11.3 Fuel Grades, Contamination, and Fuel-System Management

ACS PA.I.G.K1e is fuel, oil, and hydraulic. On a PAR trainer the fuel half is the one the knowledge test samples relentlessly: grade, color, contamination, and which tank is feeding. Oil was the engine-cooling story in 11.1. The hydraulic story on most ASEL trainers is a brake circuit, not a transport-category stack of pumps.

Grades, colors, and the fuel that must never go in a piston

Aviation gasoline is dyed so you can identify the grade in the sample cup. Grade is an antiknock (detonation-resistance) rating. Higher grade withstands more pressure and temperature before it detonates.

You identify fuel with color plus smell plus the sump, not with the truck’s painted label alone. 100LL in the cup is blue. Jet A looks like watery straw or almost clear and smells like kerosene. A piston engine fed jet fuel can detonate, seize, or stop — this is not a “it will just run rough” item.

PHAK: if the specified grade is not available, use the next higher aviation grade as a substitute. Never use a lower grade than the AFM requires. Too-low octane is a published detonation cause. Automobile gasoline (mogas) is legal only in airplanes and engines approved for it; it is not a silent stand-in for 100LL.

Unleaded transition, one verified line: in September 2022 the FAA issued an expanded approved-model-list STC to GAMI for G100UL, a 100-octane unleaded avgas, for spark-ignition piston engines and the airframes that use them. Use it only when the airplane and engine are authorized. Do not invent a dye color, a nationwide availability date, or a claim that every trainer on the ramp may burn it tomorrow.

Water, particles, and why you sump

Water is the principal fuel contaminant. It can arrive as dissolved moisture that precipitates when the fuel cools, as free water from a leaky cap or a dirty truck, or as condensation on the walls of a half-empty tank. Free water is heavier than avgas and settles. In the sample it may sit as droplets on the bottom or turn the fuel cloudy. Solid particles (rust, sand, sealant) settle with it.

Between each tank and the engine a strainer (gascolator) catches what the tank sumps miss. PHAK: to purge water you must drain the fuel tank sumps and the strainer, not one or the other. Drain until the sample is the right color and free of water, then discard it as contaminated fuel — do not pour a mystery sample back into the tank.

Fill the tanks after the last flight of the day so a large air space cannot cool overnight and condense a new water crop. Warm fuel holds more dissolved water than cold fuel; a temperature drop is when that water falls out. Ice in the system is frozen water: frozen free water can block a drain until it melts in flight; ice crystals from entrained water can block a screen. Some AFMs approve an anti-icing additive; do not add chemistry the book does not list.

Avgas planning weight is 6 lb/gal. That is the standard number for weight-and-balance and fuel-required math unless a specific AFM publishes another figure for that fuel. Jet-A density is different — another reason you do not treat the two as interchangeable gallons.

Gravity, pumps, vents, and vapor lock

A gravity-feed system (typical high-wing) puts the tanks above the carburetor or injector and lets fuel fall to the engine. A fuel-pump system (typical low-wing, and many injected engines even on a high wing) uses an engine-driven pump plus an auxiliary (boost) pump for start, for backup if the engine-driven pump fails, and — when the AFM says so — to keep pressure up when vapor might form. The primer on a carbureted trainer squirts raw fuel into the cylinders for a cold start; it is not a substitute for a boost pump.

Tanks are vented so air can replace the fuel you burn and so expanding fuel can leave without rupturing a cell. A vent may live in the cap or in a tube under the wing. A blocked vent can collapse a tank or starve the engine even when the gauge still shows fuel. Check vents on the preflight walk-around.

Vapor lock is fuel that boiled into vapor in a line so liquid can no longer reach the engine. Hot ramps, a hot restart, and running a tank dry (air ingested into the pump) are the usual setups. The AFM’s boost-pump procedure is how you keep liquid fuel moving; running a tank dry on purpose is not “good mixture management.”

Static electricity builds as fuel flows. Bond the truck or can and the airplane before the cap comes off.

Selectors, BOTH, and unporting in a slip

The fuel selector chooses which tank may feed. Common high-wing positions are BOTH, LEFT, RIGHT, and OFF. BOTH is the takeoff, climb, and landing default on those airplanes so either wing can still feed if one pickup unports. Low-wing airplanes often have no BOTH — you take off on a tank you just confirmed by quantity and a brief run on that tank.

Unporting is the pickup sucking air because a slip, skid, or steep attitude walked the fuel away from the outlet. A long slip to a short runway with the selector on the low tank, or on one tank with that tank already low, is the classic surprise. BOTH (when you have it) is the systems answer; “I still had 8 gallons indicated” is not, because certification only promises the gauge is right at empty.

Switch tanks in level, unaccelerated flight, with the boost pump on if the AFM wants it, and do not wait until the engine coughs. An unbalanced load from sitting on one tank for an hour is a lateral-CG and a surprise-empty problem. The Airplane Flying Handbook’s engine-failure flow still starts with selector, quantity, boost pump, mixture, mags — many “failures” are an OFF selector or a tank you already used up.

Scenario: Sam and the clear sample

Sam sumps the left wing of a 172 and the cup is colorless, with a sharp kerosene smell. The truck on the ramp is a Jet-A truck that also tows an avgas cart. Sam does not start. Colorless or straw plus kerosene is Jet A, not 100LL. A piston is not a turbine. He has the airplane defueled and inspected rather than “burning off a little and seeing.” Blue 100LL, 6 lb/gal, vents open, BOTH for takeoff, sumps and strainer drained — that is the PAR fuel system, not a hope that the smell was just “old avgas.”