Lean of Peak and the Engine Rebuild

Pilots come to their own conclusions about LOP/ROP operations.

Ben Younger engine rebuild
The author's engine being rebuilt at ACE Aircraft in Greeley, Colorado.Courtesy Ben Younger

Long before I became a pilot, I read something somewhere about the extreme dangers of shock cooling a piston aircraft engine. The article didn’t specify which engines. It tossed a large, wet blanket over every Lycoming and Continental ever made. The idea of cylinders cracking on a steep descent with all that cold air passing over them captured my imagination. It wasn’t until years later that the myth was finally debunked. Think of how many thousands of times a student pilot chops the power in a trainer and comes down at idle for a simulated engine out. That’s not to say shock cooling isn’t an issue for some engines, but it isn’t for the vast majority of the aircraft we fly. What began as a precautionary procedure for a small number of high-performance engines has needlessly trickled down to populate the zeitgeist.

I believed what I read because I saw it in print. The written word gives the illusion of reliability and conveys a sense of soundness. While where one finds information should have everything to do with how reliable it is, in today’s media I would argue this is a lost value for a good-size portion of the population.

Pre-internet, many crackpot ideas were kept alive through oral tradition, one misinformed individual passing drivel to the next host. The Web changed all that. Unlike ­writing a book or publishing a newspaper, which requires capital and effort, posting something online only necessitates a Wi-Fi connection and as little time and consideration as you like.

Consider conspiracy theory sites. Fake-lunar-landing proponents and flat-earthers have found a haven online. Their ideas gain traction by manipulating loosely assembled real and manufactured data in support of the author’s foregone conclusion. (To my mind, why people want to believe such things would require more than this column’s worth of words and would be better served by research in the New England Journal of Medicine.)

I find most pilots to be science- based, rational thinkers. Yet we still gravitate toward certain ideas not borne of true data, and instead support conclusions we are predisposed to somehow. No better example exists than the argument over operating a piston aircraft engine rich-of-peak versus lean-of-peak EGT. Google it and you will have enough ­reading to get you through your summer ­vacation…of 2024.

For those of you not ­familiar, a brief explanation: The mixture ­control in an airplane allows the pilot to control the fuel-air ­mixture ­introduced into the combustion chamber. As one climbs, the air becomes thinner, requiring less fuel to maintain the desired ratio. No one disputes this. The argument is about just how much less fuel.

Exhaust gas temperature is the ­primary tool in determining this ratio. As you begin to lean out the ­mixture, EGTs rise. They continue to rise until they peak, after which point they begin to drop.

It is at this moment that the two camps diverge. One side believes that further leaning will eventually turn your motor into scrap metal as it is deprived of the excess fuel to help with cooling. The other side believes that you are wasting gas and dirtying your engine with all those unburned hydrocarbons running ROP.

Where, then, to lean to? As a student I was taught to lean to peak EGT and then enrich the mixture until EGTs dropped 100 degrees F. This is ROP and under many conditions a perfectly reasonable practice. But not all.

LOP requires that once you reach peak EGT you continue to lean the mixture—usually 25 to 50 degrees F on the lean side for normally aspirated engines. There are times—very high power settings for one—when LOP can be harmful to an engine, but that doesn’t mean it shouldn’t be practiced at all. Baby, meet bath water.

Even universally agreed upon “safe” ROP operations can be harmful if one does not enrichen enough. Interestingly, some legacy ­aircraft in the 1970s came with errant ­information in their POHs calling for ­25- to 50-degree F ROP operations at high power settings. Decidedly, this is not good for one’s engine as it ­operates with excessive peak pressures inside the cylinders at those mixture and power settings. Though rare, even the above-mentioned, normally safe 100-degree F ROP operation can result in excessive cylinder-head ­temperatures in some engines at high cruise power. Caveats abound.

If you’re seeing a trend here, it’s that there is no one tool for the job. We fly at all altitudes and in all conditions. How one runs their engine in either envelope is far more important than which camp you live in. It’s not how hard you run your motor, it’s how you run it hard.

This column is not meant to be a debate on LOP versus ROP operations. My aim is to make you aware of how you come to conclusions. What may seem like an honest, objective search for the truth can be widely affected by your own personality, who gave you the information, what anecdotal evidence you have seen or experienced and a myriad of other stimuli that you may not be aware of. In the same way that most everyone thinks they’re a good driver, most also believe their judgment is sound. Of course, not everyone is a good driver. When deciding whose expertise to trust, try to remove as much bias as possible—or at the very least, identify it.


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So how does one parse the truth? As far as reading today’s news goes, it’s difficult. Bias is ubiquitous. But what about the sciences? Double-blind studies, control groups and placebos go a long way toward finding veracity. But not every theory can be tested, and often, it isn’t needed.

Certain things can be inferred by common sense. No study is needed to ascertain that kicking a hornet’s nest is not a great idea. But what happens when the questions go beyond obvious pragmatism and require expertise that we do not possess? In these cases, we rely on others to make the determination for us. We substitute the reputation of others for our own lack of knowledge.

As far as the LOP versus ROP debate: Go online, and you will find two distinct camps, each ­defending one method. Routinely, people are shamed on both sides for believing in the other. Even asking the question can bring a world of pain down on you in an internet forum. On “Beechtalk,” a forum that deals mostly with Bonanzas and Barons but covers many other facets of aviation, a question that begins with ­someone doubting the merits of LOP ­operations is dealt with swiftly. I usually find it funny, but a newcomer to such a forum can quickly be swayed—and not always for the right reasons. It’s convenient when arguing to avoid nuance and exceptions. Much simpler to draw a line in the sand. This is where the problems begin.

There are gurus in both camps, individuals with resumes that demand respect and attention. An engine builder who has seen many motors ruined from improper LOP operations is understandably going to have a general opinion about the practice and be less inclined to get into the finer debate points. An engineer who runs a motor on a test stand to exacting specifications sees the obvious benefits of LOP, espousing its use on every flight. He does not consider the errant practitioner. He imagines only someone following the practice to perfection.

So which method is best? LOP or ROP? As with many topics that have opinions living on opposite poles, the answer is: It depends. When used correctly, LOP is a wonderful method for going farther on less gas. Want to fly a bit faster and aren’t as concerned with maximizing fuel economy? ROP is your solution, though you probably need to run richer than you were taught years ago. But why listen to me? I’m just some Hollywood type who banged some keys on a computer. Go find your own answers. Read, ask and listen. Just consider what exactly swayed you. Unlike the combustion process, the thought process is far more prone to complications.