A320 Fuel Abnormals: Leaks, Imbalance and Gravity Feed

The A320 fuel system normally runs itself: pumps, valves and level sensors use the fuel in a set order and the crew mostly watch. The fuel abnormals are the cases where that order breaks down. Fuel can leave the airplane through a leak, one wing can become heavier than the other, a tank can run low, pumps can fail so that fuel must be drawn by gravity or becomes unusable, and refueling can stop or overfill a tank. Each has its own indications, and confusing one with another, above all a leak with an imbalance, has caused accidents.

The normal system is described in A320 fuel system, the general principles in fuel feed, boost pumps and crossfeed. Figures are those of the A320 FCOM for aircraft with electric center tank pumps that feed the engines directly. On the A320neo, and on A319ceo and A320ceo aircraft delivered from 2014, transfer jet pumps move the center fuel into the wing tanks instead, as on every A321, so the center tank items differ.

Fuel monitoring in flight

Abnormals are found by monitoring. The upper ECAM display shows the fuel on board (FOB), including any additional center tank (ACT). The FUEL page shows each tank and the fuel used (FU) by each engine, counted from engine start on the ground and frozen at shutdown; if fuel flow data is lost for more than a minute, its last two digits are dashed. The basic check is that fuel on board plus fuel used stays consistent with the fuel on board at departure; at the parking stand an unusual discrepancy calls for maintenance action.

The FOB display has its own codes. Two dashes across its last two digits mean that the fuel quantity indication (FQI) is degraded: the crew call the FUEL page to see which tank is affected and allow for the reduced accuracy in their planning. An amber half-box around FOB means that not all of the fuel displayed is usable.

On the FUEL page a wing pump in line and green has normal pressure, amber LO means low pressure and an amber crossline means its contactor is off. A center pump crossed in green has been stopped by the automatic logic; crossed in amber, it is off when it should run. An amber X FEED symbol means that the valve disagrees with its pushbutton, and an LP valve drawn in line and amber is open although its ENG MASTER is OFF. Fuel temperature brings an advisory in flight phases 2 and 6 above 45 °C in an inner tank or 55 °C in an outer tank, or below −40 °C, and turns amber with an ECAM caution beyond the certified limits; with JET A, whose minimum is −36 °C, the crew watch the FUEL page once TAT reaches −34 °C.

Fuel leak: detection and procedure

A fuel leak is fuel leaving the system other than through the engines' combustion: from a tank, a pipe, an engine or a coupling. It is dangerous twice over, because the fuel is lost and because fuel spraying near an engine can ignite. It is detected first by figures: fuel on board plus fuel used falling short of the departure figure, fuel on board decreasing faster than the fuel flow explains or departing from the flight plan prediction, or an imbalance that grows for no reason. A crew member may also see fuel streaming from a wing or an engine.

The trap is to treat the leak as an imbalance. A leak on one side makes that wing lighter, and the reflex is to open the crossfeed and feed both engines from the heavier wing. If the leak is downstream of the crossfeed, that sends the good side's fuel out through it as well; in 2001 an A330, Air Transat Flight 236, ran out of fuel over the Atlantic in this way. The certification basis of the A320neo, Special Condition F-13, names fuel leaks "incorrectly managed (management of the fuel imbalance instead of the fuel leak)" among the causes of fuel starvation. It requires the fuel system to alert the crew to abnormal fuel management and to possible leaks in the tanks, lines, components and engines. Means EASA has accepted for similar conditions include a fuel used/fuel on board disagree alert, a lateral imbalance alert and a comparison between the engine fuel flowmeters.

The rule is therefore to confirm and locate a leak before using the crossfeed. If the leak is from an engine, the procedure can lead to shutting that engine down. The crew then plan for the fuel that will actually remain and divert as needed. MINIMUM FUEL tells ATC that the aircraft is committed to one aerodrome and can accept no further delay; MAYDAY FUEL is declared once the calculated fuel on landing at the nearest suitable aerodrome would be less than the final reserve (see refueling safety and fuel emergencies and in-flight fuel management).

On the ground, a fuel leak from the engine drain mast reported during the start is handled differently: the engine runs at idle for 5 minutes. If the leak stops, the aircraft can be dispatched; if not, the engine is shut down and maintenance investigates.

Fuel imbalance and crossfeed

A fuel imbalance is a difference in quantity between the two wings. The FUEL page shows an advisory in flight phases 2 and 6 when the wings differ by more than 1,500 kg, the heavier wing's quantities pulsing. The FCOM limits are:

After a fuel system failure these limits may be exceeded without significant effect on handling.

The crossfeed valve, driven by a double motor, lets one side feed both engines. Pressing the X FEED pushbutton opens it: ON comes on white, and the green OPEN light when the valve is fully open. The FUEL X FEED memo is green while the pushbutton is ON and the valve not fully closed, and amber in the takeoff phases 3 to 5. With no leak, the imbalance is corrected by opening the crossfeed and switching off the wing tank pumps on the lighter side, so that the heavier side feeds both engines; once the wings are balanced the pumps go back on and the crossfeed is closed.

Low level and outer tank transfer

Each wing tank has a low level sensor of the fuel level sensing control unit (FLSCU). When a wing tank holds less than 750 kg, the wing tank low level warning comes on. It is independent of the FQI, so it also cross-checks the gauges. The minimum fuel for takeoff is 1,500 kg, and no wing tank low level alert may be displayed for takeoff.

The same level, about 750 kg in an inner tank, triggers the outer tank transfer. Two transfer valves in each wing open and the outer tank fuel drains into the inner tank; the valves latch open until the next refueling and the OUTR TK FUEL XFRD memo appears. The 750 kg assumes level flight without acceleration: in a steep descent, or when accelerating or decelerating, the transfer can start with more fuel in the inner tanks, and the low level warning can come on. If both transfer valves of one wing fail to open, that outer tank's fuel is unusable: its quantity is boxed amber and FOB half-boxed.

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Gravity feeding and its altitude limits

Each inner tank has two pumps for its own engine. A wing pump's amber FAULT light, with an ECAM caution, means that its delivery pressure has dropped; it does not come on when the pump is selected OFF. If both pumps of an inner tank fail, the engine can still draw fuel from that tank by gravity feeding, through suction valves that pump pressure normally holds closed, or the crossfeed can supply it from the other side. The center tank pumps have no suction valves, so center tank fuel cannot be gravity fed.

Gravity feeding is limited in altitude, as any suction feed is. Jet fuel holds dissolved air, which comes out of solution as the pressure falls in the climb. In a line fed by suction alone this air can collect and restrict the flow, and the low pressure at altitude can make the engine-driven pump cavitate, so at high altitude thrust can deteriorate or the engine flame out. Feeding by suction is therefore a lower-altitude means of keeping the engine running.

Pump failures and fuel penalty factors

A center pump's FAULT light means low delivery pressure while it runs. If both center pumps fail or are switched off, the wing pumps, which run throughout the flight, take over automatically, but the fuel left in the center tank is unusable: its quantity is boxed amber and FOB half-boxed. The MODE SEL FAULT light comes on when the center tank holds more than 250 kg while a wing tank holds less than 5,000 kg, a sign that center fuel is not being used. The ACT FAULT light, with the ACT pushbutton in AUTO, means that the center tank holds less than 3,000 kg while an ACT still holds more than 250 kg: the transfer is not working. FWD starts a manual transfer; the crew return the pushbutton to AUTO when the ACT is empty, or earlier if the center tank becomes full, to prevent an overflow.

Fuel temperature can also upset the feed. With JP4 or JET B and a wing fuel temperature at engine start above 30 °C, 40 °C or 49 °C, altitude is limited to 35,000 ft, 30,000 ft or 25,000 ft until the center tank is empty. Otherwise, at high altitude the center pumps would deliver less pressure than the wing pumps and the center first sequence would be lost.

Unusable fuel cannot be planned with: endurance and diversion decisions rest on the usable quantity alone. A failure may also raise the consumption. A fuel penalty factor is the increase in fuel consumption that the operator's documentation associates with a failure or an abnormal configuration, such as a landing gear that stays down. The EFB In-flight application computes time, fuel, distance and altitude for the actual configuration, and a dispatch under the MEL may require extra fuel (technical log, MEL and deferred defects).

Refueling abnormals

Automatic refueling can stop early. Switching between APU, external and engine electrical supplies can cause an electrical transient that halts the process: the green END light flashes, and the preselected fuel quantity must be entered again. Each refuel valve closes at the preselected load or when a high-level sensor detects a full tank, lighting its blue HI LVL light. Setting the TEST switch to HI LVL checks the sensors and their circuits: the HI LVL lights come on, or go out if the tanks are already full.

Overflow is the other risk. With MODE SEL at MAN, the system no longer stops a center pump when an inner tank is full, so a wing tank can overflow on the ground if the crew leave the center pumps on. Overflow shows as the inner tank quantity turning amber on the FUEL page. Excess fuel runs into the vent surge tank, which catches spillage, but an overfilled system can still spill fuel onto the ground.

Battery power refueling is possible when only the batteries are available. Momentarily setting the BATT POWER switch to ON makes HOT BUS 1 supply the FQI, which tests itself for about 40 seconds before refueling can be selected. The supply cuts off after 10 minutes without a refuel selection, or at the end of refueling. If the refuel panel is unserviceable, the wing tanks can be filled by gravity through filling points on top of the wings.

The APU may be started or shut down during refueling, but not started again after a failed start or an automatic shutdown, and a fuel spill calls for a normal APU shutdown. The HF radio must not be used while refueling or transferring fuel.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.