When a fuel stop for cheap 100LL may be an unwise choice

Ron Lee · #1 07-23-2009, 09:36 PM

Coming back from Seattle via Independence OR, I made a fuel stop in Wells Nevada (LWL). I made that choice mainly based upon price. The flight day started four hours later than desired due to a marine layer over Renton, WA (RNT).

I finally land in Wells around 430 PM so it was hot. Surprise. After fueling, using the facilities and looking for people I walk back to the plane and think that the conditions are ripe for vapor lock. What if I can't start the engine and I have to overnight here? Is there a hotel? How would I get there? This is a remote location.

The engine started on the third attempt and ran a bit rough for a while so vapor lock may have been a factor.

Based upon this event I plan on landing at larger airports while crossing remote regions. Saving a few dollars on gas would be chump change if a mechanical/engine issue developed.

Edit: Carbureted engine.

pvans · #2 07-23-2009, 09:54 PM

Ron,

Or, you can learn how to start a hot engine, maybe install a purge valve to bring cool fuel to the engine and pump the hot vapor locked fuel back to your other tank.

Buying cheap fuel is good for the aviation economy, we need to promote the less greedy fuel suppliers!

frankh · #3 07-23-2009, 10:12 PM

But us engineering types can't help it.

A hot start problem is highly unlikely to be vapour lock!..VL describes localised boiling at it occurs at the inlet to the machanical fuel pump.

What you are describing is vapourisation in the injector lines. This causes the hot start issues.

As mentioned above a purge valve takes care of this issue completely.

As for me with no mechanical pumps I can burn highly volatile fuel (mogas with ethanol) on the hottest of days at high altitude and take off without a second thought to a real VL issue.

Frank

NYTOM · #4 07-24-2009, 02:45 AM

Exactly where in the line would this valve go? Is this more or less just a by-pass to circulate fuel back to the tanks. And would that necessarily be a solenoid valve at the engine mounted fuel pump or a manual valve in the cockpit. Any links to a diagram of a system like this? Sounds like a good idea.

pierre smith · #5 07-24-2009, 04:59 AM

A purge valve is usually operated by pulling a cable. This reroutes the fuel back to a tank or the fuel selector, depending on how it is plumbed. You bring cooler fuel to the engine and the warm/hot fuel is gone.

The downside to having a purge valve is that if it is inadvertantly activated, your engine stops, since the fuel is now returned instead of going to the engine.

I bid on a -4 in that was totalled during the forced landing when this happened.

Regards,

Don at Airflow · #6 07-24-2009, 06:51 AM

When the parts are installed correctly with aircraft quality cables and components and approved methods for doing the install there?s no problem. If you pull the mixture control in flight the engine quits also.

Bottom line is, most all parts on an aircraft are critical to its safe and proper operation in flight. Pay attention to the details, instructions, and people with knowledge and experience based on facts. There?s lots of ?expert opinions? out there.

Just for the record, the purge return line can be teed into one of the fuel tank feed lines before the selector valve. You do not need a dedicated return line to the tank.

Don

Geico266 · #7 07-24-2009, 07:02 AM

Ron, what proceedure are you using for starting your hot engine? Maybe there is a better technique we can help you with. A carb engine should be pretty easy starting, even hot. That has been my experience anyway.

JDRhodes · #8 07-24-2009, 07:19 AM

Hot start techniques notwithstanding, Ron's original point is worth considering...

What happens in a remote area or an airport with no one around if ANY problem arises that would prevent you from leaving or require serious maintenance? Even simple maintenance is difficult without tools and parts.

Are we more willing to ignore pre-flight no-go items if the alternative to going is sleeping on the ground in the Nevada desert? Bush pilots deal with this risk all the time.

If you plan to stop at remote locations - or even fly over them - be prepared, Boy Scout! Tools, maintenance know-how, survival kits, etc. are required equipment. Thanks for the reminder, Ron!

David-aviator · #9 07-24-2009, 07:21 AM

Quote:

Originally Posted by NYTOM

Exactly where in the line would this valve go? Is this more or less just a by-pass to circulate fuel back to the tanks. And would that necessarily be a solenoid valve at the engine mounted fuel pump or a manual valve in the cockpit. Any links to a diagram of a system like this? Sounds like a good idea.

The purge valve allows one to send fresh fuel up to but not through the flow divider. The path the fuel takes is past the flow divider and back to a tank. This does make starting a hot engine easy but it does not prevent fuel from vaporizing after it passes the flow divider and is in the distribution (spider) lines. Fresh relatively cool fuel does lessen vaporization but not eliminate it completely. The valve is mechanical and attached to the inlet side of the flow divider on top of the engine.

I have found that opening the oil cooler door immediately after landing, say for a fuel stop, does cool the top of the engine quick. Even 30 minutes helps a lot. When the door is opened, much hot air can be felt moving out.

As Don at AFP reminded me on this issue, 100LL vaporizes at 130F. Fuel pressure up to and through the flow divider after the FM200 controller is under relatively low pressure, I believe 1-3 psi. If the lines are heat soaked and fuel is not moving much, some vaporization does occur. I have no problem getting the engine started, but keeping it running under these circumstances requires 1000-1200 rpm to provide enough fuel flow to prevent vaporization. Once up to take off power, vaporization after the flow divider is a non event. It does not happen. Fuel is moving and incoming fresh air across the area cools things down considerably. I never have a vaporization problem after landing even on the hottest days. The engine will idle very smoothly at 670 rpm for quite some time.

With regard to the engine driven pump inlet, the weakest link in the system with regard to vaporization, an electric pump providing fuel under pressure from aft of the firewall eliminates that threat completely. I run my electric pump constantly on hot days. It is no big deal, the pumps are designed for continuos operation. But it is a good idea to have pressure regulator by-pass fuel going back to a tank instead of running it in a loop. Fuel in the loop will warm up on a hot day due to internal electric pump temperatures. I use the same purge by-pass line back to a tank for this purpose. The purge by-pass line has a check valve to prevent fuel from flowing forward but it probably is not necessary.

Ron Lee · #10 07-24-2009, 07:32 AM

Larry, first attempt was normal. Start cranking the engine with mixture control normal and a short throttle pump. I may have tried the same method a second time. Then mixture full lean, partial throttle and crank while pushing in the mixture. That worked but it still ran rough a while.

Obviously re-education on hot starts may be productive but other mechanical problems (flat tire, etc) would also have been problematic. Personally, I would not make the same fuel stop decision under similar conditions.

On trips like this I carry a lot of stuff to deal with likely problems. I have spare tires, tubes, tools, hardware, etc. But I may not be able to change a tire without a jack or similar to raise the wheel. There are limits on what I can carry so I assume that a mechanic will be available to supply things I don't carry. No mechanic seemed to be around at this airport.