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Experimental designation for production airframe

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Well-known member
Jul 25, 2002
Kind of a weird question. Suppose you were going to take a production airframe and bolt a different type of engine to it that hasn't been STCd, or tried before, for that matter.

How would you go about getting an Experimental designation for an aircraft that you didn't actually build?

I have seen this done with larger companies doing research on aircraft mods, but what about a private individual? Is this possible?

If it's a minor change, I've seen people who have been able to do it with a form 337. If it's a radical change like going from an IO-540 to a PT-6, you still might be able to get away with doing it with a 337 if you back it up with an engineering analysis for the FAA (that you pay for). Generally, only a company testing a configuration for production is given permission to operate a modified aircraft under the Restricted certification, which doesn't allow passengers. And if you're really unlucky, the FAA will only allow flight under the dreaded Experimental for Exhibition certificate. That only allows flight between the aircraft's home base and airshows, over non-populated areas and no passengers.

AFAIK, the FAA doesn't allow production aircraft to be operated under the Experimental designation, except for flight test like you said.
This was an issue several years ago when a very expensive AD came out for a popular GA airplane (can't remember which one). Some owners wanted to switch the planes over to Experimental so they wouldn't have to comply with the AD. The FAA shot that idea down.

So what mod are you thinking about?
EagleRJ said:
So what mod are you thinking about?
I was doing a little web surfing and I came across a 125HP turboprop (I'll supply the link later, I'm just getting ready to leave the house :) ). It is designed to be sold with their own kit, but I was tossing around the idea of bolting it onto a run-out C-150 airframe or something like that.

Although it may be kind of cool on an RV, too!

There's an experimental research and development category. There are several categories under the experimental class. My aircraft is experimental exhibition, there is experimental amateur built. If you're going to be doing mods on aircraft and gettting the FAA involved, you need to be fluent in 14 CFR's, and inspectors handbooks. All are available on line in the www.faa.gov site.

Once an aircraft in licenced in experimental, its difficult to get it out and into a standard category.

Interesting idea! Do you know how much that engine costs?

I've seen numerous experimental (homebuilt) airplanes running turbines, but I can't say I've ever seen a one-off production plane with a turboprop outside airshow performers.
That would make for one wild 150, but you would basically gain outstanding climb performance and a few knots cruise gain for nearly twice the fuel burn. Not sure if it would be worth all the paperwork. (backing into a parking spot with beta would be neat, though!)
Maybe putting that in an Aerobat would be a better idea! Or maybe a Globe Swift? Or an Ercoupe?

Any idea if this engine was purpose-built as an aero engine, or is it a converted APU? There have been many attempts at making inexpensive turboprop engines out of APUs, but reliability and fuel burn have always been issues. The numbers look great on paper, but the reality is that APUs were designed to run at fixed RPM and load for limited amounts of time, and don't have the reliability of an aircraft powerplant.

There are some purpose-built turboprops on the market now, but you can't touch them for anywhere near the price of an APU-derived engine. A lot of people are looking at Innodyne for O-320/360 alternatives. Looks like a well designed engine, but it isn't cheap!
Actually, I just did a little more research on the engine. Price is $41K (yikes!) But the fuel burn is only 7.5 gallons an hour. Considering that an O-200 burns 5.5 - 6, you would probably make up the small gain in fuel consumption with speed, not to mention the lighter weight!

I'm not sure, but I think this was designed for use as an engine. It lists the prop RPM range as 800-2800, but I'm not sure if this is using a direct drive or an exhaust driven powertrain, nor am I sure if this is using a constant speed prop, either, so backing up may not be an option ;-) !

I would be interested to know how many of these engines are actually flying. Some of their numbers seem too good to be true.
Berkut said:
I would be interested to know how many of these engines are actually flying. Some of their numbers seem too good to be true.

There have been many start-up companies that offer "revolutionary" aircraft engines, but so far all of them have fallen short in producing a product that meets the stated performance figures and is reliable. Designing a simple, cheap to build, fuel efficient turboprop isn't easy. Like the saying goes, if it was, everyone would be doing it!
Most of the turboprop setups flying on experimental aircraft are one-off converted APU engines with custom fabricated reduction drives, and they're nothing but problems.

Many builders fall into the trap of using ideal performance numbers for an engine package as a given without considering real-world factors. Can you mount an aluminum-block Chevy 350 with a homemade reduction gearbox on your plane? Sure. Will it make 500hp at the prop? If you want it to. Will it be reliable enough for a single-engine airplane? Nope.
You can get an experimental certificate as long as you make enough changes to some specified areas. I was reading about this earlier this year either in Private Pilot I believe.

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