A senior Federal Aviation Administration official said last night that deficiencies in the engine mounting assemblies were found during inspections yesterday on 15 to 20 McDonnell Douglas DC10s.

Furthermore, he said, a study of FAA records shows that the DC10 has a history of engine and pylon support problems going back to 1974. While those problems have been decreasing in number because of a change in maintenance procedures, "the thing that disturbs me," the official said, "is that none of these old problems are related to the new ones we're finding now."

Fresh inspections of all 134 domestic DC10s were ordered Monday as a result of findings in the investigation of the Chicago air crash last Friday that killed at least 273 persons.

Most of the eight U.S. airlines that fly the DC10s reported that they were completing their inspections and, where necessary, repairs, and would have most of the troubled planes available for operations today.The inspections must be repeated every 10 days or 100 flying hours, whichever comes first. That is an unusually high rate of reinpection, aviation authorities said.

The intensive technical reviews were ordered by FAA Administrator Langhorne Bond Tuesday after he unconditionally grounded all U.S.-operated DC10s. The inspections concentrate on the attachments and supports that hold the engines to the wings of the jumbo set

The American Airlines crash in Chicago occurred just after the left wing engine fell from the plane during takeoff.

There also was growing evidence yesterday that the falling engine mortally wounded the hydraulic systems and thus robbed the pilot of the ability to control the plane.

Charles Foster, the senior FAS official who has recently been placed in charge of all flight safety and aircraft inspection matters, said in an interview last night , "We have found additional defects, primarily in the monoball joint." He said an exact count of DC10s affected was not yet possible, because several planes had more than one problem and each problem was reported separately.

"I would estimate there are 15 to 20 DC10s involved" with newly discovered engine mounting problems, Foster added.

The monoball joint is one of the key components of the front spar attach bulkhead, one of the main connecting points between the wing and the pylon. The pylon supports the engine.

A check with various airlines operating DC10s found two of them willing to state that they had discovered problems.

United Airlines, said spokesman Jim Linse, had found "a significant defect in one of two braces that support an important part in the wing-pylon assembly" in one plane. Twenty of United's 37 DC10s, Linse said, passed inspection yesterday and are to be put in service today.

On those 20, he said, mechanics made some minor bolt tightenings and adjustments.

Continental Airlines, which has a fleet of 15 DC10s, said through its spokesman that it had turned up a number of minor problems on five airplanes, including loose sheet metal fasteners, an improperly sized bushing, a small crack in the flanges of one attachment point and a crack and some corrosion in a plate in a wing-pylon attachment.

American Airlines and Northwest Orient both said they expected to have their fleets of 30 and 22 DC10s, respectively, back in service today. National Airlines restored all 16 of its DC10s to service yesterday, as did Western Airlines and World Airways, with nine and six DC10s, respectively. The FAA's computer in Oklahoma City spewed out the DC10 history sheet Friday night after the crash, Foster said.

The printout showed that from 1974 through January of this year 62 pylon or engine assembly problems had been reported to the FAA.The history also revealed that 38 such reports had been filed on the four-engine Boeing 747, which has an entirely different engine-mounting system, and that only one similar incident involving the Lockheed £1011 had been reported.

Many of the reports, Foster said, involved defects or deficiencies of a routime nature, although the word" cracked" appears on the computer printout. Cracks in metal, particularly in the skin, are not unusual in aircraft of any design, and there are standard procedures for inspecting cracks and preventing them from spreading.

Bond's order grounding the DC10 on Tuesday carried with it a notice that new inspection procedures would be developed for the engine assemblies in all wide-body aircraft, including the 747 and the £1011.

More than half of the DC10 incidents, 37, occurred in 1975. In July of that year, the FAA ordered McDonnell Douglas to amend the maintenance manual for the DC10 to stress possibility of problems with "hinge tees," a part of the pylon apparently not related in any way to the troubles discovered since the Chicago crash.

Moreover, the General Electric Corp. made a modification in its CF6 engine which powers most DC10s, that reduced hinge tee stress.

In 1978, only three of the DC10 incidents reported for the year involved the pylon and engine assembly, the history printout shows. "The DC10 fleet doubled and tripled during that time," Foster said, "so we can safely say that the situation has been improving. What's missing, however, is that this printout does not show us the problems that we are now finding."

Foster and bond are known to be planning a major shakeup in the FAA aircraft inspection programs.

The parts of the engine support assembly on the DC10s that have now been found broken in some instances and cracked or fatigued in others are in an area that was previously scheduled for inspection only once every 3,600 flying hours, or about once a year, FAA officials say.

The DC10 that crashed in Chicago went through its last 3,600-hour check in Tulsa in March, according to FAA records.

If there was a problem then, "we did not catch it, and we don't know why," Foster said. "obviously, we have to tighten up those procedures."

Maintenance procedures are established by a certification board the FAA forms with the manufacturer of each new aircraft before it is introduced into commercial service.

As a model ages and as problems peculiar to it are identified, the procedures are revised accordingly.

The National Transportation Safety Board has dispatched a special team to Los Angeles to study the maintenance procedures for the DC10 and how they were developed.

Bond's grounding order came after initial inspections he had ordered turned up new problems. A United Airlines mechanic in Chicago was credited with the discovery that resulted in the second order.

Capt. Ernest Burmeister, a United pilot who is working with the safety board's investigators in Chicago, said, "We were just lucky a very conscientious mechanic was putting forth extra effort."

As the mechanic, whom United declined to identify, was putting a plate back on the pylon, he noticed that it was not properly positioned. He then removed 100 metal fasteners and discovered a crack in a perforated steel plate, or webbing, which sits atop the pylon. That incident, added to the weight of other problem reports, led to Bond's order.

Senior investigators continued to test the theory yesterday that the falling engine damaged the hydraulic system on the ill-fated DC10 and made pilot control impossible.

When a hydraulic line is severed, valves are supposed to seal the line automatically, thus preventing the loss of fluid. The valves, however, can be reopened from the cockpit by a cable, according to sources. They are theorizing that the cable might have been pulled by the pilot or copilot, and the valve thus reopened, as the engine came off.