What happened
While parked at gate B-20C at O'Hare International Airport, the captain of a BAE 146 performed a procedure to transfer fluid between the green and yellow hydraulic systems following the Pilot Operating Handbook. After the transfer, the captain set the parking brake and verified the hydraulic system pressure.
The crew completed the before start, starting engines, and after engine start checklists. Once the chocks were removed by ramp personnel, the aircraft began to roll forward. Although all four engines were running and the parking brake was engaged, the movement continued. The captain attempted to use both the green braking system and the emergency yellow brake system, but the aircraft failed to stop. In an effort to halt the movement, the captain moved the anti-skid switch to the battery position and placed the thrust levers to fuel off. The airplane traveled approximately 50 feet before striking a lower level door and the bottom of the upper level of the terminal building.
The investigation
Post-incident inspections were conducted on the brakes, hydraulic system, anti-skid system, squat switches, and relevant electrical system components. These examinations failed to reveal why the airplane moved while the parking brake was set. Investigators noted that the initiation of the rolling event was not recorded on the DFDR, as the aircraft's design does not capture the status of the parking brake setting on that device.