Date: 201909
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: A320
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Taxi
ATC / advisory: Ground ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: Taxi
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Gate; General Flight Cancelled / Delayed
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We were taxiing out in ZZZ and had an extended delay on the ground due to thunderstorms in the area. Initially we taxied on only the number one engine. After about 20 minutes; we started the second engine. About 20 minutes later after getting a second reroute from ATC; the number 2 engine was shut down. Shortly thereafter the advisory system activated the fuel system display on the lower ECAM (Electronic Centralized Aircraft Monitor) page for a fuel imbalance. The center tank and left main tanks were full; and the right main tank indicated about 7900 pounds. I expected a lower fuel value in the left tank since we had run the APU and number one engine for an extended time; so this did not make any sense. We were in the middle of reprogramming the FMGC (Flight Management Guidance Computer) when the ECAM for Fuel Auto Fuel Feed Fault appeared. As we were doing the ECAM; the aircraft behind us reported fuel leaking from the left wing. ATC called the fire equipment; and within three minutes they were surrounding the airplane. We started the APU (Auxiliary Power Unit); and once it was online; the number one engine was shut down. I called the flight attendants and told them the fire trucks were on their way; and we are not evacuating. I then made an announcement to the passengers. Once the fuel pumps were turned off; the fuel leak stopped. After discussions with operations and the emergency personnel; we decided to tow the airplane to the gate. Once at the gate; the passengers were deplaned via the jet bridge. After some calculations and talking to ZZZ Maintenance; we determined that right main tank leaked into the center tank. Once the center tank was full; fuel got rerouted to the left main tank. Once that was full; fuel poured out the left wing ventilation system. We estimated that we lost about 1800 pounds of fuel on the taxiway.


Second narrative: Extended taxi in ZZZ (1 hour). We had pulled off onto taxiway W short of U to get a reroute from clearance. An aircraft behind us transmitted on ground frequency that we had a large fuel leak coming from our left wing. Initially we had no ECAM (Electronic Centralized Aircraft Monitor) or other warning/knowledge of this event. The CA (Captain) pulled up the fuel page and we saw a large discrepancy in the fuel totals of the wing tanks. I started the APU (auxiliary Power Unit) after confirming with the CA; because without it; the aircraft would have no electrical power once we shut the #1 engine down (#2 was not running). The Ground Controller rolled the trucks and hazmat team. We ran the ECAM; made sure the engines were secured and turned off all fuel pumps. We arranged for a tug to come to the aircraft and worked with operations to find us a gate to return to. We were towed back to the terminal; the passengers deplaned; and the maintenance team met us at the aircraft. CA worked with Maintenance and wrote up the malfunction.


Synopsis: Flight Crew taxiing A320 encountered fuel spill from wing vent system.