Here's the official version of events from the AA 777 fleet management:
Aircraft 7AD, while parking on D33 at DFW, experienced parking brake pressure failure, which caused the aircraft to roll backward, shearing the open 2L door when it contacted the jetbridge. The crew had completed the parking checklist and were preparing to exit the aircraft when they were notified the A/C was rolling backwards. The Captain quickly returned to his seat and applied brakes. The first application reportedly had little resistance and did not slow the aircraft. His second application had no resistance with pedals fully depressed. The aircraft continued to roll aft into the middle of ramp exit 122 and almost into the terminal B ramp. Once momentum slowed, the aircraft rolled forward toward terminal D finally coming to a stop.
The parking brake can lose pressure at anytime. With 3,000 lbs of pressure, the parking brake is designed to hold pressure for a minimum of eight hours. If the parking brake valve or any other pressure component fails, pressure can be lost very quickly - four minutes after the parking brake was set, in this case. We are working with Ramp Services to ensure chocks are installed as quickly as possible after arrival. Modern terminal ramp design requires a slope away from the terminal to ensure fuel runs away from the building in the case of a spill.
When the parking brake lever is set, the outboard parking brake lever position switch connects power from the hot battery bus to the parking brake valve (PBV). The PBV is then driven to the fully closed position. Once closed, an internal microswitch within the PBV applies power to the parking brake valve close relay. This relay then connects power to illuminate the parking brake set light on the NLG [nose landing gear] and provides a signal to AIMS [Airplane Information Management System] to display the parking brake set memo on EICAS [Engine Indicating and Crew Alerting System].
We must ensure the parking brake set messages is displayed on EICAS after the parking brake is set. This is our only assurance the parking brake valve is closed. A check of accumulator pressure should also be included in your scan after the parking brake is set to confirm sufficient brake pressure is available. Chocks are the only way to prevent aircraft movement if pressure is subsequently lost after parking.
Jim Dees B777 Fleet Captain
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