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Winter flying means cold starts. And cold starts often lead to engine fires.
Gas, like any other liquid, doesn't evaporate as much when it's cold. And when it's really cold, gas evaporates very slowly, making it hard to burn.
On top of that, your engine's oil is thicker when it's cold, making it harder to move when the engine is turning for start. That means your engine doesn't turn as fast while you're cranking it.
To top it all off, your battery can't produce as many electrons when it's cold, meaning your starter doesn't have as much energy to work with, causing it to crank slowly.
All of this leads to the first problem with cold engine starts: over priming.
Whether you're flying a carbureted or fuel injected engine, you're going to prime it for a cold start. And when it's really cold, it's easy to feel like you need to give your engine a little extra 'shot' of gas to get it going.
But that's where the problem starts, because over-priming is the leading cause of engine fires on the ground.
When you prime an engine, you're putting fuel into the intake manifold so the cylinders are ready to fire. And if you over-prime, you're putting too much fuel into one of three places:
For your engine to start, you need the right fuel-to-air ratio, which is roughly 15 parts air to one part fuel. If you prime your engine too much, your fuel-to-air ratio is off, and the mixture won't ignite.
On top of that, excess fuel can pool in the manifold or cylinder, leading to problem #2.
If you add too much fuel during priming, it can leave its gaseous state and become liquified.
When fuel becomes liquid in your cylinder, it starts splashing around, waiting for a spark to ignite it. As it continues splashing, it creates a smaller fuel-to-air ratio, and eventually it ignites (usually with a big BOOM).
The splashing fuel travels one of two places as it ignites:
So how do you know if you're over-primed? And if you are, what can you do about it?
If your plane is over-primed, you can usually smell fuel. And if you've really over primed, you can probably see fuel dripping from the exhaust pipe.
First off, if you've slightly over-primed, you can typically use a specialized start procedure for your plane. But if you're over-primed to the point where fuel is dripping from the cowling or exhaust pipe, you need to not start the engine.
Instead, you need to give the engine time to evaporated the extra fuel. To do that, open the throttle to full, which opens the engine's butterfly valve. By sending air through the engine, you give the pooled gas time to vaporize. How long does it take? It really depends on the engine, and how over-primed it is, but giving the engine 10-15 minutes to air out usually does the trick.
So when you're flying this winter, remember that even though you feel like you need a little extra prime, sticking to your normal priming procedure is the best bet.
Over-priming is the leading cause of engine fires on the ground. By not putting too much gas into your engine on start, you'll significantly reduce the chance of starting your plane on fire. And we'd much rather see your plane without the charred fire marks on it.
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 email@example.com.