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The Rotax 912 is a popular combustion engine in smaller general aviation aircraft in Europe. Due to a number of factors (including fuel cost), the Rotax-powered aircraft in our flight school are most often run on MOGAS (Super 95). Since smaller airfields don't always have MOGAS available, they are frequently flown on mixtures of AVGAS and MOGAS during longer flights.

Going through the technical documents of the engine, I never found a passage mentioning explicitly that these different fuels can actually be mixed. I was wondering why that is, considering the great length to which the manuals go to cover all potential combinations of operating fluids (lubricants, coolants, fuels).

The authoritative reference for the approved fuels of this engine is the Service Instruction Document Rotax SI-912-016R14 (linked here is a slightly outdated revision).

It does mention:

The following fuels can be used.

and goes on to list MOGAS and AVGAS of different octane ratings. The only passage where mixing of fuels is mentioned explicitly cautions against a potentially low octane rating of the mixture:

Any mixture of unapproved fuels and/or additives that cause lower than the specified octane rating can cause engine damage like e.g. detonation.

Not having a background in combustion engines, I can only speculate that perhaps it is implicitly clear that any mixture of approved fuels would also be safe for the operation of an engine?

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Right in the document you linked:

  1. Fuel For ROTAX® aircraft engines different fuel types are available. See Operators Manual of the relevant engine type

This means the specific manual for a specific engine and airplane (serial number specific) and its various supplements and added certificates.

Suitability of fuel system components of airframe ROTAX® urges owners to confirm with there airframe manufacturer that ethanol blended fuels of up to 10% (E10) are compatible with all fuel system components. It is the responsibility of the aircraft manufacturer to test their fuel system components and supply any further information on techniques, procedures and limitations of using ethanol blended fuel. ROTAX® recommends that aircraft manufacturer and owner/operators read the following:

  • FAA Advisory Circular Letter AC 23.1521-2
  • FAA Special Airworthiness Information Bulletin CE-07-06
  • EASA Safety Information Bulletin – SIB 2009-02 These contain details regarding the use of ethanol (alcohol) blended fuels and the type certificate requirements. It is strongly recommended that also non-certified aircraft also conform to the information given in the above documents.

This is the closest you will get to authoritative generic information related to what you are asking. The real issue is that unlike avgas, Mogas is not a consistent formulation and the engine and aircraft manufacturers cannot possibly test all mixtures of all brands for compatibility with avgas.

Not all MoGas is the same, there is traditional formulation (similar to avgas but wider tolerances and occasionally still sold for racecars) and several "reformulated" highway gasolines including various gasohols and "non-ethanol".

In the USA I have never seen an aircraft that allows gasohol, even those specifically rated for highway gasoline still call for "Non-ethanol" formulas, which use an ether compound as the oxygenator rather than alcohol. The engine may be rated to run on e10 but that does not mean the whole system is rated for e10.

Reasons include storage stability, moisture and temperature issues, corrosion, seal compatibility, and the predictability of the antiknock rating of mixed fuels. Vapor pressure curves also contribute. (Avgas has anti-oxidants and the longest storage life by far at several years in a good container. Gasohol like e10 goes bad in a couple of months, non-ethanol may be good for 6 months to a year.)

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  • $\begingroup$ Excellent point being made that the whole system is to be taken into consideration when deciding which fuel (mixtures) can be used. Knock resistance of the fuel as such is not the only concern. $\endgroup$
    – Jpe61
    Commented Aug 9, 2023 at 5:43

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