Ground school: Class 4

Tonight we talked about engines. From my years of tinkering with cars and more recently my motorcycle, I know enough to be dangerous. A small airplane engine is very much like a car or motorcycle 4-stroke engine, but it does have some important differences.

The Cessna 152 engine is a four cylinder engine with the cylinders aligned horizontally, two on each side. Each cylinder has two spark plugs (for redundancy and efficiency), and two magnetos drive the plugs; one drives one plug of all four cylinders, the other drives the other four. Unlike a car with an ignition system that requires the battery, magneto power comes from the running engine. You don’t need a battery at all for the engine to run; a complete electrical failure will not stop the engine, though you’d have to start it by turning the prop since the starter is battery powered.

Failure of one plug would reduce the efficiency of the cylinder, but it would still run fine with near full power. Failure of a magneto would be just like failure of one plug per cylinder, so it would still run fine.

The engine drive shaft is not connected to a transmission, but rather directly to the propeller, so the speed of the engine is always the speed of the prop. The engine can only go to about 2,500 RPM before the tips of the prop approach the speed of sound and make too much noise and reduce efficiency, so that’s as fast as it’s designed to run.

One particularly interesting issue with a carbureted airplane engine is the formation of carburetor ice. It doesn’t have anything to do with freezing weather; it can form when it’s as high as 70°F outside in high humidity. The physics of the air mixing causes a significant drop in temperature that can cause ice to build up. For this reason, the plane is equiped with pilot-controlled carb heat (the thing I keep forgetting to turn off when I go full power from really slow flight). It’s something every pilot should know how to use and is a regular part of basic flight procedures in a carb heat equiped aircraft.

We were told a story of one pilot that missed that lesson in his training:

A (former) club member accepted the opportunity to fly a Cessna 150 back from its required FAA annual inspection. I don’t know the details of such an inspection, but my understanding is they take apart a good bit of the plane and check it very thoroughly. This guy was a large man, the day was very nice; clear, low 70s, smooth air. On his way back to the club, the engine started to slow down. Thinking that the throttle had vibrated out a little (it can happen), he tried to push it in to recover the lost engine speed. However, it wouldn’t budge… either way. He thought the inspectors put it back together wrong (no… carb ice had frozen the throttle). Instead of using his head, he pulled and pulled on the throttle until he pulled it clear out of the panel along with the throttle cable, which he subsequently put in the passenger seat.

The engine continued to lose power, so the pilot found a place to land and made his approach. Just before he landed, the engine went to full power! He had to abort the landing. The engine wasn’t running for a short time, so the carb ice had melted on its own and returned the engine to normal operation.

Now the pilot was stuck with an engine at full power and no throttle control. As he approached the club airport, he was flying around it at full speed trying to figure out what to do. He eventually managed to reduce engine power by using the mixture control to cut the engine a little at a time so he could land.

Bottom line: if the engine is losing power, check for carb ice!