Aircraft Systems: Engine, Fuel, Electrical and Carburetor Ice

The four-stroke engine

Intake, compression, power, exhaust. Most trainers have air-cooled, horizontally opposed engines.

Ignition: magnetos

Two magnetos fire two spark plugs per cylinder, independent of the electrical system. That's why the engine keeps running if the alternator fails, and why you check each mag in the run-up.

Mixture

As you climb, air gets thinner. Leaning the mixture keeps the fuel/air ratio correct. Follow the POH. Too rich wastes fuel and fouls plugs; too lean can cause roughness, overheating or detonation.

Carburetor ice

Fuel vaporization and the venturi effect can cool the carburetor below freezing even on warm days. It's possible across a wide temperature range (roughly 20–70 °F) when humidity is high. First sign with a fixed-pitch prop: a drop in RPM. Apply full carb heat; RPM may drop further, then rise as ice melts.

Fuel system

• Gravity-fed (high wing) or pump-fed (low wing).
• Sump every drain before flight: check for water and the correct color (100LL is blue).
• Know your fuel selector positions and unusable fuel.

Electrical

An alternator charges the battery and powers avionics and lights. The ammeter or load meter shows charging; a low-voltage light warns of failure. If the alternator fails, shed load and land.