Denver, CO, USA
N469WN
BOEING 737-7H4
The passengers and the flight crew were seated, and the ground crew was preparing to push the airplane away from the gate. The snowplow was conducting snow removal operations in clean-up mode. According to a witness, the snowplow was traveling eastbound behind the airplane and just north of the vehicle service road. The snowplow passed the airplane and then initiated a gradual turn to the right. The snowplow continued around 180 degrees and hit the airplane on the left side near the empennage. The snowplow operator reported that he fell asleep while driving. The investigation revealed that the snowplow operator reported for his shift at 1730 the day before the incident and worked until 0200 the next morning. The snowplow operator slept in his supervisor’s vehicle, which was actively involved in the snow removal operations. At 0800, the snowplow operator resumed snow removal operations, and he had been working for about 6.5 hours when the incident occurred. The snowplow operator had an opportunity to sleep for 6 hours in his supervisor’s vehicle; however, since the vehicle was being operated, he probably did not get uninterrupted sleep and most likely got less than 6 hours of sleep. Although the investigation was unable to determine how long the snowplow operator had been awake on the day before the accident or how long he had slept in his supervisor’s vehicle, he was most likely fatigued. The company did not have, and was not required to have, guidance or a policy addressing fatigue management.
HISTORY OF FLIGHT On December 22, 2011, at 1437 mountain standard time, a Boeing 737-7H4, N469WN, received minor damage when it was hit by a snowplow, while parked at the gate, at Denver International Airport (KDEN), Denver, Colorado. The captain, first officer, three cabin crew, and 128 passengers were not injured. The driver of the snowplow was not injured. The airplane was registered to and operated by Southwest Airlines under the provisions of 14 Code of Federal Regulations Part 121. Instrument meteorological conditions prevailed at the time of the incident. The airplane was at the gate, preparing for departure and was en route to Los Angeles, California (KLAX). The airplane was parked at gate C-29, located at the northwest end of the C termninal. The passengers and the flight crew were seated and the ground crew was preparing to push the airplane away from the gate. The snowplow, operated by Aero Snow Removal Corporation (Aero Snow), was conducting snow removal operations. According to a witness, the snowplow was traveling eastbound, just north of the vehicle service road (VSR). The snowplow proceeded to the east, past the airplane, and then initiated a gradual turn to the right. The snowplow continued around 180 degrees and hit the airplane on the left side near the empennage. A security camera at gate C-29 captured the snowplow and a portion of the incident sequence. The snowplow entered the camera view at 1436:47. When the snowplow appeared, the blade was up. As the snowplow continued eastbound, the blade came down and rotated away from the camera. The snowplow disappeared behind the airplane at 1437:06, and at 1437:16, the blade of the snowplow appeared under the empennage of the airplane. At 1437:19, the airplane moved abruptly to the west, consistent with the impact of the snowplow. At the time of the incident, the snowplows were in cleanup mode. The snow had stopped falling and the snowplows were picking up the snow being plowed away from the gates. According to the written statement provided by the snowplow operator and the police report, the operator fell asleep while conducting snowplow operations. Despite multiple requests, the snowplow operator refused to return requests for an interview. DAMAGE TO AIRCRAFT According to Southwest Airlines, the snowplow hit the left side of the airplane on the auxiliary power unit (APU) access door and the damage extended about three feet up and forward of the impact point. The APU access door and the fuselage skin were buckled. Just adjacent to this area were three irregular shaped holes in the skin, approximately 3 to 4 inches in diameter. A stringer was broken in two. The damage was confined to the unpressurized section of the empennage. OTHER DAMAGE According to Aero Snow, the cab of the snowplow was damaged and the light at the top of the cab was bent. PERSONNEL INFORMATION The snowplow operator involved in the incident had been an employee with Aero Snow since the start of the 2011/2012 snow season. He had not been involved in any previous incidents while employed with Aero Snow. Following the incident, he reported to Aero Snow that he was not suffering from any illnesses and was not taking any medications at the time of the incident. According to Aero Snow, the snowplow operator’s responsibilities were the same as other employees at Aero Snow – he did not have a standard shift or start time and generally worked 12 to 14 hour shifts. On the day prior to the incident, he started his shift approximately 1730. Approximately 0200 the following morning, the snowplow operator reported to the supervisor that he was tired or fatigued. Aero Snow reported that he took a nap in the back seat of his supervisor’s vehicle, which was located out on the ramp and was actively supervising snow removal operations. Investigators were unable to determine why the snowplow operator slept in the back of the supervisor’s vehicle as opposed to returning to the bunk house to sleep. The snowplow operator awoke from his nap approximately 0800 on the day of the incident and felt that he could continue with snow removal operations. His supervisor concurred. He continued with snow removal operations on the north side of the C concourse until the time of the incident. Investigators were unable to determine why he started this shift or if he knew how long he would be working when he reported to the airport for work. METEOROLOGICAL INFORMATION The closest official weather observation station was KDEN, Denver, Colorado. The routine aviation weather report for KDEN, issued at 1453, reported wind 080 degrees at 4 knots, visibility 10 miles, sky condition 1,100 feet overcast, temperature minus 07 degrees Celsius, dew point minus 10 degrees Celsius, altimeter 30.28 inches. TESTS AND RESEARCH The remain-over-night parking area at DIA was used to understand the maneuverability of the snowplow. Traffic cones were laid out to simulate the empennage of the airplane and the VSR. The tail of the airplane was 47 feet from the south edge of the VSR, and the north edge of the VSR was 24 feet from the south edge of the VSR, identical to the distances at gate C-29. The snowplow used for this demonstration was the same snowplow involved in the incident. The snowplow was maneuvered along the approximate course described by the witnesses. This resulted in the snowplow meeting the traffic cones in the same location that the incident plow hit the airplane. According to the individual operating the snowplow during this simulation, one slight control input was required to maneuver the snowplow along the course described by witnesses. This involved the hand going from the 12 o’clock position to the 6 o’clock position in a slow and continuous fashion. ORGANIZATIONAL AND MANAGEMENT INFORMATION DIA is responsible for the oversight of the contract with Aero Snow. At the start of the season, DIA met with Aero Snow and reviewed the logistics, traffic patterns, and any changes for the snow removal plan for the current season. The contract between DIA and Aero Snow does not provide for oversight or authority over the hours worked by Aero Snow employees nor does it provide oversight or authority over the hours of rest required between shifts for Aero Snow employees. According to Aero Snow, the responsibility for fatigue and fatigue management comes down to personal responsibility. Each employee is responsible for obtaining the proper amount of rest before showing up for work. Employees are encouraged to report if they are ill or fatigue and there are no ramifications if they are unable to perform their duties. There is not a company specific standard or procedure regarding fatigue or fatigue management; nor are they required to have one. Aero Snow provides sleeping quarters, which can accommodate 30 to 50 people. The sleeping quarters consist of bunk beds and air mattresses located just off the dining area in the Aero Snow building at DIA. These quarters are provided to allow employees to obtain the necessary rest and not have to deal with the hazards of snowy weather or icy roads. While conducting snow removal operations supervisors are in continuous contact with their employees, via radio and telephone. Employees are continually being asked how they are doing. When Aero Snow hires a new employee, they are provided airport training for airfield operations, which covers the dangers of the airport. In addition, they are provided training that orients them to the snowplow or machine that they will be operating. They also receive safety training addressing slips and falls due to the ice and snow. Prior to the accident, the safety training did not address employee illnesses or fatigue; however, they did talk about personal responsibility in ensuring that you are fit for the job. ADDITIONAL INFORMATION The snowplows operated by Aero Snow, and the one involved in the subject incident, were designed and manufactured by Case North America. These vehicles are approximately 2 years old and accumulate approximately 150 hours per machine, per year. These machines are articulated in the front and designed to maneuver in small spaces. According to Aero Snow, these machines are very comfortable and very sensitive to the touch. The cabs are heated, comfortable, and relatively quiet. Aero Snow provides exclusive snow removal services between the main terminal and terminal A, between terminal A and terminal B, between terminal B and terminal C, and terminal C and the overnight parking area. The vehicle service roads create the boundaries for snow removal. The airlines are responsible for pushing the snow in their parking areas adjacent the terminal to and through the vehicle service roads. Aero Snow is responsible for removing the snow from this area to designated snow dump areas. Aero Snow is not responsible for removing snow along the VSR or within the parking areas for each respective airline. During snow removal operations, the snowplow operates in an east/west vector, removing snow between the edges of the VSR and the overnight parking area. The gate where the incident took place is at the end of a snow removal run – the snow is dumped just to the west of the incident gate in the Charlie North Island. Normal operations involve the snowplow pushing snow to this island, then backing up, reversing course, and continuing back to the east to the other side of the C concourse. There would be no reason for the snowplow, in normal operations, to reverse course near those particular gates, nor would there be a reason for the snowplow to enter the VSR or gate area.
The snowplow operator’s incapacitation due to fatigue, which resulted in the snowplow contacting a parked airplane. Contributing to the incident was the company’s lack of a policy regarding employee fatigue.
Source: NTSB Aviation Accident Database
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