There may be no greater debate amongst pilots than the “rich of peak” or “lean of peak” (ROP vs. LOP) exhaust gas temperature method of leaning the aircraft engine.
Fortunately, we pilots have options when it comes to leaning methods – or do we? The correct leaning procedures are generally outlined in the POH. However, many of our aircraft’s POHs were written long before the advent of today’s engine monitoring capabilities.
So that leaves us with the question; is it better to operate rich of peak or lean of peak?
Some engines achieve maximum economy range on the leaner fuel side of peak exhaust gas temperatures (EGT) and maximum power on the rich side of peak EGT. Some achieve both on the rich side of peak EGT. The ultimate answer comes from your engine manufacturer’s engine operator’s manual (EOM).
Operating an aircraft engine on either side of peak EGT is not necessarily harmful if it is accomplished within the limitations set forth by the aircraft manufacturer’s POH or the engine manufacturer’s EOM.
Many of us were not trained in proper engine leaning nor were the three possibilities explained to us during our training; rich of peak EGT, peak EGT, lean of peak EGT. This article aims to clear that up!
Table of Contents
Why lean at all? Optimizing Combustion.
Think, for a moment, about the conditions your aircraft’s engine operates – especially compared to your car’s engine.
Your piston-powered aircraft engine regularly operates (or is designed to operate) between sea-level and 12,000’ or more (way more if it is turbocharged) above sea-level across a very wide range of other atmospheric conditions; humidity, atmospheric pressure, etc.
Then, consider the requirements of combustion, an event that is necessary to make the pistons move up and down (or side-to-side) in our engines.
Not only do we need the chemical reaction known as combustion to take place, we need it to take place at the correct time (when the piston reaches the top of the stroke) and also at the correct “burn rate” so the pressures against the piston are not too great.
We need three things for combustion: oxygen, heat, and fuel. And then we need them at the correct quantities under tremendously varying conditions.
This is worth understanding in a little more detail to fully comprehend the importance of engine leaning.
Oxygen
Oxygen comes from the induction system – it is the air that enters the cylinder.
If our aircraft is normally-aspirated, the amount of oxygen present for combustion is the result of the ambient air pressure.
As we climb higher in the atmosphere, the number of oxygen molecules present in a constant volume of air (like the amount of air that enters the cylinder during the intake stroke of the piston) decreases.
In a normally aspirated aircraft, the only real control we have over the amount of oxygen available for combustion is through altitude selection.
However, we do not choose altitude based on our combustion needs (other than not choosing an altitude above the capabilities of our aircraft). The altitudes are chosen based on getting to our destination with some acceptable combination of time and fuel burned, as well as weather and obstacle clearance.
Heat
The heat comes from the spark generated by the magneto sending an electrical impulse to the spark plug. The resulting spark creates the heat needed to launch the chemical reaction we need to burn the fuel/oxygen mixture.
From the cockpit, we have little control over this aspect of the process. We really only have the ability to turn the magneto on or off.
We cannot change the amount of spark generated or the timing of that spark as we fly along.
Fuel
Last, but definitely not least, the third leg of the combustion triangle is fuel. This is where us pilots get to participate in this process.
We have a control in the cockpit that allows us to change the amount of fuel entering the combustion chamber without also changing any of the other parameters (oxygen and heat). That control is, of course, the mixture control.
“But wait”, you say, “I control the throttle – that plays a huge part in this whole combustion thing, doesn’t it?” Of course it does.
However, the throttle is controlling how much of the fuel/air mixture is making its way to the spark – the volume.
The ratio of fuel:air remains basically the same as we advance or retard the throttle. It is the aptly named mixture control that alters the amount of fuel that is then mixed with the air.
So, in order to achieve the correct burn – or chemical reaction – from the cockpit while at our desired altitude, we manipulate the mixture control.
That correct burn results in a desired result somewhere from generating the most power possible to achieving the lowest practical fuel burn and everything in between.
How do we achieve the proper fuel to air ratio: Leaning 101
As student pilots, if we were taught to lean at all, we were likely taught the following process:
- Level off at cruise altitude
- Leave climb power set until the aircraft has accelerated a bit more than cruise speed
- Set cruising power
- Let the aircraft stabilize at cruise
- Slowly pull back on the mixture control (lean) until the engine starts to run rough
- Push the mixture control back in (enrichen) until the engine is running smoothly again
That is really all there is to it and , in my opinion, there is absolutely nothing wrong with it.
Without any fancy engine analyzers, or even a relatively simple exhaust gas temperature (EGT) indicator, you’ve used your “pilot senses” to find a fuel:air ratio that the engine likes without sending vast quantities of unburned fuel out of the exhaust.
What is interesting about this “visceral” leaning process is that it does not take into account EGT at all.
Leaning Using EGT
Most, if not all, aircraft POH’s reference EGT when leaning. EGT is important because it is a much more precise way of fine tuning the fuel:air mixture.
At a full rich mixture up at altitude, there is more fuel than needed by the combustion. This results in some unburned fuel passing out the exhaust valve along with the gasses from the combustion.
This results in a relatively low EGT – that unburned fuel cools the exhaust gasses.
As the mixture is leaned, there is less unburned fuel to cool the exhaust gasses and the EGT rises.
Eventually the fuel:air mixture is optimal and everything is consumed during combustion. This is known as “stoichiometric mixture”.
Leaning the mixture further past this perfect fuel:air ratio results in too little fuel entering the cylinder for the volume of air. This results in weaker combustion (due to the fuel shortage) and the EGT starts to come down.
Typically, the “best power” setting is a bit rich of peak and “best economy” is at peak or a bit lean of peak.
These mixture settings are described as a specific number of degrees rich or lean of peak EGT.
Pilots can also utilize EGT for finding the desired fuel:air mixture at cruise power settings. This is accomplished by using a single probe EGT gauge or an engine analyzer.
I previously covered the topic of utilizing EGT gauges for leaning in “Is Operating an Aircraft Lean of Peak Harmful” – you may want to refer to that for more information on how to use EGT as a leaning tool.
In a nutshell, the mixture is leaned until the EGT reaches its highest temperature. At that point, the mixture is either enrichened or leaned until the desired temperature change has taken place.
The process of leaning is simple enough. The controversy lies in whether or not it is acceptable to operate lean of peak EGT – something that was considered taboo not all that many years ago.
Rich of Peak vs. Lean of Peak
Before entering into this discussion, I want to make it clear that your aircraft POH and EOM is the final authority on how to properly operate your aircraft.
Any deviation from those instructions places you into the “test pilot” category and should only be attempted under the watchful eye of your flight instructor.
Lean of Peak Advantages
There are benefits to operating lean of peak that have only recently been discovered.
This is due to the advanced level of engine monitoring that has only recently been obtained by the advances in engine monitoring technology.
In fact, much of what was thought to be forbidden (like how operating lean of peak is always bad) has now been proven to be false in many cases.
Mike Bush, in his “The Savvy Aviator #59” discusses leaning the mixture at great length. One of the first headings of the article is ”Forget the POH!”.
The section heading is, no doubt, there to grab the attention of the reader. But, the message is clear; there is much that has been learned about fuel:air ratios and the POH may not be the most up to date authority on the subject.
For example, Bush’s analysis of detonation and the conditions that cause detonation have shown that most POH’s recommend operating in a state that is very close to the perfect conditions of detonation.
Operating LOP actually offers more protection against the potentially catastrophic condition known as detonation than does operating at 50 degrees rich of peak.
This data points to several advantages operating LOP:
- More protection against detonation
- Cooler cylinder head temperatures (more important to engine life than most other factors)
- Less internal stress upon the engine
- Cleaner emissions (less unburned fuel)
- Less fuel burn
Rich of Peak Advantages
Operating rich of the peak EGT does have several advantages as well. Probably the most significant advantage is that ROP operations are what most aircraft POH’s use in their performance tables.
Therefore, when operating ROP, aircraft performance will likely be more predictable.
In cruise flight, for example, aircraft speed and fuel burn will likely be much more predictable.
The advantages of operating ROP:
- Cruise performance that more closely matches “book” performance
- More power – faster speeds
- Warmer CHTs (this can be desirable in aircraft that run exceptionally cool otherwise)
- Operating as described in the POH ensures that you aren’t doing anything “wrong”
- Carbureted engines may only run well at peak or ROP – anything LOP may be unacceptably rough
When understanding how to properly lean your engine, I would always offer the following advice:
Your aircraft’s Pilot’s Operating Handbook (POH) and your engine’s Engine Operator’s Manual (EOM) are the final authorities on how to properly operate your engine.
Engine Operator’s Manual?
I believe that the secrets to operating your engine properly are not necessarily found within the POH (which has likely not been substantially updated in decades) but there may be quite a bit of information located within the EOM.
I am a bit embarrassed to admit that I had never even consulted the EOM prior to a year or two ago. I had always used the POH – and there’s nothing wrong with that.
However, the EOM really is the final authority on operating the powerplant located at the pointy-end of your particular aircraft.
For example, the POH for an aircraft powered by the venerable Lycoming 0-360 series may suggest that best economy is reached at 50 degrees rich of peak EGT and best power is 125 rich of peak EGT.
However, the EOM for the engine may offer the following chart:
This representative chart indicates that best economy cruise is actually achieved at peak to 100 degrees lean of peak. Best power is more inline with what the POH may suggest. This chart indicates that the best power setting is 100-175 rich of peak EGT.
Here is what you may want to do:
- Refer to your POH and find your aircraft’s exact engine (Lycoming IO-360, for example, is not sufficient – IO-360-C1C6 is what you need)
- Proceed to your engine manufacturer’s website
- Review the operating instructions for your engine
The operating instructions will provide a plethora of information regarding the engine manufacturer’s recommendations on all aspects of operating your particular engine model including leaning specifics.
For example, in Lycoming’s EOM for the “O-360 and Associated Models” it states the following:
“Best Economy Cruise (approximately 75% power and below) – Operate at peak EGT”
This may not totally agree with the instructions located in the POH.
In my particular aircraft, a Beechcraft Sierra with a Lycoming IO-360, the POH recommends operating at 25 degrees rich of peak and warns against leaning past peak EGT. So, while the two are not exactly diametrically opposed, there is some ambiguity between the two.
Fortunately, the EOM is clear in that it makes it clear that the POH is the ultimate authority when it comes to properly operating the aircraft. One may assume that this is because the airframe plays an important role in maintaining proper operating temperatures for a desired performance.
I will also point out that the EOM for the Lycoming engine in my Sierra also spells out the recommended leaning process without an EGT gauge and it is pretty much exactly what I described in the previous section; pull the mixture control back until the onset of loss of power or engine roughness and then enrichen the mixture until power is restored or the engine smooths out.
Who to believe?
The suggestion is not to ignore the POH or the EOM. It is important, as pilots, to understand our aircraft, analyze the performance, and then ultimately make the best decision for our mission, our aircraft, and the safety of those on board.
