Fuel Systems


  • The fuel system is designed to provide an uninterrupted flow of clean fuel from the fuel tanks to the engine
  • The fuel must be available to the engine under all conditions of engine power, altitude, attitude, and during all flight maneuvers
  • Common classifications apply to fuel systems:
Pilot Handbook of Aeronautical Knowledge, Gravity Feed System
Figure 1: Pilot Handbook of Aeronautical Knowledge,
Gravity Feed System

Gravity-Feed System:

  • The gravity-feed system utilizes the force of gravity to transfer the fuel from the tanks to the engine [Figure 1]
  • For example, on high-wing airplanes, the fuel tanks are installed in the wings, and the fuel is gravity fed through the system and into the carburetor

Fuel-Pump System:

  • If the design of the aircraft is such that gravity cannot be used to transfer fuel, fuel pumps are installed [Figure 2]
  • For example, on low-wing airplanes, the fuel tanks in the wings are located below the carburetor requiring a pump
  • Aircraft with fuel-pump systems have two fuel pumps
  • The main pump system is engine driven with an electrically driven auxiliary pump 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 and is electrically driven by a switch in the cockpit

Pilot Handbook of Aeronautical Knowledge, Fuel Pump System
Figure 2: Pilot Handbook of Aeronautical Knowledge,
Fuel Pump System

Volatility & Flash Point:

  • Volatility: a fuel's tendency to vaporize
    • A more highly volatile fuel will vaporize at lower temperatures than fuel with lower volatility rating
    • An increase in temperature increases volatility, forming more vapors
    • When enough vapors are formed so fuel will ignite readily, that is the flash-point
  • Flash Point: the lowest temperature at which a combustible liquid (fuel) gives off vapors in sufficient quantity to ignite with the application of flame

Fuel Primer:

  • Both gravity-feed and fuel-pump systems may incorporate a fuel primer into the system
  • The fuel primer is used to draw fuel from the tanks to vaporize fuel directly into the cylinders prior to starting the engine
  • During cold weather, when engines are difficult to start, the fuel primer helps because there is not enough heat available to vaporize the fuel in the carburetor
  • It is important to lock the primer in place when it is not in use
    • If the knob is free to move, it may vibrate out during flight and can cause an excessively rich mixture
  • To avoid over-priming, read the priming instructions for the aircraft

Fuel Tanks:

  • The fuel tanks, normally located inside the wings of an airplane, have a filler opening on top of the wing through which they can be filled
  • The tanks are vented to the outside to maintain atmospheric pressure inside the tank through the filler cap or through a tube extending through the surface of the wing
  • Fuel tanks also include an overflow drain that may stand alone or be collocated with the fuel tank vent
    • This allows fuel to expand with increases in temperature without damage to the tank itself
    • If the tanks have been filled on a hot day, it is not unusual to see fuel coming from the overflow drain
  • Fuel/Oil coolers function by exchanging the hot oil against cold fuel for cooler oil against now warmer fuel

Fuel Gauges:

  • The fuel quantity gauges indicate the amount of fuel measured by a sensing unit in each fuel tank and is displayed in gallons or pounds
  • Aircraft certification rules require accuracy in fuel gauges only when they read "empty"
    • Any reading other than "empty" should be verified by checking the fuel level in each tank during the preflight inspection, and then compare it with the corresponding fuel quantity indication
  • If a fuel pump is installed in the fuel system, a fuel pressure gauge is also included
  • The normal operating pressure can be found in the AFM/POH or on the gauge by color coding

Pilot Handbook of Aeronautical Knowledge, Fuel Selector Valve
Figure 3: Pilot Handbook of Aeronautical Knowledge, Fuel Selector Valve

Fuel Selectors:

  • The fuel selector valve allows selection of fuel from various tanks [Figure 3]
  • A common type of selector valve contains four positions:
    • LEFT
    • RIGHT
    • BOTH
    • OFF
  • Selecting the LEFT or RIGHT position allows fuel to feed only from that tank, while selecting the BOTH position feeds fuel from both tanks
  • Varying the position may be used to balance the amount of fuel remaining in each wing tank
  • Fuel placards show limitations on fuel tank usage, such as "level flight only" and/or "both" for landings and takeoffs

Fuel Strainers, Sumps, and Drains:

  • After leaving the fuel tank and before it enters the carburetor, the fuel passes through a Fuel Strainer which removes any moisture and other sediments in the system
  • Since these contaminants are heavier than aviation fuel, they settle in a sump at the bottom of the strainer assembly
  • A sump is a low point in a fuel system and/or fuel tank
  • Fuel samples should be drained and checked visually for water and contaminants
  • Water in the sump is hazardous because in cold weather the water can freeze and block fuel lines
  • In warm weather, it can flow into the carburetor and stop the engine
  • If water is present in the sump, more water in the fuel tanks is probable and they should be drained until there is no evidence of water
  • Never take off until all water and contaminants have been removed from the engine fuel system
  • Because of the variation in fuel systems, become thoroughly familiar with the systems that apply to the aircraft being flown
  • Consult the AFM/POH for specific operating procedures

Pilot Handbook of Aeronautical Knowledge, Aviation Fuel Color-Coding System
Figure 4: Pilot Handbook of Aeronautical Knowledge,
Aviation Fuel Color-Coding System

Fuel Grades:

  • 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 [Figure 4]
  • 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 ignite at higher temperatures, but not prematurely
  • The proper fuel grade is stated in the AFM/POH, on placards in the flight deck, and next to the filler caps
    • If the proper grade of fuel is not available, use the next higher grade as a substitute
    • Never use a grade lower than recommended
    Figure 4: Pilot Handbook of Aeronautical Knowledge,
    Aviation Fuel Color-Coding System
  • This can cause the cylinder head temperature and engine oil temperature to exceed their normal operating ranges, which may result in detonation
  • Care must be exercised to ensure that the correct aviation grade is being used for the specific type of engine
  • Auto gas should NEVER be used in aircraft engines unless the aircraft has been modified with a Supplemental Type Certificate (STC) issued by the Federal Aviation Administration (FAA)
  • The current method identifies AVGAS for aircraft with reciprocating engines by the octane and performance number, along with the abbreviation AVGAS
  • Although AVGAS 100LL performs the same as grade 100, the "LL" indicates it has a low lead content
  • Fuel for aircraft with turbine engines is classified as JET A, JET A-1, and JET B
  • Jet fuel is basically kerosene and has a distinctive kerosene smell
  • Since use of the correct fuel is critical, dyes are added to help identify the type and grade of fuel
  • In addition to the color of the fuel itself, the color-coding system extends to decals and various airport fuel handling equipment
  • For example, all AVGAS is identified by name, using white letters on a red background
  • In contrast, turbine fuels are identified by white letters on a black background

Fuel Contamination:

  • Accidents attributed to powerplant failure from fuel contamination have often been traced to:
    • Inadequate preflight inspection by the pilot
    • Servicing aircraft with improperly filtered/mixed fuel from small tanks or drums
    • Storing aircraft with partially filled fuel tanks
    • Lack of proper maintenance
  • Types of contaminate:
    • Chemical: usually resulting from inadvertent mixing of petroleum products, this type affects the chemical and physical properties of the fuel and can only be detected by specific laboratory tests
    • Material (particulate): generally consists of water, microbiological growth, and particulate matter which can usually be detected visually
    • Microbiological: consists of living organisms that grow at the fuel-water interface
  • Fuel should be drained from the fuel strainer quick drain and from each fuel tank sump into a transparent container, and then checked for dirt and water
  • When the fuel strainer is being drained, water in the tank may not appear until all the fuel has been drained from the lines leading to the tank
  • This indicates that water remains in the tank, and is not forcing the fuel out of the fuel lines leading to the fuel strainer
  • Therefore, drain enough fuel from the fuel strainer to be certain that fuel is being drained from the tank
  • The amount will depend on the length of fuel line from the tank to the drain
  • If water or other contaminants are found in the first sample, drain further samples until no trace appears
  • Water may also remain in the fuel tanks after the drainage from the fuel strainer has ceased to show any trace of water
  • This residual water can be removed only by draining the fuel tank sump drains
  • Water is the principal fuel contaminant
  • Suspended water droplets in the fuel can be identified by a cloudy appearance of the fuel, or by the clear separation of water from the colored fuel, which occurs after the water has settled to the bottom of the tank
  • As a safety measure, the fuel sumps should be drained before every flight during the preflight inspection
  • Fuel tanks should be filled after each flight or after the last flight of the day to prevent moisture condensation within the tank
  • To prevent fuel contamination, avoid refueling from cans and drums
  • In remote areas or in emergency situations, there may be no alternative to refueling from sources with inadequate anti-contamination systems
  • While a chamois skin and funnel may be the only possible means of filtering fuel, using them is hazardous
  • Remember, the use of a chamois will not always ensure decontaminated fuel
  • Worn-out chamois will not filter water; neither will a new, clean chamois that is already water-wet or damp
  • Most imitation chamois skins will not filter water

Refueling Procedures:

  • Static electricity is formed by the friction of air passing over the surfaces of an aircraft in flight and by the flow of fuel through the hose and nozzle during refueling
  • Nylon, Dacron, or wool clothing is especially prone to accumulate and discharge static electricity from the person to the funnel or nozzle
  • To guard against the possibility of static electricity igniting fuel fumes, a ground wire should be attached to the aircraft before the fuel cap is removed from the tank
  • Because both the aircraft and refueler have different static charges, bonding both components to each other is critical
    • This means by contacting metal to metal, the static differential charge is equalized
  • The refueling nozzle should be bonded to the aircraft before refueling begins and should remain bonded throughout the refueling process
  • When a fuel truck is used, it should be grounded prior to the fuel nozzle contacting the aircraft
  • If fueling from drums or cans is necessary, proper bonding and grounding connections are important
  • Drums should be placed near grounding posts and the following sequence of connections observed:
    1. Drum to ground
    2. Ground to aircraft
    3. Bond drum to aircraft or nozzle to aircraft before the fuel cap is removed
  • When disconnecting, reverse the order
  • The passage of fuel through a chamois increases the charge of static electricity and the danger of sparks
  • The aircraft must be properly grounded and the nozzle, chamois filter, and funnel bonded to the aircraft
  • If a can is used, it should be connected to either the grounding post or the funnel
  • Under no circumstances should a plastic bucket or similar non-conductive container be used in this operation
  • Some sort of eye protection should be worn and refueling performed in a well ventilated area to avoid bodily harm

System Failures:

  • Running a tank completely dry may allow air to enter the fuel system and cause vapor lock, which makes it difficult to restart the engine
  • Vapor Lock: on fuel-injected engines, the fuel becomes so hot it vaporizes in the fuel line, not allowing fuel to reach the cylinders
  • Loss of fuel pressure can be caused by malfunctioning/failed pumps or cavitation

Minimum Fuel Advisory:

  • Pilot:
    • Advise ATC of your minimum fuel status when your fuel supply has reached a state where, upon reaching destination, you cannot accept any undue delay
    • Be aware this is not an emergency situation, but merely an advisory that indicates an emergency situation is possible should any undue delay occur
    • On initial contact the term "minimum fuel" should be used after stating call sign
      • "Salt Lake Approach, United 621, minimum fuel"
    • Be aware a minimum fuel advisory does not imply a need for traffic priority
    • If the remaining usable fuel supply suggests the need for traffic priority to ensure a safe landing, you should declare an emergency due to low fuel and report fuel remaining in minutes
      • Fuel Remaining: A phrase used by either pilots or controllers when relating to the fuel remaining on board until actual fuel exhaustion. When transmitting such information in response to either a controller question or pilot initiated cautionary advisory to air traffic control, pilots will state the APPROXIMATE NUMBER OF MINUTES the flight can continue with the fuel remaining. All reserve fuel SHOULD BE INCLUDED in the time stated, as should an allowance for established fuel gauge system error
  • Controller:
    • When an aircraft declares a state of minimum fuel, relay this information to the facility to whom control jurisdiction is transferred
    • Be alert for any occurrence which might delay the aircraft.

Case Studies:

  • NTSB Identification: ANC14CA014: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's failure to adequately remove water-contaminated fuel during the preflight inspection, which resulted in a total loss of engine power
  • NTSB Identification: CEN13LA354: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot’s failure to identify the water contamination of the fuel system during his preflight inspection, which resulted in a total loss of engine power during the airplane’s initial climb
  • NTSB Identification: ATL03FA133: The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's failure to maintain control of the airplane during a VFR pattern for a precautionary landing, which resulted in an uncontrolled descent and subsequent collision with terrain. Also causal was the pilot's inadequate preflight inspection of the aircraft, which resulted in his failure to secure the fuel cap