This information is provided "as is" and you use this information at your own risk. The author is not a CFI. Talk to your CFI if you have any questions about proper operation of aircraft.
Leaning the engine properly leads to lower internal cylinder temperatures, lower internal cylinder pressures, and longer engine life. Specifically, operating the engine in a lean of peak fashion is much easier on the engine while delivering great performance. Some club members have observed true airspeeds of approximately 145 knots at less than 11 gallons per hour while running lean of peak as described below.
It is the club standard to always operate the N245SR engine lean of peak during cruise.
Some members of the Cirrus Owners and Pilots Association (COPA) have reported engine lifetimes well beyond TBO using the techniques outlined in this article, and stories abound of mechanics being shocked at the great condition of engines that are run lean of peak.
This article is based on the work of longtime COPA member Gordon Feingold, who wrote a detailed article about lean of peak operations. Portions of his article included below.
Quick and Easy Method
If you want a quick and easy way to ensure the engine is running in a safe manner, without excessive cylinder temperatures or pressures, do the following once you level off from your climb:
- Reduce the throttle to 2500 RPM.
- Lean the mixture over the course of 3 to 5 seconds to 8.8 GPH.
- If engine roughness occurs, nudge the mixture forward until the engine runs smoothly again. It should be no more than 1 or 2 tenths of a gallon per hour fuel flow.
That's it! You'll be running lean of peak with the engine in a safe place.
The Red Fin
Before diving into the procedure, it's important to understand the "red fin" diagram.
The area shown in red is where the internal cylinder pressures and temperatures are the highest, and most destructive to the engine. We want to avoid operating in this area as much as possible.
The scale on the left shows degrees rick of peak EGT (positive numbers) and degrees lean of peak (negative numbers). This will be important later as you read through this article.
The Red Box
The data shown above can also be represented in table form. This table will be used during the leaning procedure.
See the checklist in the attached PDF file. Descriptions of critical portions of the checklist are described below.
Here is an image of the checklist. Click to enlarge:
The first thing you'll do is set your power. If you use the Density Altitude readout on the MFD engine page, it will be easy to adjust for cold or hot weather that may otherwise impact the engine's performance and your ability to use this procedure.
If you are at an altitude between those in the checklist, interpolate to the nearest 25 RPM. For example if I'm flying at 3000' density altitude I'd use 2550 RPM. At 3500' I'd use 2575 RPM.
BIG MIXTURE PULL
The so-called “Big Mixture Pull” is a means of setting the mixture control to “park” your engine in a safe place. This will place your engine well lean of peak by moving rapidly “through the mountain” of peak temperatures and pressures to a safe place on the lean side.
To perform the Big Mixture Pull, pull back on the mixture control smoothly and fairly rapidly until you feel the airplane decelerate. Stop! You’re there. The pull should take about 5 seconds or so. Do NOT enrichen a little when you feel the deceleration.
Any time you want to park your engine in a known safe place, such as before finding peak EGT or for level-offs during climb or descent, just perform a Big Mixture Pull.
FINDING PEAK EGT
Since you’ve just done a Big Mixture Pull after leveling off, you’re done with maintaining takeoff EGTs for your climb, so switch back to the MFD’s engine page and press the Absolute button. Give the engine a minute or two until the EGT’s stabilize.
Now we’ll use the eMax’s “Lean Find” capability to find peak EGT, but we’re doing it from the lean side of peak rather than the rich side. This is better for two reasons. First, the power curves are steeper on the lean side of peak, so you’ll be spending less time in the “red box,” and second you are looking for the first cylinder to peak rather than the last, also resulting in less time spent in the red box.
Press the “Lean Assist” button and begin slowly enriching the mixture. Watch for the first cylinder to peak (it will turn blue on the screen).
When I say “first cylinder, I do not mean cylinder #1 – I mean whichever cylinder reaches peak first! You’ll likely find it will be the same one every time.
When that first cylinder peaks, immediately press the Normalize button to “zero” that cylinder at its peak EGT. Then pull the mixture back lean again to a “safe” place well lean of peak.
SETTING MIXTURE USING THE RED BOX TABLE
(See the The Red Box section, above, for the red box referenced in this section.)
So to set your mixture so as to respect the red box limits, you first need to know, at any point, how much power you are producing. You can then look at the table, find the row for that percentage of power, and make sure you are at or outside of the limits shown for that amount of power.
For example, if I’m developing 70% power, I can look at the table and see that for that power setting I need to be 25 degrees lean of peak or leaner (i.e. -25 degrees or more negative).
It shows me that at 70% power the area between 125 ROP and 25 LOP is to be avoided – that’s the red box at 70% power as shown in the Red Fin diagram, above.
So now that you know how to get the EGT limits from the table, you need one piece of information to set your mixture control: “How much power am I developing?”
Lean of peak, the answer is surprisingly easy, because when lean of peak, power is directly proportional to fuel flow. It turns out that you can simply multiply your fuel flow in GPH by 15 to obtain horsepower. Dividing your horsepower by 200 (for the SR20) yields percent power.
To make this easy, the red box table does this for you. Find your fuel flow in the second column and look over to the first column to find your percent power (interpolating if necessary). So the table tells me that if I am lean of peak and my fuel flow is 9.4 GPH I am developing 70% power. You can use the “red fin” graph’s horizontal axis for this purpose as well. Note that the eMax display of percent power may be way off when LOP! IGNORE IT! Use the red box table (or that factor of 15 and a whiz wheel), not the eMax display, when LOP.
So how do you use all this knowledge to set your mixture in flight? Easy. Here’s an example.
Assume you’ve leveled off, done the big mixture pull, found peak EGT and normalized there, and have leaned again back to the safe region. You will use the normalized reading from the first cylinder that peaked for this procedure. Ignore the others. Let’s call this cylinder the “key cylinder.”
Look at your fuel flow and find where it is in the table. So let’s say your fuel flow is 9.4 GPH. You can quickly see from the table that you’re developing 70% power. Now look on your engine page and see where your normalized key cylinder is relative to peak EGT. Let’s say it is showing -50. The red box table says you must be at 25 degrees LOP or leaner at 70% power (in other words, ANY setting at or leaner--that is, more negative--than peak EGT is OK).
Well, you are! You are out of the red box on the lean side by 25 degrees (-50 is less than -25). So if you want to develop more power but still stay out of the red box, you can.
So you can enrichen the mixture, let’s say to 10.1 GPH. A glance at the table says you’re now developing 75% power. Say you look at your normalized key cylinder and see it now shows –40. You look at the table and you see you’re done. Your -40 degree LOP setting is right at the red box limit for 75% power.
If that key cylinder showed -45 at 75% power, you could even be a bit richer if you wanted to max out your LOP power. But if you went to, say, 10.1 GPH and the key cylinder showed, say, -10, ouch! You’re inside the red box, because at 10.1 GPH = 75% power you must be at -40 degrees LOP or leaner and you’re not!