Piston Engine Fuel System Fundamentals

021-08-01Pablo Asensio Martínez2026-03-242 min

The fuel system is designed to store and deliver fuel to the engine at the correct pressure and flow rate under all operating conditions. It also manages fuel distribution to maintain aircraft balance.

System Requirements

According to CS 25.951, the fuel system must:

  • Ensure 100% of the required fuel flow is provided in the most critical phase of flight and during any certified maneuver.
  • Deliver fuel at adequate pressure and temperature.
  • Be capable of sustained operation even with water-saturated fuel.
  • Ensure that any air introduced into the system does not cause a flameout.

Fuel Tanks

  • Integral Tanks ("Wet Wings"):
    • The tank is formed by sealing part of the airframe structure (wings/fuselage).
    • Advantage: Provides the highest volume for the lowest weight.
    • Common on modern large transport aircraft.
  • Rigid Tanks:
    • Sealed metal containers strapped into the airframe.
    • Simple to remove/repair but add weight.
  • Flexible Tanks (Bladder Tanks):
    • Reinforced rubberized bags.
    • Installation: Can be rolled up and installed through small openings (filler necks/panels).
    • Disadvantages: Prone to wrinkling (which can trap fuel, increasing unusable fuel quantity), working hardening, and cracking. They are more easily punctured than rigid tanks.

Components and Operation

  • Drain Valve: Located at the lowest point of the tank. Used during pre-flight to drain water, which is denser than fuel and settles at the bottom.
  • Vent System: Allows air to enter the tank to replace consumed fuel, preventing a vacuum (negative pressure) and assisting fuel transfer.
  • Filters: Remove contaminants before fuel reaches the engine.
  • Crossfeed System: In pressure-fed systems, it allows any engine to be fed from any tank. This is essential for correcting fuel imbalances or managing fuel during engine failure.

Fuel Management and Concepts

  • Unusable Fuel: The small quantity of fuel at the bottom of the tank that cannot be used. This amount is stated in the manual and is NOT indicated on the cockpit gauges.
  • AVGAS Grades:
    • AVGAS 100LL: Blue.
    • AVGAS 100: Green.
    • AVGAS 115: Green.
  • Circuit Breakers: In the event of a system failure, the standard procedure is often to pull and reset the circuit breaker once, unless the checklist specifies otherwise.