History of Flight
On December 22, 2011, at 1437 mountain standard time, a Boeing 737-7H4, registration N469WN, was parked at gate C-29 at Denver International Airport (KDEN) preparing for departure to Los Angeles International Airport (KLAX). The airplane was occupied by the captain, first officer, three cabin crew, and 128 passengers, none of whom were injured. The aircraft was operated by Southwest Airlines under 14 Code of Federal Regulations Part 121. Instrument meteorological conditions prevailed at the time.
The snowplow, operated by Aero Snow Removal Corporation, was conducting snow removal in cleanup mode. A witness reported the snowplow traveled eastbound north of the vehicle service road, passed the airplane, then made a gradual 180-degree turn and struck the left side of the airplane near the empennage. Security camera footage showed the snowplow entering the view at 1436:47 with its blade up, then the blade lowered and rotated. At 1437:16, the blade appeared under the empennage, and at 1437:19, the airplane moved abruptly westward consistent with the impact.
According to the snowplow operator's written statement and police report, the operator fell asleep while conducting snow removal. The operator refused interview requests.
Damage to Aircraft
Southwest Airlines reported the snowplow struck the left side of the airplane on the auxiliary power unit (APU) access door, causing damage extending about three feet upward and forward. The APU access door and fuselage skin were buckled, with three irregular holes approximately 3 to 4 inches in diameter in the skin adjacent to the impact. A stringer was broken in two. Damage was confined to the unpressurized section of the empennage.
Other Damage
Aero Snow reported the snowplow cab was damaged and the light on top of the cab was bent.
Personnel Information
The snowplow operator had been employed with Aero Snow since the start of the 2011/2012 snow season and had no prior incidents. Following the incident, he reported no illnesses or medications at the time. Aero Snow stated his responsibilities were typical; he had no standard shift and generally worked 12 to 14 hour shifts. On the day before the incident, he started his shift around 1730. At approximately 0200, he reported fatigue to his supervisor, then took a nap in the back seat of the supervisor's vehicle on the ramp. He awoke around 0800 and felt able to continue; his supervisor concurred. He worked on the north side of the C concourse until the incident. Investigators could not determine why he started this shift or his expected work duration.
Organizational and Management Information
Denver International Airport (DIA) oversees Aero Snow under contract, but the contract does not include oversight of employee hours or rest periods. Aero Snow stated fatigue management is the personal responsibility of employees. They are encouraged to report illness or fatigue with no repercussions. Aero Snow provides sleeping quarters for 30 to 50 people with bunk beds and air mattresses. Supervisors maintain continuous contact via radio and telephone during operations. New employees receive airport operations training, snowplow operation training, and safety training covering slips and falls, but not specifically fatigue at the time of the incident.
Additional Information
The snowplow involved was a Case North America articulated vehicle, approximately 2 years old, accumulating about 150 hours per year. Aero Snow described the cabs as heated, comfortable, and relatively quiet. Aero Snow provides snow removal between terminals and the overnight parking area, pushing snow to designated dump areas. The incident occurred at the end of a snow removal run near the Charlie North Island. Normal operations would not involve reversing course near the gate area.
A simulation using the same snowplow and traffic cones to represent the airplane empennage and vehicle service road distances replicated the collision course, requiring only a slow, continuous hand movement from 12 to 6 o'clock on the steering control.