Aviation investigators yesterday tentatively blamed a broken turbine fan blade for the in-flight failure of an engine of a United Airlines 727 shortly after the jet took off from National Airport Monday afternoon.

A spokesman for the federal National Transportation Safety Board termed the incident a "relatively minor failure" that did not warrant a full investigation by the board. Officials said yesterday that blade failure occurs from time to time in jets, but that the aircraft are built to withstand such malfunctions.

United spokesman Charles Novak said the engine failed at about 2,500 feet, leading the crew to shut it down. The plane landed with power from its other two engines. As it taxied toward a gate, people on the ground reported sparks, smoke or flames coming from the stopped engine. The crew tripped an anti-fire system that fed carbon dioxide into the engine.

At the gate, the 83 passengers were evacuated safely and firefighters sprayed a small amount of foam on the engine, Novak said.

Investigators believe the cause of the fire probably was the failure of one of the engine's approximately 100 turbine blades, which form a fan that is turned as burning fuel rushes past. One or more of the nickel alloy blades may have snapped off while spinning at high speed, they believe.

Investigators said the Pratt & Whitney engine appeared to have "contained" blade fragments successfully, as federal design rules require, preventing damage to other parts of the plane.

There appeared to have been no significant fire in the engine, Novak said. Investigators speculated that waste fuel in the engine's exhaust nozzle was ignited briefly by heat from the stopped engine.

Aviation experts say blades have broken periodically since the start of jet aviation, but said they are unaware of any cases in which broken blades have been the cause of an accident, because engine covers are designed to absorb the fragments.

The failure came two days after another United 727 apparently experienced blade breakage as it raced down a runway at Chicago O'Hare Airport. The crew aborted the take-off and evacuated passengers on the runway using slides after getting reports of fire in the engine.

Novak said the two incidents were not related and United did not plan any changes in procedure.

William Fay, a U.S. Tax Court judge who was aboard the United flight out of National, said he felt the crew had not adequately informed passengers about what they should do during the incident. Fay said the pilot announced an engine was out and that the plane was returning to National. Passengers heard nothing more, Fay said, until the jet reached the gate and a stewardess ordered everyone to get off and leave their belongings behind. Fay said he and others were jostled in the rush to get out.

Novak pointed out that the crew had returned plane and passengers safely.