Air carrier flight crew reported push crew failure to report breakaway from tug resulted in Captain applying brakes to regain aircraft control.
Synopsis
Air carrier flight crew reported push crew failure to report breakaway from tug resulted in Captain applying brakes to regain aircraft control.
Narrative
We had just completed the push back checklist and the Captain contacted the ramp and they instructed him to release the brakes. He released the brakes and told them to hold the push. I had my head down printing the final weights and requesting the takeoff data. Suddenly the relief pilot said 'hey are we moving' I looked up and we were indeed moving but we didn't have a clearance. The Captain quickly got on the flight interphone and said something to the effective of 'why are we moving'. He initiated the call. I didn't have flight interphone on as I was monitoring ramp for push instructions so I couldn't hear what they said to him. I was watching him as he asked a second time and stated we are mowing. He then applied brakes carefully and then said he was setting the parking brake. He asked them what happened; but they just said the plane got away from the tug. He quickly inquired if everyone was okay and then asked if had hit anything or damaged anything. We were told that all was good; and they were going to reconnect. We are not sure if the ramper meant the tow bar wasn't connected to the tug or if it wasn't connected to the plane. We heard nothing from the cockpit to indicate it had come lose from the plane so best we could it ascertain that one end wasn't connect. The ramper never yelled breakaway it was the cockpit that initiated the alert and inquiry. They then called back and said they were secure and we released the brake again right around when we were finally given taxi clearance after we the airplane we were waiting to enter the ramp passed behind us.
Second reporter narrative
After completion of the Before Pushback checklist; I coordinated with the Tug Driver to confirm that they were fully ready for brake release and pushback clearance. I received an affirmative response from the Tug Driver so the brakes were released and I informed the driver to standby for pushback clearance. A short time later my relief pilot asked why we were moving. Since we hadn't received clearance; I began to question the driver down below and was quickly told to re-set the brakes. The driver then stated the aircraft had become disconnected from the tow bar. I smoothly came to a stop; set the brakes; and began asking questions. I asked if anyone was hurt and if we had damaged any equipment or the airplane. Both answers were 'no' so they re-attached the tow bar and we completed our pushback and start. The Tug Driver never said 'breakaway' so I had to ask several questions to determine if we were being pushed when the disconnect occurred or if the aircraft simply rolled away from the Tug. Having a clear picture of the scenario downstairs is critical in this situation because applying the aircraft brakes while being actively pushed by the Tug 'could' lead to major issues and damage.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.