Another point.
As far as I can remember according to FAA rules the airframe manufacturer has to approve the fuel before you are able to use it, while under EASA it is enough to have the engine manufacturer approval.
Ben wrote:
As far as I can remember according to FAA rules the airframe manufacturer has to approve the fuel before you are able to use it, while under EASA it is enough to have the engine manufacturer approval
Conventional practice anywhere for deviation from anything listed on the TCDS (like fuel type) is a government approved STC for that aircraft type. The type specific STC does not need to come from any particular source but may reference data from e.g. the engine manufacturer. Examples would be the EAA and Petersen STCs.
Well, yes, you are right.
What I meant is that under EASA there is no need for STC. Once Lycoming published their SB stating that the engine is ok for 91UL then this is all the owner needs.
Ben
Yes, I understand that EASA at one point issued an unconventional memo unilaterally approving UL91 without regard to the aircraft fuel system design and without modifying the fuel requirements of any TCDS. Whether anybody in their right mind would proceed in the absence of an STC, without doing their own very careful experimentation and in doing so understanding that they are a test pilot is another question. There is the argument that unlike other non-TC approved fuels 91UL is just 100LL without the lead, maybe it is valid. I’d personally be careful.
Silvaire wrote:
Yes, I understand that EASA at one point issued an unconventional memo unilaterally approving UL91 without regard to the aircraft fuel system design and without modifying the fuel requirements of any TCDS.
It it not in an "unconventional"memo" but in the Standard Change document which lists a number of modifications that may be done without either an STC or a modification approval.
There is the argument that unlike other non-TC approved fuels 91UL is just 100LL without the lead, maybe it is valid. I’d personally be careful.
There is tons of experience in Sweden over many years of flying with Hjelmco 91/96UL — which is also covered by EASAs “unilateral approval” — without any issues whatsoever. On the contrary.
I think issuing a non-type specific government document which provides approval in opposition to the specific written requirements of an unchanged government maintained type certificate requiring minimum 100 octane fuel local copy is by definition unconventional. Obviously there are many features of an aircraft type which (unlike approved fuel type) are not directly documented in a TCDS. A ‘standard change’ approval would be more conventional and appropriate in relation to those features, in a similar way to how standard airframe repairs done under under approved data (e.g. AC 43.13) do not require an STC or field approval to alter the structural configuration.
Experience is certainly good, but is not always directly applicable. My questions before flying my 80/87, 100LL and Autogas STC certified aircraft with 91UL would not be answered by the EASA ‘standard change’ memo that did not require any testing on my specific airframe and fuel system type. I understand that Hjelmco 91/96 is not the only version of UL91? Is that correct, and if so is there reason to believe that experience on one (or even many) aircraft fuel system designs is adequate and complete in relation to the use of any covered 91UL in the fuel system of my specific aircraft fuel system design, which has likely never been tested with any 91UL? I’m quite unsure of that and in the absence of a type specific STC I would treat initial flights with 91UL in my plane as an experimental test program, conducted by me as test pilot, and BTW given the advantages of that fuel I would likely do it if the fuel were widely available to me.
Many will be aware that several types of aircraft proved in testing impossible to certify under an auto fuel STC, for reasons unrelated to engine compatibility.
I think most people are comfortable substituting 100LL with 91UL because the only difference is the missing TEL, so “what could possibly go wrong” 
Fortunately for most in Europe, 91UL is almost nonexistent so it’s not an issue.
Silvaire wrote:
I understand that Hjelmco 91/96 is not the only version of UL91?
91/96UL is not the same fuel as UL91, so there are a few engine models that run on one but not the other (both ways).
Just as UL91 is 100LL without lead, 91/96UL is 91/98 (according to US standard ASTM D910) but without lead.
Thanks for that explanation of UL91 versus 91/96UL!
