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Pedal to the Metal

 

July 2008

 

This month’s ShopTalk will re-address the issue on how to lean a turbocharged engine without damaging it and what tools you, as a pilot, need installed in the cockpit to complete this critical and sometimes confusing job. If you keep your MAPA magazines, there are four ShopTalk articles to review: Operational Hints and Tips for the M20K, January 2000; Mooney TLS/Bravo Operational Mechanical Tips, April 1999 and March 2000 and Aftermarket Turbo Considerations, May 2000. All engines mentioned in these articles are boosted well above 30 inches in order to make full horsepower. The TLS/Bravo has the only engine that utilizes a density controller to compensate for OAT making engine management a little easier. The information in this article is for engine operations at 70 percent and higher. Lower power settings may not result in high TIT and/or CHT indications.

 

Operating your airplane is about energy management and operating your engine is about temperature management, to protect the metal.

 

The first item to understand is that a published fuel flow number (Gallons Per Hour, GPH) at a specific RPM and manifold pressure should not be used to lean the engine. The reality with your turbocharged aircraft engine is that the supplied power charts have been corrected to standard day conditions and you almost never end up operating during standard day conditions.

 

These power charts provided with your aircraft are calculated by the engine manufacturer for standard day conditions. The airframe manufacture then flies prototype aircraft and in most cases modifies the calculated power chart. Since each engine runs a little differently (and itself changes as it wears), no engine, when correctly leaned, will ever match the fuel flow numbers published in the P.O.H. If leaned to the fuel flow number you will eventually damage the engine.

 

With this in mind, one can realize that thirty-five inches of manifold pressure on a standard day will not produce the same horsepower as thirty-five inches on a day that is above or below standard day temperature. The cooler the air below standard day the more horsepower your engine will produce for a given manifold pressure. Only the M20K POH addresses how to compensate for a non-standard day.

 

Manifold pressure and RPM directly correlate to horsepower produced, and the horsepower produced directly relates to heat generated in the engine and exhaust system. Heat (temperature) is critical to exhaust valve life and cylinder wear, especially in TCM engines.

 

For the turbocharger to maintain the manifold pressure so the engine can breathe, the turbocharger wheels spin faster as the airplane climbs to higher altitudes. As air compresses it heats up and as we already stated, cool air makes more horsepower than hot air. This is why an intercooler system between the turbocharger and the throttle body. For those of you not familiar with intercooler systems, an intercooler is basically a radiator for hot compressed air to flow through. The compressed air is cooled as heat is transferred to ambient air flowing between the radiator coils.

 

If you analyzed engine operation time one would see that most engines are operated about 80% of the time at cruise power settings and 20% at climb, descent, or at idle. Cruise is where the operator will do the most damage to their airplane engine, statistically in a very short time.

 

The POH will give you some never-exceed numbers for TIT, CHT and oil temperature but just because you don’t exceed these numbers doesn’t mean you are treating your engine kindly. The TLS/Bravo TLS/Bravo (M20M) is a classic example of an excessive red line on a CHT, Here’s why: Other manufacturers using the TIO-540 engine red line CHT at 460°. But not the M20M; it is set at 500°. Worse yet, as one travels into the flight levels, the probe, located on cylinder 3, may be on the wrong cylinder! Some data indicates that at high altitudes the hottest cylinder may be number one. I suspect that at low IAS (due to the high altitude) with the cowl flaps closed completely, insufficient air is getting through the cowling, it is blocked. When you, the TLS/Bravo TLS/Bravo pilot, adjust flight parameters (speed, power, mixture, cowl flaps) to limit CHT to red line, you may be exceeding 500° on a cylinder that is not being monitored. Continuous operation at near 500° isn’t good for the prospect of getting that engine to TBO (2,000 hours) without a few cylinder changes.

 

I am frequently asked, “Are there any EGT numbers we need to be aware of?” The answer is, “No”. No manufacturer publishes these as they will vary depending on power settings. On turbocharged engines, Turbine Inlet Temperature (TIT) is used in lieu of EGT as protecting the metal on the turbine side of the turbocharger is paramount. On the TLS/Bravo, 1750° TIT for one minute is acceptable to determine peak only, but otherwise 1650° is the maximum continuous acceptable to Lycoming and Continental. In reality we can learn from other people’s mistakes and we can recognize that there are some common sense numbers to operate your engine by.

 

You bought a Mooney to go fast and a turbocharged one to fly high, so how do you balance going fast flying high and using as little amount of fuel as possible and doing as little amount of damage as possible to the engine? By using common sense numbers that can be monitored you can learn how to properly lean your engine. Some will say, “I just run it way rich.” Well, your plane flies slower, it’ll use more oil, cylinder wear (cylinder wall washout) may increase and plugs may foul. If you run too lean you will eventually burn up exhaust valves, the waste gate, turbine wheel, the top of the cylinder bore, and damage the exhaust system. Just like a CHT has a red line, the TIT has a red line and it’s not good to run your engine only one needle width below the TIT red line.

 

So here are your common sense numbers: when in takeoff or climb-out, never exceed 1550° on the TIT and 400° on any CHT, open the cowl flaps to cool the cylinder heads and oil temp. If the TIT is above 1550°, correct by enrichening the mixture. Monitor the oil temperature, 210° should be the maximum reading on the oil temp gauge. During climb-out lower the nose and cruise climb. In the 252 and TLS/Bravo, partially close the cowl flaps. A 110 knot climb-out will keep enough air flowing through the cowling to help keep the temperatures in line. It is OK to lean the engine in cruise climb as long as you don’t exceed the above numbers. This will save fuel and prevent plug fouling and cylinder wall washout.

 

Once you reach the planned cruising altitude, accelerate and approximately set the manifold pressure per the power chart using the throttle, reduce the rpm to match that power setting at that altitude using the prop control. Continue to lean the mixture an 1/8 on a turn then wait five to ten seconds then go another 1/8 of a turn. Take your time, watch the manifold pressure gauge. Yes, that’s right; the manifold pressure gauge. As you lean the engine and you approach peak TIT, the manifold pressure gauge will go up ½ an inch (if you have a digital manifold pressure gauge you may only see .3 of an inch increase but it will go up). The manifold pressure gauge will respond much faster than any temperature gauge. If you continue to lean, it will go back down. This is called lean-of-peak. Once you get the highest needle or digital readout using the mixture control, stop leaning, this is peak TIT. Remember that the maximum manifold pressure is going to give you the maximum horse power. Let the engine run at least a minute or two to stabilize, and note the TIT reading.

 

Once you have determined peak manifold pressure and have come up with a peak TIT, a good rule of thumb is to never continually exceed 1600°. If your TIT stabilizes at peak of 1675° enriched the mixture to lower the TIT a minimum of 76° in order to not prematurely damage your engine At power settings higher than 75% you may want to enrichen more. The question “Should I lean 50° rich-of-peak or 50° lean-of-peak?” needs no answer. You must maintain TIT below 1600° continuous.

 

Now, no analog instrument is going to be accurate enough for proper turbocharged engine operation. If you don’t have a digital TIT get one even if it’s a JPI slim line digital TIT. The best scenario is a digital manifold pressure gauge and digital TIT along with individual digital readouts for each CHT. For less than $3000, a full digital engine monitoring system can be installed in your airplane. Return on investment will occur during the first 500 hours as you will improve flight efficiency and lower maintenance costs, especially on cylinders. I often see digital TIT installations where the probe has been installed in the wrong spot. This can cause you to destroy your engine because the hottest spot is not the entrance to the waste gate (where Lycoming locates it), not the collection in the manifold, it’s the entrance to the turbine wheel. Make sure your installation shop puts the probe in the right spot.

 

I get asked all the time how about lean-of-peak? This is a discussion and argument that has been going on for a long time. But really the bottom line here is not whether you run rich-of-peak or lean-of-peak, it is running TIT below 1600° continuously and the engine must run smoothly. It doesn’t mater how you cool the fire as long as the engine runs smoothly and you are below 1600°. Now I must state that no turbocharged Lycoming or Continental POH will endorse running lean-of-peak. As a mater of fact the only POH I have ever seen it in is the A36 Bonanza and that’s at 65% power or less and is a non-turbocharged engine.

 

When you attempt lean-of-peak operation and under 1600° TIT, you’ll find that few engines run smoothly. On some engines, Gami injector nozzles are definitely a benefit; on others, a waste of money. If you have a M20K with a GB or LB engine, Gami nozzles are a must-have item. With a 252 with an MB engine, Gami nozzles are a waste of money. These are specific examples. If you purchase Gami nozzles to fine-tune your engine and have no expectation of saving fuel by running lean-of-peak, you will be happy with them. One other thing about lean-of-peak is your airplane will be slower than when you run rich-of-peak and as I already stated earlier, you bought a Mooney to go fast (of course, fuel was much cheaper), your choice.

 

Once you get the TIT below 1600° continuous, your attention should turn to the CHTs. Forget the fuel flow. It is what it is; at that altitude, at that power setting, on that day, at that temperature, for your engine. It’s not going to match your published fuel flow number in your POH. It never will and it could be a couple gallons an hour higher than what is published. For CHTs, the magic number here is 400°. We never want any cylinder to exceed that on a continuous basis. We already know you can’t rely on the single probe system that’s installed in most of our planes. So, with a CHT digital readout for each cylinder, monitor and assure 400° is not exceeded (continuous) for any cylinder. The ideal is between 375° and 400°. This is where the cylinder barrels are straight and at their best running temperature. CHTs are controlled by the cowl flaps, forward speed and/or OAT. With the 231 Mooney there is limited control of the cowl flaps. With a 252 or TLS/Bravo, there is infinite control. We’ve discovered years ago that the TLS/Bravo flying in the flight levels needs the cowl flaps opened slightly to keep the CHTs below 400°. CHT control will vary depending on cowl flaps adjustment and the condition of your baffles and baffle seals. The 375-400 degree range applies to climb, cruise and descent. Proper descent planning or use of speed brakes in descent and cowl flaps in climb will allow the pilot to maintain these parameters. A turbocharged engine run at 75% or above all the time will not last as long as one that’s been run at 65%. Higher horsepower means higher temperatures and higher internal pressure; higher rpm means more revolutions and more piston travel. These items are directly related to wear and tear on your engine.

 

Now there is an extreme to what I call babying your engine, I don’t recommend for anyone to casually run a turbocharged engine below 65%. At lower power settings it is easier to load up the engine and the lower temperatures can contribute to shock cooling. It can be done efficiently, but that discussion is beyond the scope of this article. Just follow the power chart, forget about published fuel flow and never operate your engine outside of the published RPM and manifold pressure numbers. “25 square” is not an accurate way to operate a turbocharged engine.

 

If you operate your TLS/Bravo with new Lycoming cylinders as I described above, you will typically get about 1500 hours before a cylinder change. At lower average power settings you might be able to get all the way to TBO without a cylinder change. Lycoming cylinders are nitrided barrels. They are very hard and just don’t wear out. Typical failures are with exhaust valves and the seats. The TSIO 360 or 520 engines operated as suggested, with new TCM cylinders, should achieve 1100 to 1200 hours before overhaul.

 

Continental barrels are not hardened and tend to wear out at the top near the exhaust valve where the most heat is concentrated. The rings migrate to that area and oil begins to blow past the rings; compression goes down. I believe Continental engines are better run at 65 or 70%. Lycomings can be run, as suggested, at 75% and still get good longevity.

 

The other advantage of following these basic guide lines is that your turbocharger will last longer. I see some planes with 400 hours needing a new turbocharger. That’s insane. If you operate your turbocharged airplane engine as stated in this article your turbo should last 1200 to 1500 hours. The waste gate might even go longer than that before the butterfly is melted over and you can’t get the manifold pressure out of it.

 

By running with these numbers, more fuel may be burned but fuel is (still) the cheapest thing you will ever put in your plane. Today, turbochargers cost almost $2,000 and they are expensive to change. Some of the exhaust components on the TLS/Bravo are close to $4,000 to buy. If you follow my common sense numbers, your plane wont be down with burnt-up parts, you’ll get to fly it more and it will cost you less in the long run.

 

A great advantage of flying a turbo-charged airplane is the pilot has many options available. One can get high above terrain and weather, catch a strong tailwind, lower the gallons per mile. Or, if time and weather permit, stay low enough to stay off the supplemental oxygen, pull the power back and enjoy the scenery while saving some money. The turbo-charge envelope is so much greater than a normally aspirated engine; it gives a pilot much greater freedom. But with that freedom comes much more responsibility. A turbo-charged engine is more easily damaged than a non- due to greater available power, more complexity and wider environmental conditions. So fly high or low, fly fast or slow, but fly smart.

 

As always, if you have a question about this article, you may contact me at my aircraft repair shop, 307-789-6866 or via e-mail. Until the next ShopTalk, enjoy flying your Mooney.

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