It’s not until most pilots become aircraft owners that they start to even care where the mixture lever belongs. Also, most pilot’s operating handbooks advocate a cruise mixture setting of 50 degrees rich of peak exhaust gas temperature. And that is often the worst possible place you can be for cylinder longevity, according to the smart people who study these things.
In his best-selling book, Mike Busch on Engines: What Every Aircraft Owner Needs to Know about the Design, Operation, Condition, Monitoring, Maintenance and Troubleshooting of Piston Aircraft Engines, Busch takes nine pages to describe the basics of aircraft leaning, and eight more for advanced leaning. It’s considered one of the foremost resources on the topic, and despite the detailed explanations, it remains the topic Busch is asked about the most.
Learning to properly use the mixture control isn’t complicated, especially if you can learn and accept a few truths and forget a few things you thought you knew. First, cylinder head temperature, not exhaust gas temperature, is what we should be focusing on. Busch explains that although internal cylinder pressure is the best and most accurate measurement of stress on a cylinder, the best we can do is measure a close proxy, CHT. By keeping CHTs well below limits, we can hopefully extend the life of the top end. Although most pilot’s operating handbooks call for a leaning value based on EGT, Busch says we can largely ignore it during the leaning process, and focus solely on CHTs (“Obsessed with EGT,” January 2023 AOPA Pilot). Although redline on your cylinders might be 460 degrees (Continental) or 500 degrees (Lycoming), the goal is to keep the CHTs below 400 on most Continentals and 420 on Lycomings.
Second, forget that 50 degrees rich of peak EGT is the best mixture to operate. In a column written many years ago for AVweb, John Deakin makes a compelling argument as to why. In it he references a chart developed by Continental that plots fuel flow, brake horsepower, EGT, and CHT of an IO-550 on a test stand on one graph. Key to understanding the chart is knowing that the throttle and rpm were held constant throughout the test. The only thing that changed was the fuel flow via the mixture control. EGT peaks at about 1,520 degrees Fahrenheit, which corresponded to a cylinder head temperature of about 410 degrees. Critically, enriching the mixture to 50 degrees rich increased CHT to its maximum value, around 425.
The lesson is simple. Because most of us historically have had only rudimentary EGT gauges with which to monitor leaning, the handbooks described a set value based on some faulty assumptions and the spare instrumentation that what was available at the time. Focusing only on EGTs ignores the critical nature of CHTs to longevity, and the chart shows that the former is definitely not a good proxy for the later.
Third, and thanks again to the graph, flying lean of peak can make your engine run cooler and make the top end happier. CHTs drop off sharply on the lean side of peak EGT, and while there is a small horsepower decrease in a normally aspirated engine, cooler temperatures mean a longer engine life. If you’re new to flying lean of peak, maybe you’re concerned about flying leaner than the airplane manufacturer recommended. But remember, we generally only talk about EGT as part of the leaning process because that’s what was available at the time the airplane was made, not because it’s the best measurement of stress on the engine.
Finally, there is no one right way to lean. Although leaning with an engine monitor and properly tuned fuel injectors is optimal, it’s not necessary, according to Busch. Pilots of carbureted engines can do it, as can anyone without an engine monitor or an EGT gauge. If that’s you, Busch’s advice is simple: lean to the onset of roughness and enrich only enough to smooth it out again.
For those with an engine monitor, Busch recommends not using the Lean Find function and instead focusing on CHTs and fuel flow. Here’s how he describes his personal leaning procedure, which he used over thousands of hours, including taking his engines well beyond 200 percent of time between overhauls.
If he wants to go far, Busch leans to a specific fuel flow he knows will give him the range he wants. Some horsepower is lost in a normally aspirated engine when you go well lean of peak EGT, but that’s an acceptable tradeoff if your goal is range and cylinder longevity. A GPS-coupled fuel totalizer will do all the math for you. If Busch wants to go fast, he simply enriches the mixture until the hottest cylinder is less than 400 degrees (420 in a Lycoming).
The experts at GAMI developed a concept as part of their Advanced Pilot Seminars presentations on engine operation called “the red box.” The red box is a no-go zone where pilots shouldn’t ever operate the airplane for an extended period. At higher power settings the red box is large, and at lower power settings—usually somewhere between 60 and 65 percent power—it goes away completely. Essentially, the box corresponds to an area where CHT values will be more than 400 degrees. It doesn’t matter if you’re climbing, cruising, or descending, you can lean to whatever you like so long as it’s not in the red box.
Thanks to the work of the engine manufacturers and independent testers like GAMI, we know more than ever what consequences our leaning process has on engine health and longevity. Operating lean of peak doesn’t burn valves, and operating 50 degrees rich of peak is often right in the middle of the red box. The old way of doing things is not always the best way.