EMB-175 flight crew while parked at gate reported uncommanded aircraft movement.
Synopsis
EMB-175 flight crew while parked at gate reported uncommanded aircraft movement.
Narrative
Captain and I pulled into gate at ZZZ just after XA:00 pm local time. Captain taxied in using the marshaller's signals. We taxied slowly up to him until he gave us the X to stop. At which point Captain stopped the taxi and as far I can remember set the brake and then shut down both engines then went to notify the ground personnel that the brake was set. Meanwhile I had completed my flow and was waiting for the Captain to perform his flow and the shutdown checklist call out. Captain and I at this point both looked outside for the ground crew and didn't see anyone at the nose of the aircraft anymore. I then pulled the checklist from the holder and he called for the shutdown checklist. We then ran the shutdown checklist and then I inserted the checklist back in the holder. At this point nothing unusual appeared to be happening. Nothing had caught my eye or senses as to we were moving. About 5-10 seconds later I felt what felt like a tug hooking up to the aircraft as I was gathering my belongings and looked up to notice the aircraft had rolled backwards and Captain was now holding the brakes with his feet. We checked with the ground crew and Flight Attendants to confirm no injuries. After confirming we started engine one and moved forward back to the gate and shut down with no further issues.I am not entirely sure what caused the brake to be released. After we parked the Captain did his flow and as far as I can remember everything; parking brake included; seemed to be set properly before we ran the shutdown checklist. The most likely cause I can imagine is right after we completed the checklist both him and I moved our seats back giving us better access to our tote bags to start preparing to leave the aircraft since it was our last flight of the day. My guess as to how the brake ended up released again is either it never got fully engaged or during the backwards movement of his seat the Captain may have nudged the brake enough to knock it loose and release it.Suggestions: Flight crew should be more cognizant of the parking brake when parking. Ensuring the brake is set is vital to safety around the aircraft for anyone nearby. First Officer should also pay attention to these details even though they may not be on he/she's flow as it is a crew responsibility. Also before going heads down in the cockpit; both pilots should ensure a secured aircraft.
Second reporter narrative
I had just pulled into and parked at gate in ZZZ. I completed my shutdown flow and called for the shutdown checklist. As I called for the shutdown checklist I looked up for the ground marshaller to give them the parking brake set hand signal. When I gave this signal they had all ready moved from the nose of the aircraft. The First Officer then read the shutdown checklist; which we completed in its entirety. I then pushed to slide my chair to rear position; while activating the chairs release mechanism. I went head down momentarily and when I looked back up I realized the aircraft was slowly rolling backwards. I immediately applied the toe brakes and brought the aircraft to a safe stop. I then checked with ground personnel and Flight Attendants to confirm that everyone was okay. They confirmed that everyone was okay. There were no injuries and no aircraft damage. While sliding seat back and reaching for chair release lever I believe I might have inadvertently bumped the parking brake lever. Suggestions: I should pay closer attention and be more diligent when reaching for the chair release mechanism.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.