Air Carrier flight crew reported aircraft started an uncommanded roll on engine start with an expected bias for push back under control of ground crew resulted in a ground loss of control.
Synopsis
Air Carrier flight crew reported aircraft started an uncommanded roll on engine start with an expected bias for push back under control of ground crew resulted in a ground loss of control.
Narrative
On Day 0; I was the Relief Pilot of flight XXX LEBL-ZZZ.We were parked at stand XX in LEBL. The ground personnel proceeded with the standard phraseology Ground to flight deck; ready for brake release" in order to release the brake and signal an OUT time. The Captain used standard phraseology and released the brake. A couple of minutes later the flying FO requested clearance to start on the ATC Clearance frequency. We received authorization to start from clearance delivery. At this moment we accepted that an engine start was going to happen at the gate. To confirm this; the Captain relayed the "ready to start engines?" to the ground crew. They positively acknowledged the information and mentioned "cleared to start".The start procedure was initiated. At approximately the completion of the start of the first engine; the ground personnel came on the intercom. Pronouncing "ground to cockpit ... (unclear)". At that moment the Captain noticed the airplane was moving forward and immediately stopped the airplane. The Ground Crew came on the intercom acknowledging we stopped; and I realized that's what they intended to tell us on the previous call. The words "breakaway; breakaway" were not used by the ground crew; however the Captain immediately noticed the forward movement and stopped the aircraft.It's unknown how far forward the airplane rolled. Based on the Ground Crew immediately telling us; and the Captain immediately stopping upon receiving such call; the forward movement is estimated to be barely a few feet. However this information can only be confirmed by the ground crew."
Second reporter narrative
We were parked at hard stand XX and received startup clearance from LEBL clearance delivery. CA asked the Ramp Agent if they were ready for brake release and they said yes. Then CA asked the agent if we were clear to start the engines and after the verbal confirmation; I started the left engine. As the engine started; the aircraft started to move the CA caught it right away and held the brakes. Right after; he set the parking brake as he relayed that information to the agent. We weren't aware that we were not hooked to a tractor and chocked neither was that communicated to us. Per our knowledge; movement was minimal when the CA caught it.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.