On May 10, 2013, about 0907 mountain standard time, a Boeing 737-7H4, N913WN, collided with a parked, unoccupied Boeing 737-7H4, N776WN, at the Sky Harbor International Airport, Phoenix, Arizona. The airplane had one engine operating and was being pushed back for its flight by a pushback tug. The 2 airline transport pilots, 3 flight attendants and 54 passengers were not injured. There were no reported ground injuries. The airplane was registered to and operated by Southwest Airlines under the provisions of Title 14 Code of Federal Regulations Part 121. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed. The scheduled domestic passenger flight had a planned destination of the Reno International Airport, Reno, Nevada, and was originating at the time. According to company ground personnel, the airplane was parked at gate C-1, which is positioned next to the terminal building. An unoccupied, adjacently parked airplane was on its right side, at gate C-3. The parking gate required the airplane to be pushed back into an adjacent alleyway. A turn reversal unto the alleyway, commonly referred to as a "S" type turn maneuver, would be accomplished. Performing this type of turn initially required the tug operator to turn towards the airplane parked on its right side, until adequate spacing was achieved with the terminal building, and then turn in the opposite direction for nearly 180 degrees, to correctly position the airplane in the alleyway. During the turn towards the parked airplane, the wing walker reported that the pushback tug operator turned tight. Also during the turn, the wing walker further reported that they had to slow down the tug's speed in order to ensure they avoided the baggage carts located near the right engine. With their attention focused on clearing the airplane's engine from the nearby carts, the wing walker looked back, and noticed the right wing tip moving towards the left horizontal stabilizer of the adjacently parked airplane, but was unable to get the tug operators attention in time to stop. Subsequently, the airplane's right winglet struck the parked, unoccupied airplane's left horizontal stabilizer and elevator. The pushback tug operator was hired in the previous year, and was qualified on the tug. During an interview with the National Transportation Safety Board (NTSB) investigator in charge (IIC), he stated that he has performed numerous pushbacks at similar gates near the terminal building. The wing walker had 18 years' experience with the company and was qualified. The operator's pushback procedures were followed, with the exception of a pre-brief for the pushback not being accomplished. Examination of both airplanes by NTSB investigators revealed that the pushback airplane, N913WN, sustained minor damage to the right winglet while the parked airplane sustained damage to its horizontal stabilizer and elevator. The pushback tug was examined by the operator and NTSB investigators and was fully functional with no anomalies noted.