Fuel, oil, induction & cooling

Your fuel system

Definition

Fuel flows from the tanks, through the fuel selector and a gascolator (a strainer with a drain), through the fuel pump, to the carburetor or fuel-injection system, and into the engine. High-wing airplanes can gravity-feed because the tanks sit above the engine; low-wing airplanes need a fuel pump to lift fuel up. The fuel-quantity gauge tells you what the sender reads — a visual or dipstick check tells you the truth, so verify fuel before every flight. Fill in your airplane's fuel system below.

Explanation

Know your airplane's fuel exactly: usable fuel (not total — some is unusable), the grade (100LL avgas is dyed blue), your selector logic (a high-wing 'BOTH' vs. a low-wing 'LEFT/RIGHT' that you must switch), and whether it gravity-feeds or needs a pump. These are the numbers a fuel-planning error turns into an emergency.

Figure 7-30. Gravity-feed and fuel-pump systems. — Figure 7-30 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 186)

Why it matters

Fuel mismanagement — bad planning, the wrong tank, unsumped water — is one of the most common and preventable causes of engine stoppage. Knowing your own fuel system is the defense.

From the Right Seat

Learn your selector logic cold. On a low-wing airplane with no BOTH, running one tank dry while the other is full has caused countless 'engine failures' that were really a selector left in the wrong place.

A DPE might ask

Trace the fuel from the tank to the engine.
What is your usable fuel, grade, and selector logic?

PHAK ch. 7 — Fuel systems

Fuel contamination & starvation

Definition

Before every flight you sump the fuel drains — the tank drains and the gascolator — into a clear tester. You're checking for water (which sinks below the fuel and shows as a separate clear layer or bubbles), for sediment, and that the fuel is the correct grade (100LL avgas is blue). Two failures worth distinguishing: fuel exhaustion is simply running out of fuel; fuel starvation is having fuel on board that isn't reaching the engine — a selector on an empty tank or OFF, a blocked line, or fuel vapor.

Explanation

Water gets into tanks from condensation and rain; sumping until the sample is clear and the right color is a non-negotiable preflight item. The exhaustion-vs-starvation distinction matters because they have different fixes: exhaustion is a planning problem (verify quantity, plan reserves); starvation is a management problem (correct selector position, don't run a tank dry, use the boost pump per the POH).

Fuel problem	What it is → what to do
Water / sediment	Water sinks below the fuel. Sump every drain before flight until the sample is clear and blue.
Wrong grade	Confirm the correct fuel (100LL avgas is blue). Never put jet fuel in a piston engine.
Fuel exhaustion	You ran out of fuel. Prevented by verifying quantity visually and planning reserves.
Fuel starvation	Fuel is on board but not reaching the engine — a selector on an empty tank or OFF, a blocked line, or vapor. Manage the selector; don't run a tank dry.

Why it matters

These are among the most preventable engine failures there are — a 30-second sump and a correct selector position prevent most of them.

Common error

Exhaustion and starvation get used interchangeably, but they're different failures. Fuel exhaustion means the tanks are empty — you're out of fuel. Fuel starvation means fuel is on board but isn't reaching the engine, most often because the selector is on an empty tank or OFF. One is fixed by fuel planning, the other by managing the selector.

The tanks still contain fuel, but the engine quit because the selector was on an empty tank. This is:

A DPE might ask

What do you check for when you sump the fuel?
What's the difference between fuel exhaustion and fuel starvation?

PHAK ch. 7 — Fuel contamination & management

Oil system

Definition

Engine oil does more than lubricate. It reduces friction between moving parts, carries heat away from the internal engine, helps clean by carrying contaminants toward the filter, and helps seal between the piston rings and cylinder walls. Check the oil quantity and condition exactly as your POH directs before each flight, and interpret oil pressure and temperature together — as you saw in the engine lesson.

Explanation

Don't teach yourself a universal 'safe' oil quantity — the POH gives the real minimum, and your operator may set a higher practical one. Oil that's very low, very dark, or dropping between flights is telling you something. Pressure and temperature are the in-flight health signals: pressure that a system is producing flow, temperature about heat and cooling.

Oil	Detail
Lubricates	Reduces friction between moving parts.
Cools	Carries heat away from the internal engine.
Cleans & seals	Carries contaminants to the filter; seals the piston rings.
Check	Quantity + condition per the POH; interpret oil pressure and temperature together.

Why it matters

Oil starvation destroys an engine fast, and the warning signs — low quantity on preflight, a pressure or temperature trend in flight — are yours to catch. This is the maintenance discipline that keeps the engine you just described healthy.

A DPE might ask

What are the functions of engine oil?
How do you check the oil, and what are you looking for?

PHAK ch. 7 — Oil system

Induction & mixture

Definition

The induction system delivers the air-fuel mixture to the cylinders. A carbureted engine mixes fuel and air in a carburetor before it reaches the cylinders; a fuel-injected engine meters fuel directly to each cylinder, which gives more even distribution and — importantly — no carburetor-ice risk. The mixture control sets the ratio of fuel to air: as you climb and the air thins, you lean the mixture to keep it correct.

Explanation

Carbureted systems are simpler and common in trainers, but they can form carburetor ice (next slide). Fuel-injected systems avoid that and distribute fuel more evenly, at the cost of a sometimes-fussier hot start. Either way, mixture management matters: too rich wastes fuel and fouls plugs; the leaning technique you learned in the engine lesson keeps it right.

Carbureted

A carburetor mixes fuel and air
Simple, common in trainers
Susceptible to carburetor ice
Has a carb-heat control

Fuel-injected

Fuel metered to each cylinder
More even fuel distribution
Not susceptible to carb ice
Can be harder to hot-start

Why it matters

Whether your airplane is carbureted or injected changes your procedures — carb heat use, hot-start technique, and the carb-ice risk. Knowing which you fly is basic to operating it correctly.

A DPE might ask

What's the difference between a carbureted and a fuel-injected engine?
What does the mixture control do?

PHAK ch. 7 — Induction & mixture

Carburetor ice

Definition

In a carbureted engine, fuel vaporizing and air speeding up through the carburetor's venturi drop the temperature sharply — sometimes far enough to freeze moisture out of the air and build ice in the carburetor, choking off the engine. It can happen on a cool, humid day and even on a warm, humid one; the classic danger zone is roughly 20 to 70°F with visible moisture or high humidity. The first symptoms are a gradual drop in RPM (fixed-pitch prop) or manifold pressure (constant-speed) and engine roughness.

Explanation

Carb heat routes warm air into the carburetor to melt and prevent ice — it's an anti-ice system, not a warm-up knob. Apply FULL carb heat when you suspect ice: expect a small initial RPM drop (warm air is less dense), then rougher running as the melting ice passes through the engine, then a smoothing and RPM recovery as it clears. Fuel-injected engines don't have a carburetor and aren't susceptible.

Carb ice	Detail
When	Cool, humid air — even on a warm day (roughly 20–70°F with moisture). The venturi cools sharply.
Symptoms	A gradual RPM drop (fixed-pitch) or manifold-pressure drop (constant-speed), then roughness.
What to do	Apply FULL carb heat. Expect an initial small drop, then rougher running as the ice clears, then smoothing.
Remember	Carb heat is an anti-ice system, not a warm-up knob. Fuel-injected engines aren't susceptible.

Why it matters

Carb ice has caused many power losses in perfectly good engines. Recognizing the gradual RPM drop and applying full carb heat — and not backing off when it briefly runs rougher — is a core carbureted-airplane skill.

Common error

Two mistakes: thinking carb ice only happens in freezing weather (it happens on warm, humid days), and pulling carb heat back off when the engine runs rougher after applying it. That rougher run is the melting ice passing through — keep the heat on until it clears and the RPM recovers.

You apply full carb heat for suspected carb ice and the engine runs rougher. You should:

Safety

If you fly a carbureted airplane, treat a slow, unexplained RPM loss as carb ice until proven otherwise — apply full carb heat early. Waiting until the engine is running badly makes it harder to clear.

A DPE might ask

Under what conditions does carburetor ice form?
What does carb heat do, and what do you expect when you apply it?

PHAK ch. 7 — Carburetor ice & carb heat

Cooling & exhaust

Definition

Training engines are air-cooled: baffles direct outside air over the finned cylinders to carry heat away — there's no radiator. Some airplanes have cowl flaps you open to increase cooling in the climb and close to reduce drag in cruise. The exhaust system carries the burned combustion gases overboard. High power, low airspeed, hot days, and long ground operations all raise engine temperatures.

Explanation

You manage engine temperature with airspeed (more air = more cooling), power (less power = less heat), and cowl flaps if fitted. On the ground, point into the wind when practical and don't treat a long taxi or run-up as free warm-up. The exhaust's heat is also what warms the cabin — which is why a cracked exhaust is a carbon-monoxide concern (covered in the cabin lesson).

Cooling & exhaust	Detail
Air-cooled	Baffles direct air over the finned cylinders to carry heat away — no radiator.
Cowl flaps	If fitted, open them to increase cooling (climb) and close to reduce drag (cruise).
Heat stress	High power, low airspeed, and hot days raise temperatures — manage with airspeed, power, cowl flaps.
Exhaust	Carries burned gases overboard. (Cabin-heat carbon-monoxide risk is covered in Gear, Brakes, Cabin & CO.)

Why it matters

Overheating an air-cooled engine — high power at low speed on a hot day — shortens its life and can cause detonation. Managing cooling is quiet, everyday airmanship.

A DPE might ask

How is the engine cooled, and how do you manage its temperature?
What can cause high engine temperatures?

PHAK ch. 7 — Engine cooling & exhaust