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What Causes Engine Fires During Start?

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Did you know that engine fires during start are fairly common, especially in the winter months? You might be thinking "OK great, so this could happen to me?!" Yes, it can. And it can happen a lot easier than you might think.

Over-priming is the leading cause for engine fires on the ground. If your engine doesn't start, how many times are you going to re-prime? How many times does it take to over prime?

During a cold engine start, you have to prime the engine. When you prime an engine, you're putting fuel into the cylinders (or the intake manifold) so that the engine can fire. Pilots tend to over-prime the engine by priming too much or too many times (we're guilty of it too). In contrast, there are very few people who prime too little.

So what happens when you over-prime? When you prime the engine, the extra fuel goes to one of three places:

1) The intake manifold, right in front of the intake valve.
2) The valve-chamber, where the fuel will be sucked into the cylinder.
3) Directly into the cylinder itself.

For the engine to start, you need a correct fuel-to-air ratio, which is about 15 parts air to one part fuel. If you prime too much, you have too much fuel for the amount of air in the cylinder, and the fuel won't ignite. Plus, the excess fuel can pool in the manifold or cylinder. Often times, you've added so much that the fuel becomes liquified, leaving its gaseous state.

When you attempt to start the engine, this excess fuel might be splashed by the piston. Eventually, this splashing fuel creates a smaller fuel-to-air ratio and BOOM, it ignites. The real problem here is the splashed fuel, which travels to one of two places:

1) Exhaust Manifold: The ignited fuel exits through the fire-proof exhaust system, no big deal.
2) Intake Manifold: The splashed fuel can also exit through the intake manifold. If you're really unlucky, this ignited fuel might run down into the carburetor or fuel-injector assembly. These parts are not fire-proof. If ignited, the fuel can literally burn through them, igniting an entire pool of fuel. If this happens, you're in trouble.

Here's the scenario: you're starting a cold engine, so you have to prime. You prime the engine a little more than normal, maybe 2-3 times. When you try to start the engine, it won't fire up. You figure that you haven't primed enough, so you prime again. This process repeats, and results in you flooding the engine.

So how do you know if you've over-primed? And what should you do if that's the case?

1) If you're over-primed, you'll have difficulty starting the engine. You'll also likely smell fuel.
2) If you've slightly over-primed the plane, you can often use a specialized start procedure for your aircraft. But, if you've over-primed a lot, with fuel dripping from the exhaust, don't try to start the engine. Instead, make sure the engine is off, and open the throttle to full. This will open the engine's butterfly-valve, sending air throughout the engine, evaporating the excess fuel. Let it sit open for 10 to 15 minutes, or more, and go relax in the pilot's lounge. No rush!

Engine fires happen, especially in the winter, but understanding the dangers of over-priming can help you avoid them. Be careful not to over-prime your plane, even when it's bitterly cold, and your engine will thank you for it.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and commercial pilot for Mokulele Airlines. He holds multi-engine and instrument ratings, and is an aviation student at the University of North Dakota. He's the author of the articles, quizzes and lists you love to read every week. You can reach Swayne at, and follow his flying adventures at

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