reasonable compromise when the proper auto gas octane is  not available. Manufacturers of two-stroke engines and four-stroke engines  used on WSC aircraft typically recommend the use of 89  octane minimum auto fuel for their engines. Additives are  put into auto gas primarily to reduce harmful emissions rather  than boost performance. The additives are supposed to be  listed at the pump, but the accuracy of this posting should  be questioned. Clean fuel is imperative for the safe operation of a WSC  aircraft. Of the accidents attributed to powerplant failure from  fuel contamination, most have been traced to: Methanol alcohol has corrosive properties and can damage  engines. Engine manufacturers do not recommend more than  five percent methanol in fuel. Consult the POH for specifi cs  on an engine. Rust is common in metal fuel containers and is a common  fuel contaminant. Metal fuel tanks should be filled after each  flight, or at least after the last flight of the day to prevent  moisture condensation within the tank.  Ethanol alcohol is less corrosive than methanol. However, it  attracts water and is not as economical as gasoline. Ethanol  does not get very good fuel economy. Avoid fuels with any  more than 10 percent of ethanol.  Another way to prevent fuel contamination is to avoid  refueling from cans and drums. Use a water fi ltering funnel  or a funnel with a chamois skin when refueling from cans or  drums. However, the use of a chamois will not always ensure  decontaminated fuel. Worn out chamois will not fi lter water;  neither will a new, clean chamois that is already water-wet or  damp. Most imitation chamois skins will not fi lter water. Consult the POH for specifics on an engine. Manufacturers  provide specific recommendations for the percentage of  alcohol in fuel. The posting on the pump may not be accurate  and alcohol content can vary greatly between fuel brands and  stations. Additionally, higher percentages of alcohol will be  added to auto gas in the future.  A simple test can be conducted to measure the fuel’s  alcohol content to ensure the fuel used stays within the  manufacturer’s recommendations. Use a general aviation  sump collector which includes graduation marks. Add water  to a specific mark. Then add fuel to fill the collector up to  the line for gas. Cover the top and shake it vigorously. After  it settles, the water and alcohol will combine and it will  look like there is now more water in the sump collector.  The difference between the initial amount of water fi rst put  into the collector and the new level of combined water and  alcohol equals the amount of alcohol in the fuel. Compare  this amount of alcohol and the amount of fuel to determine  the percentage of alcohol content in the fuel.  Letting fuel sit for weeks without using it will cause it to go  bad. Even if gas does not go bad, it will often lose octane  with time. For premixed gasoline and two-stroke oil, there  is another set of problems. Fuel and oil are normally mixed  at a 50:1 ratio. If premixed gas sits in a plastic container for  a while, the gas will evaporate leaving a richer oil mixture  in the container. In any case, fresh gas should be used when  possible. Never mix oil and fuel in an enclosed area. Not only are the  fumes irritating, but with the right fuel/air mixture can cause  an explosion. Do all oil and gas mixing outside. Refueling  from fuel cans should also be done outside. Never smoke  while refueling. Be careful when refueling an aircraft that has  just landed. There is danger of spilling fuel on a hot engine  component, particularly an exhaust system component.  Refueling should be done using only safety-approved fuel  containers marked with the type of fuel stored in them.  Confusing premixed fuel and fuel that has no oil in it can  be disastrous.  Methyl tertiary–butyl ether (MTBE) does not have the  corrosive or water attractive properties of the previously  mentioned additives and is added to fuel to improve air  quality. It has been banned in several states because it is  carcinogenic and has been found in ground water. It does not  attract water, but it is expensive, and found only in some of  the better grade fuels. Refueling Procedures Bad Gasoline Fuel Contamination • Failure to remove contamination from the fuel system  during prefl ight. • Servicing aircraft with improperly filtered fuel from  small tanks or drums. • Storing aircraft with partially filled fuel tanks. • Lack of proper maintenance. 4-15