Narrative:

During taxi out we experienced a minor mechanical problem with bscu channel 2. We parked the aircraft in a run-up area and attempted to resolve the problem with maintenance control's assistance. The bscu channel 2 malfunction could not be cleared; and maintenance control proposed to defer it. We were agreeable. With maintenance control on the phone; we proceeded with the MEL steps necessary to confirm that the alternate braking system functioned properly; it was discovered that the engine #1 engine-driven hydraulic pump could not be depressurized if the engine was rotating; and the green hydraulic system thus remained pressurized (regardless of switch and/or circuit breaker status). We returned to the gate for maintenance action. 3 mechanics witnessed the same problem; and they could not depressurize the engine #1 engine-driven hydraulic pump or depressurize the green hydraulic system if engine #1 was rotating. Changing the switch on the overhead panel and changing a cannon plug at the pump did not change anything. A maintenance supervisor told us that he believed that this might be deferrable. I advised him that to take the aircraft in this condition would make it impossible for us to comply with the green hydraulic overheat irregular procedure; should we have that problem in-flight. Should we experience a green hydraulic system or fluid problem; even engine shutdown would leave the engine rotating (absent a core seizure); and thus the system pressurized through the engine-driven pump. I stated that if maintenance were to defer this item and leave the aircraft 'as is;' I would refuse it. Maintenance continued unsuccessfully to attempt to repair the problem; and just before 4 hours after scheduled departure the flight was canceled.

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Original NASA ASRS Text

Title: AN AIRBUS A320 EXPERIENCED A MECHANICAL PROB WITH THE BRAKE AND STEERING CTL UNIT (BSCU) DURING TAXI OUT.

Narrative: DURING TAXI OUT WE EXPERIENCED A MINOR MECHANICAL PROB WITH BSCU CHANNEL 2. WE PARKED THE ACFT IN A RUN-UP AREA AND ATTEMPTED TO RESOLVE THE PROB WITH MAINT CTL'S ASSISTANCE. THE BSCU CHANNEL 2 MALFUNCTION COULD NOT BE CLRED; AND MAINT CTL PROPOSED TO DEFER IT. WE WERE AGREEABLE. WITH MAINT CTL ON THE PHONE; WE PROCEEDED WITH THE MEL STEPS NECESSARY TO CONFIRM THAT THE ALTERNATE BRAKING SYS FUNCTIONED PROPERLY; IT WAS DISCOVERED THAT THE ENG #1 ENG-DRIVEN HYD PUMP COULD NOT BE DEPRESSURIZED IF THE ENG WAS ROTATING; AND THE GREEN HYD SYS THUS REMAINED PRESSURIZED (REGARDLESS OF SWITCH AND/OR CIRCUIT BREAKER STATUS). WE RETURNED TO THE GATE FOR MAINT ACTION. 3 MECHS WITNESSED THE SAME PROB; AND THEY COULD NOT DEPRESSURIZE THE ENG #1 ENG-DRIVEN HYD PUMP OR DEPRESSURIZE THE GREEN HYD SYS IF ENG #1 WAS ROTATING. CHANGING THE SWITCH ON THE OVERHEAD PANEL AND CHANGING A CANNON PLUG AT THE PUMP DID NOT CHANGE ANYTHING. A MAINT SUPVR TOLD US THAT HE BELIEVED THAT THIS MIGHT BE DEFERRABLE. I ADVISED HIM THAT TO TAKE THE ACFT IN THIS CONDITION WOULD MAKE IT IMPOSSIBLE FOR US TO COMPLY WITH THE GREEN HYD OVERHEAT IRREGULAR PROC; SHOULD WE HAVE THAT PROB INFLT. SHOULD WE EXPERIENCE A GREEN HYD SYS OR FLUID PROB; EVEN ENG SHUTDOWN WOULD LEAVE THE ENG ROTATING (ABSENT A CORE SEIZURE); AND THUS THE SYS PRESSURIZED THROUGH THE ENG-DRIVEN PUMP. I STATED THAT IF MAINT WERE TO DEFER THIS ITEM AND LEAVE THE ACFT 'AS IS;' I WOULD REFUSE IT. MAINT CONTINUED UNSUCCESSFULLY TO ATTEMPT TO REPAIR THE PROB; AND JUST BEFORE 4 HRS AFTER SCHEDULED DEP THE FLT WAS CANCELED.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.