The airplane was operating 2 hours behind schedule and had been dispatched with its auxiliary power unit (APU) inoperative, a condition that was permissible based on the approved Minimum Equipment List. The ramp area was covered in snow and ice, and it was snowing during aircraft loading and the subsequent pushback attempt. The captain started both engines while the airplane was at the gate, utilizing an external air-start cart in lieu of the APU. The captain stated that he started both engines due to a concern that, by starting one engine only, he would encounter control problems taxiing in the slippery conditions. He recognized that he could also have attempted to start the second engine in the alleyway after pushback using a cross-bleed engine start, but was concerned that attempting such a start would require increased engine thrust on the operating engine and could be disruptive to ground personnel and other traffic within the alleyway. During pushback, the tug was unable to gain enough traction to move the airplane and was subsequently replaced with a larger tug. The larger tug was able to push back the airplane; however, a short time later the thrust from the airplane's engines, which were running at idle, exceeded the traction available to the tug. The captain, seated in the left seat, experienced a sensation of unusual movement as the airplane began to overpower the tug, and he asked the tug driver if the driver still had control of the airplane. The tug driver confirmed that he had control; however, the airplane subsequently moved forward while still attached to the tug, which rotated to the right striking the airplane's fuselage. According to ground personnel and flight crew guidance, if the ramp surface conditions were such that adequate traction was not available, engine start should have been delayed until the pushback was complete. However, an engine start was required prior to pushback due to the inoperative APU. No guidance existed for either the flight or ground crew regarding pushback procedures in low traction ramp conditions with an inoperative APU.
The tug driver’s loss of airplane control during pushback due to a loss of traction in slippery conditions on the ramp. Contributing to the accident was the ground crews’ and airline operators’ inadequate guidance with regard to pushback procedures during low ground traction conditions with an inoperative airplane auxiliary power unit.
— 80,775 samples, 14 parameters, SRN 275,844 to SRN 279,440.
| Parameter | Max | at | Min | at | Final | at |
|---|---|---|---|---|---|---|
| Engine 1 N1 %RPM | 25.7 | SRN 277,285 | 0.0 | SRN 275,844 | 0.0 | SRN 279,439 |
| Engine 2 N1 %RPM | 25.6 | SRN 276,490 | 0.0 | SRN 275,853 | 14.8 | SRN 279,440 |
| Time | Event | Page |
|---|---|---|
| approximately 2150 mountain standard time | Delta Connection flight 4543, a Bombardier CL-600-2C10, N614SK, collided with a tug during push-back from the gate at Salt Lake City International Airport, Salt Lake City, Utah. | 1 |
Docket: data.ntsb.gov/Docket/?NTSBNumber=WPR11LA058. This brief is an analytical aid assembled from public records; it is not an investigative finding.