The Savvy Aviator #36: Temperature, Temperature, Temperature
I've had wonderful luck with piston aircraft engines throughout my 37 years as an aircraft owner. All the engines on my airplanes have made TBO with minimal maintenance along the way, and in recent years they've gone well beyond TBO.
For decades, I was convinced that the secret of my success was the fact that I "babied" my engines, typically limiting my cruise power settings to no more than 60% or 65% power. I felt that sacrificing a little airspeed in exchange for long engine life and reduced maintenance cost was a good tradeoff.
More recently, I've come to learn that such "babying" is one way to achieve long engine life, but it's not the only way. That's because it's not power that damages our engines -- it's temperature. It turns out you can run these engines as hard as you like so long as you are obsessive about keeping temperatures under control.
Or as my friend, powerplant guru and former TCM tech rep Bob "Mose" Moseley says, "There are three things that affect how long your engine will last: (1) temperature; (2) temperature; and (3) temperature!"
It's All About The Heat
Our piston aircraft engines are heat engines. They have moving parts -- notably exhaust valves and valve guides -- that are continually exposed to extremely high temperatures in the 1,200 °F to 1,600 °F range (and sometimes even hotter). Since engine oil cannot survive temperatures above about 400°F, these moving parts must function with no lubrication. They depend on extremely hard metals operating at extremely close tolerances at extremely high temperatures with no lubrication. It's nothing short of miraculous, and a testament to outstanding engineering, that they last as long as they do.
The key to making these critical parts last is temperature control, and the most important temperature is cylinder head temperature (CHT). Mose has been monitoring and overhauling these engines for nearly four decades, and he claims that an engine that is operated at CHTs above 400 °F on a regular basis will show up to five times as much wear metal in oil analysis as an identical engine that is consistently limited to CHTs of 350 °F or less. "It's amazing how much a small increase in CHT can accelerate engine wear," says Mose.
As critical as CHT is, many owners don't have a clue whether their CHTs are above 400°F or below 350°F. That's because the engine instrumentation provided by most aircraft manufacturers is pathetically inadequate. The factory CHT gauge looks at only one cylinder, and it's not necessarily the hottest one. Further, the factory CHT gauge often isn't even calibrated, and its green arc extends up to a ridiculously hot 460 °F (for Continentals) or 500 °F (for Lycomings). Those numbers may be okay as an emergency red line, but they're abusive for continuous operation. If all you have is factory gauges, you could easily be cooking your cylinders to death while blissfully thinking that all is OK because the CHT gauge is well within the green arc.
To know what's really going on in front of the firewall, you have to have a modern, multiprobe, engine analyzer with a digital readout. Such instrumentation isn't cheap -- figure $2,500 for a single or $5,000 for a twin, installed -- but if it saves you from having to replace a couple of jugs en route to TBO, it has more than paid for itself. Installing a digital engine analyzer is probably the best money you can spend on your airplane.
Fuel System Setup
For takeoff and initial climb, we normally are at wide-open throttle, full-rich mixture, maximum RPM (if we have a constant-speed prop), and wide-open cowl flaps (if we have those). So there's not much we can do from the cockpit to affect CHT during these phases of flight.
What does affect CHT is how our full-power fuel flows are adjusted. Unfortunately, it is shockingly common to see damagingly high CHTs due to improperly adjusted fuel flows, particularly in fuel-injected engines. It is not unusual for the fuel flows to be set wrong from the day an engine is installed, and never to be checked or adjusted all the way to TBO. The owner winds up going through cylinders every 500 hours and never knowing why (or blaming the manufacturer).
In part, the problem lies with mechanics who don't fully understand how critical it is to test and adjust the fuel-system setup on a regular basis. For example, TCM recommends that the fuel system setup on fuel-injected TCM engines be checked and adjusted several times a year to account for seasonal changes. Most TCM-powered airplanes go year after year without this being done, and many shops don't even have the necessary test equipment to do it.
Even when mechanics do test and adjust the fuel system, they often adjust it wrong. For example, TCM service instruction SID97-3C contains a lengthy table that specifies full-power fuel flow as a range (minimum and maximum). The "fine print" of SID97-3C instructs mechanics to adjust the full-power fuel flow to the high end of the specified range, but many mechanics miss this subtlety and adjust it to the middle of the range. Experience shows that this is simply not enough fuel flow to keep CHTs cool during hot-weather takeoffs.
Then there's the problem of aftermarket engine modifications. For example, engines that have been retrofitted with Superior's Millennium cylinders often run higher CHTs than they did with their original factory cylinders. We now know that the reason for this is that Millennium cylinders have substantially better "volumetric efficiency" than factory cylinders -- in other words, they breathe better. Since they breathe more air during every combustion cycle, they need more fuel to maintain the same fuel/air mixture. The full-power fuel flow marked on your fuel-flow gauge or specified in TCM SID97-3C may simply not be high enough if you have Millennium cylinders installed.
Even worse are turbocharged engines with aftermarket intercoolers installed. The intercooler reduces the temperature of the air that the cylinder breathes, making it denser. Denser air demands more fuel to maintain the desired fuel/air mixture, so full-power fuel flow must be increased significantly above original factory specifications. Too often this is not done, and the result is fried cylinders.
Many A&Ps are reluctant to adjust takeoff fuel flow above red-line. However, if you have Millennium cylinders, an aftermarket intercooler, or some other "mod" that allows your engine to produce more power than it did when it left the factory, that's exactly what must be done to keep your CHTs cool and avoid premature cylinder failure.
Enough Fuel Flow?
How can you tell if your full-power fuel flow is adequate? If you're limited to factory gauges, you probably can't, at least with any precision. About the best you can to is to watch your fuel flow gauge (if you have one).
A good rule of thumb is to multiply your engine's maximum rated horsepower by 0.1 to obtain the minimum required fuel flow in gallons-per-hour, or by 0.6 for pounds-per-hour. For example, if your engine is rated at 285 horsepower, your takeoff fuel flow should be at least 28.5 GPH or 171 PPH; if it's rated 310 horsepower, the minimum should be 31.0 GPH or 186 PPH. If your takeoff fuel flow is significantly less than this, have your mechanic crank it up. And don't forget that if you have Millennium cylinders or an aftermarket intercooler, your engine might be producing a few percent more horsepower than what the book says, so it might need a few percent more fuel flow.
Now if you have a digital, multiprobe engine analyzer, it's easy to tell if your fuel flow is adjusted high enough. Just make sure none of your CHTs exceed 380 °F during takeoff and climb. If they're around 350 °F, that's even better. Experience also shows that full-power EGTs should be no higher than about 1,300 °F for normally aspirated engines and about 1400 °F for turbocharged engines, although these numbers aren't nearly as important as CHTs.
What About Cruise?
Cruise flight represents the lion's share of our flying time. Just as in takeoff and climb, it's essential to keep all our CHTs at or below 380 °F during cruise to achieve good cylinder longevity, and 350 °F is even better. But hopefully we can do this without pouring 30 GPH of 100LL on the problem.
There are basically three different strategies for keeping CHTs low during cruise:
- Baby the engine
- Operate very rich
- Operate lean-of-peak
All three strategies work, and conscientious use of any of them will give you a good shot at making TBO with minimum cylinder problems. But each has its pros and cons. Let's take a closer look.
Baby The Engine
Many POHs talk about operating at three alternative mixture settings: "best power mixture" (~125 °F rich-of-peak [ROP] EGT), "recommended lean mixture" (~50 °F rich of peak EGT) and "best economy mixture" (~peak EGT). In my experience, most pilots tend to operate somewhere between best-power mixture and recommended lean mixture.
It turns out that "recommended lean mixture" (~50 °F ROP) is just about the worst possible mixture setting for keeping CHT low. If you look at the graph above, you'll see that CHT reaches a maximum very close to 50 °F ROP. So if you want to operate at "recommended lean mixture" and simultaneously keep CHT low, there's only one way to get there: reduce power dramatically (e.g., to 60% power or less). In other words, baby the engine.
Both "best power mixture" and "best economy mixture" result in somewhat lower CHTs than does "recommended lean mixture." At either of these mixture settings, you can usually operate at 65% power or so and still keep CHTs in the acceptable range.
In any of these cases, you're trading power and airspeed for reduced temperatures and increased longevity. For most of us, that's a reasonable tradeoff to make.
Operate Very Rich
But what if you are unwilling to sacrifice power and airspeed? Is it possible to go fast and still keep CHTs low?
Sure it is. We already talked about one way to do this in our discussion of takeoff and initial climb: Pour lots of 100LL on the problem. In other words, operate very rich.
How rich? The graph above suggests that to reduce CHTs by 25 °F, you need to enrichen the mixture to about 160 °F ROP. For each additional 10 °F of CHT reduction, you need to enrichen an additional 50 °F ROP. Using such very rich mixtures, you can go fast and still stay cool. But before you decide to go this route, consider the downsides.
The most obvious downside is that this strategy is very fuel-inefficient. Compared to "best economy mixture," the very-rich strategy consumes about 25% more fuel, and reduces range by a similar amount. Advocates of very rich mixtures will tell you that "fuel is cheaper than engines," but don't be so sure. At today's avgas prices, using 25% more fuel in a 300-horsepower engine can cost more than $25,000 over the engine's TBO, and that's enough to change out quite a few cylinders.
A second and less obvious downside is that very rich mixtures result in "dirty" combustion with lots of unburned byproducts in the exhaust gas. Operating this way for long periods of time tends to cause deposit buildup on piston crowns, ring grooves, spark plugs and exhaust valve stems. Do it long enough and you could wind up with stuck rings, stuck valves, worn valve guides, and fouled plugs.
The third way to reduce CHTs is to lean even more aggressively than the POH recommends and operate on the lean side of peak EGT. The graph shows that you can reduce CHTs by 25 °F by leaning to about 10 °F LOP. For each additional 10 °F of CHT reduction, you need to lean an additional 15 °F LOP. Using these very lean mixtures, you can go fast, stay cool, and obtain outstanding fuel economy, all at the same time.
What's the downside of the LOP approach? The only major downside is that if your engine has uneven mixture distribution among its cylinders, it will usually run unacceptably rough at LOP mixture settings.
Uneven mixture distribution can usually be corrected in fuel-injected engines by "tuning" the fuel injector nozzles to eliminate the mixture imbalances. GAMIjectors are tuned nozzles that are now STC'd for the majority of fuel-injected Continentals and Lycomings. TCM now offers its own version of tuned injectors on some of its premium engines.
The TSIO-520-BB engines in my Cessna T310R are equipped with GAMIjectors. As a result, my mixture distribution is near-perfect and I can usually operate extremely lean (nearly 100 °F LOP) without perceptible roughness.
If your engine is carbureted, you have no injector nozzles to tweak. If your mixture distribution is uneven, your engine probably won't operate LOP without unacceptable roughness, and there's probably not much you can do about it.
There's huge variation in mixture distribution among different makes and models of carburated aircraft engines. Most carbureted Lycomings have relatively even mixture distributions and are often good candidates for LOP operation. On the other hand, the ubiquitous Continental O-470 engine found in most Cessna 182s is notorious for having uneven mixture distribution, and most can't be leaned beyond peak EGT without getting a serious case of the shakes.
Whatever strategy you prefer, the important thing is to keep a close watch on your CHTs and ensure that they remain cool. The best way to do this is to install a multiprobe digital engine monitor and program its CHT alarm to go off at 390 °F or 400 °F.
If the alarm goes off during takeoff or initial climb, you're going to have to get your mechanic to turn up the full-power fuel flow. If it goes off during cruise, either richen (if ROP) or lean (if LOP) to bring the CHT down to acceptable levels.
If you don't have a multiprobe, digital, engine monitor, install one. The cost of such instrumentation (including installation) is usually less than the cost of replacing one cylinder. Failure to install such instrumentation is a classic case of "penny wise, pound foolish."
See you next month.
Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.