The Federal Aviation Administration said yesterday it has discovered a serious potential safety defect in the engine that powers two-thirds of U.S. jetliners. It ordered an intensive inspection program that could slow summertime airline operations.

In a legally binding order signed late yesterday, the FAA directed the airlines to inspect the jet engines on Boeing 727s and 737s and McDonnell Douglas DC9s. The order covers some 4,500 engines on about 1,650 airplanes and will cost the airline industry $10 million to $20 million to carry out, FAA engineering officials estimated.

Inspections will be required within varying amounts of time depending on the engine's age and model number.

Inspection requires removal and partial disassembly of the engine to check the suspect part or parts. If no spare engine is available, the downtime on an airplane could run from three to five days, the FAA estimated. The 727 has three engines, the 737 and DC9 two each.

The engine, built by Pratt & Whitney, is known as the JT8D and is generally regarded as one of the most reliable powerplants ever produced for aviation.

A Pratt & Whitney spokesman said the FAA action made mandatory a service bulletin the manufacturer is sending to airlines.

The investigation that led to the order began June 15, when an engine part broke as a Hawaiian Airlines DC9 was taking off from Honolulu. The pilot aborted the takeoff and the passengers were evacuated. There were minor injuries in the evacuation process. The engine fragments exploded outward, officials said, and did not penetrate the passenger cabin.

The Hawaiian Airlines engine was an older model that had undergone thousands of takeoffs and landings. Forty-five engines of that model were ordered inspected as a result of that incident.

At the same time, FAA officials began checking other JT8D models on a random basis. Two different engine models on Eastern Airlines planes were discovered to have a similar problem. That led to yesterday's sweeping order.

The problem part is known as the eight-stage compressor hub, a 59-pound wheel deep in the engine to which a cluster of compressor blades is attached. Cracking in the hub was found to be the result for the disintegration.

"We have determined," an FAA spokesman said, "that a machining error during manufacture is responsible for the hub cracking."

Technicians explained that a flange is molded on to the hub specifically so it can be machined to bring the hub into perfect round, or balance. In the grinding process, according to the technicians, forces were exerted that tended to contribute to possible cracking of the hub at some point in the engine's life.

The compressor in a JT8D engine collects air and forces it through 13 successively smaller fans into a combustion chamber, where it is mixed with fuel and ignited. The resulting gases are then exhausted to provide thrust. The eighth stage would be the eighth of the 13 fans.