Cessna 172 Engine Leaning
Intro
Correct engine leaning procedures are often overlooked in primary training as pilots are either scared by misconceptions or think it’s a fiddly distraction during short lessons when they’re trying to get in a bunch of maneuvers for a checkride. Even if you didn’t like chemistry and shudder when you hear the word “stoichiometry,” there are a few practical reasons to lean your engine:
- Maintenance and reliability: if you run full rich, the excess fuel forms carbon and lead deposits on the spark plugs and valves, leading to bad mag checks (especially on the lower plugs) and hot spots where the carbon glows. This means more time in maintenance and less time flying (or more airplanes to cover the schedule), resulting in higher costs for the flight school that get passed on to the student.
- Engine longevity: carbon and lead deposits break off and get into the rest of the engine, which means more wear. Happy engines run lean of peak and stay cleaner, which lengthens their life. More cost savings.
- Fuel costs: multiply your local price of avgas by 12 GPH, then multiply by 8 GPH and compare the difference. That’s what you save by leaning vs flying around full rich in a Cessna 172. If you have a “wet rental,” compare the avgas price x 12 to the overall rental rate: it’s probably a pretty big fraction. If everyone at the flight school leans correctly, chances are the rates might come down (or at least not rise as fast!)
Overview of Leaning
Simplified overview of EGT, CHT, and efficiency.
The Wikipedia page on stoichiometry is a pretty good primer for the basic idea at a molecular level, and Mike Busch’s video on leaning is a must see. To summarize, leaning the engine out from full rich leads to increasing exhaust gas temperatures (EGT) as more of the fuel combusts, until you reach peak EGT, at which you have stoichiometric combustion (correct proportions of fuel and air). As you get lean of peak, the EGT begins to drop again as you have excess air going through the engine. There are two points worth noting on the chart: best power mixture and best efficiency: best power is slightly rich of peak. One reason for this is that the mixing is imperfect in the cylinder, so to make sure you burn all the O2, a bit of extra fuel increases the likelihood that all of it gets combusted. Second, richer mixture will lead to a faster flame front propagation, and in turn more torque per stroke. Because the fuel is denser in mass than the air, paying a small volumetric penalty for added fuel helps get close to 100% O2 burn. If, on the other hand, you’re looking for best efficiency, then being a tad lean of peak gets extra O2 molecules that reduce the rich spots in the cylinder and combust close to 100% of that expensive fuel (the air is much cheaper) for best BSFC. Moreover, LOP also helps leave a bit of extra O2 to burn off any carbon deposits from the times where you need to run full rich. Peak cylinder head temperatures occur just ROP, so you want to avoid that.
The dreaded “red box” is the zone where your engine is at high power and a leaned-out burn would exceed CHT limits. Takeoffs and go-arounds are such instances, which is why we use fuel as a coolant during those phases. As soon as you’re at a low enough power setting, you should pull the engine back to slightly LOP for best efficiency.
The Cessna 172 manual 7-34 shows a CHT redline of 500°F, while the Lycoming manual 3-6 tells us “For maximum service life, cylinder head temperatures should be maintained below 435°F (224°C) during high performance cruise operation and below 400°F (205°C) for economy cruise powers.” Mike Busch recommends 400 as a practical limit, so let’s plan for that. Whatever else you do, keep your CHT below 400.
Cessna’s POH page 4-37 and the cruise table in 5-20 use a 50 ROP mixture as a best power setting for their numbers:
The dreaded “red box” is the zone where your engine is at high power and a leaned-out burn would exceed CHT limits. Takeoffs and go-arounds are such instances, which is why we use fuel as a coolant during those phases. As soon as you’re at a low enough power setting, you should pull the engine back to slightly LOP for best efficiency.
The Cessna 172 manual 7-34 shows a CHT redline of 500°F, while the Lycoming manual 3-6 tells us “For maximum service life, cylinder head temperatures should be maintained below 435°F (224°C) during high performance cruise operation and below 400°F (205°C) for economy cruise powers.” Mike Busch recommends 400 as a practical limit, so let’s plan for that. Whatever else you do, keep your CHT below 400.
Cessna’s POH page 4-37 and the cruise table in 5-20 use a 50 ROP mixture as a best power setting for their numbers:
Cessna's POH recommends 50 ROP, which puts us near peak CHT.
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Lycoming's POH suggests a best economy at around 30 LOP.
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That puts us right in the red box, which is where we don’t want to be. Also, there are only rare situations where we want best power in a 172. If we wanted to go fast, we wouldn’t be in a 172 in the first place. Between the 500 CHT and the 50 ROP in the Cessna manual, I’m inclined to take what they say with a generous helping of salt.
Cessna’s POH page 4-37 claims that “Operation at peak EGT provides the best fuel economy. This results in approximately 4% greater range than shown in this POH accompanied by approximately a 3 knot decrease in speed.” The Lycoming manual 3-6 tells us that for “Best Economy Cruise (approximately 75% power and below) – Operate at peak EGT.” On the very next page, they give us a chart that seems to contradict that (see above, right).
Looking at the lowest BSFC value and tracing a line up to the EGT curve, then tracing across, gives us a best economy mixture about 30 LOP. Either Cessna and Lycoming have just given us a simplified guide because they don’t want to go in the weeds, or they’re actually right for some reason. The only good reason I could think of is that the spark timing was tuned to the fixed blade pitch on the 172, and that pulling the mix LOP would result in too much delay of the flame propagation. In any case, as long as the engine’s not running rough, being slightly LOP won’t harm the engine.
Cessna’s POH page 4-37 claims that “Operation at peak EGT provides the best fuel economy. This results in approximately 4% greater range than shown in this POH accompanied by approximately a 3 knot decrease in speed.” The Lycoming manual 3-6 tells us that for “Best Economy Cruise (approximately 75% power and below) – Operate at peak EGT.” On the very next page, they give us a chart that seems to contradict that (see above, right).
Looking at the lowest BSFC value and tracing a line up to the EGT curve, then tracing across, gives us a best economy mixture about 30 LOP. Either Cessna and Lycoming have just given us a simplified guide because they don’t want to go in the weeds, or they’re actually right for some reason. The only good reason I could think of is that the spark timing was tuned to the fixed blade pitch on the 172, and that pulling the mix LOP would result in too much delay of the flame propagation. In any case, as long as the engine’s not running rough, being slightly LOP won’t harm the engine.
How To Do It
Zones on the throttle/mixture curves for certain phases of flight.
Ground Leaning
On the ground after startup, you can pull the plunger out about an inch (3/4 on cold days, 1.25 on warm), then twist until it sounds rough. Enrich just half a twist past that to keep it “rich of dead.” This has two advantages beyond the generally applicable ones for leaning:
In the climb, you will be at a high enough power setting that you’d be in the red box if you leaned close to peak EGT, so you need to use fuel as a coolant. How much? Whatever you do, keep out of the red box. The POH says to lean above 3000 feet as general guidance. To get more specific, you can start twisting the knob back cautiously as long as you keep those CHTs <400. For short climbs on training flights, the juice might not be worth the squeeze, but for long climbs you should work on it. If you took off leaned for best power, you might want to enrichen the mixture slightly to keep CHTs cool once you're out of 1000 AGL. Again, let CHT guide you.
Cruise Leaning
One challenge that I’ve seen students struggle with is the delay in the response of EGT to leaning inputs when trying to follow the POH guidance. There’s a physical probe with thermal inertia in the exhaust ports that needs to warm up or cool down as exhaust goes past it. The same way it takes a minute to figure out if you have a fever using an under-tongue thermometer, the EGT probes often lag 10-20 seconds. Where people get frustrated is watching the EGT seesaw around peak, not knowing which side they’re on, because they’ve been adjusting it and are not sure where the lag values have put them. Mike Busch, in his Advanced Leaning video, recommends against chasing peak EGT in this manner because it involves spending more time in the red box, or the “red fin,” a zone of power and mixture setting that result in CHT >400. Instead, he recommends George Braly’s “big mixture pull” to transit that zone quickly. Unfortunately, the Cessna 172 POH only has fuel flow numbers for 50 ROP, so you need to use a mixture of BMP and EGT.
The way I like to do that in the C172 is to gross tune with fuel flow, then fine tune with EGT. If you’ve got a G1000, bring up the leaning page on the MFD. On a steam-gauge, look for fuel flow and EGT. Briskly twist (avoid using the plunger in flight, if it’s bumpy you may inadvertently kill the engine) the knob back to a number that’s about a quarter to half a GPH below the book setting for your RPM. If the engine starts running rough, twist it a turn back in to get the roughness to go away quickly. Pause for 20 seconds to let the EGT probes catch up. Twist a half turn in and see if the EGT rises or falls. If it rises, you’re LOP, so give it another quarter turn at a time until you hit peak EGT, then back it off to about 30 LOP. If your first twist in after the fuel flow adjustment gets you a drop in EGT, that means you’re ROP, so lean it in half-turn increments to get to peak and then 30 LOP. As you get better at it, you’ll recognize the EGT peak more quickly and will be able to lean it under a minute.
If you’re using 65% power or less and are in a rush and won’t be cruising very long (e.g. short instrument flight or traffic pattern), you can gross-tune with fuel flow and call it “good enough” for now. You should be outside the red fin from a power standpoint, so you can afford to be a bit off the optimum mixture. “Good enough” is way better than leaving it full rich.
Descents
As you throttle back, you let less air into the cylinders, which means that the combustion pressures, and in turn temperatures, will come down as well. This cooler combustion environment will result in more deposits if you run too rich, so plan to keep it lean for as long as you think reasonable. Twist the mix in just enough to keep the engine happy on the descent. Mike Busch recommends keeping the mixtures lean all the way through touchdown, and grabs all the knobs in a go-around (it helps that his twin has all 6 in a little quadrant that you can easily advance with one hand), which is the best answer from an engine health standpoint. If you or your flight school think that students might miss the mixture knob in the heat of battle on a go around, then a reasonable compromise could be to have “full rich” in your before-landing checklist and accept the suboptimal engine conditions as a price for safety. As soon as you taxi clear, re-lean the engine to rich of dead to burn off all those deposits you just created as you taxi in.
On the ground after startup, you can pull the plunger out about an inch (3/4 on cold days, 1.25 on warm), then twist until it sounds rough. Enrich just half a twist past that to keep it “rich of dead.” This has two advantages beyond the generally applicable ones for leaning:
- Faster oil warm-up: by burning LOP, you’re going to get higher temperatures than full rich, so your engine internals and oil will get into the green earlier.
- Built-in throttle limit: some students like to give the throttle a big input for breakaway thrust. If the engine is this lean, it will cough a bit and serve as a reminder to ease up a bit. Likewise, if you re-lean to rich-of-dead after your runup (e.g. if there’s a long conga line) and somehow forget to go rich for takeoff, it will cough on the takeoff roll to draw your attention to it. That’s a much better way to catch that than to have a slight power drop because you didn’t lean it enough and you’re making partial power. If it is a hot and high day and you need best power, lean for that (POH 4-30) and watch your CHTs on climbout.
In the climb, you will be at a high enough power setting that you’d be in the red box if you leaned close to peak EGT, so you need to use fuel as a coolant. How much? Whatever you do, keep out of the red box. The POH says to lean above 3000 feet as general guidance. To get more specific, you can start twisting the knob back cautiously as long as you keep those CHTs <400. For short climbs on training flights, the juice might not be worth the squeeze, but for long climbs you should work on it. If you took off leaned for best power, you might want to enrichen the mixture slightly to keep CHTs cool once you're out of 1000 AGL. Again, let CHT guide you.
Cruise Leaning
One challenge that I’ve seen students struggle with is the delay in the response of EGT to leaning inputs when trying to follow the POH guidance. There’s a physical probe with thermal inertia in the exhaust ports that needs to warm up or cool down as exhaust goes past it. The same way it takes a minute to figure out if you have a fever using an under-tongue thermometer, the EGT probes often lag 10-20 seconds. Where people get frustrated is watching the EGT seesaw around peak, not knowing which side they’re on, because they’ve been adjusting it and are not sure where the lag values have put them. Mike Busch, in his Advanced Leaning video, recommends against chasing peak EGT in this manner because it involves spending more time in the red box, or the “red fin,” a zone of power and mixture setting that result in CHT >400. Instead, he recommends George Braly’s “big mixture pull” to transit that zone quickly. Unfortunately, the Cessna 172 POH only has fuel flow numbers for 50 ROP, so you need to use a mixture of BMP and EGT.
The way I like to do that in the C172 is to gross tune with fuel flow, then fine tune with EGT. If you’ve got a G1000, bring up the leaning page on the MFD. On a steam-gauge, look for fuel flow and EGT. Briskly twist (avoid using the plunger in flight, if it’s bumpy you may inadvertently kill the engine) the knob back to a number that’s about a quarter to half a GPH below the book setting for your RPM. If the engine starts running rough, twist it a turn back in to get the roughness to go away quickly. Pause for 20 seconds to let the EGT probes catch up. Twist a half turn in and see if the EGT rises or falls. If it rises, you’re LOP, so give it another quarter turn at a time until you hit peak EGT, then back it off to about 30 LOP. If your first twist in after the fuel flow adjustment gets you a drop in EGT, that means you’re ROP, so lean it in half-turn increments to get to peak and then 30 LOP. As you get better at it, you’ll recognize the EGT peak more quickly and will be able to lean it under a minute.
If you’re using 65% power or less and are in a rush and won’t be cruising very long (e.g. short instrument flight or traffic pattern), you can gross-tune with fuel flow and call it “good enough” for now. You should be outside the red fin from a power standpoint, so you can afford to be a bit off the optimum mixture. “Good enough” is way better than leaving it full rich.
Descents
As you throttle back, you let less air into the cylinders, which means that the combustion pressures, and in turn temperatures, will come down as well. This cooler combustion environment will result in more deposits if you run too rich, so plan to keep it lean for as long as you think reasonable. Twist the mix in just enough to keep the engine happy on the descent. Mike Busch recommends keeping the mixtures lean all the way through touchdown, and grabs all the knobs in a go-around (it helps that his twin has all 6 in a little quadrant that you can easily advance with one hand), which is the best answer from an engine health standpoint. If you or your flight school think that students might miss the mixture knob in the heat of battle on a go around, then a reasonable compromise could be to have “full rich” in your before-landing checklist and accept the suboptimal engine conditions as a price for safety. As soon as you taxi clear, re-lean the engine to rich of dead to burn off all those deposits you just created as you taxi in.
Conclusion
Leaning an engine is not that hard or scary, it just takes a bit of practice. Keeping your CHT below 400 and your engine lean with the gross-tune/fine-tune technique will save you a substantial amount of fuel money and maintenance downtime.