Which of the following sources of heat can a fuel heater use?

Fuel heaters are designed to ensure that the aircraft's fuel remains at an optimal temperature to prevent issues such as fuel icing and ensure proper flow characteristics. The correct answer indicates that a fuel heater can utilize both engine bleed air and engine lubricating oil as heat sources.

Engine bleed air is taken from the compressor section of the engine and is extremely hot. This air can be directed to the fuel heater to warm up the fuel before it enters the engine combustion chamber. This is particularly important during flight at high altitudes, where fuel can cool significantly.

On the other hand, engine lubricating oil also provides a source of heat. When the engine is running, the lubricating oil gets heated up through contact with engine components. A fuel heater can tap into this heat to ensure the fuel is adequately warmed prior to combustion.

Using both sources allows for increased flexibility and efficiency of the fuel heating system, particularly in varying flight conditions. This dual capability of heat sources therefore enhances the overall performance and reliability of the aircraft's fuel system.