Lecture on Aircraft Fuel Systems

Overview

• Combustion in engines requires a mixture of fuel and air.
• The fuel system's purpose is to deliver fuel from the tanks to the engine.

Types of Fuel Systems

1. Gravity Fed Systems

• Used in high-wing airplanes (e.g., Cessna 172).
• Fuel tanks located in wings above the engine.
• Gravity allows fuel to naturally flow from the tanks to the engine, though a fuel pump may still be present for emergencies.

2. Fuel Pump Systems

• Required in low-wing airplanes (e.g., Piper Arrow).
• Fuel tanks located below the engine necessitating pumps to move fuel upwards.
• Typically, two pumps are used: an engine-driven pump and an electrically-driven backup pump.

Fuel Priming

• Some aircraft have a fuel primer to inject fuel directly into the cylinder before engine start.
• Useful in cold conditions.
• Cessna 172 can use an electric fuel pump for priming despite lacking a dedicated primer.

Fuel Tanks

• Typically located inside the wings and filled from the top.
• Equipped with a vented cap to allow air pressure in, preventing a vacuum.
• Some aircraft also have external fuel vents for additional airflow into the tank.

Fuel Sensors and Gauges

• Fuel sensors within the tanks relay information to gauges in the cockpit.
• Important to visually verify fuel levels as gauges may not be completely reliable.

Contamination Checks

• Critical to check for contaminants in the fuel, such as water.
• Fuel sumps/drains are available for pilots to test for impurities.
• Most general aviation aircraft use 100 low lead fuel (blue).

Fuel Selector

• Controls which tank fuel is drawn from, generally has four positions: left, right, both, off.
• In Cessna 172, only three positions: left, right, both, with a separate fuel shutoff valve above the selector.

Safety Considerations

• Always check fuel visually and ensure no contamination.
• Use the fuel selector appropriately, especially in emergencies (e.g., engine fire).
• Understand the specific fuel system and controls of your aircraft model.