Vapor Lock

[gbigs]

The new owner and my old CFI were up in the CT yesterday with ground temps over 100f.  They were flying for 2 plus hours and got to 8k feet (68f oat) and the engine quit.  They restarted the engine and put on the Aux Pump and the engine ran fine after that....  Vapor lock.  

 

The wife and I never got that in the plane because we never flew in such hot temps for long periods of time.  So if flying for a long time hot and want to climb over 8k feet make sure to put on the Aux Pump.

[Ed Cesnalis]

CA gas has different winter and summer blends with different vapor pressures to prevent vapor lock.  Nevada?  not so much.

[Anticept]

The combination of hot weather and high altitude may have lead to that. The vapor pressure of car gas is quite high, so if conditions go below it, it will more readily turn to vapor.

[Jacques]

some reading here

 

http://www.lightaircraftassociation.co.uk/engineering/Mogas/OPERATING%20INFORMATION%20Iss%205.pdf

 

specially,,paragrah  5  and 7

[Roger Lee]

What were they doing when the engine quit and at what rpm? Unlikely vapor lock.

[gbigs]

What were they doing when the engine quit and at what rpm? Unlikely vapor lock.

 

I was not flying.  It was reported that they were at 9k feet and the engine quit.  He restarted the engine with the Aux pump on and it ran okay after that.  He did say they were running at 230 above and below the yellow for a long time too, climbing, descending, maneuvering....he is giving lessons to the new owner.

 

The temps here have been north of 105 most of the day...i assumed it had to do with heat.

 

They flew all day today without issue over the Sierras to California and back without issue also.  It may be the Mogas....I am sending that PDF the other commentor just posted.

[FlyingMonkey]

Isn't vapor lock more likely at low level in high temperatures than once climbed up to cooler temperatures?

[Roger Lee]

Vapor lock would have happened with either pump running. It may or may not have started right up again. The 912is engine has quite a bit more fuel recirculation vs the 912uls.

91 oct. partial pressure drop cause? possible, but not convinced.

vapor lock? possible, but not convinced.

 

Too many other variables to just jump in and just call it vapor lock.

[Ed Cesnalis]

Vapor lock is more likely to prevent a restart after landing for gas in the hot dessert, not likely at altitude I would think.

[Brucegoose]

Interesting read Jacques,

 

With ref. to Para 11, does the CTsw have a vapour return line?

[FastEddieB]

Vapor lock is more likely to prevent a restart after landing for gas in the hot dessert, not likely at altitude I would think.

Hot dessert - yum!

 

Seriously, vapor lock can happen at altitude after climbing with tanks of hot fuel.

 

Because of this, some planes with fuel injected engines recommend boost pump on in the climb, then for a certain time after leveling off at altitude, to give the fuel a chance to cool.

 

From the SR22T POH, for example:

 

"The fuel pump should be in the BOOST position during takeoff and for climb as required for vapor suppression with hot or warm fuel."

 

I had it happen once in my Cirrus at 16,000'. Lesson was when switching off the boost pump, to watch the fuel flow gauge like a hawk, and to turn the pump back on at any hint of the needle wavering. As a precaution, some Cirrus owners just leave the boost pump on at all times, though at least some Cirrus boost pumps caution they are not designed for continuous usage.

[FlyingMonkey]

As a precaution, some Cirrus owners just leave the boost pump on at all times, though at least some Cirrus boost pumps caution they are not designed for continuous usage.

 

That falls into the category of "some is good, so more must be better", like pilots who reason that if 1.3 Vso on approach is good, 1.5 Vso must be better!

[Ed Cesnalis]

Hot dessert - yum!

 

Seriously, vapor lock can happen at altitude after climbing with tanks of hot fuel.

 

Because of this, some planes with fuel injected engines recommend boost pump on in the climb, then for a certain time after leveling off at altitude, to give the fuel a chance to cool.

 

From the SR22T POH, for example:

 

"The fuel pump should be in the BOOST position during takeoff and for climb as required for vapor suppression with hot or warm fuel."

 

I had it happen once in my Cirrus at 16,000'. Lesson was when switching off the boost pump, to watch the fuel flow gauge like a hawk, and to turn the pump back on at any hint of the needle wavering. As a precaution, some Cirrus owners just leave the boost pump on at all times, though at least some Cirrus boost pumps caution they are not designed for continuous usage.

 

Thanks,  never had an FI plane. 

[Roger Lee]

This engine circulates a lot of fuel. Quite a bit more than a 912uls and it is not recommended to run the second pump. In this engine it should make no difference because of the way they are plumbed. If it was a true vapor lock this most likely would not have restarted.

[ralarcon]

This engine circulates a lot of fuel. Quite a bit more than a 912uls and it is not recommended to run the second pump. In this engine it should make no difference because of the way they are plumbed. If it was a true vapor lock this most likely would not have restarted.

Agree, in hot and humid Florida , I have never had vapor lock on my carburated CTLS,. What I often have is fuel leaking out of the winglet vent ports. Clearly no vapor lock.

 

Cheers

[FastEddieB]

My impression is vapor lock is less of a problem with carbureted engines.

 

All a carb needs is fuel making it to the float bowl rapidly enough to meet the engine's needs. A bit of vapor will usually just push through, and as long as there's enough fuel in the float bowl you'd never be aware of the bubbles of air passing through.

 

But fuel injection depends on constant high pressure fuel making it to the injectors. Any interruption in that fuel flow is immediately apparent.

 

At least that's my take, for what its worth.

[FastEddieB]

some reading here

 

http://www.lightaircraftassociation.co.uk/engineering/Mogas/OPERATING%20INFORMATION%20Iss%205.pdf

 

specially,,paragrah  5  and 7

 

Interesting.

 

Especially the part about not using MOGAS above 6,000', or over 20ºC (68ºF) which is done quite routinely without incident.

 

From the linked article:

 

5. OperatingLimitations

Unleaded Mogas fuel is restricted by CAP 747, Appendix 8, General Concessions 4 and 5 to operation with a fuel not exceeding 20° C and an altitude not exceeding 6000 ft. These additional limitations must be displayed in the cockpit using a suitable placard (see section 6 below). 

[Anticept]

Vapor lock is something that started back in the days of old automobile engines. At the time, there was no fuel pump in the gas tank. In those days, there weren't fuel pumps in the gas tanks. What would happen is the fuel would be drawn up to the engine and sometimes lower the pressure enough that vaporization occurs. The problem with the gases (the phase, not gasoline) is it takes up DRAMATICALLY more space than liquids do, raising pressure as the phase change occurs, and it's really, really difficult to draw fuel past it. That. and many pumps that work with gasoline don't work with air.

 

It doesn't matter what kind of engine it was, the problem is the fuel was not being drawn from the tank and starving the engine. That's vapor lock.

 

Low wing aircraft suffer this same problem, while high wings usually don't. Even if vaporization occurs, the high wing head pressure will USUALLY push the air out; low wings do not get this luxury. That's why low wings have boost pumps in the wings or belly, and often high wings do not. Note that I said USUALLY. High wings are NOT immune to vapor lock. Here's why:

 

Imagine an aircraft that has high wings and tractor configuration. Fuel goes from the wings, down into the belly before going back up to the engine fuel pump. In this configuration, if air forms anywhere in the system, it will either float up to the wings or up to the fuel pump and get purged by head pressure. But, what if we put a U bend in it somewhere in the belly so that there's a high point? This U bend becomes the vulnerable point to vapor lock. If air forms, it will float to the high point in that bend and get trapped. Enough air will actually act like a constriction on the line, fuel tries to push past it but you're only going to get a little bit of a trickle.

 

In order to purge that air, you basically have to either raise the fuel pressure to push it past, or open the fuel line so that the flow rate is faster than the air bubble's ability to stay in the U bend. It's vitally important to avoid having high points in a system unless you supplement it with pumps, return lines, or some kind of air bleed. Even in the 912 carb engines Rotax strongly recommends a restricted return line after the fuel pump to the fuel tank, or will accept a return line to the gascolator (Flight Design does the latter. Doing the former requires a differential fuel flow indication system if fuel flow readings are desired).

 

All that said, the answer to the engine quitting is correct: turn on the boost pumps. I'm partially with Roger; it's unlikely that vapor lock occurred, but it's still quite possible. Turning on the boost pump is an indication that this is what may have occurred, but there's still too many variables.

 

Just for reference:

 

• The EPA requires gasoline in most places in the states to have a vapor pressure of 9psig @ 100F during summer months. In some places, 7.8psig. The vapor pressure is where things will boil off if atmospheric pressure drops below that number
• HOWEVER, the EPA allows a 1 psig variance to the rule!
• The closer you get to the vapor pressure, the faster things evaporate too!
• At 8,000 feet, air pressure is at about 21-22 inches of mercury. That's about 10.5 psig. Awfully close to that 9 psig! Get a bit of a low pressure in the system somewhere and you might very well get vaporization!

[Roger Lee]

The issue here is no one know why it stopped. vapor lock would only explain one thing. It's all speculation to just point the finger at one potential cause.