The fuel system is designed to provide an uninterrupted flow of clean fuel from the fuel tank to the engine. See Chapter 3, Components and Systems, for more information on fuel tanks. See earlier section in this chapter for specifics on fuel injection systems. The fuel must be available to the engine under all conditions of engine power, altitude, attitude, and during all approved flight maneuvers. [Figure 4-17]
WSC aircraft with carburetors have engine-driven fuel pump systems. A diaphragm pump is the primary pump in the fuel system for two-stroke engines. Air pulses in the crankcase actuate a diaphragm and provide fuel under pressure to the carburetor. Four-stroke engines have a mechanical pump driven directly off the engine.
Sometimes an electric auxiliary pump is provided for use in engine starting and in the event the engine pump fails. The auxiliary pump, also known as a boost pump, provides added reliability to the fuel system. The electric auxiliary pump is controlled by a switch in the flight deck.
Fuel Plunger Primer
The optional fuel plunger primer is used to draw fuel from the tanks to supply it directly into the engine prior to starting. This is particularly helpful during cold weather when engines are hard to start because there is not enough heat available to vaporize the fuel in the carburetor. For some aircraft, it is the only way to deliver fuel to the engine when first starting. After the engine starts and is running, the fuel pump pushes fuel to the carburetors and begins normal fuel delivery. To avoid overpriming, read the priming instructions in the POH.
A choke or fuel enriching system is an alternate method to provide additional fuel to the engine for initial cold starting. Actuating the choke control allows more fuel to flow into the carburetor.
Fuel Bulb Primer
The fuel bulb primer is manually actuated by squeezing the bulb to draw fuel from the fuel tanks. This charges the fuel lines and carburetor float bowls before starting the engine the first time on a given day. After the engine starts, the fuel pump is able to deliver the fuel to the fuel bowls. An electric auxiliary fuel pump can also be used to charge the fuel lines and carburetor fuel bowls before starting. This auxiliary fuel pump is also used as a backup pump of the engine-driven fuel pump fails.
The fuel quantity gauge indicates the amount of fuel measured by a sensing unit in each fuel tank and is displayed in gallons. Do not depend solely on the accuracy of the fuel quantity gauge. Always visually check the fuel level in the tank during the preflight inspection, and then compare it with the corresponding fuel quantity indication. It is also important to track inflight fuel consumption. Be sure to consult the POH and know the approximate consumption rate to ensure sufficient fuel for flight. If an auxiliary electric fuel pump is installed in the fuel system, a fuel pressure gauge is sometimes included. This gauge indicates the pressure in the fuel lines. The normal operating pressure can be found in the POH.
After leaving the fuel tank, the fuel passes through a filter before it enters the fuel pump or carburetor. This filter removes sediments that might be in the fuel. [Figure 4-18]
Aviation gasoline (AVGAS) is identified by an octane or performance number (grade) which designates the antiknock value or knock resistance of the fuel mixture in the engine cylinder. The higher the grade of gasoline, the more pressure the fuel can withstand without detonating. Lower grades of fuel are used in lower compression engines because these fuels ignite at a lower temperature. Higher grades are used in higher compression engines, because they must ignite at higher temperatures but not prematurely. If the proper grade of fuel is not available, use the next higher grade as a substitute. Never use a lower grade. This can cause the cylinder head temperature to exceed its normal operating range, which may result in detonation. Unfortunately, AVGAS 100 Low Lead (LL) may not be recommended by two-stroke engine manufacturers and may not be preferred by the four-stroke manufactures. Even though the “LL” stands for low lead, 100LL contains more lead than the old leaded gas dispensed at automotive filling stations. The lead in the fuel leaves deposits in the piston ring grooves, freezing the rings in position and reducing engine performance. Spark plugs are also very susceptible to lead fouling. This is especially true in two-stroke engines that use cooler ignition temperatures than standard aircraft engines.
AVGAS does have some advantages. It degrades slower than auto gas, maintaining its efficiency for a full 3 months. AVGAS 100LL has no seasonal or regional variations and is manufactured according to a standardized “recipe” worldwide. If the airport has only 100LL available, it is permissible, absent any limitations of the engine manufacturer, to mix 100LL and auto gasoline for use in two-stroke engines. A 50–50 ratio will boost the octane rating and limit the amount of lead available for fouling. Generally speaking, this is a 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.
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 specifics on an engine.
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.
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 first 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.
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 groundwater. It does not attract water, but it is expensive, and found only in some of the better grade fuels.