An “unusual vibration” on an Alaska Airlines flight prompted an emergency landing at Seattle-Tacoma International Airport, airline officials said. As the aircraft was landing, a piece of metal paneling flew off on Monday, Aug. 22, Alaska Airlines said in a statement. The vibration was first heard on the left side of the Boeing 737-900ER as it departed from the Washington airport. The plane was en route to San Diego, but it never made it because it was forced to turn around. As the plane returned to Sea-Tac, the cowling — the metal paneling that covers the engine — detached.