leaning mixture prior to takeoff

C

clestudentpilot

Well-Known Member
I hope I'm posting this in the correct section. I'm just wondering if any of you guys do this. I fly out of a low elevation airport (about 1200') and my instructor has me lean the mixture in a 172 during the runup and keep it leaned like that even for takeoff. The POH I have only says to lean in at airports above 3000'. He has me lean it until the RPM's peak and start to drop, and then enrich it until it is back to the peak value. I do what he says, he is the instructor and the PIC, but it isn't in the POH so it concerns me a little. He told me that some POH's for 172's do say to lean it for maximum RPM, the copy that I have just doesn't. What do you guys think of this? I've asked some other pilots and they say they've never done this, but it may be correct that some models call for leaning to peak RPM, they just don't know.
 
There is nothing wrong with doing this below 3000'.
most people don't do it though because it really is not necessary near sea level.

It is critical though when you are above 3000' DA because then a full rich setting will actually be too rich and you are flooding the engine thus reducing maximum power available and risking an engine quiting or sputtering on you during takeoff.
 
I am under the impression that take-offs should be done (low altitude) at full rich in order to provide proper cooling for the engine. They are set up to run slightly rich on purpose at take-off power in order to provide cooling via the excess fuel taking heat away, as well as slowing the flame propogation through the cylinder and preventing peak compression to occuring at too close to top dead center (i.e. lots of stress).

Here's a recent thread where we talked about it some: The Red Knob

...and here is the article linked in that thread: AvWeb Pelican's Perch #18:
Mixture Magic

I suspect that your instructor's technique will allow the engine to develop slightly more power, but won't be good for the health of the engine.
 
I never told my students to do this as it was just something else to confuse them with, as you are. When I fly my C172S during the runup I'll lean it down to the top of the green on the fuel flow and use that as my setting. Thats just me. Is it right or wrong...I've never had problems with doing it as far as engine concerns and the engine went far beyond TBO and at overhaul the engine was in fine condition.
 
At low altitude airports (below 3000ft MSL) ALWAYS take off full rich. I have never seen ANY POH that said to take off partially lean at low altitude. You need the extra fuel to aid in cooling durring take off and initial climb.

Flying our of Denver, is a different scenerio, you need to lean somewhat for take off.

Useing parital throttle durring take off and climb is always a bad idea as well. At full throttle the power enrichment valve opens making your mixture super rich. Many people throttle back for climb, which I disagree with.

OTOH, you should always TAXI with the mixture well leaned.


Fly your plane the way the POH says, if your CFI overulles you, then follow his instructions
 
Yeah we leave the throttles wide open for the entire climb for cooling. If we brought it back to 25 squared the cylinder temps would redline after a few minutes.
As far as leaning the mixture, back at humpty diddle we used to lean it at 1,000ft AGL to top of the green arc but that was them saving money. C-172s aren't really prown to heat problems. As far as I know at least.
 
Especially on hot days where engine temps are near their upper limits, always use a rich mixture for takeoff below 3000 feet. As others have said here, leaning just rich of full lean may be necessary at higher elevation airports. The unburned gas in a lean mixture cools the engine and that is why the POH specifies to takeoff at full rich below 3000 feet. I have been an A&P for 25 years and have overhauled many engines. I can easily spot those that have been run hot. The extra gas used to run a little richer mixture is well worth it in my opinion.
 
clestudentpilot said:
He has me lean it until the RPM's peak and start to drop, and then enrich it until it is back to the peak value. What do you guys think of this?
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I think it's a very bad idea. That engine has a fuel enrichment system that kicks in at full throttle for a reason (see The Red Knob thread SteveC linked to) and leaning for takeoff defeats it. You should not lean for takeoff at low altitudes.

Does your CFI own the airplane? I always operate the way the owner wants me to, unless I think it's going to kill me.
 
clestudentpilot said:
I hope I'm posting this in the correct section. I'm just wondering if any of you guys do this. I fly out of a low elevation airport (about 1200') and my instructor has me lean the mixture in a 172 during the runup and keep it leaned like that even for takeoff. The POH I have only says to lean in at airports above 3000'. He has me lean it until the RPM's peak and start to drop, and then enrich it until it is back to the peak value. I do what he says, he is the instructor and the PIC, but it isn't in the POH so it concerns me a little. He told me that some POH's for 172's do say to lean it for maximum RPM, the copy that I have just doesn't. What do you guys think of this? I've asked some other pilots and they say they've never done this, but it may be correct that some models call for leaning to peak RPM, they just don't know.
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Like everyone else has said, you should always lean it for taxiing and after your run up. However, leaning it for takeoff really depends upon the altitude of the airport - which is in your POH (yes, I know you checked). I personally have never done that because I've never taken off at an airport with a high altitude (1,200 ft is not high, I'm talking rocky mountains high). Although what your CFI is doing is questionable, I would just do what he says for now and not make it a big argument.
 
At a high-elevation airport, on a really, really hot day, you can seriously foul the spark plugs on a short taxi to the runway without leaning.

Other than that, I think that this practice falls into the same category as turning off the landing light as soon as you're airborne.
 
Yeah, but that's leaning for taxi. His CFI wants him to lean for takeoff, which is a whole 'nother ballgame.
 
USMCmech said:
Many people throttle back for climb, which I disagree with.
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I've often heard the stress of the engine is at is max when at high angles of attack, low airflow, and high power settings which is the reason for the power reduction and setting climb power.

For instance, I often fly a turbo 182, after 1000' AGL, I bring the power back from 31+" MAP to 25" and set the prop at 2400 RPM while transitioning to a cruise climb and keeping my fuel flow around 16 GPH in the climb.
 
meritflyer said:
I often fly a turbo 182, after 1000' AGL, I bring the power back from 31+" MAP to 25" and set the prop at 2400 RPM while transitioning to a cruise climb and keeping my fuel flow around 16 GPH in the climb.
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Watch your fuel flow and CHTs VERY closely when you do that next time. Throttleing back closes the enrichment valve. This is why you see a large drop in FF, and if you watch the CHTs you will see a slight rise.


If you throttle back to 25" you will be running your engine hotter, with less cooling airflow, for a longer peroid of time. OTOH, full throttle gets you up to altitude quicker, with cooler CHTs.

Airflow, not engine power is the bigest variable controlling tmeperatures. The quicker you can get full cruise, the better. Or better yet, use full power, and climb at a higher airspeed, and still get to cruise altitude just as fast.
 
USMCmech said:
Watch your fuel flow and CHTs VERY closely when you do that next time. Throttleing back closes the enrichment valve. This is why you see a large drop in FF, and if you watch the CHTs you will see a slight rise.


If you throttle back to 25" you will be running your engine hotter, with less cooling airflow, for a longer peroid of time. OTOH, full throttle gets you up to altitude quicker, with cooler CHTs.

Airflow, not engine power is the bigest variable controlling tmeperatures. The quicker you can get full cruise, the better. Or better yet, use full power, and climb at a higher airspeed, and still get to cruise altitude just as fast.
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The POH advises of climbing this way to keep it cooler. It cautions of using MCP during climbs. In the exact words of the POH -

Normal enroute climbs are performed with flaps up, at 25 inches of manifold pressure, 2400 RPM, 16 GPH fuel flow, and 90-100 KIAS for the best combination of performance, visibility, engine cooling, and passenger comfort (due to lower noise levels). However, MCP power settings may be used for increased performed as desired. This type of climb should be of minimum duration and engine temperatures should be carefully monitored due to lower airspeed
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As for airflow. I agree, this is why after 1000' AGL is reached and no MCP climb is required per a DP, the climb power is set and the nose is lowered, increasing airflow to increase the cooling.
 
Leaning to peak for takeoff sounds like it would heat the engine a little too much. I teach the lights - camera - action check before takeoff and have them keep it lean during the taxi.
 
Thanks for all the reply's guys. Berkut, the planes are owned by the flight school, and I think other instructors practice this as well, so maybe the owner of the flight school wants this done. I flew with one other guy there, and he does the same thing. It is never much of a lean due to the low altitude. I don't know if there are any technical terms I'm not using, but about 2 to 3 turns lean of full rich is what we use. I will continue doing this with the instructor, because I recognize he is the one who knows what he's doing, and I am just beginning and he is also the PIC. I guess I wanted to make sure this isn't a dangerous practice. I know it may be dangerous to the life of the engine, but I just want to make sure I'm not setting myself up to kill myself or others
 
meritflyer said:
best combination of performance, visibility, engine cooling, and passenger comfort
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Note that it says that this is the best trade off between engine cooling, and several other factors. It does not say that this is the best profile for engine cooling.

It specifically says that MCP is allowed, just that you need to watch the temps.


Try it next time you fly, I think you will be suprised.
 
The POH's I recall did not reference "MSL", but people assumed that's what it meant. My recollection is that it refers to Density Altitude, and you can easily be above 3000' density altitude at 1200 MSL, just takes about 30c or so.
 
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