What design feature must be included with gravity-feed fuel systems to function correctly?

Gravity-feed fuel systems rely on the force of gravity to move fuel from the fuel tanks to the engine. For this system to function effectively, it is crucial that the airspaces above the fuel in the tanks are interconnected. This design feature allows for the equalization of pressure, which is essential for preventing vapor lock and ensuring a consistent flow of fuel to the engine.

When tanks are interconnected, it means that as fuel is drawn from one tank, the other tank can provide air, thus allowing more fuel to flow by preventing vacuum formation within the tank. If the tank airspaces were isolated, a vacuum could form, hindering fuel flow and potentially leading to engine performance issues or fuel starvation. This interconnectedness stabilizes the system, ensuring reliable operation of the gravity-feed mechanism throughout various flight conditions.

In contrast, while the other listed features are important in specific fuel system designs, they are not essential for the basic functioning of a gravity-feed system. The presence of multiple fuel filters, electronic monitoring systems, or manual valves does not address the fundamental principle of gravity-fed systems, which is why the interconnected airspaces are the necessary design feature.