Mastering Fuel Temperature

Why fuel temperature is critical

Fuel is integral to engine performance, and its temperature directly affects its viscosity: the resistance to flow or ‘thickness’ of the fuel. At optimal temperatures, fuel flows seamlessly into the engines, enabling smooth combustion. However, deviations from these ideal temperatures can cause significant issues.

Impact of low fuel temperature

When fuel temperature drops too low, its viscosity increases, making it thicker. Thick fuel doesn’t flow as easily, which can cause difficulty during the engine start-up process. Thicker fuel may struggle to flow through fuel lines or engine components, potentially preventing engine start-up. At extremely low temperatures, fuel can become viscous enough to cause flow issues, and while solidification is rare under normal operational conditions, it remains a serious safety concern.

Additionally, low fuel temperature can also cause engine components, such as fuel nozzles and pumps, to freeze, potentially causing damage or wear over time. These scenarios highlight the importance of monitoring fuel temperature and taking action if it reaches dangerously low levels.

Impact of high fuel temperature

On the other hand, when fuel temperatures are too high, the fuel becomes thinner, which can negatively affect atomisation in the engine. Proper atomisation is essential for efficient combustion, and poor atomisation results in incomplete burning of the fuel. Improper atomisation results in incomplete burning of the fuel, leading to inefficiency, wasted energy, and increased wear on engine components.

Higher fuel temperatures lead to reduced viscosity, which can accelerate wear on engine components such as fuel nozzles and pumps, leading to potential damage over time. In the worst-case scenario, extremely high fuel temperatures could lead to engine stalls, potentially compromising the safety of the aircraft.

Fuel temperature limitations for A320 variants

Each aircraft variant of the A320 has different fuel temperature limitations. Understanding these thresholds and knowing when to take action is critical for flight safety.

Low fuel temperature

For all A320 family variants, a drop in fuel temperature below -40 °C in any tank triggers an ECAM Advisory.

An ECAM Caution is triggered when fuel temperature drops below:

• -43 °C for A319/A320 in either outer or inner tanks
• -44 °C for A321 wing tanks

When the fuel temperature approaches critical thresholds, we are alerted to take corrective actions. These actions include monitoring fuel temperature and taking steps to warm the fuel, such as adjusting altitude or reducing the risk of additional cooling by altering flight parameters. More on that later.

High fuel temperature

High fuel temperatures are less common but can occur, especially operating around regions like the Middle East and Africa. The A320 fuel system is designed to accommodate temperature variations, including a rise of up to 2% in fuel volume as the temperature rises. However, excessive fuel temperature still poses a risk.

An ECAM Advisory is triggered when the fuel temperature exceeds:

• 55 °C for the A319/A320 outer tanks
• 45 °C for A319/A320 inner tanks and A321 wing tanks

An ECAM Caution is triggered when fuel temperature exceeds:

• 60 °C (in-flight) or 55 °C (on the ground) for the A319/A320 outer tanks
• 54 °C (in-flight) or 45 °C (on the ground) for A319/A320 inner tanks and A321 wing tanks

How we manage fuel temperature in flight

Managing fuel temperature throughout the flight requires careful attention, as the fuel’s temperature changes during various phases of flight.

Taxiing and engine start

During the taxiing phase, engine heat can raise the temperature of fuel, particularly in the inner tanks. We can mitigate this by using techniques such as alternating engine use, allowing one engine to cool while the other remains active. Reducing engine power while taxiing helps minimise the heat generated by the engines, particularly when ambient temperatures are high.

We should remain vigilant when taxiing in warmer temperatures, ensuring fuel levels are appropriately managed to avoid exceeding the maximum temperature limits.

Climb and cruise phases

As the aircraft climbs to higher altitudes, the air temperature drops. This causes the fuel temperature, particularly in the outer tanks, to decrease. We can mitigate this by adjusting our indicated airspeed. Increasing airspeed during climb increases the Total Air Temperature (TAT), which warms the fuel. The TAT is the air temperature adjusted for the heating effect of high-speed flight and is crucial because it directly affects the fuel temperature in the tanks by heating the air that flows around the aircraft.

During cruise, fuel temperature typically stabilises, but fluctuations may still come with weather systems, particularly cold fronts. We should monitor the fuel temperature closely and adjust flight parameters if needed. If the fuel temperature drops below -40 °C during cruise, a descent to a warmer air mass may be necessary to bring the fuel temperature back within safe limits.

Descent and landing

Descending into warmer air during the descent phase can naturally raise the fuel temperature, especially in the inner tanks near the engines. If necessary, we can adjust engine power to prevent fuel from overheating, especially during a rapid descent. It’s important to monitor the fuel temperature closely during this phase to avoid exceeding the temperature limits.

Dealing with low fuel temperature

If the fuel temperature falls too low, several actions should be taken to manage the situation:

1. Descent to Warmer Air: Descending can help raise the air temperature, which directly affects the fuel temperature. Below the tropopause, a descent of approximately 4,000 feet raises the total air temperature by around 7 °C (FCTM), which can be enough to prevent the fuel temperature from dropping to unsafe levels. The actual change depends on the atmosphere on the day, and in severe cases a descent to as low as FL250 may be required.
2. Adjust Mach Number: Increasing the aircraft’s Mach number also increases the total air temperature. A small change in Mach number (such as an increase of 0.01 M) can raise the total air temperature by about 0.7 °C, helping to warm the fuel.
3. Monitor and Adjust for Fuel Penalty: Although descending or changing Mach number can increase fuel temperature, we must consider the associated fuel penalty and make decisions that balance fuel efficiency with safety.

Dealing with high fuel temperature

High fuel temperatures, although less common, can still pose challenges, especially during hot weather or when the aircraft is filled to capacity. If fuel temperature starts to exceed 40 °C in any tank, we should closely monitor the situation and take appropriate action.

On the ground (A319/A320), when the caution triggers at 55 °C in an outer tank or 45 °C in an inner tank, taxi time is limited: takeoff may be performed until the fuel reaches 60 °C in the outer tank or 54 °C in the inner tank. At those values, takeoff must be delayed, and the ENG MASTER switch of the affected side is set OFF.

The procedure first asks us to switch the GALLEY pushbutton OFF (if fitted to your variant). The GALLEY pushbutton controls the electrical supply to the galley equipment (such as ovens, coffee makers, and other kitchen appliances). Switching it OFF is not about the galley’s own heat: it reduces the electrical load on the generators, so the IDGs produce less heat. The IDG oil is cooled by the fuel in its heat exchanger, and that warmed fuel returns to the tank, so a lower load means less heat going into the fuel. As the FCOM puts it: “Reducing electrical loads reduce heat emitted by IDG.”

If only one side is affected by high fuel temperature, we can adjust the fuel flow to manage the temperature. Disconnecting the auto-thrust system and adjusting the thrust lever increases fuel flow through the IDG (Integrated Drive Generator) oil heat exchanger, which helps reduce the temperature of the fuel. If the temperature exceeds 65 °C in an outer cell or 57 °C in an inner cell in flight, the ECAM will direct us to start the APU (if available) and utilise the APU Generator to allow an IDG disconnection on the affected side.

Conclusion

Effective fuel temperature management is critical to ensuring both the safety and operational efficiency of the aircraft. By understanding how fuel temperature affects engine performance, knowing the limits for each variant, and employing practical strategies for managing temperature, we can navigate various challenges that may arise during different flight phases. The actions you take in response to temperature changes can significantly impact the aircraft’s overall performance and safety, making fuel temperature a critical factor in aviation operations.