Narrative:

The aircraft was not at the gate when the crew arrived, but was towed to the gate shortly after we got there. We went aboard and I found a ramp individual, the one that was in the cockpit guarding the brakes, speaking with the first officer about battery charging. I asked him what was up and he said that the batteries were low enough that they could not get the APU started. The individual that was in the cockpit told me that after making 3 attempts to start the APU, they had decided to tow the aircraft to the gate to put external power on it to charge the battery. I asked him if they had charged the batteries before they brought the aircraft up and he told me that they had not put power on the aircraft before getting it to the gate. This is very significant because the brake pressure gauge showed no brake pressure available to stop the aircraft when we got in the cockpit. There is conflicting information here. A foreman came up to the cockpit just before departure and told us that they had 'charged the brake pressure before towing the plane.' he said that the brake pressure must have gone to zero when the brakes were parked at the gate. If the brakes had been charged before the tow, it would have required AC power, either the APU, which did not start, or a power cart to do it. Further, if the brakes had been charged, there is enough capacity in the accumulator to stop the aircraft on a landing 3-5 times. There is a significant fact here. I had a similar occurrence on a flight on may/tue/03. Possibly we need to have the batteries checked on that plane. It appears that this aircraft was towed to the gate without any brake pressure available. Had the towbar broke, one can only speculate where and against what the aircraft might have finally come to a stop.

Google
 

Original NASA ASRS Text

Title: A CAPT EXPRESSES CONCERN WITH A B757-200 BEING TOWED TO THE GATE WITH THE BRAKE ACCUMULATOR NOT CHARGED PRIOR TO TOWING.

Narrative: THE ACFT WAS NOT AT THE GATE WHEN THE CREW ARRIVED, BUT WAS TOWED TO THE GATE SHORTLY AFTER WE GOT THERE. WE WENT ABOARD AND I FOUND A RAMP INDIVIDUAL, THE ONE THAT WAS IN THE COCKPIT GUARDING THE BRAKES, SPEAKING WITH THE FO ABOUT BATTERY CHARGING. I ASKED HIM WHAT WAS UP AND HE SAID THAT THE BATTERIES WERE LOW ENOUGH THAT THEY COULD NOT GET THE APU STARTED. THE INDIVIDUAL THAT WAS IN THE COCKPIT TOLD ME THAT AFTER MAKING 3 ATTEMPTS TO START THE APU, THEY HAD DECIDED TO TOW THE ACFT TO THE GATE TO PUT EXTERNAL PWR ON IT TO CHARGE THE BATTERY. I ASKED HIM IF THEY HAD CHARGED THE BATTERIES BEFORE THEY BROUGHT THE ACFT UP AND HE TOLD ME THAT THEY HAD NOT PUT PWR ON THE ACFT BEFORE GETTING IT TO THE GATE. THIS IS VERY SIGNIFICANT BECAUSE THE BRAKE PRESSURE GAUGE SHOWED NO BRAKE PRESSURE AVAILABLE TO STOP THE ACFT WHEN WE GOT IN THE COCKPIT. THERE IS CONFLICTING INFO HERE. A FOREMAN CAME UP TO THE COCKPIT JUST BEFORE DEP AND TOLD US THAT THEY HAD 'CHARGED THE BRAKE PRESSURE BEFORE TOWING THE PLANE.' HE SAID THAT THE BRAKE PRESSURE MUST HAVE GONE TO ZERO WHEN THE BRAKES WERE PARKED AT THE GATE. IF THE BRAKES HAD BEEN CHARGED BEFORE THE TOW, IT WOULD HAVE REQUIRED AC PWR, EITHER THE APU, WHICH DID NOT START, OR A PWR CART TO DO IT. FURTHER, IF THE BRAKES HAD BEEN CHARGED, THERE IS ENOUGH CAPACITY IN THE ACCUMULATOR TO STOP THE ACFT ON A LNDG 3-5 TIMES. THERE IS A SIGNIFICANT FACT HERE. I HAD A SIMILAR OCCURRENCE ON A FLT ON MAY/TUE/03. POSSIBLY WE NEED TO HAVE THE BATTERIES CHKED ON THAT PLANE. IT APPEARS THAT THIS ACFT WAS TOWED TO THE GATE WITHOUT ANY BRAKE PRESSURE AVAILABLE. HAD THE TOWBAR BROKE, ONE CAN ONLY SPECULATE WHERE AND AGAINST WHAT THE ACFT MIGHT HAVE FINALLY COME TO A STOP.

Data retrieved from NASA's ASRS site as of July 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.