Chapter 9: Engine Fuel and Fuel Metering Systems

Chapter: Engine Fuel and Fuel Metering Systems

Overview

This chapter covers the fundamental principles, maintenance practices, and regulatory requirements governing aircraft engine fuel systems and fuel metering components. The material addresses both reciprocating and turbine engine fuel systems, emphasizing safety-critical inspection procedures, troubleshooting methodologies, and airworthiness standards. Understanding the relationship between fuel system components, their function, and proper maintenance procedures is essential for the aircraft maintenance engineer.

Fuel System Components and Their Functions

Fuel Lines and Fittings

Aircraft fuel lines transport fuel from the tanks to the engine under varying pressure and temperature conditions. These lines are typically constructed from aluminum alloy tubing, stainless steel, or flexible hose assemblies depending on their location and function.

Line Construction and Routing Requirements:

• Rigid metal lines must be installed with proper bend radii to prevent stress concentrations and flow restrictions
• Lines must be supported with approved clamps at specified intervals to prevent chafing and vibration damage
• Flexible hoses require additional support and must be inspected for deterioration, cracking, and outer cover damage
• All fuel lines must be routed away from heat sources, moving components, and areas where they could be damaged

Fitting Types and Maintenance:

• B-nut fittings (flared-type connections) are common on rigid fuel lines and require careful torque application
• AN fittings must be inspected for galling, cracks, and proper seating before assembly
• Over-tightening a leaking B-nut can deform the fitting or line, causing permanent damage
• When a leak persists after initial tightening to specification, the component must be replaced rather than subjected to additional torque

Inspection Criteria:

• Chafing marks on the outer sleeve indicate contact with adjacent structure and require line replacement
• Sharp bends restrict fuel flow and create stress fracture points
• Cracked lines constitute a serious fire hazard and must be replaced using approved methods
• Taping, sealant application, or other temporary repairs are not approved for fuel lines

Fuel Pumps

Engine-Driven Pumps:

The engine-driven fuel pump is a positive displacement device that delivers fuel from the tank to the carburetor or fuel injection system. These pumps are typically gear-type or vane-type and are driven through a coupling mechanism.

Drive Coupling Wear:

Excessive wear on the drive coupling is a critical finding that indicates impending pump failure. A worn coupling can cause:

• Intermittent fuel flow
• Low fuel pressure at the low end of the specified range
• Complete fuel starvation and engine failure

The drive coupling must be inspected during every 100-hour inspection. When wear is detected, the pump must be replaced—not lubricated, adjusted, or returned to service. The worn coupling cannot be compensated for by adjusting the relief valve or cleaning screens.

Electric Boost Pumps:

Electric boost pumps provide fuel pressure during engine start and serve as a backup to the engine-driven pump. These pumps must be certified components (TSO'd or PMA'd) when installed. Installation of non-certified aftermarket pumps without proper approval constitutes a major alteration requiring FAA Form 337 and field approval.

Fuel Pump Pressure Testing:

Fuel pumps must produce pressure within the manufacturer's specified range. Testing involves:

• Verifying pressure at idle and full power
• Checking pressure drop during high fuel flow conditions
• Confirming the pump maintains pressure without excessive fluctuation

Fuel Strainers and Filters

Fuel strainers and filters remove contaminants and water from the fuel before it reaches the metering system. These components require regular inspection and maintenance.

Strainer Maintenance:

• Sediment and water accumulation indicates contamination that must be addressed
• The strainer must be drained, cleaned, and the source of contamination investigated
• Simply replacing the element without addressing the source is insufficient
• Fuel system contamination can lead to injector blockage, fuel starvation, and engine failure

Filter Differential Pressure:

Turbine engine fuel filters are monitored by differential pressure gauges. A high reading indicates a clogged filter element that restricts fuel flow. Symptoms of a clogged filter include:

• Gradual decrease in fuel flow
• Increase in exhaust gas temperature (EGT) due to lean mixture
• Engine surging or loss of power at high power settings

The correct action is to replace the filter element and inspect the fuel system for contamination to prevent recurrence.

Fuel Metering Systems

Carbureted Systems:

Carburetors mix fuel and air in the correct proportion for combustion. Key components include:

• Float bowl and float mechanism
• Metering jets and passages
• Mixture control system
• Accelerating pump (where applicable)

Float Bowl Gasket Maintenance:

A weeping float bowl gasket requires replacement, not tightening or sealant application. Sealant can contaminate the fuel system and is not an approved repair method. This is routine maintenance within the A&P mechanic's privileges.

Fuel Injection Systems:

Fuel injection systems deliver metered fuel directly to each cylinder. Components include:

• Fuel control unit (FCU)
• Fuel manifold valve
• Injection nozzles
• Fuel flow transducer

Injector Nozzle Conditions:

• A leaking injector nozzle allows excess fuel to flow, increasing fuel flow readings without a corresponding pressure increase
• A partially blocked nozzle creates a lean mixture in that cylinder, causing rough idle and backfiring through the induction system
• Cracked nozzles are safety-critical defects requiring replacement per manufacturer's instructions—welding or epoxying is not approved

Troubleshooting Approach:

Before condemning expensive components like the FCU, check for simpler causes:

• Contaminated fuel or restricted filters can cause erratic fuel flow and surging
• Loose or corroded electrical connections can cause gauge fluctuations without affecting engine operation
• A smooth-running engine with a fluctuating fuel flow gauge points to an electrical issue, not a fuel delivery problem

Fuel Valves and Controls

Fuel Selector Valves:

Fuel selector valves control fuel flow from tanks to the engine. These multi-position, detented units require smooth operation and proper indexing.

Common Problems:

• Hard rotation typically indicates contamination, dried grease, or corrosion inside the valve
• The correct first step is disassembly, cleaning, and lubrication with approved fuel-resistant lubricant
• Replacement is only necessary if cleaning does not resolve the problem
• Forcing the valve can cause damage and compromise safety
• Improper indexing to detent positions requires linkage adjustment and verification

Fuel Pressure Relief Valves:

A leaking fuel pressure relief valve causes pressure fluctuations and creates a fire hazard. These valves must be replaced, not repaired, and verified for proper operation after installation.

Fuel Tank Vent System:

Proper venting prevents pressure differentials that can cause fuel starvation or tank collapse. A blocked vent line can cause:

• Vacuum formation in the tank
• Intermittent fuel flow and pressure fluctuation
• Fuel pump drawing fuel intermittently

The vent system must be cleared and verified functional before return to service.

Fuel Quantity and Flow Indication

Fuel Flow Transducers:

These devices measure fuel flow rate and transmit signals to cockpit gauges. Troubleshooting considerations:

• A loose or corroded electrical connection is the most common cause of gauge fluctuation
• Air in the fuel lines would cause engine surging or roughness, not just gauge fluctuation
• A worn fuel pump produces pressure drops and performance issues, not isolated gauge anomalies

Fuel Pressure Gauges:

Rapid pressure fluctuation can indicate:

• Partially blocked fuel tank vent creating vacuum
• Fuel pump malfunction
• Sticking bypass valve

Regulatory Requirements and Documentation

14 CFR 43.9 Maintenance Record Entries

After performing maintenance, the mechanic must make a logbook entry containing:

• A description of the work performed (or reference to acceptable data)
• The date of completion
• The mechanic's signature and certificate number

This entry is the legal record of maintenance and is critical for future inspections and AD compliance.

Major Repairs and Alterations

Major Repair Definition:

Replacing a fuel tank is classified as a major repair per 14 CFR 43, Appendix A. This requires:

• A logbook entry per 14 CFR 43.9
• Completion of FAA Form 337
• Submission of the form to the FAA

Major Alteration Requirements:

Installing non-original components without STC or field approval constitutes an unapproved major alteration. Examples include:

• Non-certified aftermarket fuel pumps
• Non-original quick-drain valves
• Any deviation from type design

The aircraft must be restored to its original configuration or an approved one before return to service.

Return to Service Requirements

• 14 CFR 91.7 prohibits operating an unairworthy aircraft
• Known fuel leaks, worn components, or unapproved alterations make the aircraft ineligible for return to service
• All repairs must be performed using approved methods per 14 CFR 43.13(a)
• Parts must be approved (PMA, TSO, or original manufacturer) for the application

Safety Considerations

Fire Prevention

Fuel system leaks present immediate fire hazards. Any evidence of fuel leakage requires immediate corrective action before further flight. The aircraft must be grounded and repaired per approved procedures.

Static Electricity and Bonding

Proper bonding and grounding of fuel system components prevents static electricity buildup. A spark from static discharge can ignite fuel vapors, especially during refueling operations. The primary hazard of an unbonded fuel filler cap is the potential for static spark ignition.

Vapor Lock Prevention

Vapor lock occurs when fuel vaporizes in the lines, often during climb when temperatures and altitudes change. This causes fuel starvation and power loss. Proper fuel system design and maintenance prevent vapor lock conditions.

Troubleshooting Methodology

Systematic Approach

Effective troubleshooting follows a logical progression:

1. Verify the reported symptom
2. Check the simplest and most likely causes first
3. Use available instrumentation (pressure gauges, flow meters, differential pressure indicators)
4. Isolate components to determine the source of the problem
5. Replace components only after confirming the diagnosis

Common Symptom Patterns

Pressure Testing Procedures

When a fuel system must hold pressure for a specified time (e.g., 10 minutes), a gradual pressure drop indicates a leak. The proper approach is to:

1. Isolate components to determine the leak source
2. Inspect fittings and connections
3. Repair or replace the leaking component
4. Re-test to verify the system holds pressure

Approved Maintenance Practices

Fuel System Component Replacement

Components that must be replaced rather than repaired include:

• Cracked fuel lines
• Worn fuel pump drive couplings
• Leaking fuel pressure relief valves
• Cracked fuel nozzles
• Damaged control cables
• Chafed fuel lines with outer cover damage

Inspection and Cleaning Procedures

• Fuel strainers must be drained and cleaned regularly
• Fuel selector valves should be disassembled, cleaned, and lubricated with approved fuel-resistant lubricant
• Fuel lines must be inspected for chafing, sharp bends, and proper support
• All connections should be checked for security and leaks

Documentation Requirements

Every maintenance action must be properly documented:

• Routine maintenance: Logbook entry per 14 CFR 43.9
• Major repairs: Logbook entry plus FAA Form 337
• Major alterations: Logbook entry plus FAA Form 337 with field approval

Summary

The engine fuel and fuel metering system is safety-critical and requires meticulous maintenance. Key principles include:

• Fuel system integrity is paramount—leaks and damaged components must be addressed immediately
• Approved methods and parts must be used for all repairs
• Proper documentation is legally required for all maintenance actions
• Systematic troubleshooting prevents unnecessary component replacement
• Understanding the relationship between fuel system components and engine operation is essential for accurate diagnosis

The maintenance technician must balance technical knowledge, regulatory compliance, and practical troubleshooting skills to ensure fuel systems are returned to service in an airworthy condition.