Aircraft Fuel System

Aircraft Fuel System

1. Basic Fuel System Requirements

   • Fuel System Lightning Protection
     • prevent the ignition of the fuel within the system
       • direct lightning
       • corona
         • a luminous discharge that occurs as a result of an electrical potential difference between the aircraft and the surrounding area
       • Streamering
         • a branch-like ionized path that occurs in the presence of a direct stroke or under conditions when lightning strikes are imminent
   • Fuel System Independence
     • independence between multiple fuel storage and supply systems
   • Title 14 of the Code of Federal Regulations (14 CFR)
     • 14 Part 23—Normal, Utility, Acrobatic, and Commuter Category Airplanes
     • 14 Part 25—Transport Category Airplanes
     • 14 Part 27—Normal Category Rotorcraft
     • 14 Part 29—Transport Category Rotorcraft
     • 14 Part 31—Manned Free Balloons
     • 14 Part 34- outlines requirements that fall under the jurisdiction of the Environmental Protection Agency (EPA)
       • A turbine engine fuel system must be capable of sustained operation throughout its flow and pressure range even though the fuel has some water in it
         • with water at 80 °F
           • having 0.75 cubic centimeters (cm) of free water per gallon added to it
             • then cooled to the most critical condition for icing likely to be encountered in operation.
     • Fuel
       • Pumps
       • Filters
       • Fuel Lines
       • Valves
       • Methods
       • Metering devices
       • Storage Tanks
       • monitoring devices
   • Fuel Flow
     • powerplant functions properly in all likely operating conditions
     • auxiliary power unit functions properly in all likely operating conditions

2. Fuel

   • Fuel Storage System
     • Each tank must be isolated from personnel personnel compartments and protected from hazards due to unintended temperature influences
     • must provide fuel for at least one-half hour of operation at maximum continuous power or thrus
     • fuel tanks must be designed to prevent significant loss of stored fuel from any vent system
   • Fuel Storage Refilling or Recharging System
     • must be designed to prevent:
       • contamination of the fuel stored during likely operating conditions
       • improper refilling or recharging
       • the occurrence of any hazard to the airplane or to persons during refilling or recharging
   • Types of Aviation Fuel
     • kerosene
       • fuel with a much higher flash point, so it is less flammable
       • should never be mixed with aviation gasoline
       • higher viscosity with much lower volatility and higher boiling points than gasoline
       • operate in cold environments
       • must start readily and be able to restart while in fligh
         • Fuel with high volatility makes this easier.
       • Fuel Types
         • Jet A
           • the most common turbine engine fuel available in the continental United States
             • low volatility and low vapor pressure
         • Jet A-1
           • is the most popular globally
             • low volatility and low vapor pressure
         • Jet B
           • is primarily available in Alaska and Canada
             • High volatility and low freezing point
       • Fuel Issues
         • areas where water can settle, and microbes can flourish
     • AVGAS
       • Combustion releases energy in the fuel, which is converted into the mechanical motion of the engine
       • is very volatile and extremely flammable, with a low flash point
       • has been available in different formulas
       • is a blend of numerous hydrocarbon compounds, each with different boiling points and volatility
       • Vapor Lock
         • AVGAS vaporizes in the fuel line or other components between the fuel tank and the carburetor
           • boost pumps located in the fuel tank can prevent that
       • vapor pressure be between 5.5 pounds per square inch (psi)
         • 7.0 psi at 100 °F.
       • Aromatic Fuels
         • In years past, considerable quantities of aromatic hydrocarbons were sometimes added
           • increase the rich mixture performance of AVGAS
       • Octane and Performance Number Rating
         • a single 100 octane low-lead fuel (100LL)
       • Fuel Identification
         • 100LL AVGAS
           • is dyed blue
         • 100 AVGAS
           • is dyed green
         • 80/87 AVGAS
           • no longer available
             • is dyed red
         • 82UL (unleaded)
           • is dyed purple
         • 115/145 AVGAS

3. Small Single-Engine Aircraft Fuel Systems

   • Gravity Feed Systems
     • High-wing aircraft
       • gravity is used to deliver the fuel.
         • shutoff valve has two positions:
           • fuel ON
           • fuel OFF
         • selector valve provides four options:
           • fuel shutoff to the engine
           • fuel feed from the right-wing tank only
           • fuel feed from the left fuel tank only
           • fuel feed to the engine from both tanks simultaneously
     • is the simplest aircraft fuel system.
   • Pump Feed Systems
     • Low- and mid-wing single reciprocating engine aircraft
       • one or more pumps are used to move the fue
         • fuel cannot be drawn from both tanks simultaneously
   • High-Wing Aircraft with Fuel Injection System
     • equipped with a fuel system that features fuel injection rather than a carburetor
       • It is first delivered from the fuel tanks by gravity to two smaller accumulator or reservoir tanks.
         • These tanks, one for each wing tank, consolidate the liquid fuel and have a relatively small airspace.
           • They deliver fuel through a threeway selector valve
             • LEFT
             • OFF
             • RIGHT
             • The selector valve also acts simultaneously as a diverter of air that has been separated out of the fuel in the engine-driven fuel pump and returned to the valve
               • It routes the air to the vent space above the fuel in the selected reservoir tank
   • Small Multiengine (Reciprocating) Aircraft Fuel Systems
     • Low-Wing Twin
       • t is more complicated than a single-engine aircraft
         • Two selector valves are required
     • High-Wing Twin
       • combines gravity feed with an electric fuel pump
         • The metering unit for each engine provides the proper flow of fuel to the distribution manifold which feeds the injectors
   • Large Reciprocating-Engine Aircraft Fuel Systems
     • are no longer produced
       • They are mostly carbureted and share many features with the light aircraft systems
   • Jet Transport Aircraft Fuel Systems
   • Fuel Valves
     • They are used to
       • route the fuel to a desired location
       • shut off fuel flow
     • Hand-Operated Valves
       • three basic types
         • cone-type valve
           • consists of a machined valve housing into which a rotatable brass or nylon cone is set.
         • poppet-type valve
           • Selector valves
         • Manually-Operated Gate Valves
           • can be used, especially as fire control valves, requiring no electrical power to shutoff fuel flow when the emergency fire handle is pulled.
     • Motor-Operated Valves
       • electric motors are used to actuate the units
       • plug-type valve
         • an electric motor is used to rotate the plug or drum rather than it being rotated manually.
       • gate valve
         • uses a geared, reversible electric motor to turn the actuating arm of the valve that moves the fuel gate into or out of the path of the fuel.
     • Solenoid-Operated Valves

4. Fuel System Components

   • Fuel Pumps
     • Auxiliary pumps are used on many aircraft as well
     • On many large aircraft, boost pumps are used to move fuel from one tank to another
   • Hand-Operated Fuel Pumps
     • Some older reciprocating engine aircraft
       • wobble pumps
         • double-acting pumps that deliver fuel with each stroke of the pump handle
   • Centrifugal Boost Pumps
     • electric motor driven and most frequently is submerged in the fuel tank or located just outside of the bottom of the tank with the inlet of the pump extending into the tank.
     • is a variable displacement pump
   • Ejector Pumps
     • Some aircraft use ejector pumps to help ensure that liquid fuel is always at the inlet of the pump.
   • Pulsating Electric Pumps
     • is usually used in the same manner as a centrifugal fuel pump on larger aircraft, except it is located downstream of the fuel tank outlets
     • can be used at high altitudes to prevent vapor lock
   • Vane-Type Fuel Pumps
     • the most common types of fuel pumps found on reciprocating-engine aircraft
   • Fuel Filters
     • Two main types of fuel cleaning device
     • All aircraft fuel systems have filters and strainers to ensure that the fuel delivered to the engine(s) is free from contaminants.
     • Turbine engine fuel control units are extremely close tolerance devices
     • are often used between the engine-driven fuel pump and the fuel metering device on reciprocating, as well as turbine-engine aircraft
   • Fuel Heaters and Ice Prevention
     • The formation of ice on the filter element blocks the flow of fuel through the filter
     • heat exchanger units also heat the fuel sufficiently to melt any ice that has already formed
     • The most common types of fuel heaters are
       • air/fuel heaters
         • uses warm compressor bleed air to heat the fuel
       • oil/fuel heaters
         • exchanger heats the fuel with hot engine oil

5. Fuel System Indicators

   • Fuel Quantity Indicating Systems
     • All aircraft fuel systems must have some form of fuel quantity indicator.
       • vary widely depending on the complexity of the fuel system and the aircraft on which they are installed
   • Fuel Flowmeters
     • indicates an engine’s fuel use in real time
       • The types depends primarily on the powerplant being used and the associated fuel system
         • numerous calculations can be performed to aid the pilot’s situational awareness and flight planning
   • Fuel Temperature Gauges
     • fuel temperature can be integrated into fuel flow processing calculations
   • Fuel Pressure Gauges
     • can give the pilot early warning of a fuel system related malfunction
       • A more complex aircraft may have a sensor with a transducer located at the fuel inlet to the metering device that sends electrical signals to a cockpit gauge
         • Modern aircraft may use a variety of sensors including solid state types and those with digital output signals or signals that are converted to digital output
   • Pressure Warning Signal
     • Fuel pressure is an important parameter that merits the use of a warning signal when it falls outside of the normal operating range
   • Valve-In-Transit Indicator Lights
     • valves and pumps are used to move fuel and to have it flow to desired locations in aircraft with multiple fuel tanks
     • His functioninng is critical

6. Fuel System Repair

   • Troubleshooting the Fuel System
     • Knowledge of the fuel system and how it operates is essential
       • a logical sequence of steps to narrow the problem to a specific component or location should be followed
   • Location of Leaks and Defects
     • Fuel leaks occur in fuel tanks and in the internal component
   • Fuel Leak Classification
     • Four basic classifications
       • stain
       • seep
       • heavy seep
       • running leak
   • Replacement of Gaskets, Seals, and Packings
     • a new gasket, seal, or packing must be installed when a component is replaced
   • Fuel Tank Repair
     • at the time a tank is repaired, a thorough inspection should be made
       • Rigid removable fuel tanks can be riveted, welded, or soldered together
       • Some metal fuel tanks experiencing minor seepage can be repaired with a sloshing procedure
   • Welded Tanks
     • can be constructed from steel or weldable aluminum, such as 3003S or 5052SO
   • Riveted Tanks
     • are often repaired by riveting
   • Soldered Tanks
   • Bladder Tanks
     • they are patched using patch material, adhesive, and methods approved by the manufacturer
     • ynthetic bladder tanks have a limited service life
     • are usually required to remain wetted with fuel at all times to prevent drying and cracking of the bladder material
   • Integral Tanks
     • an integral tank develops a leak at an access panel
   • Fire Safety

7. Fuel System Servicing

   • Checking for Fuel System Contaminants
     • Continuous vigilance is required
       • Keeping a fuel system clean begins with an awareness of the common types of contamination
         • Water
           • can be dissolved into fuel or entrained
           • can enter a fuel system via condensation
           • small quantities don't represent a problem
           • Large amounts can disrupt engine operation
         • Solid Particle Contaminants
           • broken-off sealant, or pieces of filter elements, corrosion, etc
           • Preventing solid contaminant introduction into the fuel is critical
           • The maximum amount of solid particle contamination allowable is much less in turbine engine fuel systems than in reciprocating-engine fuel systems
         • Surfactants
           • liquid chemical contaminants that naturally occur in fuels
           • in small quantities are unavoidable and pose little threat to fuel system functioning
           • Larger quantities of surfactants do pose problems
         • Microorganisms
           • His presence s is a critical problem
           • They form a visible slime that is dark brown, grey, red, or black in color
           • Since the microbes live in free water and feed on fuel, the most powerful remedy for their presence is to keep water from accumulating in the fuel
         • Foreign Fuel Contamination
           • Contamination of an aircraft’s fuel with fuel not intended for use in that particular aircraft can have disastrous consequences
           • Detection of Contaminants
             • Detections instruments
               • Hydrokit
       • Fuel Contamination Control
         • Contamination control begins long before the fuel gets pumped into an aircraft fuel tank
         • Filters should be changed at the specified intervals

8. Fueling and Defueling Procedures

   • Fueling
     • nozzle locks onto the fueling port at the aircraft fueling station. Fuel is pumped into the aircraft through this secured and sealed connection. Gauges are monitored to ascertain when the tanks are properly loaded
       • Aviation fuel nozzles are equipped with static bonding wires that must be attached to the aircraft before the fuel cap is opened
   • Defueling
     • Removing the fuel sometimes is necesary
       • maintenance, inspection, or due to contamination
   • Fire Hazards When Fueling or Defueling
     • the potential for fire while fueling and defueling aircraft must be addressed