It seems that automotive engine conversions are about 3 times riskier than certified engines from Ron’s chart. So then to convert to absolute risk we could multiply by the absolute rates of such accidents. Maybe the Nall report?
Again, though, my results are for Experimental Amateur-Built aircraft, for which there are NO standards that must be followed. If we're talking about an engine that will be used commercially, one would assume that, for liability reasons alone, the manufacturer would expend some effort to ensure reliability.
One thing about auto engines: The core engines
themselves don't seem to be the issue. They don't seem to suffer mechanical failure at a rate much different from traditional engines. Where they DO get bit is in the ancillary equipment necessary to make the engine work on an airplane....the propeller speed reduction unit (PSRU), the liquid cooling system, the electronic ignition, etc.
This chart shows the relative frequency of accidents due to engine-related mechanical issues.
Note that the "Engine Internal" category (which is basically referring to the core engine) is only slightly higher for auto engine conversions. And look at the failures related to systems that traditional air-cooled aviation engines DON'T HAVE TO HAVE. If it's not there, it ain't gonna cause an accident by breaking.
Again, though, a commercial activity such as Corsair has the opportunity to bullet-proof these systems. The Rotax 912 has liquid cooling, a PSRU, and electronic ignition...and failures of those systems are few and far between. The engine is just about as reliable as the O-200 it's replacing.
Now, my data above doesn't address the rate of failure COMPARED TO THE NUMBER OF ENGINES installed. Instead, it looks at the accidents involving aircraft with a given type of engine installed, and computes the percentage of those accidents that were due to a mechanical failure of the engine. It also includes only those cases where an NTSB report was generated. If the owner of a VW-powered airplane plops down in a field somewhere and trailers his plane home, it's not going to be listed.
Ideally, we'd be comparing the accident statistics to the number of engines installed. Unfortunately, the statistics just aren't out there. When you look at the FAA registry, 12% of the homebuilts are listed merely as having "AMA/EXPR" engines. That category, in fact, is the #3 most common installed engine on homebuilts.
Note that the #2 most common engine on homebuilts is a Rotax.. and half the Rotax listings don't specify the engine model. Which means we don't know how many of those ~4700 Rotax-powered homebuilts have two-stroke engines or four-stroke engine.
Note the small sample size for auto-engine conversions other than VW and Subaru. This plot cuts off at 100 aircraft. If the engine type isn't listed, that means there are fewer than a hundred on the FAA registry.
Now, there are undoubtedly MORE, since some of those "AMA/EXPR" engines are GM engines, or Mazda engines, or whatever. One noted converter claims there are thousands of their engines out there, but I only find a hundred or so actually in the FAA registry. Makes you lose faith in advertising.....
But we would like to know what are the annual odds of an engine failure are. I tackled a process to estimate how many of those AMA/EXPR engines were specific engine types...cross referencing the FAA registry cases with the NTSB accident database (the NTSB is better at identifying specific engine types). Came up with an estimated percentage of the AMA/EXPR engines that were Lycomings, Continentals, Hondas, etc. and developed an estimate as to the total number of engines installed in the fleet. THEN I compared it to the accident data....
Probably not a lot of surprise, here. I was actually surprised that the Rotax 912 came out as high as it did, but I was facing the same problems with the "Rotax" engine listings as the AMA/EXPR ones... there are thousands of homebuilts listed as merely having a "Rotax" engine without specifying whether it's a 582 or a 912.
So you can see why there's skepticism in the aviation industry as to the viability of converted auto engines in aircraft. The main question, I think, is whether the owner will be able to get insurance on an auto-engine-conversion Cessna 172....
Ron Wanttaja