Question Vapor lock: What is it and who is affected by it?

[Neal]

I've seen some YouTube videos where this occurred where a backcountry airplane had to abort a takeoff. Now I see this article in my morning email news feed. I'd like to learn more about it, who is affected by it, i.e. does it affect fuel injected engines? And how to mitigate it?

Vapor lock leads to forced landing — General Aviation News

The pilot reported that he was about halfway home when the power loss occurred.

generalaviationnews.com

 

[midlifeflyer]

It affects fuel injected engines.

 

[FlyBigE]

I dealt with this in my early cargo days and in my Flight Training days in the C172R model. In my experience it was only a start issue. I never experienced or heard of it happening in flight.

 

[belchfire]

Vapor lock is rare and difficult to positively identify. It is similar to the widely known starting issues with constant flow fuel injection systems but is not identical-though they share one thing-fuel gets vaporized in a fuel line.

When a fuel injected flat engine is shut down the fuel pump stops pressurizing the system, fuel stops flowing and cooling airflow stops. The result is that the residual fuel in the injection lines that branch out across the top of the engine vaporizes and creates an over-rich condition in the intake system with no fuel in the injection lines. The vapor can also occur in the fuel side of the Bendix type injection system which is why the procedure for hot starts varies between Continental and Lycoming engines.

In either fuel injected engine the presumption is that the engine is essentially flooded like when a girlfriend stomped on the accelerator pedal seven times before trying to start the hotrod and now your Holly 4 pumper has dumped about a pint of raw gas into the intake manifold.

In the aircraft you have an electric boost pump so the first step is to turn that on. In the Continental Constant Flow system the throttle is pushed open, the starter is engaged and when the engine starts to catch the throttle is moved to the idle position and the mixture is pushed rich and you have a running engine.

With the Bendix RSA system with it's four chamber fuel metering unit one opens the throttle and fully enriches the mixture and deliberately floods the engine, then closes the mixture and engages the starter. When the shower of sparks ignition system sets some of the fuel on fire one starts closing the throttle and carefully enriches (lest you flood the bloody thing again-and trust me, you don't want to do that!) and having previously made the proper absolutions, sacrifices, prayers and holding your mouth just right if the moon and stars align the Lycoming will cough itself to life-providing of course the battery doesn't die first!

I jest (a little) and honestly I liked the Lycs better than the more fragile Continentals but while the vaporized fuel inhibits normal starting in the fuel injected engines it is not true vapor lock because you have a fuel pump that will push liquid fuel out to the cylinders.

Vapor lock is real but it is also exceedingly difficult to prove and has likely been blamed when no other cause for engine anomaly/failure could be determined. In that respect it is sort of like carburetor ice but vapor lock is far less common.

Vapor lock is rare in modern aircraft that are running avgas. The fuel is formulated to resist it and it takes a perfect alignment of temperature, atmospheric pressure, static pressure of the fuel, rate of flow through the fuel lines, vibration or other mechanical agitation of the fuel and even fuel line routing to allow a bubble of vapor to form in a fuel line with sufficient pressure to prevent liquid fuel from getting past it.

Vapor lock also tends to occur in gravity feed fuel systems or upstream of any pump that is installed in the system...

If I am not mistaken some of the older FAA publications (think 1980's and earlier) have discussions about how to design fuel systems to prevent vapor lock and the EAA should have information or be able to direct you to better sources.

Without seeing the whole set of fuel lines and not knowing where, if any, fuel filters, fuel pumps, poorly designed bends are in the homebuilt aircraft's fuel system or even what engine was installed I really can't hazard a guess as to what was really happening. With a history of engine issues as vague as "vapor lock" I am only qualified to say the sucker is lucky to be alive!

I found this on Zenith's website...

Alternative engines will affect performance, specifications and flight characteristics of the aircraft. Also, the weight and balance of the aircraft may be adversely affected by alternative engines, and the original fuel system may not be adequate or suitable for some engines. Most alternative engines will require a custom engine mount and engine cowl. Zenith Aircraft Company does not manufacture or directly support engines.

The Zenith 750 is designed around the O-200 which is a known quantity. Any vapor lock would most likely have had to occur upstream of the engine, if that indeed was the cause as we do not know what engine was installed for sure.
Hope that helps...