The Federal Aviation Administration is investigating maintenance practices at American Airlines, the nation's second-largest air carrier, after a series of safety-related incidents, FAA and American officials confirmed yesterday.

Recent incidents that prompted the investigation include an engine falling off a Boeing 727 in flight and the use of plastic parts instead of required metal parts in the wings of McDonnell Douglas DC10 jetliners.

The American assistant vice president for aircraft maintenance has taken leave of his duties at the airline's prime maintenance base in Tulsa and the FAA is reassigning its principal American maintenance inspector.

Roger G. Knight, manager of the FAA's flight standards division at the regional office in Fort Worth, said the special inspection was started in June because "we were uncertain as to how effective the continuing maintenance program at American was." He declined to cite specifics.

He said top American officials have cooperated fully in the investigation. Robert L. Crandall, chairman of American's parent, AMR Corp., "has been involved since the first briefing on this," Knight said. He said that "we feel modifications they have made in their maintenance program have been very positive."

Crandall was not available for comment. However, Rocko J. Masiello, American's senior vice president for operations, said there is nothing unusual about the FAA's inspection. "We're going to have to be aware of the fact that as we grow we have to make changes," he said. "As the FAA talks to us about these things, we respond."

American has been one of the most aggressive carriers since airlines were deregulated in 1978 and has been a favorite of Wall Street analysts for sharp management and skillful marketing. It is now growing, Masiello said, at about 18 to 20 percent annually.

Within recent months American has announced plans to build hub-and-spoke operations at Nashville and Raleigh-Durham, where airliners from many destinations arrive and depart at about the same time of day, allowing connections to numerous places. American has a strong hub-and-spoke presence at its headquarters airport in Dallas-Fort Worth and at Chicago's O'Hare airport.

Because of that growth, Masiello said, "the number of incidents are apt to go up." Concerning possible enforcement actions, he said, "The FAA always has enforcement actions . . . . We're having a dialogue all the time. They tell us about things, we tell them about things."

A team of about 12 FAA maintenance inspectors drawn from field offices across the country has been studying American maintenance since June, Knight said. "About the second week in July we had our first session with American . . . . Now we've got almost daily meetings between their Tulsa people and our people."

"I'd like for a reader to think this is not extraordinary," Knight said. "We're not dealing with anything more than the normal function of the FAA and the normal requirement on industry to comply with the rules . . . . "

Special FAA surveillance is not an everyday occurrence, however, and American is second only to United in its penetration of the domestic travel market.

The FAA mounted a similar inspection at Republic Airlines in 1983 after that carrier suffered two in-flight emergencies because of unexpected fuel shortages. The agency has performed two special checks at Continental Airlines in response to complaints from the striking Air Line Pilots Association that Continental was operating unsafely with nonunion pilots.

Knight said he is reassigning his principal inspector at Tulsa because, "Periodically you have to have a little bit of mobility . . . . It's time to change. It's my own fault we haven't done it. We're moving him into a comparable position with another carrier. It was not a firing."

Masiello said that American's assistant vice president for aircraft maintenance "is currently on leave" and "has talked to us about semiretirement."

Crandall told Aviation Week and Space Technology in July that he had instituted an intensive look at American's maintenance and safety procedures. Of the recent incidents, he said, "As best we can tell, there is nothing to any of them in any linkage sense, nor do they represent any breakdown in procedures. It seems like nothing more or less than bad luck."

American paid a $500,000 fine "under protest" to settle all civil enforcement claims with the FAA concerning a now-banned procedure for mounting engines and engine support pylons the airline had used on DC10s before the Chicago crash that killed 273 people in 1979.

In subsequent civil litigation it was established in U.S. District Court in Chicago that American officials had ordered an internal investigation of that crash, then destroyed all copies of the report of the investigation on the advice of their own counsel and in violation of another court's order to preserve documents.

Last January, the FAA proposed a $375,000 civil penalty on American because of the plastic wing parts, and the proposal is pending. The matter came to the FAA's attention when a slat fell off a DC10 as the jumbo jet was landing at Dallas-Fort Worth. A slat is a large curved plate extended from the front of the wing to assist in slow-speed flying.

The 727 engine fell off over New Mexico after it ingested a large chunk of ice that had formed on the outside of the airplane. The ice formed from a leaky lavatory valve.