Date: 201710
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Object
Detector: Person Flight Crew
When detected: Taxi
Result: Flight Crew Returned To Gate
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: Pushback from Gate was normal. Both engines were started while pushing back. The Tug Driver stopped the aircraft at the top of the alley and asked me to set brakes; which I did. I then asked the Driver if he could push us back further or spin us around as we were in somewhat close proximity to equipment and we could get clearance from Ground onto the taxiway if needed. He stated that they didn't normally push back further or spin aircraft around anymore (due to some prior issues with other aircraft during pushbacks); and I said to standby and we would talk to Ground to get clearance onto the taxiway. While I could have made the turn out of the alley from my position at that time; I felt Safety would be improved by having the Tug Driver push us back further away from the ground support equipment; especially at night when it's harder to judge clearance distance. It took about one minute to obtain clearance from Ground to push onto the taxiway. At no time did the Tug Driver give any verbal or visual indication that any part of the towbar system had been disconnected. After receiving ATC clearance I released the parking brake and was about to tell the Tug Driver we were cleared to push onto the taxiway when the aircraft rolled forward immediately upon brake release. The aircraft movement was momentarily startling but I quickly applied the brakes as my FO called 'brake; brake; brake' however the nose gear rolled onto the towbar even though brake application was almost immediate. We first confirmed that nobody was hurt and then asked what had happened as we felt and heard that we had run over something. The Tug Driver said the aircraft had run over the towbar which was now trapped under the nose wheels. We ran through the profile and decided to shut down the engines. We then informed the Flight Attendants and Passengers that we would be returning to the gate. We contacted Station Operations and Maintenance and let them know what had happened. The Tug Driver was able to release the towbar from under the nose wheel. While we were waiting for Maintenance to arrive to inspect the nose wheel and nose landing gear area; I asked the Tug Driver what had happened as I did not understand why the aircraft had rolled forward. He said that he had not moved the tug as he knew we wanted to be pushed back further but the Guide Agent had released the lock on the towbar in preparation for disconnect. So when I released the brakes the aircraft jumped the towbar.Once Maintenance had confirmed it was safe to do so; we requested to be towed back to the gate. Once there; after the normal Shutdown Checklist; we took care of the Passengers first and kept them informed. A logbook entry was made. Maintenance did notice some scuff marks on the tires; which they changed; and a small amount of metal showing at the bottom of the strut where the towbar scraped it. They did not know when we left the aircraft for another one if it was simply paint that had come off; or if it was a gouge in the metal. I spoke with Dispatch and the Chief Pilot on Call. The Chief Pilot on Call approved continuing onto ZZZ if we felt safe and able to. We were swapped into another aircraft and the subsequent flight into ZZZ was uneventful.Clearly; my failure to communicate fully with the Tug Driver was the root cause of the incident and I take complete responsibility for that. At the gate; I am very careful to make sure the Tug Driver is ready for the aircraft before I release the brake. Had I done this at the top of the alley there is a good chance the Tug Driver would have said he was not ready to continue the push and the towbar needed to be secured. While nobody was hurt; this incident had a very serious potential for injury to the Ramp Personnel and I am immensely grateful that nobody was hurt. I am very troubled that my actions could have caused or contributed to life changing injury to another person and will always be grateful that no injury occurred. As to actions to prevent this happening again; and this is purely speculation on my part; it is possible that with a wired headset; the Guide Agent would not have released the towbar as the Tug Driver knew the push was not complete and would not have given the disconnect signal. Now with the new headsets; which of course overall are a great Safety enhancement; the Guide Agent heard the 'set brakes' and was primed to release the pin on the towbar. It might be possible to provide training (or further training) to Ramp Personnel not to release any part of the towbar system until the Pushback Driver indicates they are ready for release. For my part; I will never release the brakes again once they are placed on during a pushback without double confirming that the Tug Driver is ready for brake release. Also; although not a factor in this incident; I will be sure to triple check when the parking brake is placed on; it really is on; with the red light illuminated as this incident made it very apparent the potential for very serious injury or aircraft damage that can happen within one or two seconds if the aircraft rolls even one inch during the towbar disconnection process. In the past; I have been careful to ensure the parking brake is set when I think it is; but now I will be even more alert for possible errors regarding the setting and releasing of the parking brake.


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


Synopsis: B737 flight crew reported the aircraft rolled onto the towbar trapping it under the nose wheels when the Captain released the brakes before notifying the Tug Driver after the pushback.