NTSB Factual Report

On August 3, 2005, at 0840 mountain daylight time, an Airbus A320-232 airplane, N496UA, operated by United Airlines d/b/a Ted Airlines, sustained minor damage to its left winglet when it struck the tail cone of another Airbus A320-232, N472UA, d/b/a Ted Airlines, while taxiing to Gate A-48 on the south side of Concourse A at the Denver International Airport (DEN), near Denver, Colorado. Both airplanes were operated by and registered to United Airlines, of Chicago, Illinois. There were no injuries to the crew or passengers on either airplane. Visual meteorological conditions prevailed for the scheduled, domestic flight conducted under 14 Code of Federal Regulations Part 121. N472UA had been pushed back from Gate A-40 and was destined for Las Vegas, Nevada. N496UA had just arrived from Phoenix, Arizona.

According to a Ted Airlines ramp supervisor, since gate A-40 was angled and adjacent to the center terminal building, the pushback was "doglegged" and company policy required two wing walkers to assist the tug operator. The pushback procedure required the tug operator to push the airplane straight back 5 or 6 feet, and then turn the airplane so its main landing gear wheels were perpendicular to and on the centerline of Taxiway AA. He said the purpose of placing the main landing gear wheels on the Taxiway AA centerline was to assure adequate clearance for airplane's taxiing on Taxiway AS. In addition, he said the wing walkers only responsibility was to ensure clearance between the airplane and vehicles on the ramp. They were not responsible for the airplane's position during pushback.

In an interview, the tug operator stated that he pushed N472UA back so its main landing gear was placed just aft of the Taxiway AA centerline. After the airplane had been pushed back, and before the tug was released, ground crews placed about five more pieces of luggage into the cargo hold. After the luggage was loaded, the tug operator and wing walkers disconnected the tug and went inside the terminal. The tug operated did not see the collision and was later notified of the event.

A witness, an airport engineer, observed N472UA as it was pushed back from Gate A-40, which was an angled gate (NE/SW) located immediately east of the center terminal. He watched the tug operator push the airplane back and also observed two wing walkers escorting the airplane. Though the witness could not confirm how far back the main landing gear wheels were positioned from the Taxiway AA centerline, he did say that it was "pushed back too far." He said the tug operator and two wing walkers then unhooked the tug and walked back to the terminal. During that time, N496UA was taxing from west to east on Taxiway AS toward Gate A-48, and had just passed under the crossover bridge. The witness stated that N496UA's nose wheel was "definitely" on the Taxiway AS centerline as it taxied. As N496UA taxied behind N472UA, its left winglet cut through the bottom section of N472UA's tail cone, about one-foot aft of the APU maintenance doors.

In a written statement, the flight crew from N496UA stated that they received clearance to transition from Taxiway AS and proceed eastbound on Taxiway AA to Gate A-48, which is beyond Gate A-40. As they taxied toward the crossover bridge, they noticed that N472UA was pushed back from the gate and "appeared to be out of position" reducing the normal clearance between the two aircraft. The flight crew elected to increase their wing tip clearance by moving to the right of the centerline approximately 5-6 feet just after they passed under the crossover bridge. As they approached N472UA, the Captain slowed the taxi speed to a crawl and determined there was sufficient clearance between the two aircraft. After crossing behind N472UA, N496UA taxied to Gate 48.

The flight crew aboard N472UA stated they felt a "slight movement" as N496UA taxied behind them. Immediately, the crew looked out the right cockpit window and observed no visible damage to N496UA's left wing. Shortly after, N472UA began to taxi when they were told to stop due to damage on the tail cone.

Examination of both airplanes revealed that N496UA sustained minor damage (scratches only) to the top of its left winglet, and N472UA sustained minor damage to the tail cone. The damage consisted of a tear in the skin about three feet forward of the aft-tip of the tail cone. The auxiliary power unit was not damaged.

A review of engineering drawings and measurements taken on the ramp revealed that if the main landing gear of an Airbus A320 was parked on the Taxiway AA centerline, and the nose wheel of an Airbus A320 was on the centerline of Taxiway AS, there would have been approximately 30 feet of clearance between the parked airplane's tail cone and a taxiing airplane's wingtip.

As a result of the incident, United Airlines agreed to install stop-blocks and gate identifiers at each gate on Concourse A. They also plan to work with DEN airport operations personnel to review and modify existing airport engineering drawings to properly reflect the addition of the stop-blocks. United Airlines will also revise pushback procedures to reflect the painted stop-blocks as the new stopping point.

NTSB Probable Cause

The captain's failure to maintain sufficient clearance while taxiing to an assigned gate, which resulted in a collision with an airplane that had been pushed back (and holding) from another gate. Also causal was the tug operator's failure to follow published company procedures resulting in the holding airplane being pushed back too far, which reduced wingtip clearance of the taxiing airplane.