The Federal Aviation Administration has proposed a $375,000 civil penalty against American Airlines, the nation's second largest, for using a plastic rather than a required metal part in McDonnell Douglas DC10 jets even after the plastic part failed on two occasions.

The proposal arose from an FAA investigation after a slat on the right wing of an American Airlines DC10 fell off as the jumbo jet was landing at Dallas-Fort Worth airport last Sept. 14, the FAA said.

Slats are curved metal plates that are extended from the front of the wing. There are eight on each wing of a DC10. They are used during takeoffs and landings when planes move at slower speeds and need extra lift to gain altitude or to remain under control while descending.

The loss of a slat is not unknown and does not necessarily endanger flight, aviation experts said yesterday.

However, the loss of slats on one wing was blamed by the National Transportation Safety Board for the nation's worst aviation accident, the takeoff crash of an American Airlines DC10 that killed 273 people in Chicago in May 1979.

In the Dallas-Fort Worth incident, investigators found that a phenolic, or plastic, pulley instead of a stronger aluminum pulley was carrying the slat retraction cable.

The plane involved there and two others cited in the FAA penalty letter to American made at least 1,165 flights with the wrong pulleys, and thus were in "an unairworthy condition" on those flights, the FAA said. On two occasions -- including the Dallas-Fort Worth incident -- the plastic pulley failed, the FAA said.

"What concerns us is that this happened, they had a warning, and it wasn't caught," a federal source said.

Last April, a DC10 wing was seen drooping while the plane was on the ground at Toronto. The plastic pulley was replaced with the proper aluminum pulley and the incident was not reported as a "service difficulty," the FAA said. "Service Difficulty Reports" are required by the FAA. One of the violations cited in the FAA letter to American was the absence of that report.

The Dallas-Fort Worth incident occurred five months later. That time the phenolic pulley was replaced with another phenolic pulley and operated for 22 flights.

The phenolic pulley on the third plane was installed in January 1984, but was not discovered for 496 flights.

American Airlines spokesman Lowell Duncan read this statement:

"The issue concerns a parts identification problem which American discovered, fixed and itself reported to the FAA. The problem at no time represented risk to flight safety.

"American is dismayed at the FAA's approach to this matter. It is a highly technical issue which we discovered and we remedied. For the FAA to come in after the fact and levy the fine does nothing to promote the cause of safety.

"American's emphasis on safety is well known and in the FAA's last audit in the fall, American was cited for its outstanding safety and maintenance procedures."

The FAA said it was alerted to the problem only by the Dallas-Fort Worth incident.

American has several options, ranging from paying without comment to seeking a lower settlement or a dismissal. No decisions have been made, Duncan said.

C.R. Melugin, chief of the FAA's Southwest Region in Fort Worth, said that "the FAA determined the cause of the failures to be in the American Airlines supply system when American Airlines combined two separate parts under one part number."

Both phenolic and aluminum pulleys are used in the DC10 slat system, but at high-stress locations aluminum ones are required. The pulleys in question are almost 10 inches in diameter, carry a 3/8-inch cable, and pull the slat back into the wing.

A McDonnell Douglas spokesman said that phenolic pulleys cost about half as much as aluminum ones and are lighter, an important consideration in aviation. He placed a "ballpark" price of about $200 on an aluminum pulley.

The 1979 Chicago crash occurred when the left wing slats retracted as the plane's nose was lifting off the ground. The slats on the right wing remained extended, thus giving it considerably more lift than the left.

The left-wing slats retracted after the left-wing engine and its supporting pylon fell off and, in so doing, ripped loose the hydraulic lines and other devices that controlled the slats. The plane climbed briefly, rolled to the left and crashed.

The engine and pylon fell off as a result of "damage by improper maintenance procedures," the National Transportation Safety Board concluded.

American Airlines paid the FAA a $500,000 civil penalty "under protest" for violations stemming from the investigation of that accident. Its insurers and those of McDonnell Douglas have paid millions of dollars in liability suits stemming from the accident.