Southwest Airlines is performing a fleet-wide check of its baggage tugs after a runaway vehicle damaged the engines and fuselage of an aircraft leaving the gate at the Los Angeles International Airport on 9 April.
Southwest ordered the check after investigators found a faulty safety switch on the errant tug and others at Los Angeles.
According to the US National Transportation Safety Board's initial report on the incident, Southwest Flight 579, a Boeing 737-300 (N624SW) with 109 passengers and crew was being pushed back from the terminal on a flight to Tucson when the incident occurred Friday evening.
The airline tells the NTSB that the tug driver exited the vehicle to handle bags without first engaging the emergency brake or turning off its electric motor.
"The employee stated that he picked up one of the bags and placed it on the passenger's seat of the unoccupied tug," says the NTSB. "Then, as the employee was handling the second transfer bag, he observed that the tug had moved, and it was proceeding in the direction of a hydrant fuelling cart that was parked between gates 3 and 5."
According to Southwest employees on the scene, the tug initially hit the hydrant fuelling cart, then continued at what some said was "full speed" until hitting N624SW's left engine, impacting the left side of the aircraft and belly as it passed underneath. The tug then stopped after hitting the aircraft's right engine. The captain shut down the right engine and the aircraft was towed back to the terminal, according to the report.
NTSB says damage to the aircraft includes "dented, punctured, and lacerated skin panels in the left wing-to-body nacelle and in the airplane's belly", adding that "no definitive determination has been made regarding the extent of internal structural damage".
On the tug, the NTSB determined that a "deadman" switch, a weight-activated device that opens an electric circuit when the tug's driver's seat is unoccupied, had failed.
"The initial Safety Board conducted survey of other tugs operated by Southwest Airlines revealed similar malfunctions of deadman switches in baggage tugs at LAX. Southwest Airlines is performing a fleet-wide survey of its tugs to ascertain whether similar anomalies exist at other airports it services."