterday.

The order takes out of service for an unknown period 12 percent of the seats available on domestic airlines. However, according to the Air Transportation Association's William Jackman, most DC10s have been committed to transcontinental flights and service between New York and Chicago, Chicago and the West Coast, New York and Miami and the West Coast and Hawaii.

When asked how he thought his grounding order would affect air travel, Bond said, "I hope it will not disrupt U.S. air traffic, but if that's necessary that will be the case."

The three airlines that would appear to be most affected are United, American and Northwest Orient. Their fleets have 37, 30 and 22 DC10s, respectively. United, however, is just resuming service after a lengthy strike, and had few reservations.

American said that about 100 of its scheduled 1,060 departures every day were made by DC10s, and that its DC10s were in heavy use in the Chicago-New York/La Guardia corridor.

"We canceled flights and have actually suspended service," said Joe Moran, and American Spokesman. "We're trying to use backup equipment, either our own or by transferring people to other flights."

United said that it had flown eight of its scheduled 50 DC10 flights yesterday before the ban came. As with other domestic airlines, those flights were permitted to continue to their next scheduled destination.

Northwest Orient refused to release any statistics on cancellations, but said that some substitutions were made with other aircraft.

Northwest was reported by the FAA to have found three planes with wing assembly problems during the first round of DC10 inspections. The airline had no comment, but reported that in inspecting the bolts for metal fatigue with a penetrant dye, "Any bolts which showed the slightest indication of wear were replaced with new bolts." Northwest would not say how many bolts showed wear.

National Airlines, with 16 DC10s, appeared to have a major problem because the only planes it has that are capable of crossing the Atlantic are DC10s.

Last night National was holding two flights for Europe in Miami and New York pending receipt of the FAA's order and, it hoped, a successful inspection.

Bond was asked yesterday how the cracks and fatigue points had been overlooked or disregarded in earlier inspections. "There's no good explanation for that, somewhere along the way, we didn't do it right."

While airlines and passengers were busy changing their plans, the safety board investigation was continuing in Chicago and expanding to other centers of air travel, such as Tulsa and Washington.

Board experts, it was learned, are undertaking an extensive examination to the crashed DC10's hydraulic systems to test a hypothesis that they were knocked out when the left engine fell off the jet just as it lifted off the runway.

There is no question that one of the systems was knocked out, because each of the plane's three systems is gear-driven from the engines. When one engine goes, so does one system.

The remaining two systems are supposed to function independently and redundantly. That is, if one goes out, a backup is available. The hydraulic systems cannot flaps, slats, ailerons, rudders and horizontal stabilizers, all the structural features that make an airplane controllable and help keep it aloft.

If, however, the other two systems were slowly drained of their hydraulic fluid, the plane would become unflyable. Board experts are trying to find out exactly what hydraulic lines were severed as the engine came off the plane.It is known that some hydraulic lines were cut.

The board experts are also looking into the possibility that the left engine, which is known to have rotated up and over the wing, may have slammed against the slats and the slat actuators, damaging the hydraulic system there. Slats are devices on the leading edge of the wing that give a plane extra-lift during takeoff.

Complicating the search for the causes of hydraulic failure is the fact that few large pieces of ths airplane survived the impact; reconstruction is difficult. It is known that after the left engine fell off the plane continued to climb normally to an altitude of about 600 feet before things started to go dramatically wrong. That sequence is typical of what could happen if hydraulic control were gradually lost, aviation experts said.

The three engines have been recovered and are being shipped to the American Airlines maintenance base in Tulsa for extensive study, although there is no indication that they did not perform properly. Much of the debris will be brought to Washington for examination in the safety board's labs.

Board specialists are scheduling a number of DC10 simulations to discover what would happen under various hydraulic pressure loss conditions.

A special board panel has been set up to study a number of engineering questions at McDonnell Douglas headquarters in Long Beach.

The board yesterday also discovered a new noise on the tape in its continuing study of the cockpit voice recorder on the crashed plane. The recording stopped immediately after the plane's nose lifted off the ground, and the last word on it was a crew member saying "damn." At that moment, board spokesman Robert Buckhorn said, a thud was heard.

"We don't know what it means," Buckhorn said. CAPTION: Picture 1, An American Airlines DC10 sits at Dulles International Airport after FAA chief Langhorne Bond ordered grounding of all 134 of the McDonnell Douglas wide-body jets in domestic service. By Ken feil - The Washington Post; Picture 2, FAA Administrator Bond announces grounding of DC10s until after examination. UPI; Picture 3, Mechanics inspect the engine mounting of an American Airlines DC10 at Los Angeles International Airport. AP;