Air carrier flight crew reported aircraft rolled back uncommanded during pushback when the ground crew failed to advise pilots to set brakes.
Synopsis
Air carrier flight crew reported aircraft rolled back uncommanded during pushback when the ground crew failed to advise pilots to set brakes.
Narrative
Ground crew after moving the aircraft to a spot did not ask to set parking brake. After a short pause; I wondered if we had lost comm so asked the ground crew for a response with no answer. Both me and the LCP (line check pilot) Captain in the right seat attempted to reach the ground crew. I could not see anyone out front. I saw the wing walker and smoothly applied toe brakes and attempted further contact. With no comm; I could not tell if the ground crew wanted to further move the aircraft so eased off the brakes; could see the aircraft move and quickly reapplied the brakes. By this time the ground crew was holding the bypass pin assuming all is well. LCP opened the FO (First Officer) window and informed them without comm; they should move to hand signals and not assume all is well without verbal or hand signal confirmation of Parking Brake Set. And we were telling them for their safety. The Ground Crew reconnected the Bluetooth headset; apologized and proceeded with normal callouts.
Second reporter narrative
Tug headset failed on pushback after cleared to start engines. After the aircraft came to a stop we were not told to set brakes either via the headset or hand signals. We could hear what we thought was the tow bar being disconnected. The aircraft began an uncommanded roll. We smoothly applied brakes and stopped the aircraft. I attempted to get the attention of the ground crew by pressing the ground call button and later flashing the taxi light. The ground crew did not respond. I could then see them start to pull away from the aircraft. I opened my window and flashed the lights again and was finally able to get their attention. The ground crew reconnected the headset. I told the crew that they never told us to set brakes and we never stated the brakes were set yet they disconnected the tow bar. She apologized.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.