The fuel tanks are rigid tanks fitted in the wings and filled by the overwing method. The fuel is drawn from the tanks by a mechanical or electrical fuel pump through a tank selector and filter before being delivered to the carburettor.

Atmosphere Exclusion: Once the fuel is in the aircraft fuel tanks, the main source of water contamination is the atmosphere that remains within the tank. If the tanks are topped up to full then the atmosphere is excluded together with the moisture it contains, thus minimising the likelihood that the fuel will be contaminated.

Water is always present in fuel, the amount will vary according to the efficiency of the manufacturer’s quality control and the preventive measures taken during storage and transfer. Further measures can be taken to minimise water accretion once the fuel has been transferred to the aircraft tanks.

Water Drains: If the fuel can be allowed to settle after refuelling then the water droplets, being heavier than the fuel, will fall to the bottom of the tank and can then be drained off through the water drain valve.

The highest absorption of humidity in fuel can be observed in almost empty tanks.