Narrative:

I was pilot in command of flight aircraft X when the rollback situation happened on gate X at ZZZ. We touched down in ZZZ and taxied to our assigned gate. The gate was occupied when we arrived. We were instructed to hold on the taxiway until our gate became available. We held for about 25 minutes and then we were given a new gate from operations. We taxied to [new gate] and parked using the self-parking system; as there were no ramp agents present. Once we were parked; I shut the engines down and called for the shutdown checklist. All items on the shutdown checklist were completed with the exception of the parking brake. We held the shutdown checklist pending the setting of the parking brake. I was manually on the brakes with my feet firmly on the rudder pedals. We were in this position at the gate for about 5 minutes while the gate agent maneuvered the jet bridge to the aircraft. The aircraft suddenly started rolling back and it was brought to my attention by my first officer. At first it appeared to me as an optical illusion caused by the moving jet bridge as I was; and had been; firmly on the brakes the entire time. When I realized that it was in fact the airplane that was rolling backwards; I instantaneously applied greater pressure to the rudder pedals and immediately applied the parking brake when I noticed the manual brakes were ineffective in stopping the aircraft's backwards roll. The aircraft was towed forward by ramp personnel to the proper parking line. The jet bridge was subsequently attached to the airplane and passengers were deplaned. At the time of the rollback; no ground equipment was attached to the aircraft. The aircraft was not chocked. Since at the time of the rollback no equipment was attached; a post flight walk around inspection of the aircraft confirmed no damage. The onboard ACARS recorded an 'in the gate' time of xa:36 pm local time. 39 minutes after touchdown. Suggestions: I was under the incorrect impression that the brake system would hold pressure even after engine shutdown. More specifically after hydraulic system deactivation. After the incident; while searching for clues as to why the airplane rolled back even while my feet were firmly on the brake pedals; the problem became clear. While the hydraulic system does hold pressure through the accumulators; this pressure is only available via the emergency/parking brake and not to the manual brakes via the rudder pedals. I understand that a 'loss' of hydraulic systems 1 and 2 renders the brakes inoperative. In my case; a 'loss' put me in the mindset of a catastrophic loss; when in reality a 'loss' occurs every time the engines are shutdown and system pressure is lost. As far as the checklist is concerned. Even though I held the completion of the checklist pending the setting of the parking brake; due to the brake system logic the parking park should have been engaged immediately.

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Original NASA ASRS Text

Title: EMB175 flight crew reported the aircraft rollback shortly after parking at the gate.

Narrative: I was Pilot in Command of flight Aircraft X when the rollback situation happened on gate X at ZZZ. We touched down in ZZZ and taxied to our assigned gate. The gate was occupied when we arrived. We were instructed to hold on the taxiway until our gate became available. We held for about 25 minutes and then we were given a new gate from operations. We taxied to [new gate] and parked using the self-parking system; as there were no ramp agents present. Once we were parked; I shut the engines down and called for the shutdown checklist. All items on the shutdown checklist were completed with the exception of the parking brake. We held the shutdown checklist pending the setting of the parking brake. I was manually on the brakes with my feet firmly on the rudder pedals. We were in this position at the gate for about 5 minutes while the gate agent maneuvered the jet bridge to the aircraft. The aircraft suddenly started rolling back and it was brought to my attention by my First Officer. At first it appeared to me as an optical illusion caused by the moving jet bridge as I was; and had been; firmly on the brakes the entire time. When I realized that it was in fact the airplane that was rolling backwards; I instantaneously applied greater pressure to the rudder pedals and immediately applied the parking brake when I noticed the manual brakes were ineffective in stopping the aircraft's backwards roll. The aircraft was towed forward by ramp personnel to the proper parking line. The jet bridge was subsequently attached to the airplane and passengers were deplaned. At the time of the rollback; no ground equipment was attached to the aircraft. The aircraft was not chocked. Since at the time of the rollback no equipment was attached; a post flight walk around inspection of the aircraft confirmed no damage. The onboard ACARS recorded an 'in the gate' time of XA:36 pm local time. 39 minutes after touchdown. Suggestions: I was under the incorrect impression that the brake system would hold pressure even after engine shutdown. More specifically after hydraulic system deactivation. After the incident; while searching for clues as to why the airplane rolled back even while my feet were firmly on the brake pedals; the problem became clear. While the hydraulic system does hold pressure through the accumulators; this pressure is only available via the emergency/parking brake and not to the manual brakes via the rudder pedals. I understand that a 'loss' of hydraulic systems 1 and 2 renders the brakes inoperative. In my case; a 'loss' put me in the mindset of a catastrophic loss; when in reality a 'loss' occurs every time the engines are shutdown and system pressure is lost. As far as the checklist is concerned. Even though I held the completion of the checklist pending the setting of the parking brake; due to the brake system logic the parking park should have been engaged immediately.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.