Eureka
I was cleaning the bottom of my pool this afternoon. Normally I attach the hose to it's location about 20cms below water level, open the waste valve (Located well below the pool floor level) and switch on the pump. A huge amount of costly water is lost in the time taken to clean the dirt from the pool bottom. So today I decided to follow the same procedure without the pump running. The pressure differential was still more than adequate to suck the dirt from the bottom with about 50% less water loss. The only problem I see in my A320 is if the gravity feed flow rate is too low for the demand from the engine, could a flameout result at a high thrust setting?