Buying a Used Decathlon
By Greg Koontz
(Editor's Note: The following article by Greg Koontz is an excerpt from the feature story that will appear in its entirety in the April issue of Sport Aerobatics magazine.)
Buying a used aircraft brings tough questions. How was it flown? Did the previous owner ride fast in rough air? Did they hurry down and shock cool? How many times has it been slammed into the pavement? If you've flown with a lot of pilots (and I know I have), you have a good idea how rough some pilots can treat a poor ol' airplane.
Now consider that the airplane you want to buy is aerobatic. You just popped open a whole new can of worms. In the course of my business I buy and sell a few aerobatic planes, almost exclusively Decathlons. The type has been around since the Bee Gees. They've been used, cruised, and often abused. Most have been staying alive (couldn't resist that). Finding a good one is tricky business. I've been burned. Sellers often avoid telling all or just don't know it. Here are my thoughts for success.
The second thing that comes to mind (we'll get to the first later) is the engine. The AEIO-320 and 360 series have an odd and often misunderstood time between overhauls (TBO). Lycoming considers these engines to be submitted to a lot of sudden engine power changes. These cause forces and temperature changes that in turn accelerate wear on parts, so they throw out 1,600 as TBO.
But they also say that, because every engine might have had more or less exposure to these circumstances, the operator should make their own determination of TBO with a little interpolation. Wow! So if you find a plane that was never flip-flopped, or say, flip-flopped half the time, you just do the math. I assume you can just compare it to a normal IO-360, which is recommended for a 2,000-hour TBO, and adjust down from there. That leaves you a little investigation to do. (Read more about this.)
It is my experience that Super Decathlons, especially with constant-speed propellers, don't get as much of this "throttle-jockeying" as one might imagine. Most of us fly acro with a "set it and forget it" attitude, so our engines just hum along mostly at a 25/25 power setting. Mixture gets set to a conservative place that keeps temps in a small envelope of content. I pay the bills, so I never end a flight with a sudden reduction in power followed by a screaming dive at the airport. I usually avoid the big temperature swings this way. It gives me great life out of my rugged little Lycomings.
But beware! I rarely see this habit in those who fly these machines, so do a good compression check on your pre-buy inspection. It says a lot. Check the oil for metals and the plugs for how the plane has been leaned.
While you expect a 400-hp Pitts S-2 with 10-to-1 pistons to gyrate and hover until the CHTs cry uncle, the ol' Decathlon usually doesn't live that life. So the Pitts could have a questionable engine at mid TBO, and a Decathlon probably just needs that hottest cylinder worked on at around 1,400 hours. It depends if it's been flown in intermediate category by an 18-year-old or occasionally taken off to get a hamburger by a retired accountant, so ask.
What you are really looking for are those prop strikes that are so common to this type or planes that have sat for years; you know, the ad that says 900 hours total, 150 since overhaul. It isn't aerobatic wear; it's a pilot who didn't hold the back pressure in a bounce! It tells a story on how it's been treated. So what I got to say is: Check that engine well. It's the big-ticket item.
Let's move on to the first thing: the wings. So, Greg, they ask, what is your take on wood spars?
My reply: "All I can say is I have a 1939 Lipert-Reed Clipped Wing Cub with an original wood spar that I love to fly in Sportsman Competitions." (Please see the April issue of Sport Aerobatics magazine to read more.
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