Date: 201606
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Aircraft: B777-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
Aircraft component: Nose Gear
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Troubleshooting
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Ground Event / Encounter Other / Unknown
Detector: Person Ground Personnel
Result: Flight Crew Returned To Gate; General Maintenance Action
Contributing factors: Equipment / Tooling; Procedure
Primary problem: Procedure

Narrative: The aircraft was pushed back tail south and engines were started during pushback. The parking brake was parked and I gave clearance to disconnect. After several moments the ground crew advised they were having trouble disconnecting the tow bar. I was given instructions to release the parking brake and I did. I expected them to move the airplane to realign the tow bar. Several moments later the aircraft moved forward and stopped. I was given instructions to set the parking brake and I did. I was then advised that maintenance would need to inspect the nose gear for possible damage. Several minutes passed and I elected to shut down the engines to conserve fuel. We called maintenance on the radio and were advised that we would need to return to the gate for inspection. A mechanic plugged in asked me to rotate the nose wheel to the left and right turn limits which I did. He then confirmed that they would need to perform a 'card' and we would need to return to a gate. We restarted the engines and taxied to gate.A mechanic arrived to the cockpit and commented that this was yet another event where the ground crew had difficulty disconnecting the tow bar because it was jammed. I understood him to mean that the tow bar was under stress and would not allow its removal. We discussed the benefits of delaying engine start during pushback. Maintenance performed their inspection of the nose gear and a Ground Interrupt ETOPS inspection. No damage was found and the logbook was signed off.As we were prepared to depart my First Officer received a phone call from [the] Chief Pilot and was told that the ground crew had stated that we had rolled forward into the tow bar when our brakes should have been parked. I elected not to have a conversation at that moment because it would be a distraction. This was the first time anyone in the cockpit had heard this information. All of my cockpit crew members plus an FAA inspector in the cockpit jumpseat had been listening to my ground communications during the incident and agreed that I had followed the ground crew instructions.I think this was a serious incident. A ground crew member could have been hurt or the aircraft could have been damaged. I think that we are at risk any time we release the parking brake with the engines running on the 777 because the airplane will move without advancing the throttles. It might be a good idea to change our procedures so that the engines are not started until after the tow bar is removed and the salute is given with the tug and tow bar safely away from the airplane. In this particular incident the aircraft moved forward and it was assumed that the tug was in control of the movement. Unless the ground crew specifically states what they are doing the cockpit crew has no view and no way of knowing what is happening at the nose gear.


Second narrative: [Report narrative contained no additional information.]


Synopsis: The ground crew had difficulty disconnecting the tow bar. They asked the Caption to release the brakes; saying they had a problem disconnecting. The Captain did and the aircraft moved forward.