What happened
A Delta Air Lines aircraft arrived at the gate at 0951, following a delay caused by a reported takeoff slat warning light indication problem. After maintenance completed an initial check, pushback for the scheduled flight occurred at 1206. During taxiing and pre-takeoff checks at 1224, the captain reported a recurring takeoff slat warning light issue. Maintenance requested the aircraft return to the gate for further inspection.
At 1241, a maintenance technician assessed the fault. The procedure required advancing one or both engine throttles to verify the operation of the warning horn. Following this check, the slats were set in the takeoff configuration. During the maintenance process, the captain and first officer remained in their seats. At 1251, the first officer received pushback clearance.
The engines were started during the pushback sequence. The captain noted that the initial stages of the engine start appeared normal and did not direct the crew to re-accomplish the before-start checklist prior to the sequence. However, shortly after engine start, while the tug was still attached, the aircraft began moving forward under its own power. The captain observed that the throttles remained in the advanced position. As both the captain and first officer attempted to pull the throttles back to idle, the aircraft moved forward sufficiently to cause the tug and tow bar to jackknife to the left. This movement struck the Delta Air Lines aircraft, resulting in a 4-foot long gash in the fuselage. The captain subsequently shut down the engines and secured the cockpit.
The investigation
The investigation focused on the maintenance check procedures and the state of the engine throttles during the engine start sequence. It was determined that the maintenance check required advancing the throttles to verify the warning horn, and the aircraft moved forward because the throttles were left in that advanced position.