One of three engines fell off an American Airlines plane carrying 90 people from Dallas-Fort Worth to San Diego yesterday afternoon, but the plane landed safely.

Linda Johnson, a spokesman for the airline, said the captain of Flight 199 radioed to controllers that "he felt a shudder and thought an engine had seized," or frozen, as the plane flew west of Phoenix.

The captain followed the procedure for shutting down an engine and continued to San Diego without incident. "As far as I've been able to determine they didn't know the engine was gone until they landed," Johnson said. The airline withheld the names of the flight crew.

"The plane landed, taxied to the gate and shut down," said Buck Jones, assistant airport manager in San Diego. "It was an uneventful landing."

The plane was a Boeing 727, which has three rear-mounted engines, one on each side and one in the tail cone. The whereabouts of the engine, which fell off the right side, were unknown. "We think it's somewhere in Arizona," Johnson said.

Such events are rare but not unprecedented, according to aviation experts. In a famous incident more than a decade ago, an engine fell off an airplane after ingesting a chunk of ice that had built up outside the plane because of a leaky lavatory tank.

The National Transportation Safety Board sent an investigator. The Federal Aviation Administration and American Airlines also said they were investigating.