Engine, propeller & ignition

Your engine, described

Definition

The training airplane's engine is a horizontally opposed, air-cooled, four-stroke reciprocating (piston) engine. You don't need to be a mechanic — but you do need to know your engine and what its normal indications look like. Fill in your airplane's engine below (you'll find it in Chapter 1 of your POH), and tap any term to learn what it means.

Explanation

The engine turns fuel and air into rotation at the crankshaft, which spins the propeller. 'Horizontally opposed' describes the cylinder layout; 'four-stroke' describes how each cylinder makes power; 'air-cooled' and 'fuel-injected' (or carbureted) describe cooling and fuel delivery. Knowing your specific engine — and its normal oil, fuel, and temperature indications — is what lets you notice when something's wrong.

Figure 7-4. Main components of a spark ignition reciprocating engine. — Figure 7-4 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 163)

Why it matters

Every abnormal indication you'll learn to catch is measured against your engine's normal. Knowing your specific engine is the baseline for all of it — and it's a standard checkride-oral question.

From the Right Seat

On an early lesson I have students recite their engine from memory — make, model, and what each part of the designation means. Filling this card in is the first step toward that.

A DPE might ask

Describe your engine — make, model, and configuration.
What does the designation (e.g. IO-360) tell you?

PHAK ch. 7 — Aircraft powerplant

The 4-stroke cycle

Definition

A four-stroke engine makes power in four steps, repeated in every cylinder. Intake: the piston moves down and draws in a mixture of air and fuel. Compression: the piston moves up and squeezes the mixture. Power: the spark plug fires, the mixture burns, and the expanding gases drive the piston down. Exhaust: the piston moves up and pushes the burned gases out. The crankshaft turns that up-and-down motion into the rotation that spins the propeller.

Explanation

Only the power stroke makes power; the other three prepare for it or clean up after it. Both valves are closed during compression and power — that seal is what lets the mixture compress and burn. A four-cylinder engine staggers its cylinders so there's always a power stroke happening, which is what makes the power delivery smooth.

Figure 7-5. The arrows in this illustration indicate the direction of motion of the crankshaft and piston during the four-stroke cycle. — Figure 7-5 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 164)

Why it matters

The cycle is the foundation for everything else — detonation, pre-ignition, mixture, and ignition all act on this cycle. Examiners expect you to know the four strokes in order.

A DPE might ask

What are the four strokes, in order?
Which stroke actually produces power?

PHAK ch. 7 — The four-stroke cycle

Detonation & pre-ignition

Definition

Two abnormal-combustion problems can damage an engine fast. Detonation is when the fuel-air mixture explodes instead of burning smoothly — and it happens after the spark. It's caused by using too low a fuel grade, running high power with poor cooling, or too lean a mixture at high power. Pre-ignition is when the mixture ignites before the spark, set off by a hot spot in the cylinder such as a glowing carbon deposit or a cracked spark plug. Both drive cylinder temperatures up and can destroy an engine.

Explanation

The fix for both is similar: enrich the mixture (fuel cools), reduce power, open the cowl flaps and increase airspeed to improve cooling, and make sure you're using the correct fuel grade. The key distinction for the oral is the timing: detonation is an abnormal, explosive burn AFTER the spark; pre-ignition is ignition BEFORE the spark, from a hot spot.

Detonation

The mixture explodes instead of burning smoothly
Happens AFTER the spark
Causes: low fuel grade, high power + poor cooling, too lean
Fix: enrich, reduce power, open cowl flaps, increase airspeed, correct fuel

Pre-ignition

The mixture ignites BEFORE the spark
From a hot spot — a glowing deposit or cracked plug
Sudden roughness, high CHT, power loss
Fix: reduce power, enrich, cool the engine

Why it matters

These can turn an engine into scrap in minutes, and the corrective actions are yours to take from the cockpit. Knowing the difference — and the shared fix — is real airmanship and a common oral question.

Common error

Detonation and pre-ignition get swapped constantly. Anchor it on timing: detonation is the mixture EXPLODING after the normal spark; pre-ignition is the mixture igniting BEFORE the spark, from a hot spot. Detonation can lead to pre-ignition and vice versa, but the timing is what defines each.

The mixture ignites BEFORE the spark plug fires, set off by a glowing deposit. This is:

A DPE might ask

What's the difference between detonation and pre-ignition?
What would you do if you suspected detonation?

PHAK ch. 7 — Detonation & pre-ignition

Propeller

Definition

The propeller is a rotating airfoil — each blade is a little wing that turns the engine's torque into thrust. Trainers usually have a fixed-pitch propeller: the blade angle is fixed, the design is simple, and RPM is your reference for power. Higher-performance airplanes often have a constant-speed propeller: a governor automatically changes the blade angle to hold whatever RPM you select, so you manage manifold pressure and RPM together.

Explanation

With a fixed-pitch prop, one lever (the throttle) controls everything and RPM tracks power. With a constant-speed prop, the throttle sets manifold pressure and a second (prop) lever sets RPM; the governor does the rest. A high RPM is not automatically a high power setting — on a constant-speed airplane you read power from manifold pressure and RPM together.

Fixed-pitch

Blade angle is fixed
Simple — common in trainers
RPM is your power reference
One lever: throttle

Constant-speed

A governor varies blade angle
Holds the RPM you select
Manage manifold pressure + RPM together
Two levers: throttle + prop

Why it matters

You'll train on a fixed-pitch prop, but understanding constant-speed sets you up for the complex-airplane endorsement and commercial work. Either way, knowing what your RPM is telling you is basic engine management.

A DPE might ask

What's the difference between a fixed-pitch and constant-speed propeller?
On a constant-speed airplane, how do you read power?

PHAK ch. 7 — Propellers

Dual ignition & magneto failures

Definition

The engine has two magnetos — engine-driven generators that make the high voltage for the spark plugs. They're independent of the battery and alternator, and each one fires its own spark plug in every cylinder (two plugs per cylinder). That redundancy is why the run-up includes a magneto check: you're verifying that each ignition system can run the engine by itself. In flight, a single magneto failure usually feels like slight roughness and a small RPM drop — the engine keeps running on the other mag — so it can be subtle.

Explanation

Reading the mag check: a small RPM drop on each mag is normal (you've dropped one plug per cylinder, so the burn is slightly less efficient). If the engine QUITS on one mag, that magneto is dead — no-go, get maintenance. If there's NO RPM drop at all, the magneto isn't being grounded when you switch it off — usually a broken P-lead — which means that mag is HOT and live even with the switch off. Treat the propeller as live, and get maintenance before flying.

Mag-check result	Meaning → what to do
Small RPM drop on each mag	Normal — both mags firing. Good within POH limits.
Engine quits on one mag	That magneto is dead — no-go, maintenance.
No RPM drop at all	Mag not grounded (broken P-lead) — hot/live. Treat the prop as live; no-go, maintenance.
Rough or excessive drop	Fouled plugs or a failing mag — troubleshoot per POH; maintenance if it persists.

Why it matters

The mag check is a 10-second run-up item that catches a dead mag before takeoff — and a no-drop is a genuine safety hazard, because a 'hot' mag means the prop can fire even with the switch off. Knowing what each result means is real systems airmanship.

Common error

A no-RPM-drop mag check feels like good news — but it's the opposite. No drop means the switch isn't grounding that magneto (a broken P-lead), so the magneto is live even in the OFF position. The propeller must be treated as capable of firing. It's a maintenance no-go, not a pass.

During the mag check you get NO RPM drop on the right magneto. This means:

Safety

Always treat a propeller as if the mags are hot — never move or stand in the arc of a prop you wouldn't want to start. A broken P-lead makes that danger real even with the key off.

A DPE might ask

Why does the airplane have two magnetos?
What does a no-RPM-drop mag check tell you? What if the engine quits on one mag?

PHAK ch. 7 — Ignition system & the magneto check

Engine indications & leaning

Definition

Your engine gauges are trend monitors, not decorations. Scan them after start, at run-up, in the climb, and in cruise, and compare them to the POH's normal ranges. A single number matters less than an unexplained change. The big ones: oil pressure and oil temperature tell you the engine is being lubricated and cooled, and EGT (exhaust gas temperature) is what you use to lean the mixture.

Explanation

Oil pressure dropping toward zero is an emergency — possible loss of lubrication: reduce power and land as soon as possible, prepared for the engine to fail. High oil pressure is usually just cold oil on a cold start and normalizes as it warms; if it's abnormal when warm, reduce power and monitor. Rising oil temperature calls for enriching, increasing airspeed, and reducing power — and high oil temp with low oil pressure together is serious, so land. For leaning, bring the mixture toward peak EGT, then enrichen to run rich of peak — this keeps cylinder temperatures down and preserves engine life.

Indication	What a change means → action
Oil pressure drop / zero	Possible loss of lubrication — reduce power, land ASAP / prepare for engine failure.
Oil pressure high / spike	Often cold oil on a cold start (normalizes); if abnormal when warm, reduce power + monitor.
Oil temperature rising	Enrich, increase airspeed, reduce power; high oil temp with low oil pressure is serious — land.
EGT (leaning)	Lean toward peak EGT, then enrichen to run rich of peak to protect the engine.

Why it matters

These gauges are your early warning system. Catching an oil-pressure or temperature trend early — and leaning correctly — is the difference between a precaution and an engine failure.

From the Right Seat

I teach leaning to peak EGT then enrichening to run rich of peak. It costs a little fuel economy versus lean-of-peak, but it keeps cylinder temperatures down and is the conservative choice for engine longevity in most trainers.

A DPE might ask

How do you use the engine instruments to monitor the engine?
What would you do if oil pressure dropped to zero? How do you lean?

PHAK ch. 7 — Engine instruments & leaning