On September 21, 2003, about 1610 mountain standard time, a deHavilland DH-8-202, N455YV, struck a parked and unoccupied Bombardier CL-65 airplane, N7358, when ground personnel were pushing the deHavilland for departure at Phoenix Sky Harbor Airport (PHX), Phoenix, Arizona. The elevator of the Bombardier sliced through the rudder of deHavilland. The deHavilland sustained substantial damage. The airline transport pilot, commercial co-pilot, 1 flight attendant, and 29 passengers on the deHavilland were not injured. Mesa Airlines, Inc., d.b.a. America West Express, was operating the flight under the provisions of 14 CFR Part 121. Visual meteorological conditions prevailed for the scheduled domestic passenger cross-country flight that was destined for Yuma MCAS-Yuma International (YUM), Yuma, Arizona, with a 1605 departure. An instrument flight rules (IFR) flight plan had been filed. The operator reported in a written statement that a ground agent was pushing the deHavilland from the gate and did not clear the 30-foot turn line. The ground agent then pushed the tail into the Bombardier; as a result, the elevator of the Bombardier sliced through the rudder of the deHavilland. The operator felt the accident could have been prevented if the ground agent would have watched his wing walkers during the push back and cleared the turn line, prior to turning the airplane.