Air carrier flight crew reported they were disconnected from the tug during pushback and rolled backwards for a few feet before regaining control.
Synopsis
Air carrier flight crew reported they were disconnected from the tug during pushback and rolled backwards for a few feet before regaining control.
Narrative
The aircraft was completing normal pushback from gate XX in ORD. In adherence to new company policy; engines were not started during pushback. Upon pushback completion; the crew stated; 'Pushback complete; set parking brake.' However; simultaneously; Ramp Control called us and asked us to pull forward abeam XY to allow space for an additional aircraft behind us. I set the parking brake and told the pushback crew; 'Parking brake set; standby.' My First Officer responded appropriately to the call from Ramp Control. After his response; I stated to the pushback crew; 'Ramp Control is asking us to pull forward abeam XY.' Their response was 'Copy that; Captain'. I interpreted this as they were ready to pull the aircraft abeam XY; and since I had set the parking brake; I did not want the pushback tug to pull on the aircraft with the brake still applied. I then stated 'standby;' to which there was no response. I released the parking brake and stated 'parking brake released.' At this time; we slowly began moving backward to which I believed was the pusback crew misunderstanding the instructions; so I restated 'pull the aircraft forward abeam XY.' In response; the pushback crew stated 'Captain; set your parking brake.' I applied the parking brake slowly and we came to a mildly abrupt stop. It was at this moment that we realized that we had been disconnected from the tug and were rolling backwards freely. We rolled approximately 15-20 feet before stopping. After confirming with the pushback crew that no injuries had been sustained and that no damage occurred; we reconnected with the tug and were pulled forward abeam XY. At this point; we were disconnected from the tug and began normal engine start. The flight continued without any further issues.Cause: Congested traffic in a busy ramp space causes Ramp Control to desire to move our aircraft not under our power to save a few minutes before we start our engines. Potential communication misunderstanding with the pushback crew.Suggestions: If I could go back; I would not have set the parking brake and stated 'parking brake set; standby' after the initial pushback. I believe if I had not set the brake and communicated with the pushback crew; they would not have disconnected from the aircraft. Instead; I would've stated the instructions from Ramp Control without ever setting the brake. I believe this would have mitigated any chances for miscommunication with the pushback crew.
Second reporter narrative
We were cleared to push by Ramp Control from XX (no specific disconnect location assigned.) We were pushed onto the north line abeam approximately XZ/XA. Just as the guideman began saying 'set brakes' and more the Ramp Controller requested that we disconnect at a different location. The 2nd calls at the same different blocked each other; so I had to clarify with Ramp where they wanted us to disconnect; and Ramp instructed us to disconnect abeam XY. While I was clarifying the instruction with ATC; Captain had set the parking brake. Captain instructor the rampers to move the airplane forward abeam XY; they responded with 'copy that Captain;' the Captain released the parking brake; and communicated this fact to the guideman. We slowly started inching backwards deeper into the alley (contradictory to the instruction); and both me and the Captain noticed immediately. Captain clarified with the guideman to again move the airplane forward to XY. After a few second delay; the guideman instructed us to 'set the parking brake.' We set the parking brake and came to an abrupt stop. The aircraft didn't roll a significant distance and I would estimate the parking brake was set while traveling ~2-3 kts. The guideman indicated that the airplane was disconnected from the tow bar. Captain ensured that everyone on the ground was safe and unharmed. The rampers reconnected the bar; moved us forward; and the rest of the pushback process was completed as normal.Cause: There was a miscommunication about the status of the push during the initial stop of the aircraft between the Captain and the guideman. The rampers presumed that they were cleared to (and it was safe to) disconnect the tow bar. Since I was distracted talking to Ramp Control for about 10-15 seconds I didn't hear what the Captain told the guideman initially; but afterwards indicated he said 'parking brake set; standby.' The guideman (from what I heard) never stated that they were disconnecting the tow bar and completing the pushback process. They understood that we needed to be moved towards XY; but never explicitly told us they needed to reconnect the tow bar or that the tow bar was disconnected in the first place.Suggestions: Clearer communication from both parties may have prevented the occurrence. It was a nonstandard situation and a late request/ change in clearance from Ramp Control; but additional details could have been included. Potentially; the Captain should have elaborated why we were standby but I know he was listening to Ramp Control briefly like me trying to figure out the updated clearance. Presumably; the guideman just assumed that the push was complete once the parking brake was set; but should have queried the Captain what he meant by standby.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.