How is the fuel warmed to reach 5 degrees or greater?

• A. Fuel/Oil heat exchanger
• B. Aircraft fuel heater
• C. Engine bleed air heater
• D. Fuel warming via ambient air

Answer: (A)

Warming the fuel to at least 5°C is done to prevent wax formation and ensure proper fuel viscosity for reliable pumping in cold conditions. The fuel/oil heat exchanger achieves this by using hot engine oil to transfer heat to the fuel as it flows through the exchanger. Since the engine oil is already circulating at elevated temperatures, it provides a steady, reliable source of heat that raises the fuel temperature before it reaches the pumps and fuel control unit. This method works well across a wide range of ambient temperatures and engine conditions, and it relies on the engine’s own heat rather than ambient air or additional heater sources. Other options exist, but they’re either less reliable in cold, low-speed conditions or add extra complexity and bleed-air usage.