Fuel tanks on Airbus aircraft are designed so that fuel spills into the vent tank through their vent line in the event of a tank overfill. Each vent tank is equipped with a NACA duct where fuel can flow out  and onto the ground if the vent tank is full. 

On A320 family aircraft, the refuel valve for a fuel tank will automatically close when the targeted fuel quantity is reached, or fuel is detected by the high level sensor fitted in the tank during refuelling. 

(fig.1) Functional schematic of the refuelling and venting system of an A320 aircraft

A monitoring function was first introduced on A321 aircraft to trigger a FUEL L(R) WING TK OVERFLOW ECAM alert in the case of a fuel overflow detected in the vent tank by the overflow sensor. The ECAM alert (Modification 154427) is now also installed in production on A319/A320 aircraft since May 2013 (aircraft serial number 5597 and onward). It can be installed on previously built A318/A319/A320 aircraft with Service Bulletin A320-28-1216.

Transfer jet-pumps were introduced on the A321 aircraft at its entry into service to simplify the fuel system, replacing the center tank electrical fuel boost pumps previously used on A320 family aircraft (fig.2). To take advantage of this simplification, the transfer jet-pumps were introduced in the center tank of A319ceo and A320ceo aircraft delivered from 2014. and are standard on all A319neo, A320neo, and A321neo aircraft. 

(fig.2) Comparison of the fuel feeding system of A320 family aircraft equipped with or without fuel transfer jet-pumps

The center tank transfer mode selector should be in the AUTO position (no light) during the refuelling operation. This means the left and right transfer valves will remain closed. The fuel pumps and the Left and Right CTR TK XFR pushbutton switches can be ON or OFF. The switches will be OFF (no light) if the flight crew has already performed their cockpit preparation.  

The center tank and the outer wing tanks are filled first in a normal refuelling operation. Once the outer tank is full of fuel, the inner wing tank will be filled with fuel passing through the spill pipe (fig.3). 

(fig.3) Normal refuelling on A320 family aircraft equipped with transfer jet-pumps (Cockpit preparation already done in this example)

Cases of fuel spillage were reported to Airbus during the refuelling of A320 family aircraft equipped with transfer jet-pumps in the center tank. The transfer mode selector was left in the MAN position during the refuelling operation of the reported events. This condition forced the transfer valves to open and fuel was supplied to the jet-pumps (fig.4). With the jet pumps ON and fuel flow provided by the fuel pumps, fuel was moved from the center tank to the wing tanks. Even though the refuel valves automatically closed when the preselected quantity of fuel was reached, the transfer valves remained open and the jet-pumps continued to transfer the fuel out of the center tank into the inner tank. When the wing tanks were completely full, fuel was pushed through the wing tank vent lines into the vent tanks and eventually overflowed from the NACA vent causing fuel to spill to the ground. 

(fig.4) Fuel spillage due to mode selector left on the manual position on A320 family aircraft equipped with transfer jet-pumps

The parking SOP requests the flight crew to set the fuel pumps and center transfer valves OFF. This means that the fuel pumps and the Left and Right CTR TK XFR pushbutton switches should be in the OFF position. The fuel mode selector should be in AUTO mode.

(fig.5) Configuration of the FUEL Overhead panel as per PARKING SOP

COCKPIT PREPARATION procedure

Before the next flight, the flight crew must extinguish all white lights from the overhead panel during the cockpit preparation following the “dark cockpit” philosophy. The fuel pumps are switched ON and the Left and Right CTR TK XFR pushbutton switches should be pushed to extinguish their OFF light (fig.6). If the flight crew observes the FUEL MODE SEL MAN light ON, it must be switched OFF at this stage.

(fig.6) Configuration of the FUEL Overhead panel as per COCKPIT PREPARATION SOP (all lights OFF)

In addition, SOP requests specifically to check that the FUEL MODE SEL is set to AUTO in the FUEL section of the cockpit preparation  (fig.7). A dedicated caution warns the flight crew of the potential fuel spillage if the FUEL MODE SEL is set to MAN position on ground with the CTR TK L XFR and TR TK R XFR pushbutton switches not in the OFF position.

(fig.7) Extract from the A320 FCOM PRO-NOR-SOP Cockpit preparation - Fuel

In the event of a fuel spillage, Airbus recommends the following actions:

1. Perform TSM Task “Refuel - Fuel Spillage From NACA Vent During Refuel’ to identify the root cause of the spillage
2. Notify Airbus about the fuel spillage event and provide the following information:

• Fuel Spillage Sheet (available in the TSM task)
• Fault Reports
• Shop Reports of replaced components if any, including Part Number and Serial Number.