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A fuel tank where the fuel pickup (the ‘clunk’) is at the end of a flexible tube. Gravity ensures that the ‘clunk’ is always at the bottom, enabling
fuel to be collected whatever the orientation of the fuel tank. Clunk tanks are notorious for getting twisted up resulting in a restricted or no fuel being
If an aircraft undergoes a heavy landing the clunk often flies forward and gets twisted up around the vent and filler pipes. It should be noted that the filler and vent/pressure pipes point upwards and away from the fuel level during normal flight, pointing the pressurisation pipe into the fuel can introduce bubbles into the fuel. Bubbles may also be introduced due to severe engine vibration. The resultant frothing of the fuel may also cause fueling problems. A possible fix is to provide some isolation between the engine and the airframe. However, first check that the engine is running correctly and that the propeller is balanced.