History of Flight
On November 23, 2010, about 2220 mountain standard time, a Bombardier CL-600-2C10 (CRJ), registration N614SK, operated by SkyWest Airlines as Delta Connection flight 4543, collided with a tow tractor during pushback from gate B4 at Salt Lake City International Airport (SLC), Salt Lake City, Utah. The two flight deck crewmembers, two flight attendants, 65 passengers, and ground handling crew were not injured. The airplane sustained substantial damage to the lower fuselage skin and multiple belly stringers. The scheduled passenger flight was operating on an instrument flight plan to Will Rogers World Airport (OKC), Oklahoma City, Oklahoma, under 14 CFR Part 121. Night instrument meteorological conditions prevailed. The flight's scheduled departure time was 2000, but it was delayed.
SkyWest Airlines, a regional airline headquartered in St. George, Utah, operated the flight under a code-sharing agreement with Delta Air Lines, Inc. Ground handling was performed by Delta agents. The airplane was dispatched with the auxiliary power unit (APU) inoperative, a condition permissible under the Minimum Equipment List, requiring one or both engines to be started using an external air-start cart or precharged air bottle.
The airplane load agent, driving the tow tractor, stated that snow was falling during loading and pushback. During loading, he observed a flight crew member performing an external preflight inspection. After loading, he prepared for pushback, seated in a tow tractor configured as a pushback tug connected to the nose wheel via a tow bar. He communicated with the flight crew via interphone. The captain requested an air-start at the gate and reported starting both engines. The captain then reported brakes released and clear to push back. The driver began pushing but the tug wheels slipped and the airplane would not move. He informed the flight crew and decided to use a larger tug. With the larger tug attached, pushback began. As they backed out toward the alleyway, the airplane started to skid left. The airplane then moved forward, pushing the tug to the right. The driver stated that about this time, the captain asked if the airplane was pushing the tug, to which he responded yes. The airplane continued forward, overtaking the tug, which remained connected to the nose wheel. The tug struck the belly of the airplane about the same time as the tow bar severed at the nose wheel connection.
The tug driver reported that two wing-walkers were present, and neither tug was equipped with snow chains or traction devices.
The captain recalled similar observations. He became concerned that disconnecting the tug with both engines running could cause the airplane to move forward in slippery conditions. He considered shutting down an engine but the driver had already left to get a larger tug, and the air-start equipment was removed. The airplane appeared firmly in position, so he kept both engines running. When the larger tug returned, pushback was successful initially, but then the captain felt a turning and slipping sensation. He asked if the driver still had control, and the driver replied yes. He then saw the tug rotate right and felt a collision. He could not definitively confirm whether the tug struck the airplane or vice versa.
Regarding starting both engines, the captain stated he made a judgment call based on concern about control problems taxiing in slippery conditions with only one engine. He considered a cross-bleed start in the alleyway but was concerned that increasing power on the running engine could be disruptive to ground personnel and traffic.
The captain stated that although snow was falling, the ramp at the gate was plowed and mostly clear, while the alleyway was contaminated with snow and ice. His view was not obscured.
Post-accident examination of a photograph showed the airplane's nose gear canted to the right over-steer limits, with the tow bar bracket partially attached to the left side of the wheel. The bracket separated from the tow bar at its shank, which remained in the tow bar collar. The tow bar remained attached to the tug, which was impinged against the right forward fuselage, opposite the direction of travel. Tire-shaped snow tracks were observed in the snow behind the nose wheel, sweeping in an arc aft toward the left side. No snow disruption was present forward of the nose wheel.
Meteorological Information
A special automated surface weather observation for SLC at 2222 indicated wind from 020 degrees at 6 knots; visibility 2 miles with light snow and mist; broken cloud ceiling at 1,600 feet, scattered at 700 feet, overcast at 6,000 feet; temperature minus 7°C; dew point minus 9°C; altimeter 29.81 inches of mercury.
Flight Recorders
The airplane was equipped with a Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR). The CVR was not de-energized after the collision, so the accident sequence was overwritten. The FDR contained about 77 hours of data, including the accident. It recorded both engines started about 2158, powered to 25% N1 while the airplane remained on a heading of 142 degrees magnetic. At 2220, the heading decreased to 57 degrees over 45 seconds, with a coincident jump in lateral and longitudinal acceleration of 0.07 and -0.08 g, respectively. Brake pedals were applied about 2 seconds after the acceleration event.
Additional Information
SkyWest CRJ Standard Operating Procedures (SOP) required that when ramp surface conditions would prevent adequate traction for ground equipment, engine start be delayed until pushback is complete, and the pushback operator notify the captain. The Below-Wing Stations Operating Manual contained a similar warning. Neither document provided guidance for pushback procedures with an inoperative APU during low traction conditions.
Following the accident, SkyWest issued a Ground Operations Bulletin and Delta issued an Immediate Action Bulletin requiring that when ramp surface conditions prevent adequate pushback unit traction, only one engine may be started prior to pushing back a regional jet aircraft.