What happened
The pilot reported that he was attempting to start the engine of an unoccupied airplane by manually rotating the propeller, a procedure known as hand propping. To secure the aircraft during this process, he had chocked the nose wheel and engaged the parking brake. However, he noted that the parking brake was ineffective at holding the aircraft in place.
During the start sequence, the engine successfully ignited. Immediately following the startup, the airplane began to move forward despite the chocks and the attempted use of the parking brake. The aircraft accelerated until it collided with a parked airplane nearby. After striking the second aircraft, the unoccupied plane continued its momentum, hitting a vehicle before finally coming to rest after impacting a hangar door.
The investigation
The primary mechanical finding related to the failure of the braking system to hold the aircraft stationary during the engine start procedure. The nose wheel chocks were in place but did not prevent the forward movement once the engine provided thrust or if the brakes failed to engage sufficiently.