Unit 4 · Engine and Systems · Lesson 2 of 4

Ignition, fuel and carburetor ice

10 min readLesson 18 of 593D lab

By the end of this lesson you'll be able to:

Explain how magnetos work and why there are two
Describe carburetor icing and how carb heat fixes it
Explain mixture, detonation and preignition

Magnetos

The ignition system uses magnetos, engine-driven generators that make their own electricity for the spark plugs. That means the engine keeps running even if the airplane's electrical system fails completely. There are two magnetos and two spark plugs per cylinder, for redundancy and better combustion. In the run-up, you check each one separately: the RPM should drop a small amount on each, within the POH limits.

A "hot" prop. If a magneto ground wire breaks, that magneto stays live even with the key off. Treat every propeller as if it could start.

Carburetors and fuel injection

A carburetor mixes fuel and air using a narrow venturi. The fuel's evaporation and the pressure drop in the venturi cool the air sharply, sometimes by 60°F or more, and moisture in the air can freeze in the throat: carburetor ice.

Carb ice is most likely below about 70°F with high humidity, but it can form at temperatures as high as 100°F with humidity as low as 50%, and it's most likely at low power settings, like a descent. The first sign in a fixed-pitch airplane is a drop in RPM, then rough running.

Carburetor heat sends warm air into the carburetor. When you apply it with ice present, RPM first drops further (warm air is less dense, and melted ice goes through the engine), then rises as the ice melts. Fuel-injected engines don't have a carburetor, so they're much less prone to induction icing.

Mixture

As you climb, the air gets less dense but the carburetor sends the same amount of fuel, so the mixture gets too rich. You use the mixture control to lean it, following the POH. Too rich wastes fuel and fouls plugs; too lean can overheat the engine.

Detonation and preignition

Detonation is the fuel-air charge exploding instead of burning smoothly. Causes include fuel grade too low, an overly lean mixture and high engine temperatures. Fix it by enriching the mixture, reducing power and lowering the nose for cooling.
Preignition is the mixture igniting before the spark, often from a hot spot like a carbon deposit.

Always use the fuel grade the POH specifies. If the correct grade isn't available, use the next higher grade, never a lower one.

Try it in 3D: How the engine works, in 3DWatch intake, compression, power and exhaust in a see-through cylinder.

Key takeaways

Two magnetos make their own electricity; the engine runs without the electrical system.
Carb ice: first sign is an RPM drop; carb heat makes RPM drop, then rise if ice was there.
Detonation: enrich, reduce power, cool. Never use a lower fuel grade.

What the examiner may ask

Practice answering out loud before you open each answer.

Why does your airplane have two magnetos?

Redundancy and better, more complete combustion with two spark plugs per cylinder.

What are the conditions for carburetor ice?

Most likely below about 70°F with high humidity, possible up to about 100°F with humidity as low as 50%, and especially at low power settings.

What's the first indication of carb ice in your airplane?

In a fixed-pitch prop airplane, a drop in RPM, followed by roughness.

Common mistakes

Forgetting carb heat before reducing power for a descent (if the POH recommends it).
Leaning aggressively at high power during a climb, risking detonation.
Using a lower fuel grade than specified.

Read it in the FAA sources

PHAK · Chapter 7Aircraft SystemsOpens the PHAK at this chapter ↗

Go deeper on the site

Prep: Before-Takeoff Check (Run-Up)

Lesson check

Get every question right to complete the lesson. Take your time; wrong answers explain why.

Check: If the airplane's alternator and battery both fail, what happens to the engine?Answer: It keeps running, because the magnetos make their own electricity. Magnetos are self-contained, so ignition doesn't depend on the electrical system.

Check: You apply carb heat in a fixed-pitch airplane and RPM drops, then slowly rises. What does that tell you?Answer: Carburetor ice was present and is melting. The initial drop comes from less dense warm air and melting ice; the rise shows the ice is clearing.

Check: If the specified fuel grade isn't available, what should you use?Answer: The next higher grade. Never use a lower grade; the next higher grade is the accepted alternative.

Check: What's a common way to correct detonation?Answer: Enrich the mixture and reduce power. Enriching the mixture, reducing power and cooling the engine help stop detonation.

Check: If a magneto ground wire breaks, what happens?Answer: The magneto stays 'hot' even with the switch off. A broken P-lead means the magneto can fire even with the key off: treat every prop as live.

Skip for now

Something wrong or out of date? Report an error