On aircraft engines, part 2

A couple of weeks ago, I wrote a post about piston aircraft engines (tl;dr: ancient and expensive technology but generally very reliable). The fact that the general aviation fleet is still powered almost exclusively by these engines may have surprised you, and I wish I could say that it’s getting better right away.. but it’s not. There are some encouraging signs on the horizon, though.

One alternative is to just replace the engine (or its components). This can be done through a process known as supplemental type certification (STC), an existing airframe/engine combination can be changed, often in significant ways, provided you can prove to the FAA’s satisfaction that the changes are not unsafe. For example, there is a well-known STC for many models of Cessna 182 that allows you to run plain auto gas in the engine. There are others covering all sorts of engine upgrades and replacements: Electroair makes an electronic ignition system, Peterson, Texas Skyways, and P.Ponk make kits to replace the 182’s engine with larger and more powerful versions, and there’s even an STC to put an SMA diesel engine up front. At the high end, O & N Aircraft will happily sell you a turbine engine that will turn your Cessna 210 into a real beast (and set you back several hundred thousand dollars, too.)

The problem with STCs is that they tend to be expensive (since the manufacturer has to run the entire FAA approval gauntlet) and very specific (the STC allows you to make the specified changes only to the exact make and model specified in the STC). The expense of STC engine swaps raises the question of how much sense it makes to put an expensive engine into an inexpensive airframe, e.g. Peterson quoted me more than $80,000 to put a new engine into a 1969 182 with a market value of just under $50,000. That didn’t seem to make a lot of sense to me. Less expensive STCs, such as the Electroair electronic ignition, may have reliability or efficiency benefits that make sense, but it’s hard to see that happening for an entire engine.

A few manufacturers have made other attempts to give us better engines. One that I remember well was the Mooney PFM, a collaboration between Porsche and Mooney that put an air-cooled Porsche flat-six into the Mooney M20. The PFM had a single-lever throttle (with no manual mixture or prop adjustment), was fuel-injected, and could optionally be turbocharged. However, it wasn’t very successful in the marketplace despite its advantages.

My longtime friend Phil asked a great question in a comment to the previous post: what about turbine and diesel engines? Why don’t manufacturers just use them instead? Well, they do in new aircraft. For example, Piper will happily sell you a Meridian (with a Pratt and Whitney PT6 turbine, the gold standard in turboprop engines) starting at about $2.2 million dollars or a Mirage, which is about 40 knots slower, uses a piston engine, and costs roughly half as much. Turbine engines, of course, are mechanically and operationally simple and very robust, but they are expensive to acquire and maintain, which pretty much rules them out for the class of airplanes that most GA pilots have access to. Diesels are starting to make inroads too; the only model of Cessna 182 you can now buy is the Cessna 182 JT-A, which replaces the old-school piston engine with a 227-hp SR305 diesel (the same as the one available via STC for older 182s). The history of diesel engines for general aviation is long and complicated; suffice to say that Cessna and Diamond are the only two manufacturers I can think of who are currently selling diesel-powered aircraft despite their efficiency advantages. However, the idea of a drop-in diesel STC replacement for the O-470, IO-540, and other popular engines is gaining traction in the market, with both Continental and Lycoming developing products.

More interestingly, Redbird’s RedHawk project is converting Cessna 172s by putting diesel engines and improved avionics in them; I suspect that Redbird will be very successful in selling these refurbished aircraft as primary trainers, and that may serve as an effective tipping point both for generating demand and demonstrating the potential market for diesel STCs for other lower-cost/older aircraft. We can only hope…