Aircraft RV-12, 912ULS, mogas, mechanical fuel pump about 200 hrs since new.

Can anyone explain this?

Today I performed a test that I have not tried before under these same conditions. I was at 6,800’ with a DA of 9,700’, OAT 82f. I was at 5400 rpm and turned off the electric fuel pump, after about 15 seconds the fuel pressure stabilized at .8 psi and the flow was 2.2 gph. At about 30 seconds into the test the engine started stumbling from fuel starvation. It never quit running but lost RPM and was very rough. I could smooth it out by dropping the RPM to 4900, (less fuel demand), or turning on the electric fuel pump.

Once I was back at my home airport with a generous altitude I tested again, same altitude and DA as before. The results were the same.

I have on occasion performed this test on cooler days at lower altitudes and never had a problem, engine runs fine and maybe a drop in fuel pressure to 3.5 psi.

Keeping the previous good test in mind I dropped to 4,000’ with a DA of 6,700, the OAT was 89f, engine RPM 5400.This time it was normal, no engine starvation, fuel pressure drop but not serious.

So my question is why does altitude have an effect on the pressure output of the fuel pump. This is clearly not an instrumentation issue since the engine was starved at 10,500 DA and not at 6,700.


my blog; waltsrv12.com

  • Re: Low low fuel pressure and flow at high altitude

    by » 2 weeks ago

    The 912ULS is designed to run on the engine driven mechanical pump alone. 

    If this pump is not delivering the required pressure/volume of fuel I would suggest:

    # Fuel filter(s) inspection/clean/replace as required. Dont forget the filter in the pump itself and in tank "finger" filters .

    # Fuel supply system - check for kinks or other potential obstructions. If not already fitted consider a Boost Pump bypass circuit.

    # Consider the fitting of a smaller fuel return line restrictor jet (discussed elsewhere on this Forum)

    # Unlikely but always possible that the 200 hr mechanical pump, is not delivering rated volume/pressure - replace. (If you have an old, known to be good, at time of replacement pump - fit it, go flying and note performance)

  • Re: Low low fuel pressure and flow at high altitude

    by » 2 weeks ago

    HI Walt

    There are some good papers on this from carb engines in the past in aviation.  Several factors are happening.  First the relative pressure we know drops with altitude and if we have higher temperatures that is even worse.  The pumps then lose the pressure at the tank over the fuel.  Now we know that pumps would much rather push fuel than suck them, it is just a matter of the dynamics of moving fluids.  In an RV12 the boost is near the fuel tank, that's a good thing.  When you shut it down with higher altitudes you will see the relative drop due to the atmospheric pressure drop and the mechanical pump can't suck as well from the tank.  

    Now a big consideration here is if you are using auto fuel...well at that density altitude you stated you simply should not.  Auto fuels vapor pressure is not good enough for that condition.  I suspect you are experiencing vapor lock, or signs of it, by the high altitude.  Low fuel pressure as you described in those conditions is common when you have the onset of fuel vaporization, fuel boiling, due to lack of pressure in the system.  This is why Avgas has a RVP of 7, auto fuels are much higher.  If I am right try filling up with some 100LL or UL94, both aviation fuels with much higher RVP. 


  • Re: Low low fuel pressure and flow at high altitude

    by » 2 weeks ago

    Hi RW,

    In Australia, VFR aircraft don't rotinly fly over 10,000ft , no high mountain ranges, so the altitude  limits above this, of automotive petrol, are rarely if ever explored.

    My experince and limited knowledge, I have never heard of an  inflight fuel vaporisation incidents ( this is for 912 carburettor engines). Ground and take-off,  definitely, occasionally may be some effect on climb-out - should be considered possible for every TO where ambient temperature is approaching 30C and above..

    This may be due to autofuel differences  eg I have never heard of anyone using ethanol blends in their aircraft.


  • Re: Low low fuel pressure and flow at high altitude

    by » 2 weeks ago

    Hi Sean

    I don't believe you have as many fuel variables as they see in the USA market.  Under 10000 feet is for sure less risk.  In the light sport rules they can follow a 2000 ft above ground level,, AGL, legally.  For a large number of areas that then puts people over 10k for the Western USA and Canada.  We also see very high operations in Latin America.  (Medellin Colombia city airport is 5500 on the runway and the international airport is 8500 ft on the ramp.  Quito Ecuador is some 9000 ft on the ramp and have a 15000 ft runway for that reason) Indeed even Avgas can vaporize in the right conditions but it is rare.  Best suggestion I give is mix Avgas with your Mogas or run just Avgas at high density altitudes.  For sure high temperatures will really add to the problem.  

    In discussion with the STC holder (EAA in the USA for auto fuel in piston Cessnas) it is not recommended to use auto fuels above 1000 ft.  Some even suggest 8000 is more appropriate.  in Europe it is all over the place on auto fuel in aircraft.  Some countries had a 6000 ft restriction on auto fuels and a 20 C limit on temperature.  (UK and Sweden I believe)  

    We get into a very grey area with this question  given there is little empirical data, it is almost all anadotale from what I have seen.  Vans aircraft did a good study a few years ago and I believe they now state switch to Avgas above 8000 ft.  I would have to ask if the study is available for public release however.  One last note, low content ethanol has no effect on vaporization from what i can see.  Rotax allows only 10% in its current fuel recommendations.  The real risk with oxygenated fuels is they will cut your storage life in half according to the fuel suppliers.  Ethanol blended fuel in the USA and Canada are very common and used frequently.  If you have an option to avoid it sure why not.  It is always best to use fresh fuel.  



    Thank you said by: Sean Griffin

  • Re: Low low fuel pressure and flow at high altitude

    by » 2 weeks ago

    Could it be a problem with the vent hole on the spring side of the diaphragm? The Rotax expert Conrad Beale talks about it here: https://www.rotax-owner.com/en/912-914-technical-questions/4952-fuel-pump-pressure-variation

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