Fuel System: Distribution, Balance, and Backup

Every flight depends on one critical resource: fuel. But managing fuel in the A320/A321 isn't just about having enough to reach your destination—it’s about understanding a sophisticated system that automatically balances loads, prevents structural damage, cools vital components, and maintains redundancy for safety. How does your aircraft ensure optimal fuel distribution while you focus on flying? The fuel system solves this through intelligent automation and multiple layers of protection.

How the System Works

Fuel Storage Architecture

Generally, the A320/A321 stores fuel in multiple interconnected tanks designed for both capacity and structural support. On the A320, the wing tanks are divided into inner and outer sections, with the outer tanks serving dual purposes: fuel storage and wing bending relief during flight. The center tank provides additional capacity, while Additional Center Tanks (ACTs) can be installed in cargo holds, extending range for long-haul operations.

Each wing includes vent surge tanks located outboard of the outer tanks or wing tanks, allowing for 2 % fuel expansion from temperature changes without spillage. This design prevents fuel loss during normal thermal cycles while maintaining proper venting and pressure relief throughout the system.

Engine Feed System Logic

The fuel feed system follows a strict hierarchy designed for efficiency and safety. Under normal conditions, each engine receives fuel from two pumps—either from the center tank or from its respective wing tank, depending on the fuel consumption sequence and aircraft type. All wing tank pumps run continuously throughout flight, while pressure relief sequence valves ensure center tank pumps take priority when multiple pumps operate simultaneously.

The crossfeed valve provides critical redundancy, allowing any pump to supply any engine. If both pumps in a wing fail, that engine can operate on gravity feed from the wing tank, provided you remain below the gravity feed ceiling—a crucial backup that maintains engine operation even with complete pump failure.

Managing Fuel Distribution and Transfer

Automatic Fuel Consumption Sequence

Your aircraft consumes fuel in a carefully orchestrated sequence that optimizes weight distribution and structural loads, for example, on the A320:

1. ACT Transfer (if installed): Additional center tanks transfer fuel to the center tank automatically after takeoff when the slats retract, provided the center tank high-level sensor has been dry for 10 min.

2. Center Tank: Supplies fuel to the engines until about 500 kg has been consumed from each inner tank.

3. Inner Tanks: Supply the engines until reaching 750 kg per tank.

4. Outer Tanks: Transfer fuel to the inner tanks to continue engine feed.

This sequence ensures optimal center of gravity throughout flight while maintaining structural wing loading. The system uses underfill sensors to trigger transitions between fuel sources, creating seamless fuel management without crew intervention.

Center Tank Transfer Logic

The Fuel Level Sensing Control Unit (FLSCU) manages center tank fuel transfer through an elegant jet pump system, e.g., on the A321neo. When the transfer valve opens, wing tank pumps create suction that draws fuel from the center tank. The system automatically closes the transfer valve when the wing tanks are full and reopens it after consuming 250 kg from the wing tanks.

In manual mode, transfer valves remain open, requiring crew intervention to prevent wing tank overflow. This becomes critical when the center tank is full or empty—the transfer switch must be turned off to prevent system damage.

ACT Operations

ACTs operate through pressurization using cabin air. During transfer, the ACT vent valve closes while the air shutoff valve opens, pressurizing the tank to push fuel toward the center tank. The system includes multiple configurations, e.g., on the A321neo:

• Single ACT: Transfers automatically after takeoff when conditions are met

• Dual ACT: ACT2 transfers first, followed by ACT1 when ACT2 empties

• Triple ACT: Forward ACT transfers first, then AFT2, finally AFT1

Transfer stops automatically when the center tank fuel reaches 5750 kg and resumes when it drops below 5000 kg, preventing overflow while maintaining optimal fuel distribution.

When Things Go Wrong

Fuel Leak Detection and Management

Fuel leaks present serious safety and operational challenges. The system provides multiple indicators: decreasing total fuel quantity, fuel imbalance development, unusual fuel flow patterns, or the dreaded "FUEL F.USED/FOB DISAGREE" alert. Your primary defense is vigilant monitoring—compare fuel on board plus fuel used against initial fuel loading every 30 min or at waypoints.

When leak indications appear, isolation becomes critical. Keep the crossfeed valve closed, turn off the center tank pumps, and ensure each wing feeds its respective engine. If one wing tank decreases faster, you've identified the leak location. For confirmed engine or pylon leaks, shut down the affected engine to stop fuel loss and prevent fire risk.

Fuel Temperature Management

Extended cruise flights, particularly at high altitudes, risk fuel freeze when the temperature approaches -47 °C for Jet A1. Fuel temperature gradually approaches Total Air Temperature (TAT), cooling at 3 °C per hour normally or up to 12 °C per hour in extreme conditions.

When fuel temperature approaches minimum limits, ECAM displays a caution. Corrective actions include descending to warmer air (a 4000 ft descent increases TAT by approximately 7 °C below the tropopause) or increasing the Mach number (a 0.01 Mach increase raises TAT by 0.7 °C). Allow up to one hour for fuel temperature stabilization after implementing corrections.

Fuel Quantity Fluctuations

Low fuel quantities (up to 6 t) create normal fluctuations during takeoff and climb. Acceleration and high pitch angles cause fuel movement within tanks, resulting in temporary FOB decreases for about 2 min, then increases before stabilizing within 6 min. Maximum fluctuation reaches 400 kg during normal operations and 300 kg during go-around. Higher fuel loads may show increases up to 1 t after takeoff before stabilizing on some aircraft types.

What This Means for You

Operational Implications

Understanding fuel system logic helps you make informed decisions during abnormal operations. When center tank pumps fail, you know inner tanks will automatically take over. When outer-to-inner transfer occurs early during descents, you recognize this as a normal system response, not a malfunction.

Fuel imbalance limits vary by flight phase and fuel quantity. At takeoff with full tanks, maximum imbalance is up to 600 kg, increasing up to 1500 kg in flight. These limits exist for structural reasons, but exceeding them during emergencies doesn't significantly affect handling—your aircraft remains fully controllable.

Refueling Operations

The refueling system operates automatically with preselected quantities, but electrical transients during power source changes can interrupt the process, requiring quantity re-entry. Refueling follows a specific sequence, e.g., on the A321: wing tanks first, then center tank if needed, and finally ACTs. The system includes multiple safety features, including high-level sensors and automatic valve closure to prevent overflow.

Performance Considerations

Fuel system design directly impacts performance calculations. The IDG cooling system recirculates fuel through heat exchangers, affecting fuel flow and temperature. During low thrust operations with high IDG oil temperatures, up to 600 kg/h may recirculate for cooling, impacting fuel planning on some aircraft types.

The fuel system represents sophisticated engineering that largely manages itself. It allows you to focus on flying while ensuring optimal fuel distribution, structural protection, and operational safety. Understanding its logic helps you recognize normal operations, identify abnormalities quickly, and make informed decisions when the unexpected occurs.

How the A320 fuel system manages distribution, transfer sequencing, and balance automatically – plus leak detection, temperature limits, and failure logic.