Flight crew reported the aircraft moved after the brakes were set and the tug was disconnected. Captain applied brakes to stop aircraft.
Synopsis
Flight crew reported the aircraft moved after the brakes were set and the tug was disconnected. Captain applied brakes to stop aircraft.
Narrative
After Pushback was complete; the command for 'Set Brakes' was given to the Captain. Captain set brakes and waited for pushback crew to clear to the left; while I completed the engine start. After approximately 20-30 seconds; pushback crew said 'brakes; brakes'. Captain engaged manual brakes realizing that we were rolling forward. Pushback crew asked if we were OK and Captain inquired if everyone was ok on the ground to which we received a positive response. Captain continued to hold manual brakes and attempted to set parking brake 3 times. Each time; the parking brake handle would return to the down position. Captain passed the manual brakes to me while he assessed the situation. After the Captain took the manual brakes back over from me; the parking brake engaged normally. We contacted Maintenance to let them know there was a parking brake anomaly and cycled the parking brake 3 times in the line to ensure that they were functioning normally. After conferring with Maintenance and ensuring parking brakes were functioning; we taxied to the active runway. Taxi to runway was uneventful. At the hold short; we once again checked the parking brake and they functioned correctly. Once airborne we decided we would write an information writeup in the maintenance logbook for crew awareness and also complete Reports. Approximately 1 hour from ZZZ we received a message from Dispatch that we possibly made contact with the tug. We also received a message to contact the Duty Officer; which we did upon arrival to ZZZ. At no point did we realize that we made contact with the tug; we were under the impression that the aircraft was stopped with the brakes. Cause: Parking brake anomaly. In addition; my scan was slow to realize we were moving while I completed the startup sequence.Suggestions: Improve my scan inside and out. Better communication with Push Crew to ascertain that we possibly made contact with the tug.
Second reporter narrative
After push back from Gate XX I was told to set the brakes by the ground crew. I set the brakes and confirmed they were set by the brake light; brake handle and the EICAS message. The First Officer also said he confirmed that the brakes were set. I told the ground crew they were cleared to disconnect. Maybe 30 seconds or so went by and we heard the ground crew say brakes; brakes." I noticed we had begun to roll and immediately step on and held the brakes. I tried to set the parking brake 3 times and they would not set. The brake handle would just slide down and would not lock the brake pedals. I checked in with the ground crew to make sure everyone was alright and they were. The ground crew said there was no damage. We told the Ramp we needed to stay in the taxi lane for a bit and talk with Maintenance. The First Officer offered to relieve me on the brakes; because I was holding the brakes with a lot of force. So; I transferred the brakes to him. I cleared off the ground crew. Once the ground crew was clear; the First Officer transferred the brakes back to me and I tried to set the brake again. The brakes set this time. We decided to try the brakes again. I let the aircraft roll for maybe a foot and set the brakes; they set and held. After talking with Maintenance; they were ok with us continuing if we were ok with it. The First Officer and I both agreed and felt it was alright to continue. We tested the parking brake one more time holding short of [Runway] XXR and the brakes worked. Enroute to ZZZ we received a ACARS message from Dispatch saying that Maintenance was going to inspect the nose wheel when we arrived in ZZZ because we may have bumped into the tug and to call the Duty Officer once on the ground. We did not think we bumped into anything. The first we heard that we might have been from the Dispatcher's message. At the gate in ZZZ I set the brakes and they worked. I guarded the brakes and didn't stop guarding them until both engines were shut down and was told to release the brakes.Cause: The First Officer and I both seen that the brakes were set and then somehow released. Movement of the aircraft was first noticed by the ground crew. The ground crew quickly made us aware and I immediately applied the brakes. Thankfully no one was injured or worse. Suggestions: Never stop watching outside for aircraft movement even when the brakes are set. Extra vigilance when ground personnel are around during push back or tow in."
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.