Vapour Lock?

[aro]

3 hours ago, turboplanner said:

The reason it is normally off is that it can cause a disaster if there is an engine bay fire and fuel lines melt or fracture, and also if the carburetor float valve is leaking

Unlikely. If there is a leak in the fuel line before the engine driven pump in a low wing aircraft, it will suck air and the engine will lose power/stop. At which point the first action is turn the pump on...

 

If there is a leak after the engine driven pump e.g. carb float valve, it probably doesn't make much difference if the aux pump is on or not because the engine driven pump is supplying fuel pressure.

 

You quoted the actual reason from the POH in your post.

 

3 hours ago, turboplanner said:

In GA students are taught why the electric fuel pump is there and when to turn it on or off.

 

What they are taught isn't always correct - it seems like there are as many myths in GA as any other level of aviation.

[Thruster88]

9 hours ago, RFguy said:

Alan, vapour lock on a heat soaked Jabiru engine  is not uncommon, the fuel pump gets absolutely cooked by the adjacent cylinder barrel.

Many over put in a blow pipe (like 3/8" ID)  from the rear of  cooling plenum  to blast the fuel pump. Some put a shiny aluminium heat shield  between the cylinder and the fuel pump.

Glen are you saying the jabiru electric pump fed with about 1.2psi of gravity input pressure cannot push fuel to the carburetor on a heat soaked engine?

 

Heat soaking would be more severe than the 15 minute idle experienced by the OP. If that loss of power was caused by vapour lock they would all be doing it. If that loss of power was caused by vapour lock nobody will be able to fly in hot conditions with any confidence.  

[turboplanner]

9 hours ago, aro said:

Unlikely. If there is a leak in the fuel line before the engine driven pump in a low wing aircraft, it will suck air and the engine will lose power/stop. At which point the first action is turn the pump on...

 

If there is a leak after the engine driven pump e.g. carb float valve, it probably doesn't make much difference if the aux pump is on or not because the engine driven pump is supplying fuel pressure.

Leaks after the engine pump are the problem; attached below  is a first hand example of one:

 

 

9 hours ago, aro said:

 

You quoted the actual reason from the POH in your post.

I was going to take the Warrior POH attachment out in case it confused people, but it had a lot of good detail. It gives a succinct reason but there are tnousands of different aircraft fuel pump systems.

 

9 hours ago, aro said:

What they are taught isn't always correct - it seems like there are as many myths in GA as any other level of aviation.

Better than someone leaving it turned on for the duration of the fight, or leaving it turned off; and not getting the safety benefits.

[RFguy]

Thruster-

Heatsoak and regular possible vapor lock - that's anecdotal in the experimental aircraft community- Jabiru engines in mostly non-Jabiru airframes.  A group I belong to that is mostly airframe builders and engine re-buiilders .
am unsure where  / if external fuel pump is in those scenarios without enquiry.

 

We all know the Jabiru high wing setup with the header tank is pretty bombproof if kept clean and sealed.

And where the electric aux / boost pump is in a Jabiru airframe- that's not an area that usually gets hot (behind pilot) , so I would say should be nil issues there.

 

IMMSMC all Jabiru POH will specifiy aux fuel pump ON from roll to top of climb. If you have a fuel leak in that region you'll likely get a fire.

Edited January 9, 2022 by RFguy

[kgwilson]

The 10 second booster pump prior to all starts eliminates this. I have forgotten to do the 10 sec booster a few times after a flight with the engine hot & on at least 2 occasions the engine fired immediately & then died. A restart (also without booster pump) & it stared & ran as per normal. Gravity had then pushed fuel into the mechanical pump & the float bowl quickly filled & all was well.

[lee-wave]

On 08/01/2022 at 2:19 AM, Alan said:

Jab 230. Engine Gen 3 with all latest mods (18 hours since overhaul). Running on BP95 Auto Gas (Avgas is now too hard to get) . CHTs pretty even and don't exceed 170°C.  No previous problems experienced. 

 

Until a few days ago - Air temp 35°C, all checks normal, put out taxing call and held short of runway while student in 150 landed who then came in off the runway onto the taxiway and turned back towards the runway in front of me.  Then I waited and waited while monitoring CHTs. Eventually the instructor got out, gave me a wave then leaned in to the door for further chat.  OK so a student doing a solo.

 

What this meant was I was idling for about 15 to 20 mins total, but the CHTs all looked OK.  The student cleared and I back tracked and took off.  Just after rotation (app. 150-200ft) my engine started running really roughly, revs dropping and loosing height. I was preparing for a cane field arrival when sufficient power came back to hold altitude, not too turn back, but I had more options.  After gaining circuit height I prepared for a normal landing, but with a now normal engine behaviour, I gained further height but remained local within gliding distance of the airport. 

 

Plugs, leads, filters checked. Fuel checked for water and any solid residues after evaporation and then donated to the lawnmower.  The engine has behaved normally for about 5 flying hours since.  

 

Lesson learned, I was not far from flattening the sugar cane. I believe the rough running was a vapour lock caused by the long waiting time and high ambient.  I had the electric fuel pump selected and could hear it running. 

However, I was thinking it would be pretty simple to set up a temperature monitoring of the firewall forward fuel lines, possibly on the inlet to the mechanical pump or inlet of the carb, but what temperatures would be regarded as critical?  I'll take some of the fuel into the lab and look at its boiling point but that would not test actual vapour pressure nor allow for the 4psi of the fuel system. 

 

Any thoughts out there?  

More then likely carb icing due to prolonged running at idle.....