Thirty-two elements, eighteen of them skills, and only three need a bench. Most of this subject is inspection, documentation and diagnosis.
1. R4 — fuel tank entry
Fuel system maintenance requiring fuel tank entry.
This is among the most dangerous jobs in aviation maintenance, and you will not do it.
A fuel tank is a confined space. The hazards compound:
- Vapor — flammable, and an ignition source is anything from a tool spark to static.
- Oxygen displacement — vapor and purge gas displace air, and a tank can be lethal to enter without any fire at all.
- Toxicity — prolonged vapor exposure.
- Rescue — a person who collapses inside a tank cannot easily be recovered, and would-be rescuers are the commonest secondary casualties in confined-space incidents.
So it is done with a permit system, a standby attendant outside who does not enter, atmospheric monitoring, forced ventilation, bonded and intrinsically safe equipment, and a rescue plan. Not one of those is optional and none of them substitutes for another.
Why you learn it if you will not do it: because the failure mode is somebody deciding it looks manageable. The rule to carry is simple: not without the permit, the attendant and the monitoring. A tank that has been open for a week and smells of nothing is still a confined space.
R3 fuel spills and R5 defuelling are the same family at lower intensity — and both are live
issues in GEN-F.
2. Three simulator elements, and what that means for you
S6 a fuel valve problem, S15 a fuel pressure warning system, S18 a fuel quantity indicating
system — three troubleshooting elements delivered on the simulator. Only landing gear has more in
Airframe. S3, troubleshoot and repair a fuel system, you do in the shop on the practical-project
aircraft, because the repair includes replacing and adjusting components.
There is an open question about whether simulated troubleshooting transfers to hardware. The credential records it (the landing gear station specification), and our capital estimate carries it as a contingency for the credential's own apparatus.
For you it changes nothing. The diagnostic reasoning is identical whether you practice on a simulator or on hardware: know the system, predict what each fault would produce, measure where the prediction differs. GEN-A Concepts §2 is the general form.
3. Contamination is the fault you will actually meet
R2, and it connects straight to GEN-F Concepts §3.
Water is the commonest contaminant and it gets in by condensation on tank walls as temperature falls. It sits at the bottom — which is where the sumps are — and it enters the engine as a slug that stops it.
Microbial growth occurs at the fuel–water interface in tanks that hold water for long periods. It looks like dark sludge, it blocks filters, and it corrodes the tank structure underneath it, which makes it a structural problem as well as a fuel one.
Rust, sealant debris, and rubber from deteriorating cells are the rest.
The defense is boring and effective: sump every drain regularly, keep tanks full where the aircraft's operation allows it to reduce condensation, and investigate rather than dismiss anything found in a sample.
4. Selector valves fail internally
S4, S5, S6, S17, S29.
A valve that turns is not a valve that seals. The detents can feel positive while the internal porting is worn, cracked or partially blocked, and the failure modes are specific:
- Internal leakage between ports — feeding from a tank you did not select, which shows up as a fuel imbalance nobody can explain.
- Restriction — insufficient flow at high power, which appears only under load.
- Incorrect detent alignment — the handle says one tank and the valve is between positions.
So checking a selector means checking flow and position, not feel. S5 — checking manually
operated valves for proper operation — is exactly this.
5. Quantity indication: two technologies, two failure signatures
K9, S9, S18.
Float systems are mechanical: a float, an arm, a variable resistance. They fail mechanically — a stuck float, a bent arm, a worn resistance track — and they fail at a position, so the reading sticks or reads wrong over part of the range.
Capacitance systems measure the dielectric between probes, which changes with fuel level. They fail electrically — contamination on a probe, water in the tank changing the dielectric, a wiring or ground fault — and they typically read wrong across the whole range rather than at one point.
Knowing which technology you have tells you where to start, and it is the difference between a systematic diagnosis and swapping parts.
6. Six Locate elements, and why that is right
S10–S14 and S16 cover crossfeed, defuelling procedures, fuel temperature systems, required
placards and operating instructions — systems a light aircraft does not have.
The ACS handles this the way it handles rotorcraft in AF-N: documentation satisfies the element. You must be able to find and explain the procedure, not perform it on hardware nobody has.
No transport-category fuel system is required to satisfy this subject.
7. What consumes this unit
| This unit | Is consumed by |
|---|---|
| Fuel grades, sumping, contamination | Arrives from GEN-F Concepts §3 |
| Lines, fittings and seals | Arrives from GEN-D |
| Quantity indicating faults | PP-D — the engine end |
| Tank structure and sealant | AF-A, AF-B — integral tanks are structure |
| Fuel to the engine | PP-I — where it goes next |
License
CC BY-SA 4.0 (Licensing).