The engine mounting assemblies on 68 DC10 jumbo jets - nearly half of the U.S. fleet - have been found to have problems ranging from cracked supports to improperly adjusted bolts during intensive inspections conducted since the planes were grounded earlier this week.

"We have found a total of 93 problems on 68 airplanes," a top Federal Aviation Administration official said yesterday, adding, "A number of the planes had more than one problem." The FAA ordered the inspections after it grounded the plane Tuesday as a result of early findings in the Chicago plane crash May 25 that killed 274 people. It was the worst crash in U.S. history.

Many of the DC10s have been repaired and returned to service. The FAA said said yesterday that 118 were back in the air and 19 were still on the ground. U.S. airlines own 134 DC10s and lease three from foreign owners.

The senior FAA official said there had been no breakdown yet on the ages of the 68 airplanes, their series or how many of the problems were significant. There are three models of DC10s - the DC10-10, the 10-30 and the 10-40.

All of those planes back on flying status will have to be reinspected every 100 flying hours or every 10 days, whichever comes first.

The Airline Passengers Association, a Texas-based group that claims 50,000 members, wrote FAA administrator Langhorne Bond yesterday and urged him to ground the DC10 "until the reason for the failure of certain parts in the engine support system . . . is determined.

The APA said it was asking for that action because "it is unclear whether the failure . . . was the result of metal fatigue or a design flaw causing an immediate catastrophic failure.

The National Transportation Safety Board which is charged with investigating the Chicago accident and determining its probable cause, has been trying to determine what part of the engine assembly structure failed first in the sequence of events in the crash.

It is known that the engine on the left wing fell off the plane just as it was lifting off the runway at O'Hare International Airport. The plane rose about 600 feet, then rolled sharply to the left and fell from the sky. Board experts theorize, but do not know yet, that the engine mortally damaged the hydraulic system when it came off and thus rendered the plane uncontrollable.

FAA inspectors, working on information developed by the safety board's investigating team, then began examining the complex support structure that holds the engine to the wing. Fractures in bushings and bolts, cracks and signs of metal fatigue in bulkheads have been found during those inspections.

Bond said yesterday that "there is no evidence it's a design problem, although the safety board will address that." He said the FAA is concentrating on its maintenance and inspection procedures for the DC10 to determine how the engine support problems escaped detection earlier.

The safety board has assigned a special team to the Long Beach, Calif., DC10 factory, to study the certification and design questions that have arisen since the Chicago accident. The board can only recommend to and prod the FAA, it has no regulatory powers of its own.

The board is also seeking help from National Aeronautics and Space Administration officials to study the effects of vibration on metal in the DC10 engine mounting assemblies. Sources of vibration include the engine itself as well as air flowing over the wings at high speeds.

We've gone through the inspection process and found broken parts," Bond said. After the parts were replaced Bond said, "we know it's back in its original state: sound."

James B. King, chairman of the safety board, said he could not comment on what the FAA has turned up in inspections of other airplanes because "I don't have anybody out there." The only plane his team has seen, he said, is the one that crashed. He said the FAA has been responsive to board recommendations "concerning the one accident we're dealing with."

Safety board statistics compiled from 1968 through 1977 show that the DC10 had a low rate of fatal accidents per 100,000 hours of flight time among jetliners commonly in use today.

The three aircraft with better rates were the Boeing 737 (0.04 fatal accidents per 100,000 hours of flight); the Boeing 727 (0.05); the Boeing 720 (0.05). The DC10 rate was 0.06, the best rate among the U.S. jumbo jets, including the Boeing 747 (0.08) and the Lockheed £1011 (0.12).

Like all statistics, however, these do not give a totally unbiased picture. The DC10 looks twice as safe as the £1011, for example, but during the period the statistics were kept the DC10 saw twice as much use. Each plane had only one fatal accident.

Both United and American Airlines - the two biggest DC10 users - said yesterday that they were carrying normal loads on their DC10s. Both also said that some few passengers had requested reassignment to other planes.