Fuel Systems

Fuel Systems: Inspection, Servicing, and Operational Checks

Aircraft fuel systems deliver clean, uncontaminated fuel at the correct pressure and flow rate to the engine under all normal and emergency flight conditions. Because fuel contamination and system failures have been causal factors in many accidents, the Aviation Maintenance Technician (AMT) must approach every fuel system task with precision, disciplined procedure-following, and a safety-first mindset.

System Overview

A typical reciprocating-engine light aircraft fuel system includes tanks (usually integral wing tanks or bladder tanks), tank vents, fuel lines, a main strainer or gascolator, a fuel selector valve, a boost (electric) pump, an engine-driven pump, a carburetor or fuel-injection unit, and quantity indicators. Turbine systems add components such as fuel-oil heat exchangers, flow dividers, and dump valves, but the inspection fundamentals are the same. Every component must be serviceable and every joint must be fuel-tight before the aircraft is returned to service.

Fuel Types and Grades

Aviation gasoline (avgas) for piston engines is most commonly 100LL (low-lead), dyed blue. Other grades such as 100 (green) and 80 (red) are now rarely encountered in the field. Aviation turbine fuel (Jet-A) is clear to straw-colored and has a distinctive kerosene odor. Using the wrong grade or type—especially Jet-A in a piston engine or avgas in a turbine—is a serious airworthiness hazard. The color-comparison card used during sump sampling gives a fast visual reference; when in doubt, confirm fuel type and grade from the placards, fuel receipt, and the POH.

Contamination: Detection and Hazards

Water is the most common contaminant. Water in avgas sinks to the lowest point of the tank or strainer because it is denser than fuel. A small amount of free water can cause engine power loss or stoppage; even suspended water in cold conditions can form ice that blocks filters. Sediment (rust, dirt, tank sealant particles) clogs fuel nozzles and strainers. Microbial growth (rare in avgas, more common in Jet-A) forms a dark slime at the fuel-water interface.

Drain every sump—all tank low points and the main strainer—into a clear sampling container. Examine the sample against white background and compare to the color card. Any visible water (which separates and sits on the bottom as a distinct layer), sediment, or color change is a failed sample. Continued draining and re-sampling is required until clean, on-color fuel flows free of contamination. Document every sample on the fuel-sampling log: sump location, quantity drained, color, odor, presence of water or sediment, and pass/fail determination.

Fuel System Operational Check

Before activating any electrical component, confirm all fuel caps are secure and no personnel are standing in the propeller arc. Follow the Aircraft Maintenance Manual (AMM) exactly—improvising sequence is not permitted. A typical operational check sequence includes:

1. Select the appropriate fuel tank per the AMM.

2. Activate the electric boost pump and verify fuel pressure rises to the specified range on the cockpit gauge.

3. Check all accessible lines, fittings, and connections for seeping or dripping fuel.

4. Rotate the fuel selector through each placarded position (BOTH, LEFT, RIGHT, OFF) and verify detent engagement and placarded alignment.

5. Verify fuel quantity indicators correspond to known fuel load.

6. Annotate every step—results, discrepancies, and corrective actions—on the work order as you go. Incomplete work orders are not acceptable for maintenance records.

Any fuel leak, quantity discrepancy, or indicator malfunction is a write-up requiring a corrective maintenance action before the aircraft can be returned to service.

Defueling and Decontamination (Wrong-Fuel Scenario)

If the wrong fuel grade has been introduced, defueling is mandatory. This procedure introduces major fire and static discharge hazards. Follow the AMM and POH defueling procedure without deviation:

Bonding and Grounding: Bond the aircraft to the defueling receptacle or bowser before any fuel cap is opened. Ground the fuel receptacle to an approved earth ground. Bond the dispensing nozzle to the aircraft. This three-point bonding and grounding sequence dissipates static charge and is the single most important fire-prevention step during fuel handling.

Fire Extinguisher: A serviceable dry-chemical or CO₂ extinguisher of the type and minimum rating specified by your facility's fire safety plan must be positioned within reach—typically within 10 feet—before fuel caps are opened. Assign a dedicated fire watch if required by local procedures.

Defueling: Use approved fuel hose and pump equipment. Drain into approved, labeled containers or a fuel truck designated for contaminated fuel. Keep drip pans under all points where fuel may spill. Wear nitrile (or other chemical-resistant) gloves throughout.

Disposal Documentation: Contaminated fuel may not be dumped on the ground or into a drain. Document the quantity removed, the reason for disposal, and the disposal method on the work order and any facility hazardous-material disposal forms required by local regulation. Retain records per your facility's procedures.

After defueling, flush the system per AMM guidance, refuel with the correct, verified fuel, and perform a full contamination inspection before returning the aircraft to service.

Personal Protective Equipment and General Safety

• Wear nitrile gloves when handling fuel to prevent skin absorption of hydrocarbons.
• Safety glasses protect against splashing fuel during sump draining.
• Work only in well-ventilated areas or outdoors; fuel vapor is heavier than air and accumulates in low spots.
• No open flames, sparks, or smoking within the fuel-handling area.
• Keep cell phones and other ignition sources away from open fuel.
• Dispose of fuel-soaked rags in approved, self-closing metal containers.