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We run a pair of 337s for aerial survey work and because of the huge problems getting avgas out here in Africa, we have been tracking both the alternative engine and alternative airframe possibilities for years.
First off, if HAWK are quoting $340k for their Skymaster conversion, I can’t see that they are going to get many takers. Adding in something between $100k and $200k for the basic aircraft means you are throwing around an investment figure above the half million mark and at the end of the day you still have a forty year old airplane no matter how well it has been checked out. For that sort of price you can buy a brand new out of the box Diamond DA42 twin diesel that offers comparable range and operating cost. It makes no sense.
Diesel does seem to be the inevitable way forward but (unfortunately) it is still early days. So far there have been only two certified engine manufacturers, Thielert and SMA, and they have both experienced massive technical difficulties in making their engines reliable. They do now seem to be getting on top of it but it would be naïve to expect any new manufacturer entering the market to instantly have a product that is problem free.
Of the two existing engines we much prefer the SMA. It was designed from the start as an aero engine as opposed to a converted car engine, it has been around for thirteen years, has a lower reliance on electronic wizardry and the major aerospace company behind it (Snecma) has stood solidly behind their warranties. One of the owners of a diesel 182 out here has had his engine replaced twice without quibble.
The SMA is also the closest potential fit to a 337. Compared to the IO360C it is 8” taller, 6” wider and 2” shorter without any sacrifice of horsepower. It weighs 100lbs more but this is largely offset by the lower sg of JetA and the reduced fuel burn. Some years ago we were informed by SMA that the tractor/pusher issue was not a design limitation but merely a matter of testing and certification but of course this carries a cost implication as well.
Cheap it is not. About $65k a pop the last time I looked. Throw in new props, mounts, instrumentation and multiply by two and you can’t be much shy of $180k before STC costs. A few years ago a firm in the USA quoted me $100k to do the STC work so there you are, back in the ball park of the HAWK price with all the same arguments against it.
It was a very smart move by TCM, or maybe it was the new Chinese owners, to buy the SMA technology. It could be they will be able to leapfrog over all the teething problems and they have the muscle and know-how to get a lot of engines onto the market quickly. Originally they were promising a certified, retrofittable 4 cylinder this year and a 6 cylinder in 2013. For which airframes they did not say and whether their plans will include the relatively small fleet of 337s remains to be seen but to my mind this is the only practical hope on the horizon.
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