Refer to figures.
The twin engine helicopters usually have:

• two tanks which are called “feeder” or “supply” tanks; or
• two groups of fuel tanks respectively for each engine, with the relevant supply tank included located in various parts of the aircraft, such as the main fuselage or the sponsons.

The corresponding tanks of each group can feed each other through several types of valves, while each group is designed to ensure the supply of the related supply tank which in turn supplies the engine (s) with clean fuel.

Thus, the groups are interconnected through a fuel system that allows fuel to be transferred manually or automatically mainly between the supply tanks and by extension to the whole groups during the helicopter’s normal operation.

The process of transferring fuel from one tank to another or from one group to another, to ensure that the fuel levels are balanced and to prevent fuel exhaustion in one of the “feeder” or “supply” tanks is called “cross-feeding”.

However, in case of an engine failure in order to exploit the total amount of fuel through one engine operation, the pilot must operate it manually.

The ability to cross-feed is important in ensuring that the helicopter has an adequate fuel supply and is able to maintain stable flight, even if one of the fuel tanks becomes low on fuel.

Regarding the helicopter’s engines, they are supplied individually from their respective fuel tank group through booster pumps which are located in the corresponding “feeder” or “supply tank”.

Therefore, a fuel cross-feed system allows any engine to be fed from any fuel supply tank and by extension from any fuel tank.