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What Is Vapor Lock, And Why Does It Happen?

Boldmethod

At some point, you've probably had a hard time starting your plane's engine. If you're flying a fuel-injected airplane and it's warm outside, chances are the problem you had was related to vapor lock.

What Is Vapor Lock?

Vapor lock happens when the fuel in your fuel lines vaporizes, turning from a liquid to a gas. In other words, your fuel has boiled.

When your fuel lines are full of vaporized 100LL, your fuel pump can't get liquid fuel to the injector nozzles, which makes starting your aircraft frustrating, difficult, or impossible.

Why Does Vapor Lock Happen?

Before we get into the details of exactly how your fuel vaporizes, it's important to understand what a typical fuel-injected engine system layout looks like.

You'll notice that many components are similar to that of a carburated aircraft. There are still filters, sumps, and fuel pumps. But there are a series of lines that branch out from the center of the engine that leads to each cylinder. These are the fuel lines, sometimes referred to as "the spider" for their shape.

Vapor lock is most likely to occur in the fuel injector lines after the flow divider, as fuel is sent to the injector nozzles to be sprayed into the intake manifold.

This is because the heat from the engine cylinders and crankcase radiates outwards and up toward the fuel injector lines.

Ahunt / Wikipedia

So if you're doing a quick turn or just shutting down to refuel, it's important to be aware that you might be at risk of vapor lock when trying to start back up.

The Problem With Fuel

The fuel that you burn in your engine also can affect your chances of getting stuck with air inside your fuel lines.

The greater the volatility of fuel, the higher the probability of vapor lock happening. While 100LL is less volatile than the gasoline that you put in your car, it can still easily vaporize when subjected to high heat (Avgas has a Reid vapor pressure range of 5.5 to 7 PSI. Automotive gasoline has a range of 8 to 14 PSI).

Volatility is the measure of how likely a substance is to evaporate at a specific temperature. The irony of this situation is that higher volatility fuel makes starting in normal conditions easier.

Put simply, if the volatility of fuel is too low, it might not start, and if the volatility is too high, it can cause vapor lock.

When Is Vapor Lock Most Likely To Happen?

Vapor lock typically starts when you shut down an engine. As you shut down, the heat rises up toward the fuel lines. Without air moving through the cowling, fuel in the lines can start to vaporize.

Generally, if you experience vapor lock, it's when you try to start your plane after a quick turn (shut down and restart within ~30 minutes). But in some rare cases, vapor lock can occur in flight.

How To Mitigate Vapor Lock

The best way to avoid or minimize vapor lock is before it happens. Once you shut down, open the cowling or oil vents. While this might not eliminate the risk of vapor lock, it can greatly reduce it.

Also, if it's possible, point your airplane into the wind. Even a small breeze through your cowl and over the engine can help cool it and reduce the chance of vapor lock.

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Vapor Locked? Here's How To Get Started Again.

What happens if you did everything to keep your engine cool, but your engine is vapor-locked?

The only real solution is to push the air bubbles out of the lines by pumping a bunch of fuel through with a boost pump. Unfortunately, there's no specific way to know if you have pushed out all of the vapor from your fuel lines. It just comes with experience.

Start by running your fuel pump for a short amount of time (5-10 seconds), try to start, and if your engine doesn't fire up, try again. Also, check between starts to see if you're dripping fuel onto the ramp below your engine. If you are, your engine is flooded.

Starting a flooded engine can also be difficult, and can also increase your odds of an engine fire.

What techniques do you use to prevent vapor lock? Tell us in the comments below.

Nicolas Shelton

Nicolas is a private pilot from Southern California. He is currently studying at Purdue University, where he is working on advanced pilot ratings. You can reach him at nicolas@boldmethod.com.

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