Piston-Aircraft Fuel Systems

A light aeroplane's fuel system can be as simple as a tank above a carburettor, and every complication after that exists for one of three reasons: the tank is below the engine, the fuel is dangerous to mishandle, or the pilot needs to know what is left. This lesson covers the fuels a piston engine may drink, the plumbing that delivers them, and the measurements that keep the arithmetic honest.

The fuels

Four fuels serve the piston fleet, each with its limitations:

AVGAS — aviation gasoline, the classic spark-ignition fuel, graded by octane and dyed for identification. Its limitation is its chemistry: high volatility and a leaded formulation the world is slowly retiring.
MOGAS — motor gasoline, approved for some engines by specific certification only: cheaper, but more volatile still, so more prone to vapour problems, and of less controlled quality.
Diesel — for compression-ignition aero engines: no ignition system to fail, efficient, but heavier fuel and engines.
JET-A1 — kerosene, burned by most aviation diesel (high-compression) engines: available at virtually every airport in the world, which is much of the aviation diesel's reason to exist. Its limitation is absolute: it must never be fed to a spark-ignition gasoline engine, which will detonate itself to destruction on it.

The characteristics the exam wants as typical values:

AVGAS — a density of about 0.72 kg per litre, a flash point around −40°C (its vapour can be lit by a spark at almost any ambient temperature, which is why gasoline handling is so strict), and a freezing point below about −58°C.
JET-A1 — denser at about 0.8 kg per litre, much harder to ignite in bulk with a flash point of at least +38°C, and specified to stay liquid down to −47°C.
MOGAS and diesel — MOGAS shares AVGAS's numbers, only more volatile still; diesel fuel is denser again, with a flash point even higher than JET-A1's.

The pattern to remember: gasoline is lighter and far more flammable; kerosene is denser, safer to handle, and the one with the freezing point that matters operationally.

What the system must do

The tasks of the fuel system read simply: store the fuel safely, deliver it to the engine cleanly and at the right pressure in every normal attitude and manoeuvre, allow the pilot to select and monitor it, and keep water and dirt out of the engine's share. The components that do it:

Tanks (wing, tip, fuselage) — wherever structure and balance allow, wing tanks being the default (the bending relief of the structures module included);
Lines — routed away from heat and ignition sources, bonded against static;
Boost pump — the electric pump that pressurises the line before start, backs up the engine-driven pump, and guards against vapour lock at high temperature and altitude;
Pressure valves — relief and check valves keeping the delivery pressure inside the carburettor's or injection system's tolerance;
Filter and strainer — coarse mesh at the tank outlet, fine filter (and the gascolator bowl) before the engine, so dirt and water are caught where they can be drained, not burned;
Vent system — lets air in as fuel leaves, and lets vapour and warming, expanding fuel breathe out; a blocked vent lets the emptying tank pull a vacuum that first starves the engine and can crush the tank;
Sump and drain — the low point where water, denser than fuel, settles overnight, and the valve through which the pre-flight sample removes it;
Fuel-quantity sensor — float or capacitance senders feeding the gauges;
Fuel-temperature sensor — where fitted, watching the margin above the fuel's freezing point and the vapour risk at the hot end.

Gravity or pressure

Two architectures deliver the fuel:

Gravity feed system — needs the tank above the engine, the high-wing arrangement: fuel falls through the selector and strainer to the carburettor, no pump needed, nothing to fail.
Pressure feed system — what low tanks force: an engine-driven pump sucks the fuel up, and the electric boost pump stands behind it for starting, take-off, landing and failures.

The split is why high-wing trainers can have gloriously simple fuel systems and low-wing aircraft always carry pumps.

Tanks, in three constructions

Drum (rigid) tank — a separate metal or composite container fitted into the airframe: simple to make and replace, but its weight and its volume are both paid twice, once for the tank and once for the structure around it.
Bladder tank — a flexible fuel-proof bag in a structural cavity: light, shapes itself to awkward spaces, tolerant of structural flexing, but ageing rubber wrinkles (trapping water and hiding quantity) and replacement is laborious.
Integral tank — the sealed structure itself holds the fuel (the wet wing): no tank weight at all and the most volume from a given space, at the price of sealing the structure fuel-tight and keeping it that way through years of flexing.

Selection, crossfeed and the fuel you cannot use

With more than one tank comes selection, and with more than one engine comes crossfeed: the valve arrangement that lets any engine draw from any tank, so an engine failure does not strand the failed engine's fuel, and an imbalance can be corrected by feeding both engines from the heavy side. Unusable fuel is the certification term for the fuel that cannot be relied upon to reach the engines: the dregs below the outlets and in the plumbing, measured in the worst approved attitude. It is counted in the aircraft's empty weight and subtracted from every endurance sum, because fuel the pumps cannot reach is ballast, not range.

What is monitored

Two parameters are monitored for the fuel system: fuel quantity, with a low-level warning, and fuel temperature. The quantity gauges carry the endurance arithmetic and the low-level warning backstops them, catching leaks, mis-set selectors and optimistic flight planning alike; the temperature (where fitted) watches both ends of the fuel's liquid range, freezing margin in the cold and vapour risk in the heat. The gauges' certified accuracy is finite, which is why dipping the tanks and cross-checking against fuel burned remains part of the discipline.