Top engineers from Northwest Airlines and the McDonnell Douglas Corp. yesterday joined federal safety investigators in Washington to determine the cause of an explosion on a DC10 flying from Dulles International Airport that dislodged part of the huge cowling from the plane's right engine and sent it plummeting into the front yard of a Leesburg home Saturday evening.

The incident occurred shortly after takeoff at 6 p.m. as the Seattle-bound jumbo jet carrying 43 passengers climbed to 7,000 feet. A spokesman for Northwest Airlines described the explosion heard by crew members as a "backfiring." The engine had just been shut down, he said, because it showed signs of overheating.

The plane's pilot "felt the plane shudder," an FAA spokesman said. No injuries were reported and the plane returned safely to Dulles, where emergency vehicles lined the runway.

The airline spokesman described the three-engine craft as being "certified airworthy with only two engines."

G. T. McCarthy, an air safety investigator for the National Transportation Safety Board, said it may be "a couple of weeks" before the cause of the accident is known. Inspection of the damaged engine and cowling by the NTSA and the Federal Aviation Administration will be followed, McCarthy said, by an examination of maintenance records at the airline's Minneapolis headquarters.

"We're going to take a look at the engine and make some decisions as to where we're going to go from here," McCarthy said.

Though it is not uncommon for aircraft to lose parts in flight, McCarthy said he had "never heard" of such an incident occurring before. Two years ago, a DC10 crashed and killed 273 persons in Chicago when an engine tore off its moorings on the plane's left wing.

A man who witnessed Saturday's mishaps said he saw "a ball of fire and huge shower of sparks." Driving along Rte. 15 to his Leesburg home, he said "it looked like a fantastic 4th of July fireworks."

The cowling ring, about 12 feet in diameter and eight feet long, landed on a front lawn amidst a Leesburg housing development. A resident watching television at the time said it sounded "like two cars hitting each other."

"I think what [the cowling] did," said investigator McCarthy, "is almost flew, or glided down. Because it has some aerodynamic characteristics."

The cowling, intact but battered, was later hauled back to the airport. The damaged plane was parked on the Northwest Airlines tarmac, positioned in a way that kept the engine from public view.