Light Aircraft Fuel Systems

A piston engine needs a steady flow of clean fuel of the right grade, at a positive pressure, in every attitude the airplane is flown. The fuel system of a light aircraft provides it with a small number of parts: tanks with vents, a selector, a strainer, drains, and in many airplanes an engine-driven pump backed by an electric one. The parts are simple, but each has a way of stopping the engine, and most of them are checked by the pilot on every walk-round.

A fuel problem does not need an empty tank. A selector on an empty tank, a blocked vent, water in the strainer or the wrong fuel can each stop an engine with fuel still on board. EASA's CPL and ATPL syllabus (021 08 and 021 10), the PPL's aircraft general knowledge and the FAA knowledge tests all cover the system, and the same knowledge carries over to the larger fuel feed systems of transport aircraft.

Gravity-feed and pump-feed systems

How the fuel reaches the engine depends on where the tanks are. In a gravity feed system the tanks sit above the engine and the height of the fuel alone supplies the pressure. Many high-wing trainers, such as the Cessna 152 and the carburetor versions of the Cessna 172, work this way: fuel flows from the wing tanks through the selector and the strainer to the carburetor's float chamber, with no fuel pump at all. The head of fuel is small, so gravity feed suits a carburetor but not a fuel injection system, which needs a higher pressure.

In a low-wing airplane the tanks are below the engine and the fuel must be lifted. An engine-driven fuel pump does it, and because it is a single point of failure an electric fuel pump, the auxiliary or boost pump, backs it up and can supply the engine on its own. The flight manual sets its use, typically:

• engine start and, on injected engines, priming;
• takeoff and landing, so that a failed engine-driven pump cannot stop the engine at a critical moment;
• tank changes;
• whenever fuel pressure fluctuates or falls, the first response to rough running with low fuel pressure;
• hot conditions at altitude, to prevent vapor lock.

Continuous use in the cruise can overpressurize some carburetors, so the flight manual governs. A fuel pressure gauge shows that the pumps are delivering; on an injected engine a fuel flow or fuel pressure gauge shows the metered fuel and is used for leaning (see carburetors and fuel injection). A carbureted engine usually also has a hand-operated primer that injects fuel into the induction manifold or cylinder inlet ports for a cold start; it must be pushed in and locked after use, or it keeps feeding fuel and makes the mixture over-rich.

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Tanks, vents and caps

EASA's syllabus names three kinds of tank: the rigid drum tank, the flexible bladder and the integral tank formed by the sealed structure. Light aircraft usually have rigid metal tanks in the wings, at the wingtips or in the fuselage. Each tank's outlet stands a little above its bottom, so that settled water and sediment stay below it. The fuel left beneath the outlet, which cannot be relied on to reach the engine in all attitudes, is unusable fuel; it is part of the basic empty mass and does not count toward endurance.

As fuel is used, air must replace it. The vent keeps the space above the fuel at outside pressure, through a vent line or a vented filler cap. If the vent is blocked by ice, insects, dirt or a wrongly fitted cap, that space falls below ambient as the tank empties, the pressure difference opposes the flow, and the engine first runs rough and then stops although fuel remains. In a severe case the tank skin is drawn inward. The symptom, fuel pressure slowly falling while one tank is selected, is easily mistaken for a pump failure. Vents and caps are therefore walk-round items, and a cap with a poor seal also lets rain into the tank.

Fuel selector and tank switching

The fuel selector chooses which tank feeds the engine. High-wing airplanes often offer OFF, LEFT, RIGHT and BOTH; low-wing types commonly offer LEFT, RIGHT and OFF only, so the pilot must change tanks on a schedule to keep the fuel balanced. OFF is also the fuel shut-off of the engine fire drill. The handle is set firmly in its detent, because a selector left between positions can restrict or cut off the flow, and its position is confirmed by sight and feel.

Takeoff and landing are made on the fuller tank, or on BOTH, because the outlet of a low tank can be uncovered, or unported, in a prolonged sideslip or uncoordinated flight. Tank changes are made with the boost pump on where the flight manual calls for it, and the fuel pressure is watched afterward. Light twins add a crossfeed that lets the live engine use the fuel on the failed engine's side; it is intended for single-engine cruise, commonly in level flight only, while takeoff and landing are flown with each engine on its own tank.

Strainers, sumps and water checks

Water is the commonest contaminant. It enters as condensation in partly empty tanks, through poor caps and seals after rain, or with the fuel itself. It does not mix with avgas and is denser, so it collects at the lowest point of each tank, the sump, where a sump drain valve is fitted. Between the tanks and the engine, at a low point of the system, the fuel strainer, usually a gascolator, combines a filter with a sediment bowl and its own drain, and catches water and dirt that pass the tank outlets.

The pre-flight fuel check follows a fixed pattern:

1. Allow a few minutes after refueling for any water to settle.
2. Drain a sample from every tank sump and from the strainer into a clear tester, before the first flight of the day and after each refueling.
3. Look for water, as clear beads or a layer at the bottom, for sediment, and for the wrong grade, shown by color and smell.
4. Keep draining until the sample is clean, then check that each drain has closed.

Draining the strainer alone is not enough, because water settles in each tank separately. Water that keeps reappearing points to a leaking cap or seal and is a matter for maintenance; the tanks may need to be defueled. Water in the fuel can stop the engine, most often shortly after takeoff. Topping up the tanks limits condensation.

Fuel grades and colors

Aviation gasoline, avgas, is dyed so that a drained sample shows its grade:

Avgas 100LL, where LL stands for low lead, has the same resistance to detonation as grade 100 with less tetraethyl lead. The lead that remains still fouls spark plugs after a long idle with the mixture fully rich, which shows as an excessive drop at the magneto check. All avgas grades have a specific gravity of about 0.72, and the FAA uses 6 lb per US gallon for weight and balance (see aviation fuel grades and properties).

The rule is to use the grade on the placard, a higher grade only where the flight manual permits it, and never a lower one, which brings detonation at high power. Jet fuel in a spark-ignition engine is the dangerous misfueling: the engine usually starts and may run at low power, then detonates and fails when takeoff power is applied. A ground run proves nothing, and the whole system must be drained and inspected before flight (see refueling safety and fuel emergencies). The opposite case is legitimate: aviation diesel piston engines are designed to burn Jet A-1, and EASA's syllabus also lists MOGAS, motor gasoline, among piston engine fuels. The flight manual says which fuels an engine may use.

Vapor lock and fuel-injected engines

Avgas is volatile, which helps a cold engine start but means it boils readily. A liquid boils when its vapor pressure reaches the pressure on its surface, so the boiling point falls as the airplane climbs. When fuel boils in a line, vapor displaces the liquid and interrupts the flow: a vapor lock. Warm fuel at altitude is the classic case, and a pump that sucks fuel from the tank lowers the line pressure and makes it worse. The electric boost pump prevents it by pushing the fuel toward the engine under positive pressure.

Fuel-injected engines are the more prone to it. After shutdown on a hot day, heat soak from the engine vaporizes the fuel standing in the lines, pump and metering unit, so a restart twenty or thirty minutes later can be difficult. Flight manuals give a hot-start procedure, commonly running the boost pump for a short time with the mixture at idle cut-off and the throttle open to purge the vapor with cool fuel, then starting with a lean mixture. Too much fuel floods the engine, and the procedure differs between types. Fuel injection is, in exchange, free of carburetor icing, though not of impact ice on the air intake.

Fuel management and fuel starvation

Light aircraft fuel gauges are indications, not measurements. Under the FAA certification rule that most light airplanes in service were built to, the former 14 CFR 23.1337(b), a gauge had to read zero in level flight when only the unusable fuel remained; at other levels it may be well out. Since the 2017 rewrite of Part 23, 14 CFR 23.2430 requires a means for the flight crew to determine the total usable fuel available. Pilots therefore confirm the contents before flight visually or with a calibrated dipstick, then manage the fuel by time and known consumption. A fuel totalizer subtracts measured fuel flow from a starting figure entered by the pilot; it is precise but cannot see the tanks, so a wrong entry or a leak passes unnoticed. The sound practice is to cross-check gauges, totalizer and the fuel log.

Fuel planning sets the reserve to be in the tanks on landing. For non-commercial VFR flight in airplanes, EASA's AMC1 NCO.OP.125(b) gives 30 minutes by day and 45 minutes by night, at holding speed at 1,500 ft above the destination, or 10 minutes at maximum continuous cruise power for a day flight that takes off and lands at the same aerodrome and stays in sight of it; 14 CFR 91.151 gives 30 minutes by day and 45 by night at normal cruising speed (see fuel and energy planning for general aviation).

Fuel exhaustion means that no usable fuel remains, through poor planning or monitoring, or an unnoticed leak. Fuel starvation means that usable fuel remains but cannot reach the engine. Its causes are the subject of this article: a selector left on an empty tank or between detents, an outlet unported in a slip with a low tank, a blocked vent, a failed engine-driven pump with the boost pump off, vapor lock, or water or debris in the strainer. Each first shows as a loss of power, often with falling fuel pressure.

The response follows from the causes. With rough running and falling fuel pressure, the boost pump goes on; after a power loss, the checks start with the fuel selector and pump, then the mixture, carburetor heat and magnetos, if height and time allow (see forced landing and ditching).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.