Aircraft Fuel System Notes

Aircraft Fuel System Overview

  • Definition: The aircraft fuel system manages the loading, storage, and delivery of fuel to the engines. It varies in complexity based on aircraft size and type.

  • Significance: The fuel system affects performance significantly; an empty tank leads to forced landings and potential damage to the aircraft.

Types of Aviation Fuel

  • Main Types:

    • AVGAS: Aviation gasoline for conventional piston engines.

    • AVTUR: Aviation turbine fuel (kerosene).

    • AVTAG: Aviation turbine gasoline.

AVGAS Characteristics

  • Specific Gravity (SG):

    • AVGAS: 0.74 at 15 °C.

    • Avtur SG range: 0.75-0.84.

  • Fuel Grades:

    • 80 Grade: Red

    • 100 Grade: Green

    • 100LL Grade: Low lead, blue

Fuel Properties

  • Volatile: Changes from liquid to vapor readily.

  • Vapor Pressure: Pressure at which fuel vaporizes.

  • Flash Point: Minimum temperature to ignite vapors without a flame.

  • Fire Point: Minimum temperature to sustain combustion.

  • Auto-Ignition Temperature: Point of self-ignition without an external source.

  • Freezing Point: Temperature below which ice crystals start to form.

Specific Fuel Types and Grades

  • Jet Fuels:

    • Jet A1: Widely used due to low freezing point.

    • Jet A: Slightly higher freezing point than Jet A1.

    • Jet B: Very low freezing point, more volatile, used in cold climates.

    • JP-5: Military fuel with a higher flash point for safety.

Fuel Delivery Systems

  • Gravity Fed Systems:

    • Utilizes gravity to move fuel, commonly in high-wing aircraft like Cessna 152 and Cessna 172.

  • Pump Feed Systems:

    • Used in low/mid-wing aircraft where tanks are below the engine. Powered by electric pumps to supply required pressure.

Fuel System Components

  • Fuel Strainers: Remove impurities from fuel.

  • Fuel Selector Valve: Directs fuel flow from chosen tank.

  • Primers: Used to introduce fuel into the engine for start-up.

Fuel Tank Types

  • Bladder Tanks: Made of rubber; prone to wear but replaceable.

  • Rigid Removable Tanks: Aluminum tanks; removable for repairs but heavier in weight.

  • Integral Tanks (Wet Wing): Use the aircraft's structure; common in large aircraft, lightweight.

Collector Tanks

  • Prevent fuel starvation during maneuvers by collecting and ensuring a steady supply to the engine.

Precautions

  • Fuel splashing must be controlled to prevent hazards. Dampening devices are essential in wing tanks during the aircraft's attitude changes.

Conclusion

  • Understanding the fuel systems of aircraft is critical for operational safety and efficiency during flight operations.