Federal Aviation Administration chief Langhorne Bond said tonight that he is considering grounding all DC10 jumbo jets and is ordering an immediate inspection of engine mounting points on DC10s as a result of the investigation into the nation's worst aviation disaster.

A support bolt, investigators here have found, cracked and broken on the American Airlines DC10 that crashed on takeoff at O'Hare International Airport here Friday. The death toll stands at 273 - everyone on the plane and two persons on the ground.

The broken bolt was discovered about 8,000 feet from the beginning of Runway 32 Right. A team led by Robert Gordon, a structures expert with the National Transportation Safety Board who has been involved in dozens of major crash probes, found the bolt, the nut and a sleeve in the grass along the left side of the runway.

The broken bolt explains why the engine came off the DC10 just as its nose was lifting off the ground, safety board vice chairman Elwood Driver said in a press conference. But it does not explain why the plane crashed. "The loss of that engine should not normally have caused that crash," Driver said. "Our investigation is continuing."

Driver recommended the inspections to FAA chief Bond moments before his announcement. It is Bond who must take action to force airlines and McDonnell-Douglas, manufacturer of the DC10, to conduct the inspections. There are 280 DC10 flying worldwide and 50 more on order. It has become one of the workhorses of the domestic widebody fleet and is used heavily by such major lines as American, United, and Northwest Orient.

In a telephone interview during Driver's press conference, Bond said, "I just told Woody that we are preparing an airworthiness directive calling for a comprehensive inspection of mounting bolts." An airworthiness directive is a mandatory order which carries the force of law. It is automatically transmitted to all owners and operators of the aircraft involved, including foreign owners.

Bond said the particulars of the inspections are being worked out. Grounding all DC10s "is an issue we're considering" and will decide Monday, Bond said.

Bond said he did not feel that "the evidence we have in hand indicates the need to ground immediately."

Later, he called a reporter and reiterated that grounding all DC10s is under "active" consideration. "We'll work on this through the night," he said.

American Airlines has already begun inspecting the bolts, according to Art Jackson, an American spokesman here. "As of yesterday morning," he said, "we inspected 12 of our fleet of 30. We're not just inspecting those bolts; we're replacing them." He said the rest of the fleet would be checked by Monday.

Driver in a prepared statement read at the beginning of his press conference said the board's investigation "has disclosed that the No. 1 engine (left wing) and pylon separated from the aircraft during the takeoff roll.

"Although the exact trajectory of the engine has not been determined, the evidence indicated that the engine and pylon rotated up and over the wing.

"The investigation revealed the presence of a fatigue fracture of the No. 1 pylon forward thrust link attach bolt, part No. RA 2300-18." The pylon, a long support structure, connects the wing and engine.

Driver had announced Saturday that his investigators were searching for a missing bolt. They found it at 2:30 p.m. this afternoon. The bolt is made of some kind of "ferrous metal," Driver said, about 3/8 inch in diameter and about 3 1/2 inches long.

The missing bolt was one of two in the middle of one of three support structures that connected the engine assembly to the wing. The bolt held in position a steel sleeve 1 1/4 inches thick that carried the General Electric engine's thrust - some 40,000 pounds at maximum power.

There should be two such sleeves, driven through the ends of a link about 6 inches long, 3 inches wide, and 1 inchthick that connected the center of the engine assembly to the left wing.

The bolt on the engine assembly end was the one the safety board could not find. With that bold missing, the sleeve could have worked its way loose. In fact, there was a visible fracture on the flange that held the sleeve, with also was found.

With the center thrust support gone, the loads that it was supposed to carry were transferred to the front and rear engine supports. Those were designed to carry sidewise and updown pressure, not forward thrust.

Driver said the two remaining supports "can carry that load for some period of time, but not forever."

William Schaefer, director of quality assurance for American Airlines here, said the bolts would normally be checked only during major maintenance, probably about once a year. The last major maintenance check on the ill-fated DC10 was late in March at American's maintenance base in Tulsa.

The bolts and other engine supports are hidden from view when the plane is standing at a loading ramp, and thus would not be seen in a routine walk-round pre-flight check.

Many in the aviation establishment feel the DC10 has proven itself over nine years of service. Nonetheless, it now has been involved in two catastrophic single-plane accidents.

Furthermore, commercial airline accidents caused by mechanical and structural failures are now so rare as to be extraordinary. Most accidents - such as the midair collision over San Diego that killed 144 people last September - involve a series of factors that come tragically together. Human failure by pilots or air traffic controllers or both is regularly at fault.

Structure failure was the factor in the first DC10 catastrophe. Near Paris in March 1974, all 346 aboard a Turkish airlines plane were killed in a crash blamed on an improperly locked cargo door which opened explosively when the cabin was pressurized after the plane had reached thinner air at about 12,000 feet. When the cargo door blew, the floor between the passenger and cargo sections collapsed and severed the controlling cables and critical hydraulic lines. Since then, the cabin floors of all widebodied jumbo jets have been strengthened.

The structural failure of a bolt has now been identified as the reason the engine fell off the American Airlines DC10, although it has not been established as "the cause" of the accident

Part of the training of American Airlines DC10 pilots, according to spokesman Lawrence Strain, is to practice a landing and a go-around with one engine out. The plane "should fly" in such a situation, Strain said.

However, "I can't think of a more critical time to lose an engine than right at liftoff," a safety board technical expert said tonight. "You have no altitude to give away, you're heavy because of the fuel load needed for a long flight, and you have no time to think."

FAA administrator Bond may have put it best in an interview tonight. "I hate to speculate," he said. "But my jackleg opinion is that it's one think to take off without thrust from one engine; it may be something else to take off without that engine."

While investigators were continuing to study metal fragments, Cook County and Illinois state police tried to control growing crowds of sightseers who began driving by the crash site almost immediately after road-blocks were lifted late this morning. It was the first time the street had been open since the accident Friday afternoon.

"I don't know why those people are coming out here; there's nothing to see that hasn't been on television," groused Mike Mitchell, Chicago policeman guarding an entrance.

Touhy Avenue, on the south side of the crash site, was filled with autos. People parked and walked over to a large chainlink fence bordering the scene. Young parents carried small children and pointed. A piece of yellow metal was stuck, incongruously, in the fork of a tree that had been turned char black by Friday's explosion and fire.

Late in the afternoon after Touhy Avenue had become a four-lane parking lot, police closed it once again."These people are crazy," a state trooper said. "We're had accidents, fights, everything."

The crash site was once a small airport; two old hangars were destroyed in the fire and their metal skeletons dominate the scene. An old weathered sign just inside the chainlink fence reads "Airplane Parts" and an arrow points in the direction of the wreckage. CAPTION: Picture 1, Elwood Driver displays sheared bolt found in grass alongside runway at O'Hare. AP; Picture 2, A sleeve bolted through these holes help anchor DC10's left engine under wing. UPI