question Rotax 912 Carburettors

[M61A1]

Thanks for relating your experiences and the symptoms relating to this issue , this is precisely what I have spoken about when asking for real world experience. Are all Bing carburettors subject to this float problem or is it just Rotax equiped motors. Are two stroke motors effected. From memory my Jab has a different float set up.my BMW 1976 R 750 and so far no problems, is it a fuel related problem or is the material they make the floats from the issue

In the SB the specifically refer to floats with part number 361184 marked on them.

[skippydiesel]

My 912 ULS is 20 years old, 920 hrs and the original floats are still fully functional/operational BUT "water logging" it is something that needs to be regularly monitored. I do my checks every 50 hrs

[facthunter]

Paul, I do base this stuff on personal flight experience and also from research with oil companies and I will specifically mention BP who put their Chief Fuel Technologist on the matter for me relating to vapourisation, float buoyancy and fuel formulations causing flooding and subsequent fire issues with fuel venting on float equipped carburettors. . I thought also the Rotax AD required "replacement" of the specified suspect floats. CASA promulgated an AD with the same info. Nev

[perrynz]

Hi Skippy. Your 20yr old floats aren’t affected by the SB, as it kicks in from 2012.

Perry

[facthunter]

Ah.. The benefit's of age. Very rare and elusive. Lots of "plastic" floats do strange things when we expect them to cope with "unknown' chemicals. or even sunlight.(not usually a carburettor problem). Nev

[skippydiesel]

Hi Skippy. Your 20yr old floats aren’t affected by the SB, as it kicks in from 2012.

Perry

Thanks for that but will still keep an eye on them all the same.

[dlegg]

In regards to the Hacman type device, I have been using one on a Jab 6 for 5 years and over 500hrs experience now. They do work well, but must have full EGT, CHT and fuel flow monitoring to get the best out of the system, and to avoid damage. The valve itself is quite sensitive and only small incremental openings are required. When confidently used, benefits are enormous. I can easily run LOP, fuel flows dropping down from 22LPH or so to only 15LPH, with correspondingly lower CHT's, of course an increase in EGT's.

The caveat here is that the engine must already have close EGT's when set up in the cruise mode, a well matched prop etc.

This system is only of benefit above 5000ft or so, and a long cross country flight, other wise the bing does a pretty reasonable job in the lower altitudes.

[R Austin]

Hi IBob.

Just stumbled across your query earlier re 24000’ in SGS.

Yes, it was a bog standard 912 ULS, except for a home made leaning system.

This combined with a low drag airframe and CS prop to keep rpm and hp up. Actually went to about FL 245, can’t remember exact number. Would have gone a bit more if light, but it was a few hundred miles inland and was a bit heavy as I was carrying fuel to fly back to coast afterwards. Plus reserve, plus oxygen and a few other items.

For a bit more perspective, that aircraft ran out of puff at about FL 185 with fixed pitch prop and no leaning.

The leaning provided a bit more boost than the extra rpm gain. Subjectively probably about 60/40 respectively.

The biggest problem encountered at FL 245 was fuel vaporisation. Over 60% of the atmosphere is below you. The atmospheric pressure there is getting low. Total of about 11.6” as I recall. Even less, only 11” of manifold pressure at wide open throttle! If you are constant speed prop rated, this will be meaningful.

So not much power when all is going right. Less with fuel vaporisation and fuel pump cavitation. Rotax will not run without a well designed fuel system and auxiliary pump capable of operating at low NPSH (net positive suction head) pushing fuel to the Rotax pump.

Hopefully that makes some sense.

Cheers

Robin

[IBob]

Hi Robin, thank you for the detailed explanation of your high altitude excursion/s!

While my own aircraft is totally unsuited to any such thing, it caught our attention here, and has generated a fair bit of conversation.

And I think you could also say that it has shifted our perception of some of what is possible.

Perhaps at some point I shall see just how high I can go with my fixed pitch prop and no means of leaning..........)

[R Austin]

Good luck. Be sure to post your results!

May not be an issue, but unless you know your personal sensitivity to hypoxia, be very careful above 10,000’.

Many younger healthy persons may be fine up to about 16K (without oxygen), but the signs are NOT obvious. It’s very insidious, creeps up on you with NO notice in most cases. And can be fatal, mostly indirectly by inappropriate pilot input/decisions/actions, including as the top speed eventually decays to equal the stall speed and the stall recovery may be substantially different at the service ceiling from what you are used to.

Also, it’s easy to exceed the Vne on descent, noting that most Vne’s are based on TAS, not IAS.

On the other hand, it’s intriguing, educational and definitely furthers your pilot experience and with safety disclaimers, I encourage anyone interested to learn more about their aircraft’s operational envelope.

Cheers.

[Thruster88]

Hi IBob.

Just stumbled across your query earlier re 24000’ in SGS.

Yes, it was a bog standard 912 ULS, except for a home made leaning system.

This combined with a low drag airframe and CS prop to keep rpm and hp up. Actually went to about FL 245, can’t remember exact number. Would have gone a bit more if light, but it was a few hundred miles inland and was a bit heavy as I was carrying fuel to fly back to coast afterwards. Plus reserve, plus oxygen and a few other items.

For a bit more perspective, that aircraft ran out of puff at about FL 185 with fixed pitch prop and no leaning.

The leaning provided a bit more boost than the extra rpm gain. Subjectively probably about 60/40 respectively.

The biggest problem encountered at FL 245 was fuel vaporisation. Over 60% of the atmosphere is below you. The atmospheric pressure there is getting low. Total of about 11.6” as I recall. Even less, only 11” of manifold pressure at wide open throttle! If you are constant speed prop rated, this will be meaningful.

So not much power when all is going right. Less with fuel vaporisation and fuel pump cavitation. Rotax will not run without a well designed fuel system and auxiliary pump capable of operating at low NPSH (net positive suction head) pushing fuel to the Rotax pump.

Hopefully that makes some sense.

Cheers

Robin

Hi Robin, thanks for posting, very interesting. Have you thought about using the Rotax 915 in a soneri. TAS at FL245 would be around 250 knots ? We know they have rated power, 135hp at 15,000 should still have a good amount at Fl245.