Home

Print

POWER PLANT CONSIDERATIONS
ShopTalk - April 1999


Editor's Note: "ShopTalk" is a new column designed to give you more technical/mechanical information about your Mooney. We will be drawing upon the expertise of several recogonized Mooney maintenance professionals to bring you timely and accurate mechanical hints and tips. Kicking off this new column is Kerry Mclntyre, an A&P and IA with 20 years of maintaining Mooney airplanes. He also owns and flies a M20M TLS.  As the owner of KNR Inc., a shop in Evanston, Wyoming specializing in aircraft rebuilding, major alterations and annual inspections, Kerry has hands-on experience with most models of Mooney airplanes. In Wyoming, Kerry can be reached at his shop at (307) 789-6866.


As an IA and A&P with 20 years experience, I have owned and rebuilt a lot of different types of airplanes. However, I can tell you that my current airplane, a Mooney TLS, is the best cross-country aircraft I will probably ever own. Nothing in the general aviation single-engine, piston-powered fleet will travel at 200 knots on 20 GPH and reach flight level 250 with such ease. It is a remarkable and fun airplane to fly.


But because the TLS has such a huge operating range, it has its own type of quirks that you don't see in any other type of piston-engine airplane. Mooney and Lycoming did a fine job of putting this airplane into production, but the M20M has had its share of growing pains over its first three or four years. Primarily, these problems have centered in the powerplant area and in this column I would like to discuss some of these TLS engine issues with you.


One of the most troublesome engine situations in the TLS has been the exhaust valve/ guide wear problem. Let's begin a discussion of this situation by looking closely at the power chart provided for the TLS in the pilot's operating hand­book (POH).


If you have ever spoken with Lycoming or Mooney as I have, you will discover that no party has all the answers for the power chart in your TLS POH (page 5-20). I have looked at all the Lycoming power charts for the TIO-540- AFlA/AFlB. None of these Lycoming charts have the notes 4 or 5 on them that appear on the chart in the M20M POH.


These notes in the POH say that cruise fuel flows will decrease .5 gph for every 20 degrees Celsius above standard day OAT. However, in the real world, I have found that during a 2.5 hour cross-country flight at 16000 feet or higher, the fuel flows decrease approximately .5 gph as the OAT decreases. Most everyone agrees that this occurs because the wings are cold soaking, which causes the fuel to cool which changes the fuel density.


Because the TIO-540-AF1A/AF1B engines have an automatic density altitude controller incorporated with the turbo/intercooler system, the engine cannot tell the difference in atmospheric pressure no matter what altitude you are at, up to its critical altitude of 22,000 feet. The beauty of this sysrem 1s that the engine will always deliver the brake horsepower you set it at no matter what the OAT/pressure or fuel density. You get the idea, it's automatic.

So why am I writing about this issue with the power chart? Because most owners/ pilots believe that the POH is always correct, but from our example, we can see that this is not always so.


As another example, I can tell you that as long as I own my TLS it will never be operated at peak TIT above 70% BHP as the POH suggests. Why? Because horsepower develops heat and fuel is a coolant in an air­cooled engine. You can cool the fire by running at best power mixture instead of peak TIT above 70% power without having to open the cowl flaps, which makes for a noisy and slower airplane.


But the big savings from operaring at a best power mixture instead of peak TIT at higher power serrings is reduced maintenance costs. We can protect our investment by using a little common sense and not run our engines at peak TIT just because it says we can in the POH. I can't tell you how many cylinders and exhaust system components I have changed over the years, but the scars on my hands are a good indicator that it's been a lot. Its just plain and simple that warped exhaust valves, melted wastegate butterfly valves, blistered exhaust pipes and fried turbo exhaust wheels are all signs of excessive leaning at high power sectings. '.


In conclusion, if you like spending thousands of dollars every year with your favorite A&P, then go ahead and run 34 inches and 2400 RPM at peak TIT. But my maintenance costs will probably be lower because I will not run peak TIT above 70% BHP. Fuel is the cheapest thing you will ever put in your airplane. Using a little more extra fuel to keep your TLS  engine  cool  at higher power settings is probably the cheapest preventative maintenance you will ever buy.

If you have questions about this or any other ShopTalk article, please e-mail me at shoptalk@knr-inc.com or call me at my aircraft repair shop, 307-789-6866. All ShopTalk articles can be read here at www.knr‑inc.com. Until then, enjoy flying at 200 knots. Remember, not everybody can.

========================================================================