The Northwest Airlines DC10 jetliner that dropped part of a disabled engine on Leesburg Saturday night had a second engine and one wing damaged in the accident, investigators discovered yesterday.

The second engine, one of three on the jumbo DC10, continued operating normally. Had it been disabled, the pilot would have faced a critical but not impossible control situation. The plane, with 43 passengers and a crew of 13, landed safely at Dulles International Airport shortly after the incident and federal and industry experts began crawling over the aircraft almost immediately.

They now believe that the incident started when, for unknown reasons, a fan blade broke on the engine under the jetliner's right wing. Some pieces of that blade, which was spinning at high speed, broke other fan blades, apparently struck and severed the large metal ring to which the rest of the cowling is attached, and penetrated to the heart of the engine.

Other pieces struck the right wing, causing slight damage to three of the four leading-edge slats, the devices that give an airplane extra "lift" during critical, low-speed periods of flight such as takeoffs and landings.

The cowling, an 8-foot by 12-foot covering for the engine, drifted to the ground and landed in a Leesburg front yard. The cowling has been recovered, but investigators are still looking for the metal retaining ring. Most of the engine remained with the McDonnell Douglas jetliner as it returned to Dulles.

In the cockpit, crew members heard what they described as an explosion and noted a drop in power for the No. 3 (right wing) engine. They turned off the engine. They did not know of the damage to the No. 2 engine, the one that is suspended uniquely in the tail of the McDonnell Douglas DC10, because it functioned normally and gave no instrument readings to indicate distress. Investigators said there was damage both to the outside covering of the No. 2 engine and to some of the fan blades.

The root of the fan blade that apparently started the sequence of events has been taken to the National Transportation Safety Board's metallurgical laboratories here for tests, board officials said yesterday. The engine itself will be taken to Minneapolis, the maintenance base for Northwest Airlines, for further study. m

Federal Aviation Administration officials said that no extraordinary orders for inspections or checking of DC10 engines have been issued. "We don't know exactly what happened yet," an official said.

Federal regulations require that engines be located in such a way that "the failure or malfunction of any engine . . . will not prevent the continued safe operation of the remaining engines . . ." while the plane is in "at least one configuration."

"You could end up in a situation where the failure of one engine could result in the failure of another engine," said Walter S. Luffsey, the FAA's associate administrator for aviation standards. But such a situation would be highly improbable, he said. "I think with a DC10 you could fly on one engine and control it, but could not maintain level flight indefinitely," he said.

The Northwest Airlines fleet of 22 DC10s is powered with the Pratt and Whitney JT9D engine. DC10s operated by other U.S. airlines use General Electric engines.

Pratt and Whitney experts are participating in the investigation. A spokesman for Pratt and Whitney said yesterday that there have been 12 known cases of fan blade failure since the engine was introduced 12 years ago. The same engine is also used on some Boeing 747s and Airbus Industries A300s, all jumbo jets.

The fan itself is located in the front of the huge engine. It is the large spinning set of blades that can be readily seen by anybody. The fan forces air back into the engine.

There is no apparent technical similarity between this incident and the DC10 accident in Chicago that killed 273 people in 1979. The Chicago crash was caused when the rear engine mount gave way under the left wing just as the jumbo was taking off. The engine rotated up and over the wing, carrying with it the rest of the engine mounting and vital hydraulic and electrical control lines. That accident was subsequently blamed on a faulty maintenance procedure that generated cracks in the engine mounts.