On June 24, 1994, about 0804 eastern daylight time, a British Aerospace BAE Jetstream 3201, N495UE, registered to First Security Bank of Utah, operated by Atlantic Coast Airlines (ACA) dba United Express, collided with a ground power unit while arriving at the gate after landing at the Orlando International Airport, Orlando, Florida. The flight was operating under 14 CFR Part 135 as flight 5500, a scheduled, domestic, passenger flight. Visual meteorological conditions prevailed at the time and an IFR flight plan had been filed. The airplane was substantially damaged and the airline transport-rated pilot, first officer, and 14 passengers were not injured. The flight originated from the Jacksonville Municipal Airport, Jacksonville, Florida, about 0710. The captain stated that after stopping the airplane at the parking spot at the gate, he advised the first officer that "Cool down's complete, parking brake is set." When the engine instruments indicated that the engines were at 50 percent rpm, the power levers were moved to the full reverse position during which the left propeller contacted the ground power unit (GPU) that was prepositioned at the gate. The passengers were later deplaned and the airplane was secured. The cockpit voice recorder (CVR) was removed and sent to the NTSB laboratory in Washington, DC. The CVR was read out at the NTSB laboratory in Washington, DC. Review of the transcript revealed that the captain did call out over the interphone system, "cool down's complete-brake's set." About 1 minute 5 seconds after the propeller contacted the GPU, the captain was heard stating "the brake is set." About 1 second later the first officer responded by saying "I just set it." About 4 seconds later the first officer stated "released it when you..." The maintenance manual for the airplane states that testing of the parking brake is accomplished by verifying that the airplane remains stationary with both engines operating to full power with the handbrake fully applied. Damage to the left engine propeller precluded performing this test; however, a tow bar and tug were attached to the nose landing gear of the airplane and with the parking brake handle engaged, attempts to move the airplane were unsuccessful. Additionally, the parking/emergency system was inspected according to the maintenance manual with no discrepancies noted. The parking/emergency system is described as incorporating a brake lever mounted on the left side of the center console. The brake lever is horn shaped with a notched pull-on shaft, the notches locating the shaft at selected pull-on brake settings. A trigger release serves to release the handle and also retain the shaft in the required setting. The handbrake shaft is painted orange on the top and sides to provide a warning indication that the handbrake is on. A spring assists in returning the shaft to the off position. Immediately following the operational test of the parking brake it was discovered by the Regional Flight Manager of ACA and the FAA inspector that the handbrake could be disengaged by movement of only the handbrake shaft. The handbrake assembly was removed from the airplane for examination. Examination of the handbrake housing assembly revealed evidence that the second notch from the bottom of the handbrake housing was slightly worn. There were no other discrepancies noted. The painted orange stripe was visible. Review of the maintenance manual regarding adjustment of the handbrake cable revealed that the handle should be pulled out three notches and beyond this point the handle should pull solid. According to the airplane manufacturer, there have not been any reported problems associated with inadvertent release of the handbrake assembly. Review of the FAA Service Difficulty Reports data base revealed no records regarding the handbrake assembly used on Jetstream 3201 aircraft. Review of the maintenance completion report records for the accident airplane from January 1, 1994, to the date of the accident revealed no discrepancies regarding the parking brake system.