Most training airplanes are powered by a four-stroke, horizontally opposed, air-cooled piston engine, the same basic design that has powered general aviation for decades. Here is how it turns fuel into thrust, and the systems you manage as the pilot.
Part of our How Airplanes Work guide.
The four-stroke cycle
Each cylinder repeats a four-stroke cycle, often called the Otto cycle:
- Intake: the piston moves down, drawing in a fuel and air mixture.
- Compression: the piston moves up, compressing the mixture.
- Power: the spark plugs ignite the mixture, and the expanding gases drive the piston down.
- Exhaust: the piston moves up again, pushing out the burned gases.
Multiple cylinders fire in sequence to turn the crankshaft smoothly, which turns the propeller.
The fuel system
The engine is either carbureted or fuel-injected. A carburetor mixes fuel and air in a single venturi; fuel injection delivers fuel to each cylinder more precisely. You control the fuel-to-air ratio with the mixture control, leaning the mixture as you climb because the air thins with altitude.
The ignition system
Aircraft piston engines use two independent magnetos that generate their own electrical current, so the engine keeps running even if the aircraft’s electrical system fails. Each cylinder has two spark plugs, which improves combustion and provides redundancy. You check both magnetos during the runup before takeoff.
Cooling and lubrication
Most trainers are air-cooled, with cooling fins and baffling directing airflow around the cylinders. An oil system lubricates and also helps carry away heat. Monitoring oil temperature and pressure is part of every scan.
Carburetor ice
In carbureted engines, the temperature drop inside the carburetor can form ice even in warm, humid weather, restricting airflow and reducing power. It is most likely in moist air at outside temperatures of roughly 20 to 70 degrees Fahrenheit, and it often appears at low power settings such as a descent, not just on the ground. Carburetor heat redirects warm air to prevent or clear it. Knowing when to use carb heat is a core skill.
Detonation and preignition
Two abnormal combustion events are worth knowing for the oral exam. Detonation is an uncontrolled explosion of the fuel-air mixture, often from using fuel of too low a grade, an excessively lean mixture, or high power with high temperatures; it can damage the engine. Preignition is the mixture igniting too early, typically from a hot spot in the cylinder such as a glowing deposit. Using the correct fuel grade, managing the mixture, and keeping engine temperatures in the green help prevent both.
What you'll need
The FAA handbooks that explain how your airplane works, from PilotMall.com.
Related: Manifold pressure: what the gauge tells you.
Frequently asked questions
What kind of engine do training airplanes use?
Most use a four-stroke, horizontally opposed, air-cooled piston engine.
Why are there two magnetos and two spark plugs?
For redundancy and better combustion. The magnetos generate their own current, so the engine runs even with an electrical failure.
What is carburetor ice?
Ice that forms inside a carburetor from the temperature drop, reducing power. Carburetor heat prevents or clears it.
Why do you lean the mixture?
Air thins with altitude, so you lean the fuel-air mixture to keep it efficient and prevent fouling.