B777 flight crew reported an uncommanded movement of the aircraft during a push back when the tow bar disconnected from the aircraft. The aircraft rolled onto the tow bar and damaged the nose gear tire where it stopped and no injuries occurred.
Synopsis
B777 flight crew reported an uncommanded movement of the aircraft during a push back when the tow bar disconnected from the aircraft. The aircraft rolled onto the tow bar and damaged the nose gear tire where it stopped and no injuries occurred.
Narrative
We started a normal pushback from XX for our flight to ZZZ1. At the designated spot for release the pushback crew said that the pushback was complete and to 'set brake' I responded with the proper verbiage..'Brake set; cleared to disconnect inter phone' It became apparent that the ground crew was having trouble disconnecting the towbar and I noticed that the pushback truck was off at a large angle to the right of the aircrafts nose. I was able to still talk to the tug driver and I told him that it would make disconnecting the towbar easier if he straightened us out to relieve the pressure on the connection. He agreed and asked me to release the brake and I did that. During this time we had been given permission to start the engines and they were running. For some reason as we were going backwards the ground crew disconnected the towbar! The tug driver started shouting to me on the headset to stop! stop! set brake! Because we were now disconnected and the engines were running; we rolled forward a few feet and ran over the now disconnected towbar before I could get the brake set. At no time during the 'straightening out of the pushback truck' did the driver ever ask me to set the brake. I had no idea we weren't connected till we started rolling forward. Maintenance was called out to inspect the damage and I was told that we were on top of the towbar with the right nose tire flat with a long gash in it. The towbar was then partially disassembled and pulled out from under the nose gear. Staying in the same spot; maintenance jacked up the nose gear and replaced the flat tire. I was concerned about damage to the other tire and strut so I asked for; and got; a super tug to pull us back to the gate. Upon further inspection maintenance found damage to the underside of the nose strut and the aircraft was taken out of service.Cause: I have no idea why the push crew thought it was a good idea to disconnect the towbar while we were still moving. I'm guessing that their experience was low and some training might be in order.
Second reporter narrative
Aircraft pushed back from XX and started engines. At completion of pushback brakes were set. We heard thumping and the tug operator called on the headset and stated they were having a lot of difficulty disconnecting the towbar. The Captain told him that he might need to straighten the towbar and tug; if it was at an angle sometimes there was just too much pressure to remove the towbar. The tug operator stated he would do that and asked the CA to release the brakes. I was Relief Pilot sitting in the observers' seat; I looked out both side windows to ensure the ramp was still clear and they pulled us forward to straighten the tug. It felt like we stopped and the Captain asked the tug driver if he wanted him to set brakes. The tug operator said I'll tell you when to set them." We heard more thumping from below; then the tug operator yelled into the interphone "Brakes; brakes; brakes! Set your brakes!" We got a tire pressure EICAS message and noticed the pressure on the right nose tire had dropped to 1. The tug operator then came on the interphone and stated that the towbar had dropped when they were trying to remove it and the aircraft ran it over. The CA asked if everyone below was okay and the tug operator said yes; everyone was fine.Cause: The ground crew attempted to remove the towbar without the aircraft being stopped with brakes set. Following SOP will mitigate the risk."
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.