19980415008799I

Narrative: at the direction of air traffic control (ATC), the aircraft was holding for departure clearance at runway 28R ramp area referred to as the "penalty box." Due to an anticipated hold of several minutes, the flight crew elected to shutdown both engines and utilize the auxiliary power unit (APU) to provide electric and pneumatic power, thereby, conserving fuel. After approximately thirty (30) minutes, the aircraft began to roll backwards even though the parking brake had been set. The flight crew immediately turned the right hydraulic system electric pump (acmp) on, however, the aircraft continued to roll. The flight crew then started the left engine, at which time, the aircraft came to a stop. The flight crew then attempted to taxi forward, however, the aircraft would not move. Upon inspection by ground personnel, it was determined the aircraft had rolled back to the point that the right main gear had exited the pavement and had sunk into soft ground. All flight crew and passengers were subsequently deplaned via the forward galley door (R1) and transported to the terminal via bus. The aircraft was extricated from the area and towed to the maintenance hangar. Detail ed inspection revealed minor damage to the right main gear assembly. Additionally, minor damage was inflicted to the right engine cowling during the extrication process. As stated in the B757 maintenance manual (m/m), the parking brake system keeps the brakes set for up to 12 hours when normal brake system pressure is removed, or indefinitely when the normal brake system pressure is available. In this incident the normal brake system supply pressure was removed by the flight crew for approximately 30 minutes with the engines shutdown and the electric hydraulic pumps turned off. With the parking brake still set, the brakes released. Subsequent maintenance checks have determined the subject aircraft had a brake pressure decay well beyond allowable limits. Detailed troubleshooting has determined both "normal" brake metering valves were bypassing fluid internally back to the return system due to improper input cable rigging. At this point in the investigation, it appears the discrepancy occurred during the manufacture of the aircraft in 1995. Us airways has instituted a three point plan to preclude future incidents in this area. Short term: a) B757/B767 flight operations personnel will ensure the normal brake system is pressured with the electric hydraulic pump(s) any time the engines are shutdown while using the parking brake system. B) all B757/B767 aircraft in their fleet will have brake pressure decay checks done by maintenance with noted discrepancies required on an asap basis. Long term: us airways will introduce into their routine maintenance program a scheduled task to routinely perform a brake pressure decay check. The frequency of the scheduled check is yet to be determined. It should be noted that neither the B757/B767 maintenance review board (mrb) report or the B757/B767 maintenance planning document (mpd) addresses the parking brake system, other than the requirement to service the brake system accumulator. This office has contacted the seattle aeg regarding this incident and discussed the fact that it could be a world fleet matter.