# Turbine Engine Fuel System Fundamentals

> Jet fuel properties, boost and HP pumps, crossfeed management, and turbine fuel system architecture.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-08 — Fuel System
- **Topic code:** 021-08-02
- **Last reviewed:** 2026-03-24
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/fuel-system/turbine-engine/

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Turbine aircraft fuel systems are designed to handle high flow rates, widely varying temperatures, and critical weight management. They use specific types of kerosene-based fuels.

## Jet Fuel Characteristics

- **Jet A:**
    - Freezing point: **-40°C**
    - Flash point: **+38°C**
    - Common in the USA.
- **Jet A-1:**
    - Freezing point: **-47°C**
    - Flash point: **+38°C**
    - Density: Approx **0.8 kg/l** (Specific Gravity ~0.8).
    - The standard fuel for international commercial aviation.
- **Jet B:**
    - "Wide-cut" fuel (mixture of gasoline and kerosene).
    - Freezing point: **-60°C** (Ideal for extremely cold climates).
    - Flash point: **-20°C** (Highly flammable/volatile).
    - **Comparison:** Jet A has a higher freezing point and a higher flash point than Jet B.
- **Additives:**
    - **FSII (Fuel System Icing Inhibitors):** Prevent water freezing/ice crystals. Mandatory in military jets; civil jets usually use fuel heaters instead.

## System Components

- **Pumps:**
    - **LP Booster Pumps (AC):** Centrifugal pumps located in the tanks. They pressurize the fuel lines to preventing **vapor lock** and **cavitation** at the engine-driven HP pump inlet.
    - **HP Pump:** Engine-driven. If booster pumps fail, the HP pump can often "suck" fuel (gravity feed), but operations are restricted.
    - **DC Pump:** Primarily used to supply fuel to the **APU** for starting.
- **Crossfeed Valve:**
    - **Purpose:** Allows **any fuel tank to feed any engine**. It maintains aircraft stability and balance.
    - **Operation for Balancing:** To correct an imbalance (e.g., Low fuel in Left tank), the pilot **opens the crossfeed valve** and turns **OFF** the pumps in the low (Left) tank. The engines will then feed from the high (Right) tank pumps.
- **Vents:**
    - Allow air to replace consumed fuel to prevent a vacuum.
    - Often use **ram air** to provide a small positive pressure head to the pumps.
- **Baffles:**
    - Internal barriers to prevent fuel "sloshing" during maneuvers, which could cause rapid CG shifts or unporting of pumps.

## Advanced Features

- **Trim Tanks:**
    - Located in the **tail** (horizontal stabilizer).
    - Fuel is transferred aft to move the CG rearward during cruise.
    - **Benefit:** Reduces the aerodynamic downforce required from the tail, thereby **reducing trim drag** and fuel consumption.
- **Fuel Jettison (Dumping):**
    - Allows the crew to reduce aircraft weight to the **Maximum Landing Mass** quickly in an emergency (e.g., engine failure shortly after takeoff).

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.