Overview

This chapter covers the design, operation, inspection, and maintenance of aircraft fuel systems. It encompasses the full range of system components from storage tanks through delivery mechanisms to engine metering devices, including fuel quantity indication, venting, pumping, filtration, and safety systems. The material addresses both piston-engine and turbine-engine aircraft fuel systems, with emphasis on the differences in their design philosophies and operational characteristics.

Key Concepts Explained

Fuel Storage Systems

Integral Fuel Tanks are sections of the aircraft structure—typically the wing box or center section—sealed with fuel-tight sealant to contain fuel. These are common in larger aircraft and require careful inspection for sealant degradation and leaks.

Bladder Fuel Tanks are flexible fuel containers installed within a structural cavity. They are common in helicopters and some fixed-wing aircraft. Bladders can be repaired using manufacturer-approved repair kits, provided the damage is within specified limits. Chafing against structural supports must be addressed by both repairing the bladder and correcting the cause of the chafing.

Rigid Fuel Tanks are typically made of aluminum or stainless steel and are installed as separate units within the airframe.

Fuel Vent Systems

The fuel vent system serves three critical functions:

Allows air to enter the tank as fuel is consumed, preventing vacuum formation
Allows fuel vapors to escape, preventing pressure buildup
Prevents over-pressurization during refueling

A blocked vent system is a critical safety issue. The most immediate consequence is that a vacuum will form inside the tank as fuel is consumed, potentially causing the tank to collapse or implode. During refueling or in hot weather, pressure can build up, potentially damaging the tank. Secondary effects include fuel pump cavitation and engine fuel starvation.

Fuel Pumps

Boost Pumps provide positive pressure to the engine-driven fuel pump inlet, preventing cavitation. Two common types exist:

Centrifugal boost pumps are not positive displacement pumps. They use a rotating impeller to create flow and pressure. Their key advantage is the ability to handle air in the fuel without damage, making them ideal as boost pumps.

Vane-type boost pumps are positive displacement pumps that provide constant flow against varying back pressures.

Engine-Driven Pumps are typically gear-type positive displacement pumps. They draw fuel from the tank through the boost pump and deliver it to the fuel control unit at high pressure.

Transfer Pumps move fuel from auxiliary or reserve tanks to the main tanks, as opposed to boost pumps which provide pressure to the engine feed system.

Scavenge Pumps remove residual fuel from the engine's fuel manifolds and nozzles after shutdown to prevent coking (varnish formation from heat) and reduce the risk of engine fires.

Fuel System Components

Fuel Strainers (Gascolators) serve multiple functions in light aircraft fuel systems: they filter fuel, provide a water drain point, and trap contaminants before they reach the carburetor.

Fuel Filters remove solid impurities from the fuel. Some filters also have water-separating capability. A fuel bypass valve opens when the filter becomes clogged, allowing unfiltered fuel to flow to the engine to prevent fuel starvation. This is a safety feature, not a normal operating condition.

Fuel Pressure Regulators maintain constant pressure in the fuel system, ensuring consistent fuel delivery regardless of changes in flow demand.

Fuel Pressure Relief Valves protect the system from over-pressure, regulate pressure, and return excess fuel to the tank.

Fuel Shutoff Valves (firewall shutoff valves) allow the crew to shut off all fuel to the engine in the event of an engine fire or other emergency.

Fuel Selector Valves allow the pilot to select which tank supplies fuel to the engine. A stiff valve indicates internal contamination or seal deterioration and requires disassembly, inspection, and lubrication with approved fuel-compatible lubricant.

Fuel Cross-Feed Valves allow fuel to be transferred between tanks or allow an engine to draw fuel from a tank on the opposite side. A leaking cross-feed valve can cause fuel imbalance between tanks, affecting lateral balance and reducing usable fuel in one tank.

Fuel Quantity Indication

Resistance-Type (Float and Rheostat) Systems use a float that rises and falls with fuel level, moving a wiper arm across a resistive element. If the float sinks (fills with fuel), it will not rise with the fuel level, and the gauge will read empty regardless of actual fuel quantity.

Capacitance-Type Systems measure fuel quantity by sensing changes in the dielectric constant between probe plates. These systems are sensitive to water or other contaminants, which have different dielectric constants than fuel, causing erratic or inaccurate readings.

Troubleshooting Approach: Before replacing components, verify the accuracy of the indication by comparing cockpit readings with manual fuel stick measurements. This helps determine if the problem is with the indicating system or the actual fuel quantity.

Fuel Return Systems

In turbine engine fuel systems, the fuel return line serves multiple purposes:

Returns excess fuel from the engine-driven pump to the tank, maintaining constant pressure at the pump inlet and preventing cavitation
Returns hot fuel from the fuel control unit to the tank, preventing FCU overheating
Cools engine oil through the fuel/oil heat exchanger

The vapor return line returns fuel vapor and separated air back to the tank, preventing vapor lock in fuel lines, especially at high altitudes or in hot weather.

Fuel System Contamination

Water Contamination is a primary concern. Water enters through cracked fuel cap gaskets, condensation in partially filled tanks, and during refueling. Water in fuel can cause corrosion, microbial growth, and ice formation at altitude.

Microbial Growth (fuel bugs) appears as a dark, slimy substance in fuel sump drain samples. These microorganisms live at the fuel-water interface and can cause corrosion and fuel system blockages.

Fuel System Icing Inhibitor (FSII) is an additive that lowers the freezing point of water present in the fuel, preventing ice crystal formation that could block filters.

Fuel Nozzles

The primary function of fuel nozzles in turbine engines is to atomize fuel into a fine mist for rapid mixing with air and efficient combustion. A leaking fuel nozzle can cause incomplete combustion and higher exhaust gas temperature (EGT).

Fuel Jettison Systems

Fuel dump systems allow the crew to reduce aircraft weight to maximum certified landing weight in emergencies. These systems typically require fuel boost pumps to be operating to provide positive pressure for fuel expulsion.

Important Regulations and Procedures

CARs Standard 571 Requirements

571.02 - Maintenance Performance: Maintenance must be performed following the manufacturer's recommendations. Using substitute parts or materials not listed in the manufacturer's manual is not permitted unless specifically approved through a design change or alternative method of compliance.

571.03 - Maintenance Recording: Required elements include product identification, description of work, date, and employee identification. Cost of parts is not a required element.

571.05 - Maintenance Control: Maintenance on aircraft operated under Part VII (commercial operations) must be performed under the control of an approved maintenance organization (AMO).

571.06 - Acceptable Data: FAA Advisory Circulars such as AC 43.13-1B are considered acceptable data for repairs when manufacturer-specific instructions are not available.

571.08 - Used Parts: Used parts must be accompanied by a maintenance release from an AMO or manufacturer confirming serviceability.

571.09 - Life-Limited Parts: Strictly prohibits installation of a life-limited part that has reached or exceeded its life limit. No exceptions are permitted. The part must be removed from service and typically destroyed or quarantined.

571.10 - Maintenance Release: Must be signed by the person who performed the maintenance or a person authorized under 571.11.

Fuel Tank Repair Procedures

Hot Work Requirements: Before any welding or soldering on fuel tanks, the tank must be purged of all flammable vapors. This is typically done by steam cleaning or purging with inert gas (nitrogen or carbon dioxide).

Sealant Cure Times: After sealant application, cure times vary depending on sealant type, temperature, and humidity. Manufacturer's instructions must always be followed before pressure testing.

Integrity Testing: Fuel tank integrity checks typically require zero or near-zero pressure loss over a specified time. Any measurable pressure drop indicates a leak that must be found and repaired.

Fuel Line Standards

AN Fittings: Use a 37° flare, the standard for aircraft fluid systems. 45° flares are used for automotive applications.

MS Flareless Fittings: Commonly used for high-pressure fuel and hydraulic systems, providing more reliable, leak-free seals than flare fittings.

Tubing Materials: High-pressure fuel lines use stainless steel (304 or 321). Aluminum tubing is used for low-pressure supply lines.

Hose Specifications:

MIL-H-8794: High-pressure rubber fuel hose (up to 1,500 psi)
MIL-H-6000: Low-pressure rubber fuel hose

Minimum Bend Radius: Generally 10 times the hose outside diameter per AC 43.13-1B.

Dent Limits: Minor dents not exceeding 20% of tube diameter and not in bend areas are acceptable per AC 43.13-1B.

Common Relationships Between Concepts

Pressure and Flow Relationships

Low fuel pressure with normal fuel flow typically indicates a failing fuel pump (worn gears)
Low fuel pressure with low fuel flow typically indicates a clogged filter or restriction
Low inlet pressure to the engine-driven pump with boost pump operating indicates a restriction in the supply line or failing boost pump
Fluctuating fuel pressure indicates air in the system (vapor lock or cavitation)

Temperature Relationships

Hot fuel return lines in turbine engines are normal, as fuel serves as a heat sink for engine oil and FCU cooling
Fuel heaters prevent ice formation as fuel temperature drops at altitude
Fuel/oil heat exchangers use cool fuel to absorb heat from hot engine oil

Vent System Relationships

Blocked vent → vacuum in tank → tank collapse
Blocked vent → pressure buildup during refueling → tank damage
Blocked vent on one tank → engine runs rough when switching to that tank

Electrical System Relationships

Short circuit to ground in common wiring → full-scale reading on all fuel quantity gauges
Open circuit → zero reading on fuel quantity gauge
Broken bonding strap on filler neck → static discharge risk during refueling
Broken bonding strap on access panel → static discharge risk in fuel vapor area

Contamination Relationships

Water in fuel → microbial growth → filter blockage and corrosion
Water in fuel → ice crystals at altitude → filter blockage
Water in fuel → erratic capacitance gauge readings
Microbial growth → dark slimy substance in sump samples

Maintenance Action Relationships

Fuel staining at properly torqued coupling → requires pressure test to verify seal integrity
Chafed fuel hose with outer layer worn through → replace hose and address root cause
Fuel filter bypass indicator popped → replace filter and investigate contamination source
Stiff fuel selector valve → disassemble, inspect, lubricate with approved lubricant
Cracked fuel cap gasket → water ingress risk, replace gasket

Chapter 10: Aircraft Fuel Systems