Private Pilot · Unit 3 · 30 min

Fuel, Electrical, and Other Systems

The systems around the engine

The previous lesson, The Engine and Propeller, followed the energy of combustion from the fuel entering the cylinders out to the propeller. This lesson covers the systems that surround that engine: the fuel system that delivers clean fuel to it, the electrical system that supplies the airplane's lights, radios, and instruments, and the small systems built onto the same hardware, including cabin heat, pitot heat, and the stall warning. None of these produces thrust, but each one keeps the engine fed or the cockpit working, and a failure in any of them changes how you fly the airplane.

The fuel system: tanks to engine

The fuel system has one job: deliver clean fuel to the engine without interruption, at every power setting and in every normal attitude. How it does that depends on where the tanks sit relative to the engine, and that is the first place a high-wing trainer and a low-wing airplane differ.

In a high-wing trainer such as the Cessna 172, the fuel tanks sit in the wings, above the engine. Fuel flows down to the engine under its own weight through a gravity-feed system, with no pump required. Because both tanks sit above the engine and drain downhill together, the airplane can draw from both at once, which is why the fuel selector has a BOTH position alongside LEFT and RIGHT. This is the quiet advantage the cockpit lesson mentioned when it introduced the selector.

A low-wing airplane cannot feed by gravity. Its tanks sit below the engine, so the fuel has to be lifted. An engine-driven pump does that work in flight, and an electric boost pump is plumbed in the line as well: it supplies fuel pressure for starting, before the engine-driven pump is turning fast enough, and it serves as a backup if the engine-driven pump fails. A low-wing selector typically reads LEFT or RIGHT with no BOTH position, because a pump drawing from two tanks at once could pull air from whichever tank empties first.

Between the tank and the engine, both systems share the same parts. Each tank is vented, either through the filler cap or a separate vent line, so that outside air replaces the fuel as it drains; a blocked vent lets the tank pressure fall below the outside air and can slow or stop the fuel flow. A fuel strainer at the low point of the system traps water and sediment before the fuel reaches the engine. The fuel primer draws fuel from the line and sprays it directly into the cylinders' intake ports to help start a cold engine; it locks in place when it is not in use, because an unlocked primer can draw fuel into the intake in flight and make the mixture too rich.

Federal rules require a fuel quantity gauge to read accurately at only one point: empty, when the usable fuel reaches zero. Between full and empty the gauge is allowed to be approximate. So the accurate measure of how much fuel you have is the preflight visual check, where you look into each tank and read the level against a calibrated tab or a dipstick. In flight the gauge shows a trend, not a number to plan a flight around.

PHAK Ch 7, "Fuel Systems"

Fuel grades and contamination

The engine is built for a specific fuel. Using a grade lower than the engine requires invites the explosive combustion the previous lesson described as detonation, which spikes cylinder pressure and can destroy the engine. To make the grade easy to confirm, aviation gasoline is dyed a distinct color for each grade, and jet fuel is left nearly colorless so it stands out from all of them.

The color is a safety check you make with your eyes. A piston trainer runs on 100LL, which is dyed blue. Jet fuel is colorless to straw-colored and smells of kerosene rather than gasoline, so if the fuel drawn from the tank is clear and smells of kerosene, the airplane has been fueled with the wrong product and must not be flown until it is corrected. Misfueling a piston engine with jet fuel is a known cause of engine failure shortly after takeoff.

The main contaminant to look for is water. Water is denser than fuel, so it settles to the lowest points of the tanks and lines. Those low points have drains, called sumps, at each tank and at the strainer. Before every flight you drain a sample from each sump into a transparent container and inspect it: the fuel should be the right color, clear rather than cloudy, and free of water droplets and sediment. Water shows up as beads or a separate layer at the bottom of the sample. You keep draining until the sample comes out clean. To limit the water in the first place, top the tanks off after the last flight of the day, because full tanks leave little air space for the moisture in humid air to condense out overnight.

One more step protects against fire during refueling. Fuel flowing through a hose and nozzle builds up a static electric charge, and a spark from that charge near fuel vapor could ignite it. To prevent this, the airplane is grounded, connected to the fuel source with a ground wire before fueling begins, so both are at the same electrical potential and no spark can jump between them.

PHAK Ch 7, "Fuel Grades" and "Fuel Contamination"

Leaning and the mixture in cruise

The mixture control is the one fuel-system control you adjust continuously in flight: it sets how much fuel the carburetor meters into the incoming air. The previous lesson explained that the carburetor keeps metering fuel for dense sea-level air, so as air thins with altitude the mixture grows too rich until you lean it back to the correct ratio. This lesson takes that from the reason the control exists to how you use it in cruise, and to the gauge that guides the setting.

Running too rich has a cost even after the engine is started. Excess fuel lowers the cylinder temperature and does not burn completely; the unburned fuel deposits carbon that fouls the spark plugs, the engine runs rough and loses power, and the fuel that never burns is pumped overboard as waste. That waste matters because the tanks are finite and, as the fuel section explained, the gauges are trusted only at empty. Your endurance depends on a known, predictable fuel burn, and an over-rich mixture makes the burn higher than planned.

Leaning corrects this, but it can be overdone in the other direction. If you lean in cruise and then descend without enriching the mixture again, the air grows denser as you lose altitude while the fuel stays reduced, and the mixture becomes too lean. An over-lean mixture at high power raises the cylinder temperature and can bring on the same detonation the previous lesson warned about, reached this time through too little fuel rather than the wrong grade, along with rough operation, overheating, and lost power. This is why the mixture is enriched before a descent and before any high-power operation near the ground.

The reference for setting the mixture is the exhaust gas temperature (EGT) gauge. It measures the temperature of the exhaust gas leaving the cylinders at the exhaust manifold, and that temperature depends on the fuel-air ratio: as you lean from a rich setting, combustion becomes more complete and the EGT rises to a peak; leaning past the peak, each charge carries less fuel, so less heat is released and the excess air absorbs part of it, and the EGT falls again. Because the temperature moves with the ratio, the gauge lets you set the mixture against a measured value rather than by feel, and leaning to the value your handbook specifies reduces fuel consumption for a given power. The specific offsets from peak EGT and the choice between a best-power and a best-economy setting belong to the performance unit, Unit 9; here the point is that the EGT gauge is the instrument that makes leaning precise.

PHAK Ch 7, "Mixture Control" and "Exhaust Systems"

The electrical system

Two sources of electrical power work together. The battery is a chemical store of energy: it turns the starter to crank the engine, supplies power before the engine is running, and serves as a reserve if the other source fails. The alternator is turned by the engine and produces the airplane's electrical power once the engine is running, both supplying the loads and recharging the battery.

Light airplanes use either a 14-volt or a 28-volt electrical system, paired with a 12-volt or a 24-volt battery. The alternator's output is held a little above the battery's voltage by the voltage regulator, and that difference is what drives charging current back into the battery. The regulator holds the system voltage steady as engine speed and electrical load change. Power from the alternator and the battery meets at the bus bar, a common conductor that distributes it to every circuit in the airplane. Each circuit is protected by a fuse or, more often, a resettable circuit breaker that opens when the current exceeds a safe value, isolating a fault before it can overheat the wiring. A breaker that has opened may be reset once; a breaker that opens again indicates a persistent fault; leave it open and have the circuit inspected.

The lights, radios, fuel gauges, flap motor, pitot heat, stall warning, and starter all run on electricity. The ignition deliberately does not: as the engine lesson established, the magnetos generate their own spark from the engine's rotation. Because of that, a complete electrical failure stops the radios and the electric instruments but does not stop the engine.

The cockpit lesson introduced the master switch as a split rocker with BAT and ALT halves that you flip together in normal use, but it did not say why the switch is split. The two halves control the battery and the alternator separately so that a failing alternator can be switched off on its own. With ALT off the airplane runs on the battery alone, and you shed electrical load, turning off lights, unneeded radios, and pitot heat, to make the battery's limited charge last long enough to land. In the schematic, the BAT half closes the battery contactor that connects the battery to the bus, and the ALT half enables the alternator's field so it can produce power.

The ammeter tells you whether the battery is charging or discharging. It reads zero at the center. A needle on the plus side means the alternator is producing more current than the loads require and the surplus is charging the battery, which is normal after starting. A needle on the minus side means the loads exceed what the alternator supplies and the battery is making up the difference. A full-scale minus reading points to alternator failure, with the battery carrying the entire load; a full-scale plus reading points to a regulator malfunction overcharging the battery. Some airplanes fit a loadmeter instead, which reads from zero upward and shows the total current the alternator is supplying: a higher reading means more electrical load, and a drop to zero means the alternator has stopped producing. A low-voltage warning light on the panel comes on when the bus voltage falls below the alternator's normal output, usually the first sign that the alternator has stopped charging.

Starting the engine draws far more current than any other load, so it has its own heavy path. Turning the ignition key to START energizes the starter contactor, a heavy-duty relay that connects the battery directly to the starter motor. Once the engine is running you release the key, the contactor opens, and the starter drops out of the circuit while the alternator takes over supplying the airplane.

PHAK Ch 7, "Electrical System"

Cabin heat and the other small systems

Cabin heat burns no fuel of its own. Outside ram air is routed through a shroud, a thin metal jacket around the engine's exhaust muffler, where the hot exhaust passing inside warms it. A cabin heat valve in the duct controls how much of that warmed air reaches the cabin. Because the air is heated by flowing around the outside of the exhaust, this arrangement carries a specific hazard: a crack in the muffler or the exhaust pipe can let exhaust gas, which contains carbon monoxide, leak into the air on its way to the cabin.

Two more small systems run off the electrical bus you just met. Pitot heat is an electric heating element inside a small pressure probe, the pitot tube, that feeds the airspeed indicator. The element keeps the probe's opening free of ice so the airspeed reading stays valid in visible moisture near freezing. The probe and the instruments it drives are the subject of the next lesson, The Pitot-Static Instruments. The stall warning is a small vane or tab on the wing's leading edge wired to a horn or light in the cabin. As the angle of attack increases toward the wing's critical angle, the airflow shifts on the leading edge and lifts the vane, closing an electric circuit that sounds the horn. It is set to trip just below the critical angle of attack; in level flight that margin gives you the warning a few knots above the stall speed, as Stalls and Spins described. Recall that the stall is defined by that angle, not by a fixed airspeed.

One engine-driven accessory serves the instruments rather than the engine: a vacuum pump turned by the engine drives a stream of air that spins the gyroscopes inside some of the flight instruments. How those instruments use those spinning gyroscopes is the subject of The Gyroscopic Instruments and Compass, later in this unit.

PHAK Ch 7, "Exhaust Systems" and "Electrical System"; PHAK Ch 8

Check your understanding

Answer from memory, without scrolling back up. Recalling it yourself is what makes it stick.