FTW98FA273

The catering truck driver parked directly behind the airplane, awaiting the arrival of another flight at an adjacent gate. He set the parking brake, turned off the engine, but left the headlights on. He said he was not concerned about parking behind the airplane because the jetway was still in position, the airplane's rotating beacon was not flashing, and another catering truck was servicing the coach section. The other truck then moved around to the front of the airplane to service the first class section. After baggage had been loaded in the forward cargo bin, the belt loader was backed away from the airplane. Normally, the belt loader would then be positioned on the vehicle service road to warn traffic that an airplane was being pushed back. This was not possible because the belt loader was blocked by the catering truck that had backed away from the airplane. The tug operator had made a previous walk-around inspection of the airplane and observed no hazards. He said that prior to pushback, he looked under the airplane one more time and saw no obstacles. He said he did not see the parked catering truck. As the catering truck was backing away from the airplane, pushback began and the airplane collided with the parked catering truck.

History of flight

On June 17, 1998, at 2046 mountain daylight time, a Boeing 737-322, N364UA, registered to the Wilmington Trust Company and operated by United Airlines, and a 1992 International truck, registered to and operated by Dobbs International Services, Inc., were both substantially damaged when the airplane was pushed back from its gate and collided with the parked truck at Denver International Airport, Denver, Colorado. There were no injuries to the airline transport rated captain and first officer, 3 flight attendants, and 65 passengers aboard the airplane. The two occupants of the truck received minor injuries. Visual meteorological conditions prevailed, and an IFR flight plan had been filed for the flight that was to be operated under Title 14 CFR Part 121. The flight had not yet originated when the accident occurred.

United Airlines flight 475, scheduled domestic passenger service from Denver, Colorado, to Ontario, California, had been cleared by ramp control for pushback from B53 (gate 53, B concourse). According to the captain's safety report, the airplane had moved backwards about 20 feet when the captain felt "a thud." The tug operator advised him that they had struck a catering truck. The APU DET INOP light illuminated and the APU (auxiliary power unit) was secured. The passengers and crew deplaned via mobile airstairs. Postaccident inspection revealed the airplane's tailcone had struck the Dobbs International Services truck, knocking the truck over on its right side.

The accident occurred at a location of 39 degrees, 51.51 minutes north latitude and 104 degrees, 40.02 minutes west longitude.

Damage to aircraft

According to United Airlines, N364UA (s/n 24533) struck the truck just aft of BS (body station) 1016. This tore a 15 inch hole between stringers 16L (left) and 26R (right). Stringers 26L, 26R, and 27R were also damaged. Four inches aft of BS 1016, between stringers 22R and 23R, there was a 3 inch x 3 inch hole in the fuselage. The skin and stringers 26L and 26R, from BS 1033 to BS 1066, were damaged. BS 1156 frame was crushed and there was a large hole in the APU access door. There was an 11 inch x 5 inch hole in the aft pressure bulkhead. Three chords and BS 1016 frame were damaged in this area The fiberglass tailcone was crushed. Cost of repairs was estimated to be $33,000.

According to Dobbs International Services, most of the damage to the airplane was caused by the truck's stabilizers that were in the retracted position. In addition to peripheral body damage, the frame of the truck (VIN 1GTSCPMN1NH453389) was sprung. Cost of repairs were estimated to be $5,000.

Personnel information

According to the Federal Aviation Administration, Captain Scott C. Volk, First Officer Christine R. Shanholtzer, and the three flight attendants aboard flight 475 were properly certificated and qualified in their respective positions to conduct the flight.

Documents submitted by United Airlines indicate the tug operator, Aircraft Maintenance Technician Gary Weekly, had been trained and found qualified to perform his assigned duties.

Dobbs International Services' documents indicate Customer Service Representative Kim Ngo and Customer Service Assistant Leroy Prince had both been trained and found qualified to perform their assigned duties. Mr. Ngo had 9-1/2 years of airline catering experience: 6 years with Chelsea Airline Catering (owned by Continental Airlines), and 3-1/2 years with Dobbs. During those 9-1/2 years, he had been cited only once by Denver International Airport. On January 18, 1996, he failed to yield the right of way to a Sierra Pacific Airlines Convair 580.

Meteorological information

The accident occurred in visual meteorological conditions during the hours of dusk (for details, see page 4 of this report). According to NTSB's Operational Factors Division, sunset occurred at 2037, and twilight ended at 2110.

Aerodrome information

The accident occurred in a non-movement area (not under air traffic control). Directly behind the parked airplanes are two lines painted on the tarmac to delineate traffic lanes used by ground support and other motorized vehicles. These lanes are referred to as the vehicle service road (VSR).

Medical and pathological information

Toxicological screening was not performed on the crew of flight 475.

Toxicological screening was performed on Mr. Ngo and Mr. Prince. According to a report from SmithKline Beecham Clinical Laboratories, both Mr. Ngo and Mr. Prince tested negative for ethyl alcohol, and basic, acidic, and neutral drugs. Specimens were taken six hours after the accident.

Toxicological screening was also performed on Mr. Weekly. According to a report from Quest Diagnostics, the drug test was negative. Specimens were taken 18 hours after the accident.

Tests and research

At the request of the investigator in charge, a human performance investigator was dispatched from NTSB headquarters to assist in the investigation. She assisted the investigator in charge in conducting interviews with the principals that were involved in the accident. The interviews were conducted on June 22, 1998, at Denver International Airport. The following based on those interviews, and is chronologically formatted (stated times vary between interviewees and are therefore approximate).

2030: Dobbs International Services (DIS) truck #53389, manned by Customer Service Representative (CSR) Ngo and Customer Service Assistant (CSA) Prince, was dispatched to gate B51 to service United Airlines (UAL) flight 1144, scheduled to arrive at 2053. The drive from the kitchen to the gate took approximately 10 minutes.

2035: Meanwhile, DIS truck #246 arrived at gate B53 to service UAL flight 475. Access to the airplane's first class (right front) galley was blocked by ramp service vehicles, so CSR Nauzad and CSA Eskeias began servicing the coach (left aft) galley. It was about this time that Aircraft Maintenance Technician (AMT) Weekly said he conducted a walk-around inspection of the airplane, and noted that the accident truck (#53389) had not yet arrived. His walk-around inspection was confirmed by Ramp Serviceman (RSM) Steven Kirsch.

2040: DIS truck #53389 arrived at gate B51, but UAL flight 1144 had still not arrived. CSR Ngo parked his truck directly behind flight 475 parked at gate B53, setting the parking brake and turning off the engine but leaving the headlights on. He said he was unconcerned about parking behind the airplane because the jetway was in place, the airplane's rotating beacon was not flashing, and another DIS truck (#246) was in the process of finishing service to the coach section (left aft galley). At about this time, DIS truck #246 backed away from the airplane and drove around the right side of DIS truck #53389. Both CSR Nauzad and CSA Eskeias noticed the parked truck (#53389) behind the airplane. They stopped briefly at the airplane's right wing tip to allow a baggage cart to be moved, then positioned their truck to service the first class galley.

2043: RSM Steven Kirsch finished loading baggage into the forward cargo bin and backed his belt loader away from the airplane. Normally, he said, he would drive the belt loader to the vehicle service road and block traffic, an indication that an airplane was being pushed back. He was unable to do so this time because he was blocked by DIS truck #246. CSR Nauzad and CSA Eskeias said they noticed AMT Weekly sitting in the tug. CSR Nauzad said he never saw AMT Weekly get out and make a walk-around inspection of the airplane. At about this time, AMT Weekly said he "looked under the airplane one more time," then got into the tug.

2043: Ramp controller Carl Noda said he is responsible for airplanes taxiing in the non-movement area. He keeps airplanes separated from airplanes, not from ground vehicles. From his position in the ramp tower, he was unab