Every pre-flight walk-around includes a step during which the pilot must drain a sufficient amount of fuel into a container to check for the presence of water in the tanks. Many an aeroplane accident has resulted from water entering an engine. Therefore, if any trace of this undesirable hydrous component exists, ridding it from your tanks is crucial prior to a flight. Once eliminated, you are safe to fly ... or are you?
A recent Transport Canada Service Advisory addressed an issue of which few pilots may be aware: there is an inherent tendency for hydrocarbon fuels to retain water. Thus, aviation fuels can contain minute quantities of dissolved water irrespective of processes used to remove it, whether at the refinery stage or right through to its residency under ground at a filling pump. As the Service Bulletin states: “In-service experience reveals that numerous difficulties and accidents have been traced to blockages of airframe and engine fuel systems by frozen contaminations of water, precipitated from the fuel stream.”
Fuel tank sumps sometimes freeze in cold weather. If nothing can drain from the sump, it may be that water in the fuel has turned to ice and blocked the sump. Above freezing, and because water is heavier than fuel, water sinks to the bottom of the tank where it can be removed with normal sump drainage. However, at temperatures below freezing, minuscule water droplets in the fuel sometimes freeze into tiny ice crystals and remain suspended in the fuel. Normal draining of the sumps will not remove these ice crystals which may then collect in such places as fuel filters and selector valves, restricting the flow of fuel.
The accident that lead to the issuance of this particular Service Bulletin involved an Aero Commander, a well-known high-wing twin- engined aircraft, that began to suffer abnormal fuel flow indications, combined with dropping rpm, on its right engine during a flight. Shortly after the first engine began to show signs of trouble, the left engine began behaving in the same way. The crew, unable to maintain altitude, was required to do a forced landing that ended fortuitously with no fatalities, though a badly damaged and irreparable aircraft was the result. The investigation into the accident revealed that the fuel supply to both engines had been blocked. The blockage was attributable to traces of ice that affected the fuel injection system on the aircraft’s Lycoming engines.
Any small amount of dissolved water, as previously stated, can be present in fuel, though not in particulate form. As an aircraft gains altitude, the outside air temperature goes down. The fuel's capacity to hold water goes down too. Particulate water can separate out, thereafter becoming a serious problem if it freezes, blocking fuel flow and shutting down the engine. Tests conducted by the Transportation Safety Board of Canada have found that the ice resulting from as little as two small drops of water can have a nefarious effect on an aircraft engine’s fuel injection system.
Anti-icing additives can cure the problem, but they must be used with caution and only as exactly specified by the manufacturer. Such additives have the effect of lowering the freezing point of water in the fuel to a temperature point below which the aircraft is not likely to encounter during flight. If incorrectly used, however, anti-icing additives may damage fuel tanks and filters, carburetors and fuel injectors.