The Federal Aviation Administration, citing dozens of examples of deferred airplane maintenance and inadequate safety documentation, threatened in a letter to ground Eastern Airlines unless it committed itself to "complete corrective action."

The FAA said it withdrew the threat before yesterday's noon deadline. The agency's decision followed a letter from Eastern's chairman, Frank Borman, and its president, Joseph B. Leonard, saying they were providing a "detailed explanation of changes that have been, are being or will be made to the . . . maintenance program" as a result of a special FAA inspection. In a letter to the FAA regional office in Atlanta, they said that Eastern is "fully dedicated" to "the finest and safest operation."

The deadline came in a 24-page letter alleging 78,372 violations of 30 different sections of the Federal Air Regulations. The alleged violations were compiled by a team of FAA inspectors during a three-month examination of Eastern triggered in part by passenger and Eastern employe complaints about flight delays. The FAA has proposed a record $9.5 million civil penalty against Eastern.

Anthony J. Broderick, the FAA's associate administrator for aviation standards, said that, despite the findings of the inspection team, "We wouldn't allow Eastern to fly if we thought there was an unsafe condition in any of the airplanes." A special team of inspectors will monitor Eastern's compliance program, the FAA said.

Some allegations from the FAA letter:

*One airplane made 10,506 flights with a landing gear-part subject to special inspection requirements that were never met. The landing gear was damaged when the part failed as the plane landed at Norfolk.

*Pilots reported at least nine times that there were problems on the left-wing engine of an A300 Airbus. The engine was replaced only after an aborted takeoff at New York's John F. Kennedy International Airport.

*A hydraulic system leak on a Boeing 727 was reported nine times by pilots and was so severe that it required service every day. It was not fixed permanently, however, for 246 flights.

*During a five-month period, a Lockheed L1011 produced 24 false fire warnings mid-flight, none of which was reported to the FAA as required.

*Eastern used "an alternative means of compliance" for an FAA-required inspection of Boeing 727s. The practice is sometimes permitted, but Eastern had not obtained the required FAA approval, the agency said.

Each flight made with a safety deficiency is technically in federal violation, and the maximum penalty per violation is $1,000. The final penalty actually paid is typically much less than proposed. Eastern can negotiate a settlement with the FAA or appeal the proposed penalty, first to the National Transportation Safety Board, then to the courts. The record penalty paid, $1.5 million, came from American Airlines last year after another special FAA inspection.

The FAA letter in some ways says as much about the FAA as it does about Eastern, because the agency is supposed to monitor airline practices and check airline manuals to assure compliance. The FAA confirmed yesterday that Jonathan Howe, director of the regional office responsible for Eastern, had been reassigned to Washington, D.C., but said the change was not related to the Eastern investigation. Howe could not be reached.

Many alleged violations at Eastern are of the paperwork category, but the FAA has maintained for years that the only way it can adequately police airline safety is by checking records against facts.

One of the more contentious issues between the FAA and Eastern concerns "airworthiness directives." A typical directive would require a new part for a plane or a more frequent inspection of a hydraulic system or a landing gear. Records showing compliance with "ADs" are thus an important part of an airplane's safety history.

"Eastern Airlines was unable to show the current status of airworthiness directives compliance for even one aircraft for at least seven days," the FAA said.

Eastern Vice President Jerry Cosley said that "It took several days because we don't keep ADs by airplane number, but by AD number . . . . When you look at the specific violation, you have to say it's not a violation but a missed communication."

Cosley said that the FAA had denied Eastern "due process" by releasing the penalty letter before discussing it or negotiating a settlement, as has been done in some cases in the past.

According to several sources, the major problem at Eastern was one of deferring maintenance for many flights. Each airplane has a "minimum equipment list," which tells a pilot whether he can fly when a specific instrument or system is broken.

Pilots call such items "MELs" or "squawks." They record inoperative equipment in the airplane's log book, which lets mechanics and the next pilot know what is going on with the plane. It is not uncommon or necessarily unsafe for a plane to have several "open" items and still be flown.

But one Eastern pilot interviewed yesterday said, "It was not unusual several months ago to find a dozen MELs when you picked up a plane . . . . When you start running into five, six, or seven items a day on different planes, you can get in trouble trying to remember it all." He said he canceled several flights for that reason.

Some pilots complained about the problem internally, and some complaints found their way to the FAA, according to sources.

Larry Schulte, chairman of Eastern's Air Line Pilot Asssociation group, said yesterday that he had written Borman to complain about the situation. He said that Borman responded that the situation was improving.

Eastern recently changed its fleet-use schedules to hold more planes out of service on a given day and assure that items on the open list were reduced, pilots said, and Cosley confirmed.