Narrative:

Aircraft was pushed back from jetway. Normally flight crew was in contact with ground crew via intercom. Engine # and 4 had been started during pushback. Captain was told to set brakes by ground crew, parking brake was set normally. As engine #2 was being started tug and tow bar were disconnected. Suddenly ground crew exclaimed, 'the aircraft is moving' whereupon the captain applied toe brakes but aircraft continued to move, whereupon I applied toe brakes also without success as captain was turning on all hydraulic switches for engine driven pumps and also ptu (power transfer unit). By this time aircraft nose cone contacted tug and aircraft stopped. Apparent cause of problem was brake accumulator failed to maintain sufficient pressure for parking brakes to hold. A remedy would be to pressurize hydraulic system with AC pump during pushback or in this particular situation the 'push emergency yellow brakes' could have been employed from either seat to stop the aircraft. If the pilots had been 'heads up' the 'push emergency yellow system' has never been used on one of my flts nor even in a simulator check so I'm not surprised that our split-second thoughts did not include this capability. Callback conversation with reporter revealed the following: reporter states that he believes that SOP's should requires the aircraft hydraulic system be pressurized during pushback so that brake pressure is assured. He states that most flight crews use this technique and he knows of no reason why it could not be made a procedure.

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Original NASA ASRS Text

Title: AFTER PUSHBACK AND AFTER THE PARKING BRAKES HAD BEEN SET ACFT ROLLED FORWARD AND CONTACTED PUSHBACK TUG.

Narrative: ACFT WAS PUSHED BACK FROM JETWAY. NORMALLY FLT CREW WAS IN CONTACT WITH GND CREW VIA INTERCOM. ENG # AND 4 HAD BEEN STARTED DURING PUSHBACK. CAPT WAS TOLD TO SET BRAKES BY GND CREW, PARKING BRAKE WAS SET NORMALLY. AS ENG #2 WAS BEING STARTED TUG AND TOW BAR WERE DISCONNECTED. SUDDENLY GND CREW EXCLAIMED, 'THE ACFT IS MOVING' WHEREUPON THE CAPT APPLIED TOE BRAKES BUT ACFT CONTINUED TO MOVE, WHEREUPON I APPLIED TOE BRAKES ALSO WITHOUT SUCCESS AS CAPT WAS TURNING ON ALL HYDRAULIC SWITCHES FOR ENG DRIVEN PUMPS AND ALSO PTU (POWER TRANSFER UNIT). BY THIS TIME ACFT NOSE CONE CONTACTED TUG AND ACFT STOPPED. APPARENT CAUSE OF PROBLEM WAS BRAKE ACCUMULATOR FAILED TO MAINTAIN SUFFICIENT PRESSURE FOR PARKING BRAKES TO HOLD. A REMEDY WOULD BE TO PRESSURIZE HYDRAULIC SYSTEM WITH AC PUMP DURING PUSHBACK OR IN THIS PARTICULAR SITUATION THE 'PUSH EMER YELLOW BRAKES' COULD HAVE BEEN EMPLOYED FROM EITHER SEAT TO STOP THE ACFT. IF THE PLTS HAD BEEN 'HEADS UP' THE 'PUSH EMER YELLOW SYSTEM' HAS NEVER BEEN USED ON ONE OF MY FLTS NOR EVEN IN A SIMULATOR CHECK SO I'M NOT SURPRISED THAT OUR SPLIT-SECOND THOUGHTS DID NOT INCLUDE THIS CAPABILITY. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: REPORTER STATES THAT HE BELIEVES THAT SOP'S SHOULD REQUIRES THE ACFT HYDRAULIC SYSTEM BE PRESSURIZED DURING PUSHBACK SO THAT BRAKE PRESSURE IS ASSURED. HE STATES THAT MOST FLT CREWS USE THIS TECHNIQUE AND HE KNOWS OF NO REASON WHY IT COULD NOT BE MADE A PROC.

Data retrieved from NASA's ASRS site as of August 2007 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.