Date: 201506
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Dusk
Aircraft: Regional Jet 200 ER/LR (CRJ200)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown; Ground Event / Encounter Other / Unknown
Detector: Person Passenger
When detected: Taxi
Result: Flight Crew Returned To Gate; General Maintenance Action
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: We were delayed for departure because of a late arriving aircraft. When our airplane arrived the [inbound] Captain advised me that the engines needed oil and called maintenance to coordinate that. Oil was put in the aircraft engines and the maintenance can was signed off and the aircraft was put back in service. The First Officer had completed his aircraft external preflight and the aircraft was being refueled. While we were doing our preflight preparation and checks in the flight deck; we noticed a fuel imbalance of between 900 and 1;000 pounds. The fueler approached us shortly after and mentioned the problem he had balancing the fuel. He asked us if he needed to over-wing fuel the left side. We told him that we had enough fuel and the fuel system was cross feeding to bring both tanks into balance and that it would not be necessary. What we did not know; however; was that he had over-wing fueled the right side without bringing it to our attention. There were no MELs for the fuel system and it was working fine from our vantage point. The First Officer checked the single point fuel cap and the fuel panel as per the SOP; the fuel was balanced and we finished up our checks and proceeded. On the taxi; the flight attendant called the flight deck and advised us that a passenger saw something fall off the right wing. We asked Ground Control to pull off somewhere to further investigate and as soon as we stopped; the First Officer went into the cabin to investigate and look at the right wing. He discovered the over-wing fuel cap was not secure. He came back to the flight deck; told me and we communicated to operations and then Ground Control that we would need to return to the gate and requested a fueler to be on hand. We did the shutdown checks after arriving at the gate; opened the door; I apologized to the passengers about the incident; and advised the fueler of the situation; the cap was secured; we added a couple hundred  pounds of fuel; did our checks; and proceeded the reminder of the flight and arrived safely in [destination]. Two of the passengers stated they would be filing reports to the FAA. One; who was very angry and a retired doctor; who did not provide a name asked if we had risk management programs in place and reporting programs to which I responded that we did.The fueler over wing fueling the airplane with no fuel system problems and not consulting the Captain prior was one factor. Another was poor communication after the aircraft was refueled. It could have been stated more clearly that the aircraft had been refueled over wing on the right side. Also I as a Captain; having seen the imbalance could have inquired better as to why we had the imbalance and determine what had been exactly done. I have never had a fueler decide to over-wing fuel an airplane that had no MELs on the fueling system without first asking me. Having known that over wing fueling had been done would have allowed us to be more aware of the need in this case to check the relevant over wing fuel cap.With significant fuel imbalances; more thorough investigation by the flight crew to more thoroughly understand the general processes that led to the imbalance would have helped to promote better awareness of the situation and allow for better mitigation of potential problems. Also; despite having done a preflight already; another look at the other fuel caps in addition to those normally used; i.g. the single point fuel cap and the external fuel control panel; could have further mitigated these problematic situations.


Second narrative: The fueler approached us in the flight deck and mentioned that he had completed fueling; but it was imbalanced. We noted a roughly 900 pound right wing heavy imbalance. We interpreted this as his mistake; and did not note any mechanical abnormalities with the aircraft. He asked us if we wanted him to over wing fuel us to even it out. The aircraft was already balancing the fuel load and we had the total fuel that we needed; so we told him no. After this conversation I went outside and checked the fuel panel door and the single point refueling cap and door. All was normal. We continued our normal preflight actions. At departure time our fuel load was balanced; we pushed back from the gate; and started our taxi.


Synopsis: A CRJ-200 flight crew reported several passengers saw an over wing fuel cap fall off the wing during taxi out. The flight returned to the gate to have the cap re-installed.