A321 crew reported a tow bar detached from the aircraft during the push back resulting in uncommanded movement. The Captain immediately applied the brakes and stopped the aircraft and no injury or damage occurred.
Synopsis
A321 crew reported a tow bar detached from the aircraft during the push back resulting in uncommanded movement. The Captain immediately applied the brakes and stopped the aircraft and no injury or damage occurred.
Narrative
The preflight was carried out normally with nothing unusual occurring. All flows; briefings; and checklists were accomplished per the AOM and we were ready for a normal pushback from Gate XX. Just prior to pushback; I could tell that the Captain was reciting the standard phraseology with the ground crew; though I was not directly listening as I had the intercom turned off per the SOP. Cross cockpit; I heard the Captain say 'Brakes released; cleared to push.' Moments later; the aircraft started moving backwards away from the gate. Initially; nothing seemed out of place since this was exactly what we were expecting. I soon noticed that one member of the ground crew was holding his hand up in a fist to give the 'Set Brakes' command; and another held his arms in an 'X' to command 'Stop.' The most obvious indicator however was that the tug remained stopped at the gate while the aircraft rolled backwards. It quickly became obvious to me that we were not attached to the tug and that we were rolling backwards freely. I called out; 'Brakes! Brakes!' The Captain quickly diagnosed the problem too and applied brake pressure and we came to a gentle stop still in the vicinity of the gate. Once stopped; the Captain reapplied the parking brake and we waited for the ground crew and tug to approach the aircraft again. The ground crew explained to the Captain that a pin in the towbar had come loose and caused it to detach from the nosewheel. We verified with the ground crew that no damage had been caused to either the aircraft or the push equipment; then the Captain instructed the ground crew to reattach the towbar and tug to complete the remainder of the push. Once attached and ready; the ground crew accomplished the pre-push procedures and again I heard the Captain say; 'Brakes released; cleared to push.' The pushback was then performed the remaining necessary distance without incident. Before taxiing out; the Captain called operations on the radio and asked to speak with the push crew supervisor. The supervisor ensured that there was no damage caused to aircraft or equipment. Aside from momentarily disconnecting from the tug earlier; everything appeared to be normal to us. We therefore decided to continue on normally. We taxied out; took off; then flew to ZZZ1 uneventfully.I have two takeaways from this event. First; I am reminded to maintain the utmost vigilance in every phase of flight - even at the gate. It took us several seconds to spot that something was wrong because moving backwards away from the gate was exactly what we expected. The lesson here is that even if something appears normal; don't let your guard down. The second is that I wasted time by calling out 'Brakes! Brakes!' to the Captain when I could have immediately applied them myself. Yes; the communication worked. And even though there was only a 2-3 second delay; the Captain understood what was happening and stopped the aircraft uneventfully. The better course of action would have been to step on the brakes myself right away; then discuss why after. If something were behind us; that short delay caused by the communication could have made a significant difference.Cause: It appears that operator error caused the towbar to become detached from the aircraft nosewheel; but that is not totally clear. I do not know for sure what really caused it. It's possible that expectation bias played a role here. Once the command of 'Brakes released; cleared to push' was given; we totally expected to begin moving backwards. However; I still think we caught the problem quickly and did well to stop the aircraft where we did.
Second reporter narrative
While preparing for departure from ZZZ; operating from an uncontrolled ramp; all standard pre-pushback announcements and coordination with ground crew via interphone were completed. After receiving confirmation to release brakes; the parking brake was released and Ground was advised we were cleared to push.Pushback was initially delayed; and when asked if there was any traffic behind us; Ground confirmed no. A few seconds into the push; while scanning the ramp area; I observed a ground crew member in the tow vehicle signaling us to stop. I immediately applied the brakes and set the parking brake.At that time; we noticed additional ramp personnel in orange vests present in the pushback area; suggesting training activity. I contacted the ramp supervisor via interphone and asked what had occurred. He informed us that the towbar had disconnected but confirmed there was no equipment malfunction.I reconfirmed this with the supervisor a second time via VHF operations. Aircraft systems were normal; and after discussing with my First Officer and the jumpseater; we determined it was safe to continue. Ground re-connected the tow equipment and completed the pushback without further issues.Cause: Contributing Factors Uncontrolled Ramp Environment Training Activity on the Ramp Corrective actions. Immediate stop of the aircraft movement upon signal. Verified situation with ramp supervisor via interphone and VHFConfirming equipment serviceability Crew coordination and sedition making prior to resuming push Safety considerations. The importance of continuous ramp scanning during push back in controlled and uncontrolled ramps as well as following SOPs
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.