Aircraft Fuel Systems

Fuel Systems Overview

• Fuel systems are designed to store and deliver fuel to aircraft engines and auxiliary power units (APUs).
• Aircraft must carry enough fuel for extended operation.
• Fuel is stored and supplied to engines safely and at a controlled rate.
• Systems include tanks, lines, connections, and fittings compatible with all fuel types.
• Sub-systems include storage, refueling, distribution, transfer, venting, and indication.

Types of Fuel Systems

• Gravity Feed: Fuel flows by gravity from higher-level tanks to the carburetor, avoiding pumps
• Pump Feed: Employs pumps to deliver fuel under pressure to the engine.

Fuel Tank Categories

• Rigid: Constructed from metal or plastic, installed where space allows
• Flexible: Thin, flexible walls (bag-style tanks), or thicker, can be folded.
• Integral: Integrated into the aircraft structure

Fuel Feed Components

• Fuel Pumps (Booster Pumps): Two centrifugal pumps per tank, driven by 115-volt, three-phase motors. Output is approximately 250-300 liters/min, and maximum pressure at zero flow is around 38 psi. Each pump has a non-return and bypass valve.
• Jet Pumps: Used to deliver fuel to different parts of the system.
• Sequence Valves: Automatically transfer fuel between tanks, limiting outer tank pump pressure to 17.5 psi (priority for inner tanks).
• Transfer Valves: Transfer fuel between tanks.
• Low Pressure Shutoff Valve: Isolates the fuel system for maintenance or fire.
• Cross Feed Valve: Transfers fuel between different sections of the system.

Fuel Tank Construction

• Rigid tank structure, interior attachments, vent connections, and attachment rings for fittings.

Water Draining

• Drain valves situated at the lowest point of each tank, for water and sediment removal. Valves are typically opened with standard tools. Outer seal is replaceable without emptying the tanks.

Fuel Tank Drain Points

• Design allows for easy opening and closing of the valves

Fuel Collector Tanks

• Used in systems for coordinated fuel collection from various tanks.

Fuel Lines and Fittings

• MIL-spec lines and fittings (MIL-18794) define fuel line safety and durability.

Fuel Filler Caps

• Designs for filling aircraft fuel tanks are safety critical and must not permit fuel leakage.

Hand Operated Valves (Cone Type)

• Components including handle, detent plate, spring, brass cone, valve housing are involved in manually controlling fuel flow.

Hand Operated Valves (Poppet Type)

• Components such as spring, poppet valve, and cam, control fuel flow in the valve mechanism.

Motor Operated Valves

• Components such as a gate, and a motor direct fuel flow to the engine.

Solenoid Operated Valves

• Components such as spring, Solenoid are used to electronically control fuel flow.

Hand Gasoline Pump

• Components such as handle, outlet valve, housing, vane shaft, and inlet valve in the manual fueling pump.

Boost Pumps

• Motor-driven pumps for fuel delivery that are used in aircraft systems to increase pressure.

Fuel EJectors, Strainers

• Components used in fuel system to filter and manage flow.

Fuel Cross-feed System

• Coordinated transfer and distribution of fuel to various aircraft engines, and systems.

Vent System

• System elements such as vent float valve, breather assembly, and overpressure protector, direct air flow into and out of the fuel tank.

Drip Sticks

• Devices for measuring fuel levels in specific locations.

Flow Indicators

• Electronic gauge for measuring and displaying various fuel flow and remaining data.

Fuel Dumping (Jettison)

• Method to jettison fuel in emergencies for weight reduction. System components include jettison valves, drive motors, and piping.

Fuel Jettison Control

• System elements used for controlling fuel jettisoning systems, including separate valve controls and electrical connectors.

The Vent Sub Systems

• System components that direct airflow into and out of fuel tanks to safeguard against pressure build-up or excessive pressure (inwards and outwards venting).

Venting Due to Heat

• Fuel systems must dynamically adjust to variations in temperature-driven expansion and contraction. Vents adapt to varying atmospheric pressures.

Unpressurized System Venting

• Venting of fuel tanks directly to the atmosphere.

Reid Vapour Pressure (RVP)

• Rate at which fuel vaporizes.

Float Valves

• Float valves adjust vents to regulate air flow in and our of tank at different flight levels.

Fuel Level Sensing

• Modern systems use sensors (thermistors) for precise fuel level readings. Older aircraft utilize float switches.

High Level Sensing

• Trigger mechanisms that close refuelling valves, and light-up 'FULL' warning on instrument panel in the case of over-fueling.

Overflow Sensing

• In case of over-fueling, the associated 'FULL' error warning will signal to the aircraft operator.

Low Level Sensing

• System that detects low fuel levels in outer tanks.

Calibration Sensing (Fuel Trim Only)

• System used to accurately measure the fuel levels in different tanks for quality assurance purposes.

Under Full Level Sensing

• System that detects low fuel levels.

Fuel Quantity Measurement and Indication

• System components and their related functions designed to quantify and display current fuel levels.

System Components

• Components employed in the system that quantify and display fuel levels.

Principle of Capacitance Gauging

• Principles describing how fuel level is measured based on capacitance variations.

Capacitance Gauging

• System components that facilitate sensing of fuel levels based on variations in electrical capacitance.

Fuel Quantity Indicating System

• System for measuring and displaying fuel quantity in aircraft tanks.

Basic Fuel System

• Overview of the core components making up a fuel system.

Refueling and Defueling

• Procedures for fueling and emptying aircraft fuel tanks.

Safety Precautions When Refueling

• Safety measures to prevent accidents during fueling, such as no smoking, appropriate gear, and proper positioning of equipment.

Defueling

• Techniques to empty aircraft fuel tanks, such as using a fuel tanker and aircraft booster pumps.

Longitudinal Balance Fuel System

• Aircraft configuration that dynamically adjusts to changing fuel levels to maintain stability and balance while flying.

Longitudinal Fuel Transfer

• System elements involved in adjusting fuel distribution for better aircraft trim during different phases of flight(e.g., take-off, landing, cruising)