A header tank is a small fuel tank fitted between the main tank and the engine carburetor, whose job is to ensure consistent fuel delivery. The header tank has it's own short clunk line, so any small amounts of air picked up from the main fuel tank clunk is released into the header tank and hopefully not passed on to the engine, which could cause it to flameout.
When fuelling the helicopter, the fuel is introduced via the header tank so that it completely fills with fuel and then overflows into the main tank. During operation, the engine draws fuel from the header tank, which then continuously refills from the main tank. Any air picked up from the main tank (for example, towards the end of the tank, or during violent maneuvers) will be trapped in the header tank and hopefully not picked up from the header tank clunk.
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