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Why Is Starting An Aircraft Engine So Hard?

Boldmethod

When's the last time you worried about starting your car or SUV? Probably never.

You get in, turn the key, and boom (or VROOOM), you're running, without even thinking twice.

So why is it so hard to get an aircraft engine going? It seems like it doesn't matter whether it's hot or cold, there's always something causing you problems.

For the most part, the problems come down to the weather, and your engine's design: carbureted, or fuel injected.

Carbureted Engines: Tough In The Cold

You've probably experienced this before: you hop into a carbureted aircraft on a cold day, prime then engine, and start cranking. And you crank some more. And some more. And the engine still doesn't want to start.

For the majority of carbureted engines, cold starts are the most difficult. And it's because of the way a carbureted engine is primed.

When you prime a carbureted engine, fuel is sprayed into the intake manifold for one cylinder (sometimes more than one cylinder). This is part of the problem.

If your engine is designed to only prime one cylinder, only one cylinder (out of four) has close to the right amount of fuel to fire up and get the engine running.

Another problem is fuel atomization. When you're starting a cold carbureted engine, the precision of fuel/air mixture isn't very accurate. Which means the fuel/air mixture that's reaching the cylinders isn't optimal for getting the engine started, compounding your problem of getting the engine going.

So what's the solution for getting a cold carbureted engine started? There isn't a perfect one, but getting the engine warmed up can help. Putting on an engine pre-heater, or pulling the aircraft into a heater hangar will (slowly) warm up the engine. And the warmer the engine, the better your fuel atomization will be.

Iditarod.com

Fuel Injected Engines: Tough In The Heat

Where carbureted engines fall short in the cold, fuel injected engines have their problems in the heat. And it's because of vapor lock.

Avgas is volatile, which means when it's heated up, it changes from a liquid to a gas.

When that happens, the fuel pump isn't able to do its job (it's hard to pump vapor through a tube, and much easier to pump liquid through).

So when does vapor lock happen? Typically after the engine has been running, and you shut it down after landing.

When you look at the location of fuel injector lines on an engine, they're typically on top of the cylinders. Engineers don't put the injector lines there there to frustrate you, but when you're dealing with an air-cooled engine with a very tight fitting cowling, there aren't a lot of options.

The problems start at engine shut-down. When you shut down a hot engine, the heat rises, it heats up the fuel lines, and it vaporizes the fuel.

All of the sudden you no longer have liquid in the injector lines, and that's a problem.

Ahunt / Wikipedia

When you have vapor lock, the only solution is to pump a bunch of fuel through the lines, typically with a boost pump, and push the vaporized fuel out of the lines. But that brings up another problem: flooding the engine.

Since there's no way to know exactly when the vaporized fuel has been purged from the injector lines, it's easy to flood the engine in the process. And flooded engines can be tough to start as well, not to mention the possibility of an engine fire during start.

So what's the best solution for vapor lock? If you're going to "quick turn" an airplane after you land, get the engine cool by opening the cowl after you shut down. And if that's not an option, open any doors you can to let the heat escape, like the oil door on a Cirrus. While it may not be the best ventilating option, every little bit of cooling helps when you're getting ready for your next engine start.

Getting Started

Knowing the common problems for engine starting on both carbureted and fuel injected engines is half the battle.

For cold starts on carbureted engines, prime first, but if you're not having any luck, try to get the engine warmed up with a preheat or hangar. And for fuel injected engines, try to cool the engine as much as possible after shutdown to prevent fuel vaporization.

If you know the keys to getting your engine started, chances are you'll have a much easier time when you turn the key and start cranking.


What are your secrets to getting a hot or cold engine started? Tell us in the comments below.

Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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