When routing a fuel line between two rigidly mounted fittings, what should be done?

When routing a fuel line between two rigidly mounted fittings, incorporating at least one bend is recommended primarily to accommodate movement and vibrations within the aircraft structure. Rigid fittings can cause stress concentration in a straight line configuration, especially during operations where the aircraft experiences various forces such as altitude changes, turbulence, or engine vibrations.

A bend allows for some degree of flexibility, which can help to manage thermal expansion and contraction of the fuel line material, as well as mitigate the risk of fatigue failure due to repeated stress cycles. It also provides some leeway in case of slight misalignments between the two fittings, ensuring that the line is not under constant stress, which would otherwise compromise its integrity.

In the context of the other options, a direct straight line may seem intuitively simple, but it can lead to potential issues as mentioned. Insulating the fuel line is beneficial for managing temperature but is not directly related to the physical routing between fixed points. While flexible tubing can be useful in certain situations, rigidly mounted fittings typically require rigid tubing with properly designed bends for effective and safe routing.