Fuel Systems

[Ref. Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 7-25, 7-26, 7-27, 7-28]

Fuel Systems

Functions of Fuel Systems

  • provide an uninterrupted flow of clean fuel from the fuel tanks to the engine (via induction system)

(2) Two general types of fuel systems in small aircraft:

  1. gravity-feed
  2. fuel-pump

Gravity-Feed System    (e.g high-wing airplanes)
utilizes the force of gravity to transfer the fuel from the tanks to the engine.
(As the fuel tanks are installed in the wings – this places the fuel tanks above the carburetor)

Fuel-Pump System    (e.g low-wing airplanes)
utilizes the fuel pumps to transfer the fuel from the tanks to the engine.
two fuel pumps – main pump and auxiliary pump
main pump system is engine driven while auxiliary pump (aka boost pump), which is controlled by a switch in the flight deck, provides added reliability to the fuel system

general Components of Fuel System

  1. Fuel pump
  2. Fuel Primer
  3. Fuel Tanks
  4. Fuel Gauges
  5. Fuel Selectors
  6. Fuel Strainers, Sumps, and Drains


  • Fuel Primer
    both gravity-feed and fuel-pump systems may incorporate a fuel primer;
    used to draw fuel from the tanks directly into the cylinders prior to starting the engine, particular during cold weather.
    (fuel primer is needed because there is not enough heat available to vaporize the fuel in the carburetor)
  • Fuel Tanks
    normally located inside the wings;
    are vented to the outside to maintain atmospheric pressure inside the tank
  • Fuel Gauges
    DON’T depend solely on the accuracy of the fuel quantity gauges. ALWAYS visually check the fuel level in each tank during the preflight inspection, and then compare it (X-CHECK) with the corresponding fuel quantity indication.

Fuel capacitance probes – AC v.s DC

  • Fuel Selectors
    allows selection of fuel from various tanks.
    Running a fuel tank dry (fuel starvation) does not only cause the engine to stop, but running for prolonged periods on one tank causes an unbalanced fuel load between tanks. 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.
    [read more: # fuel starvation v.s fuel exhaustion]
  • Fuel Strainers, Sumps, and Drains
    • Strainer, is located somewhere between fuel tank and carburetor, removes any moisture and other sediments in the system –  fuel strainer should be drained and fuel samples should be checked visually for water and contaminants before each flight.
      (NEVER take off until all water and contaminants have been removed from the engine fuel system !)
    • Sump is a low point in a fuel system AND/OR fuel tank. The fuel system may contain a sump, a fuel strainer, and fuel tank drains, which may be collocated.

Read More:

# Fuel Grade

# Fuel Contamination

# Fuel System Icing

# Refueling Procedures

# Engine System

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