Narrative:

When we arrived at the aircraft I noticed there was only one set of chocks on one main gear; so I asked the mechanic to find some more before they hooked up the fuel truck as it was fairly breezy. He did find some and had them put in place.we went up to the flight deck and did our normal preflight duties; got all the paperwork; clearances; got fueled; preflight checklist was completed; and were ready to go with the exception of catering. We had seven people onboard and had half the catering we needed. So we were all standing around waiting for the food to show up. The aircraft was a [B747] with plenty of room on the upper deck and there were somewhere around 12 people onboard.I was toward the rear of the upper deck talking with one of our jumpseaters and I felt a very large bump; I thought something had hit the plane. I immediately ran up to the cockpit and the first officer was sitting in his seat and he said the aircraft had rolled. I sat down and looked at the parking brake; which was set; the brake pressure; which was at 3;000 psi; then I looked up and saw the number 4 aux pump on. I then ran downstairs and the mechanic was standing at the L1 door looking out at host of people outside who were looking up at us. No stairs in sight; just an open L1 door.I yelled down and asked if everyone was ok outside; they all said no one was hurt and we didn't hit anything. I asked who pulled the chocks; and one of the guys outside said 'I did'; at which point the mechanic who was standing right next to me in the open doorway said 'I told him to'. He said he went up to the cockpit; saw the parking brake set and decided to have the chocks pulled. The gentleman on the ramp then said he saw the a/c (aircraft) rolling so he threw a chock under a main tire to try and stop the plane but it rolled right over it; which was the bump I had felt.I returned to the cockpit and talked to the first officer. He said he was towards rear of the cockpit and heard someone yelling that we were rolling so he jumped up in his seat and stepped on the brakes. He heard the parking brake lever release when he applied the brake but there was little resistance in the brake pedals and no braking as a result. He then turned on the number 4 aux pump; had to wait for that to pressurize the system; then stopped the plane. This is when I showed up.I will say the first officer saved the day. We had rolled backwards roughly 150 feet. Had we rolled another 150 feet we may well have been pushing a line of [aircraft] with our tail. First officer's fast thinking and immediate use of his systems knowledge saved us from a potential disaster.as for why the chocks were pulled to begin with; I have no idea. The mechanic inspected the tires and landing gear that had rolled over the chock; as did I; and no defects were noted.

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Original NASA ASRS Text

Title: B747 Captain reported issues with ground personnel while parked on the ramp resulting in spontaneous movement of the aircraft.

Narrative: When we arrived at the aircraft I noticed there was only one set of chocks on one main gear; so I asked the mechanic to find some more before they hooked up the fuel truck as it was fairly breezy. He did find some and had them put in place.We went up to the flight deck and did our normal preflight duties; got all the paperwork; clearances; got fueled; preflight checklist was completed; and were ready to go with the exception of catering. We had seven people onboard and had half the catering we needed. So we were all standing around waiting for the food to show up. The aircraft was a [B747] with plenty of room on the upper deck and there were somewhere around 12 people onboard.I was toward the rear of the upper deck talking with one of our Jumpseaters and I felt a very large bump; I thought something had hit the plane. I immediately ran up to the cockpit and the First Officer was sitting in his seat and he said the aircraft had rolled. I sat down and looked at the parking brake; which was set; the brake pressure; which was at 3;000 PSI; then I looked up and saw the number 4 aux pump on. I then ran downstairs and the mechanic was standing at the L1 door looking out at host of people outside who were looking up at us. No stairs in sight; just an open L1 door.I yelled down and asked if everyone was ok outside; they all said no one was hurt and we didn't hit anything. I asked who pulled the chocks; and one of the guys outside said 'I did'; at which point the mechanic who was standing right next to me in the open doorway said 'I told him to'. He said he went up to the cockpit; saw the parking brake set and decided to have the chocks pulled. The gentleman on the ramp then said he saw the a/c (aircraft) rolling so he threw a chock under a main tire to try and stop the plane but it rolled right over it; which was the bump I had felt.I returned to the cockpit and talked to the FO. He said he was towards rear of the cockpit and heard someone yelling that we were rolling so he jumped up in his seat and stepped on the brakes. He heard the parking brake lever release when he applied the brake but there was little resistance in the brake pedals and no braking as a result. He then turned on the number 4 aux pump; had to wait for that to pressurize the system; then stopped the plane. This is when I showed up.I will say the First Officer saved the day. We had rolled backwards roughly 150 feet. Had we rolled another 150 feet we may well have been pushing a line of [aircraft] with our tail. First Officer's fast thinking and immediate use of his systems knowledge saved us from a potential disaster.As for why the chocks were pulled to begin with; I have no idea. The mechanic inspected the tires and landing gear that had rolled over the chock; as did I; and no defects were noted.

Data retrieved from NASA's ASRS site 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.