What happened
On April 9, 2010, at approximately 19:30 PDT, a Boeing 737-3H4, registration N624SW, was undergoing pushback for a scheduled flight to Tucson International Airport at Los Angeles International Airport (LAX). During the procedure, the aircraft was struck by an uncontrolled baggage tug.
An airline ground service employee stopped the tug near gate 3, terminal 1, to retrieve two bags from the ramp. The operator exited the vehicle while the gear selector remained in the forward position, the parking brake was not set, and the electric motor had not been turned off. The operator placed one bag on the passenger seat of the unoccupied tug. While the operator was retrieving a second bag, the first bag slid from the seat onto the accelerator pedal, causing the tug to move forward.
The tug first struck a hydrant fueling cart located between gates 3 and 5. It then veered toward the aircraft, impacting the number 1 engine cowl, the left lower fuselage, and the belly. The tug continued underneath the aircraft until it struck the number 2 engine. There were no injuries to the two pilots, three flight attendants, or 104 passengers, and no ground injuries were reported.
The investigation
Maintenance inspections of the aircraft revealed various dents and skin punctures on the fuselage, a bent internal frame shear tie, and damage to the number 1 engine cowl. The number 2 engine sustained damage to the engine itself, components, cowl, and the engine pylon, including the wing fairings, pylon structure, and mid-spar fitting to under-wing fitting fuse pins.
An investigation of the tug revealed that the seat switch, which is designed to disengage the electric motor when weight is removed from the driver's seat, was inoperative. Testing confirmed that the tug would continue to move under accelerator pressure even if the parking brake was applied or the gear was in neutral, provided the seat switch failed to open the circuit.