Fuel system maintenance

Six procedures. Everything here involves a flammable liquid and its vapor, and vapor is the part people underestimate.

Procedure 0 — Make it safe

1. Bond the aircraft and any container to a common point.
2. Ventilate. Vapor is heavier than air and collects in low points and in the aircraft.
3. Remove ignition sources — no power on, no non-intrinsically-safe tools, no static-prone clothing.
4. Fire extinguisher present, correct class, and you know where it is.
5. Containment for spills, and a plan for the fuel you are about to have in the open.
6. PPE per the safety data sheet.

Check — say where the vapor will go. Fuel vapor flows downhill and pools. If the answer is "into the hangar", ventilate before you start rather than after.

Procedure 1 — Drain and inspect a fuel sample (S1, with GEN-F Procedures Procedure 6)

1. Sump every drain point, not just the accessible one.
2. Use a clear sampler against a light background.
3. Look for water at the bottom, sediment, microbial growth — dark stringy sludge at the interface — and any color change.
4. Confirm grade by color and smell.
5. Continue until clean.
6. Investigate anything found rather than sumping until it goes away — repeated water means a cap seal, a drain, or condensation that needs addressing.

Check — water and fuel can be hard to tell apart in a sampler. Tilt it and watch how the interface moves, or use a water-detecting capsule.

Procedure 2 — Inspect a tank (S2)

Externally, and without entering it (AF-J Concepts §1).

1. Metal tanks: corrosion, especially at the bottom and at seams; cracks at fittings and mounting points; security.
2. Bladder cells: hardening, cracking, wrinkling, chafe at supports, condition of the cavity — a wrinkle traps water and the cavity beneath corrodes.
3. Integral tanks: sealant condition at seams and fasteners, evidence of seepage, corrosion, structural condition. An integral tank is structure, and its inspection is AF-A Procedures Procedure 1 as well.
4. Vents clear — a blocked vent collapses a tank or starves the engine.
5. Caps and seals — the commonest water ingress path.
6. Drains functional and not weeping.

Check — check the vent. It is easy to overlook, and a blocked vent produces symptoms — fuel starvation, tank deformation — that look like something else entirely.

Procedure 3 — Service a strainer or filter (S8)

1. Procedure 0.
2. Note the fuel selector position and the manual's requirement — some strainers drain the tanks if the selector is on.
3. Remove, catching fuel.
4. Inspect what is in it: water, sediment, rubber, sealant, metal, microbial growth. Identify what you find — it tells you what is happening upstream.
5. Clean or replace per the manual, with a new gasket.
6. Reassemble, check for leaks with the system pressurized, and safety as required.

Check — look at the contents before you wash them away. It is the only direct evidence you get about the inside of the tanks (PP-G §1 — the same reasoning in oil).

Procedure 4 — Check a fuel selector valve (S4, S5, S17, S29)

1. Operate it through every position, feeling for positive detents.
2. Confirm the handle position matches the placard and the internal position — check against the manual's markings, not the handle alone.
3. Check flow at each position where the procedure allows, or check that the correct tank feeds.
4. Look for external leakage at the shaft seal.
5. Check stops and detents for wear.

Check — check flow or feed, not feel. A valve with positive detents and worn internal porting feels perfect and cross-feeds (AF-J Concepts §4).

Procedure 5 — Inspect a quantity indicating system (S9)

1. Identify the technology — float or capacitance (AF-J Concepts §5).
2. Check indication against known quantity: empty, a known partial fill, and full where the procedure allows.
3. Float systems: check the float moves freely through range, arm not bent, no fuel in the float.
4. Capacitance systems: check probe condition, connections, grounds and wiring insulation.
5. Compare indication to actual across the range, not just at one point.

Check — an error at one point suggests a mechanical fault; an error across the range suggests an electrical one. Which one you have tells you where to look.

Procedure 6 — Remove and install a fuel quantity transmitter (S17)

1. Procedure 0, and defuel or lower the level below the unit per the manual.
2. Disconnect electrically, capping the connector.
3. Remove fasteners evenly; support the unit.
4. Fit a new gasket or seal, correct part number.
5. Install, torque evenly and per the manual — uneven torque distorts the flange and it leaks.
6. Reconnect, leak check with fuel present, and functionally check the indication.

Check — leak check with fuel actually against the seal. A dry installation proves nothing, and a fuel leak into a wing is not something to discover in flight.

The checks, collected

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