Aircraft Fuel Systems: Procedures and Components

Learning Objectives

• This lesson will enable you to analyze accidents related to fuel systems and determine potential causes.
• This lesson will enable you to articulate the differences in fuels while identifying aircraft categories that commonly use them.
• This lesson will enable you to learn the critical components of aircraft fuel systems.
• This lesson will enable you to describe guidelines for aircraft fueling safety.

Aircraft Fuel Systems

• Aircraft fuel systems must function in all operating conditions.
• A fuel's grade, any contaminations, and how the aircraft is angled can affect functioning.
• The two main types of fuel systems are gravity feed and fuel pump driven.

Gravity Feed Fuel Systems

• Gravity feed systems rely on gravity to deliver fuel to the engine.
• Gravity feed systems are the simplest fuel system type.
• High-wing aircraft commonly use gravity feed systems.
• The fuel sits in the wings above the carburetor, which allows gravity to draw the fuel from tanks into the engine.

Fuel Pump Systems

• Fuel Pump systems use two pumps.
• The main pump sits near and is driven by the engine.
• The auxiliary pump (or boost pump) is electrically driven.
• Auxiliary pump is for added reliability.
• Auxiliary pump provides fuel pressure for engine start.
• Auxiliary pump is controlled by a switch in the cockpit.
• Auxiliary pump may need to be on for takeoff, landing, priming, or high-altitude flight.

Inverted Fuel Systems

• Aircraft that fly upside down use inverted fuel systems with a flop tube or a header tank.
• A flop tube is a flexible tube with a weight on one end, and sits within the fuel tank.
• The weighted end of the flop tube always falls to the lower side of the tank, which draws fuel.
• A header tank is a small tank located low in the cockpit or fuselage.
• When the aircraft is upside down, the header tank sits above the engine, and allows gravity to pull fuel into the engine.

Fuel System Components

• This includes all components needed to store fuel and deliver it to an engine.
• Common components include:
• Tanks
• Pumps
• Gauges
• Strainers
• Drains
• Filler Systems
• Primers
• Selectors
• Sumps

Primer

• Primers are implemented in both gravity fed and pump systems
• Primer vaporizes fuel and pushes it directly to the cylinders.
• Primers help to start, especially in cold weather where there is not enough heat to vaporize the fuel in the carburetor.
• Overpriming can flood the engine, making it harder to start
• Primer should be kept locked during flight to prevent inadvertently creating an overly rich fuel-air mixture

Finding The Fuel Tanks

• Fuel tanks typically sit inside the wings
• Some aircraft also have supplemental tanks to extend range.
• Tip tanks are permanent tanks that sit at the end of each wing.
• External tanks are temporary tanks fitted to the outside of the aircraft for certain operations.
• Ferry tanks are temporary tanks, and may be located inside the passenger compartment during long flights.

Fillers

• Fuel filler openings are typically located on top of each wing
• Fillers are used to fill tanks
• High wing and larger aircraft may need a ladder to reach filler
• Fillers are covered by a cap, which may sometimes be locking.
• Fillers may have a vent which must be kept clear.

Vents and Overflows

• Aircraft fuel tanks vent to the outside.
• Vents maintain atmospheric pressure inside the tanks.
• Maintaining atmospheric pressure is important as pressure changes significantly with altitude.
• Vents may be part of the filler cap or a small tube below the wing.
• Overflow drains allow fuel to expand at high temperatures.
• Overflow drains allows excess fuel to flow in order to prevent damage to the tank.
• Overflow drains and vents may use the same tube

Types of Fuel Tanks

• Bladder tanks
• Look like large bags
• Made with heavy rubberized material
• Can be removed for replacement
• Rigid tanks
• Look like boxes
• Made of metal, plastic, or composite
• Can be removed for replacement
• Wet wings
• Sections within the wings are sealed to hold fuel.
• Common on larger aircraft

Quantity and Pressure Gauges

• Fuel quantity gauges are typically supplied for each tank.
• Fuel may be measured in pounds (especially in larger aircraft) or gallons.
• FAA regulations only require that the gauge reads accurately when the tank is empty.
• Pilots must visually check tanks.
• Good fuel planning is critical.
• Fuel pump systems may also have a pressure gauge.
• Some fuel systems have a fuel flow gauge to help the pilot determine how quickly fuel is being used.

Selectors

• Selectors allow choosing the fuel source among multiple tanks.
• Selectors often have four settings: Left, Right, Off, and Both.
• When set to Both, fuel from both tanks is used.
• A selector set to Both does not guarantee fuel will be used equally.
• Use of the Both setting is often mandatory for takeoff and landing (placards show requirements.)
• When set to Left or Right, only the chosen tank supplies fuel to the engine.
• Selectors are useful for balancing fuel load in flight.
• When no Both setting is available, pilots must switch tanks often in order to maintain balance.
• Pilots may set a timer as a reminder to switch.

Running on Empty

• Aircraft fuel gauges must provide an accurate reading only when the tank is empty.
• Never run a fuel tank dry.
• Running a tank dry could allow air to enter the tank.
• Running a tank dry can lead to vapor lock.
• Running a tank dry may cause the engine to stop and be difficult or impossible to restart
• Restarting is especially hard in fuel-injected engines because fuel can become so hot that it vaporizes in the fuel line without reaching the engine.

Fuel Strainers

• Fuel Strainers contain screens or filters.
• Fuel Strainers help remove contaminants from the fuel before they reach the engine.
• These contaminants can include dirt, debris, and moisture.
• The strainer sits between the fuel tank and the carburetor.
• Strainer's drain sits on the belly of the aircraft

Sumps and Drains

• A fuel sump is a low point in the fuel tank.
• Fuel Sumps sit under each wing.
• Fuel Sumps sit under the belly.
• Sumps allow contaminants to collect in one location.
• Water, dirt, and most other contaminants are heavier than fuel, and sink into the sump.
• Sumps are equipped with drains.
• It is important to drain a small amount of fuel from each sump and the fuel strainer drain before every flight.
• This ensures the pilot will see the correct grade of fuel in each tank.
• Drain until there are no signs of contamination

Contamination Hazards

• Contaminants can be pumped into tanks through the fueling source, or they can come from the tanks themselves.
• Water is the most common contaminant, and can leak into the tank through the filler cap, or by condensation in partially filled tanks.
• Water can freeze causing fuel system icing.
• Water may black fuel lines or be ingested by the engine.
• Fuel additives can reduce icing risk.
• Water can displace fuel in the carburetor, leaving the engine with nothing to burn.

Fuel Grades

• Using the right type of fuel for an aircraft's engine is critical.
• Turbine engines use jet fuel while reciprocating engines use aviation gasoline (avgas).
• Reciprocating engines may also use diesel or automobile gasoline.
• Aviation fuels are color coded with dye.
• This reduces the likelihood of misfueling.
• Placards near the fuel filler indicate the type of fuel required.

Jet Fuel

• Jet fuel is kerosene based.
• Jet fuel is classified as Jet A or Jet B.
• Jet fuel is clear or straw colored.
• Jet fuel has a higher flash point than gasoline-based fuels.
• Jet fuels thus require higher temperatures to ignite.

Avgas

• Avgas is gasoline based.
• The most common is 100LL.
• LL stands for "low lead".
• 100LL is pale blue in color.
• Other variants of avgas are less common.
• 80 octane avgas is red.
• 100 octane avgas is green.
• If the recommended fuel is unavailable, substitute the next higher octane (e.g., 100 octane may sub for 80 octane fuel).
• A small number of aircraft can run on automobile gasoline.
• The industry is moving to find an unleaded replacement for 100LL avgas.

Fueling Safety

• Improper fueling can create major consequences.
• Misfueling may require draining the fuel system or it may lead to an engine failure during flight.
• If someone else is fueling your aircraft, always remain present and observe the process.
• If you are fueling your aircraft, double check that you are pumping the proper grade and quantity.

Fueling Guidelines

• Fuel grade and quantity
• Always ask the employee to read back your fuel order, and match the fuel truck with your fuel order.
• If self-fueling, verify the fuel pump in use is the correct grade.
• Fuel Nozzle
• Verify the fuel nozzle is compatible with aircraft fuel filler.
• Avgas nozzles are small and round.
• Jet fuel nozzles are larger with a flat end.
• Aircraft Grounding
• Always ground aircraft to the fueling source.
• This Prevents that static discharge that can cause explosions.
• Fuel trucks and fuel pumps utilize a special wire on a reel that connects to the aircraft to offer a ground.
• Payment
• Check that the invoice's fuel grade and quantity matches what was ordered or intended.
• Preflight
• Visually check the tanks for quantity and fuel color.
• Drain a sample from each tank sump.
• Check the sample for water and contaminants, and note the fuel smell and color.
• Do not depart if there is doubt about the fuel type in the tanks.
• Fuel Caps
• Ensure fuel caps are secure.
• Fuel can siphon out of tanks at a fast rate if the cap comes off in flight.

Explore aircraft fuel systems, including procedures like filling tanks after flight to prevent condensation, proper fuel draining techniques, and the importance of using the correct fuel grade. Learn about components such as fuel tank vents, boost pumps, and inverted fuel systems.