Aircraft Fuel Systems: The A&P Exam Guide
Fuel system questions on the Airframe test are rarely about obscure hardware. They ask whether you understand how fuel moves from a tank to an engine without leaking, without picking up water, and without the pilot running a tank dry at the wrong moment.
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What AM.II.J Covers on the Airframe Test
Aircraft fuel systems are Subject J of the Airframe section (AM.II) of the Aviation Mechanic ACS. The knowledge items run from fuel grades through tanks, lines, valves, pumps, filters and quantity indication. The skill items are direct: inspect a fuel system, inspect a metal, bladder or integral tank, and troubleshoot and repair a fuel system. The Airframe written test is 100 questions in two hours, with 70 percent as the passing grade under 14 CFR 65.17(b).
Fuel Types and Grades
Two families matter: aviation gasoline for reciprocating engines, and turbine fuel for jets and turboprops. Grades are defined by specification and identified by dye color, which is how a line technician confirms what is in the truck.
| Grade | Specification | Color | Notes |
|---|---|---|---|
| Avgas 80 | ASTM D910 | Red | Low compression piston engines, largely retired |
| Avgas 100 | ASTM D910 | Green | High compression piston engines |
| Avgas 100LL | ASTM D910 | Blue | Same ratings as grade 100, lower lead, most available avgas |
| Jet A | ASTM D1655 | Colorless to straw | US turbine fuel, freeze point minus 40 C |
| Jet A-1 | ASTM D1655 | Colorless to straw | International turbine fuel, freeze point minus 47 C |
The numbers in avgas grade names are performance ratings, not a measure of energy content. Turbine fuel is kerosene based, has a higher flash point than avgas, and carries no octane rating. A piston engine will not run on Jet A, and a turbine will not run on avgas.
Why Mixing Grades Is Prohibited
The engine and the airframe are certificated for a specific fuel, and 14 CFR 91.9 requires operation inside the limitations of the approved flight manual. A lower octane fuel can detonate in an engine built for grade 100, and lead from avgas has no place in a turbine system, where no filter removes it.
Anti-misfueling hardware is the second line of defense. AC 20-122A describes the devices that keep a jet nozzle out of an avgas filler neck: the avgas nozzle is small and round, the jet nozzle larger and flattened. Color is a guide, not proof, so confirm the delivery ticket. If a misfueling does happen, do not start the engine. Defuel, flush and document it as the manufacturer's instructions require.
The Components the Questions Target
- Tanks. Integral tanks are structure sealed to hold fuel, bladder cells sit in a cavity, rigid tanks are bolted in. Every tank needs a vent, and a blocked vent can starve an engine or overstress a tank.
- Pumps. The engine driven pump is the primary supply. A boost pump, usually electric, covers starting, vapor suppression at altitude and the failure case. Check valves prevent reverse flow.
- Selector, shutoff and crossfeed valves. The selector chooses the tank, the firewall shutoff valve stops flow to the engine, and the crossfeed valve lets either engine draw from either tank.
- Strainers, filters and drains. A finger strainer at the tank outlet, a main strainer or filter before the metering unit, and a drain at every low point.
- Indication. Float and resistance senders on light aircraft, capacitance probes on transport aircraft, plus a pressure gauge and flowmeter.
Water, Microbes, and Sump Checks
Water is heavier than avgas and settles at the lowest point, which is why every tank sump, gascolator and line drain sits where it does. Drain into a clear cup: a few drops of water means draining until the fuel runs clear. Every drain gets checked, not just the easy one.
Turbine fuel holds more dissolved water than avgas. As the fuel cools, that water comes out of solution and can freeze in a line or feed the microbes that live at the fuel and water interface. The common organism, Hormoconis resinae, formerly Cladosporium resinae, forms dark sludge that plugs filters, fouls quantity probes and corrodes aluminum. Biocide treatment follows the manufacturer's instructions, but removing the water is what keeps it away. More on tank condition is in our corrosion control guide.
Inspection and Leak Checks Under Part 43 and AC 43.13-1B
14 CFR 43.13(a) requires each person performing maintenance to use the methods in the manufacturer's maintenance manual, or the methods in AC 43.13-1B where the manufacturer gives none. Chapter 8, Section 2 of AC 43.13-1B covers fuel systems in paragraphs 8-30 through 8-38: lines and fittings, tanks and cells, caps, vents and overflow lines, crossfeed and shutoff and selector valves, pumps, filters and drains, and indicator systems. Paragraph 11-191 covers bonding.
The annual or 100-hour inspection required by 14 CFR 91.409 is performed to the scope in Part 43 Appendix D. On the fuel system, look for:
- Leaks. Blue or red staining at seams, rivets, fittings and drain valves, and the wet film of a slow seep.
- Line condition. Chafing, work hardening at a bend, loose clamps, over-torqued fittings.
- Vents and caps. A blocked vent, hardened cap seal or missing overflow line shows up as a feeding problem before it shows up as a leak.
- Bonding. Resistance across the tank, line and structure path that carries static to ground.
A repaired tank is leak checked at the pressure the manufacturer specifies, never higher. Integral tank sealant work depends on surface preparation more than on the sealant, and a repair applied over a wet or contaminated surface will fail.
Quantity Indication, Crossfeed, and Fuel Management
The cockpit gauge is an indication, not a servicing tool. Level is established with drip sticks, sight gauges or dip tables, then compared with the indication. A capacitance system is calibrated at empty and full points per the maintenance manual, and a sender that reads correctly on the ground can still be wrong in the air.
Crossfeed lets either engine draw from either tank, which is how a crew corrects an imbalance and keeps the center of gravity where the flight manual wants it. It is not a normal feed configuration, and the valve position and AFM procedure matter. Fuel is also the largest variable weight on most flights, so the numbers belong with our weight and balance guide.
Answering Fuel System Questions
- Name the function before the part. A question about a vent is really about pressure equalisation, and the answer follows.
- Separate the two feeds. Engine driven pump for normal supply, boost pump for starting and backup, and never the other way round.
- Treat contamination as a chain. Water, microbes, filter plugging and gauge error arrive together, so a question about one tests your grip on all four.
The same pressure and flow reasoning drives our aircraft hydraulic systems guide, and these inspection habits carry into the oral and practical exam.
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