Strictly speaking, approval is aircraft type-specific (not engine-specific), but in many cases the approval can be automatically derived from other existing approvals – see CS-STAN sections CS-SC202b and CS-SC203b. A list of approved fuels for specific Lycoming engine types appears in the service instruction 1070.
skydriller wrote:
I have noticed that the further north in Europe you go, the more prevalent UL91 appears to be – in fact there are some small aerodromes that dont sell 100LL, just 91LL.
Not just AVGAS UL91, but in Sweden AVGAS 91/96UL from Hjelmco is even more very common.
So with that in mind, a few questions:
1. Where does one find out if an aeroplane can use 91UL instead of 100LL.
I assume that the POH of the aircraft doesn’t say anything about UL91 or 91/96UL. If you have a Lycoming engine, check the latest version of Lycoming Service Instruction SI 1070 “Specified Fuels”, which will tell you for each engine model if it can use UL91 and/or 91/96UL. I imagine that Continental and other engine manufacturers provide similar information.
If you have an EASA aircraft, there is a Standard Change that permits the use of UL91 or 91/96UL subject to the engine manufacturer’s approval.
2. Are there any issues with mixing UL91 & 100LL in a tank? By which I mean arriving having used 100LL and filling up with UL91, then the reverse.
None.
3. Are there any limits to using UL91 – such as the temperature issues with MoGas?
For Lycoming engines, none. I don’t see any reasons for restrictions for other engine makes either. The temperature issues with MOGAS are because it is more volatile. UL91 and 91/96UL are “proper” aviation fuels that shouldn’t have any such issues.
Do note that the lead in 100LL provides some valve lubrication. If you switch to unleaded fuel you may need to use some additive to compensate. Lycoming specifies that an additive must be used with the engine oil. All major aviation oil brands have oil grades that already include such additives. (E.g. Aeroshell “plus” oils.)
In Sweden, a “Hjelmco Avgas 91/96 UL” is quite common, especially at smaller airfields. You see it in the table from Lycoming posted above and can also read about it here: http://www.hjelmco.com/pages.asp?r_id=13395
EDIT: Ha…Airborne_Again beat me to it!
Airborne_Again wrote:
Not just AVGAS UL91, but in Sweden AVGAS 91/96UL from Hjelmco is even more very common.
AndersB wrote:
In Sweden, a “Hjelmco Avgas 91/96 UL” is quite common, especially at smaller airfields. You see it in the table from Lycoming posted above and can also read about it here: http://www.hjelmco.com/pages.asp?r_id=13395
Gentlemen, this is exactly why Im asking!! 
Thanks, SD..
Just wondering if this alkylate gasoline we can purchase for garden equipment and burners is more or less the same as UL91. It’s expensive, because you can only buy it in small quantities (typically 4l is more than enough for a season), so the price doesn’t really matter all that much. It burns perfectly clean (no soot, no deposits), low vapor pressure and high enough octane for UL91 91/96. It can also be stored for ages, not like gasoline that is more like fresh food.
Do note that the lead in 100LL provides some valve lubrication. If you switch to unleaded fuel you may need to use some additive to compensate. Lycoming specifies that an additive must be used with the engine oil. All major aviation oil brands have oil grades that already include such additives. (E.g. Aeroshell “plus” oils.)
In a mid-life Lycoming engine using Total 15W50 oil and run for the last year almost exclusively on UL AVGAS, I have seen oil analysis results which are not inconsistent with slightly higher wear of piston rings, cylinders and/or valves and guides. I wouldn’t put it any stronger than that. It’s just one engine and there could be a dozen other reasons for an apparent moderate increase in Fe and Cr in the oil as the lead content has more than halved.
In countries where use of unleaded fuel is more prevalent, has anyone seen similar oil analysis data trends?
Interesting!
lead content has more than halved.
Should it not be zero?
I think that the implication of Lycoming’s SL on unleaded AVGAS is that it takes time to rid the combustion chambers of lead deposits acquired over hundreds of hours.
Yes that would make sense. But you should see the (normally hugely high, thousands of ppm) lead levels fall rapidly, on oil analysis.
Bathman wrote (seven years ago ):
friend of mine has put 250 hours on his o-200 using ul91 and he claims he no longer suffers lead fouling of the plugs.has anyone else noticed anything similar. i all so wonder if the oil is cleaner as it is no longer loaded with lead does this mean and reduced engine wear
Yes, when running predominantly on UL Avgas, I find that lead fouling of spark plugs is all but eliminated.
As for engine wear, I don’t know for sure, and I would stress that oil analysis data from just one mid-life engine is more “anecdote” than “evidence”.
With that in mind, in my O-360-C1F I have seen an increase in Fe and Cr after changing to 91/96 UL. While probably far from significant with respect to engine longevity, the increase is well in excess of any sample-to-sample or lab-to-lab scatter of oil analysis results.
In my engine the oil analysis shows a progressive reduction in lead content. This is partly because one can’t always refuel with UL when away from home, but it may also indicate just how much lead sludge can accumulate in the sump and oilways of an aero engine:
100LL_to_91_96UL_oil_analysis_graph_pdf
Footnote: The above engine has run exclusively on Total 15W-50 with no aftermarket additive. I use the airplane like a car, with lots of relatively short runs, and occasionally for towing gliders. Compressions remain 72-76/80, borescope inspections normal and there has been no discernible metal in the oil filter or suction screen. Even if the “increased wear” is real, and even if it is significant WRT engine life (which I very much doubt), I’m happy to continue running with UL Avgas. However, I’m minded to chuck some Camguard in the sump and report back in a year or so – if I’m spared…
