Date: 201901
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: EMB ERJ 170/175 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Normal Brake System
Component problem: Design; Improperly Operated
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Human-Machine Interface; Confusion; Situational Awareness
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: Aircraft In Service At Gate
Result: Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Human Factors
Primary problem: Human Factors

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.


Second narrative: We taxied into the gate by an electronic marshal. After the captain stopped the aircraft and began to perform his duties; the captain called for the SHUTDOWN check list. I took out the checklist card and read so the captain could confirm. I called out; PARKING BRAKE. Captain responded; do not remember exact phrasing; he was holding on setting the brake but to continue on with the check list. I read through all items then placed the checklist on my lap and said holding for brake. The gate agent had begun to maneuver the jetway towards the airplane and as I watched then the jet way seemed to move forward of the flight deck. I said to the captain; I think we are rolling. The aircraft then came to a sudden halt. I asked the captain did he set the parking brake and he confirmed that he set the brake. I put away the completed checklist and for a short time I thought perhaps the gate agent had moved the jetway forward of the flight deck which gave an illusion that we had moved back. When a gate agent signaled she could not reach the passenger door I realized we had rolled back. I asked a ramp worker where we were positioned and he said we were behind the E175 line. We asked him to tug us forward so the gate agent could finish moving the jet bridge to the passenger door and deplane the passengers.


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