That's very helpful, thanks for sharing. Any idea of the absolute numbers of Rotax and auto conversion experimentals out there? It would be nice to have an approximate specific risk for comparison. I suspect that being auto conversions a very heterogeneous group, their specific risk must be higher than Rotax. I believe that the number of 172 flying during the period you examined must be significantly higher than experimentals, which makes their reliability stand out even more.
I gave a whole talk on this at the EAA Online Homebuilt Week last month. Where were you?
I've got the charts available for free download:
http://www.wanttaja.com/eaa23.pdf
As you'll see in the presentation, the question of "How many Rotax 912s or VWs are in the homebuilt fleet" can't really be answered, as the FAA data is not precise. This is a plot similar to what I show in the presentation. It shows only engine makes that have 100 or more entries:
About 13% of all homebuilt aircraft are listed as having "AMA/EXPR" engines. The FAA registry is not one discrete file; it combines an aircraft definition file and an engine definition file with an owner data file. When a new homebuilt is presented for registration, a new entry is made in the aircraft definition file even if other aircraft of that type exist*. If the engine corresponds to an entry in the engine database, that engine is then listed...but if not, the FAA may list it as "AMA/EXPR" instead. So we don't KNOW how many of those 3500 AMA/EXPR engines are Lycomings, Continentals, Rotaxes, etc.
This especially came into play with the rise of what I call "Aftermarket Traditional" engines... the Superiors, Mattitucks, etc. When these engines were first introduced, all the aircraft were listed as AMA/EXPR engine. As time went on, though, and more of these engines appeared, the FAA relented and assigned a designator to them. But it *didn't* change the registry entry for existing planes.
The plot above show by engine make; model is another kettle o' fish entirely. Until about 2008, all Rotax engines (be they two stroke, four stroke, etc.) were just listed as "ROTAX" (or BOMBARDIER). So there are 4700 Rotax engines in the registry, of which ~2200 or so don't list the engine model. Almost 1400 do list the Rotax 912 engine, but there are undoubtedly hundreds more in service. Note that this doesn't include Special Light Sport Aircraft or Experimental Light Sport Aircraft.
Ron Wanttaja
* I've seen some multiple uses of entries for common homebuilt types, such as the RV-7
