Leaning for high altitude takeoff...

00Dog

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There has been a big debate at work (after a crash at another company) on how to lean an engine for a high altitude takeoff (above 5000'). Do you lean based on the field elevation or the density altitude at the time of takeoff?
 

Avi8tor2000

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First of all, check the specific aircraft's POH. Some makes and models require that the mixture be full rich for takeoffs with density altitudes below 3000'; others 5000'.

A good way to do it for high elevation/density altitudes is in the run up area, put the mixture full rich (as it should have been leaned for taxi), apply full throttle, and slowly lean the mixture until you achieve peak RPM. Leave it at this setting.

NOTE: this procedure should not be used for leaning the mixture at cruise. This allows for maximum power which is needed for takeoff but continued operation will run the engine too hot. At cruise, lean for peak RPM then enrichen the mixture a half inch or so (or, again, as recommended by the POH).
 

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Avi8tor2000 said:
A good way to do it for high elevation/density altitudes is in the run up area, put the mixture full rich (as it should have been leaned for taxi), apply full throttle, and slowly lean the mixture until you achieve peak RPM. Leave it at this setting.
Now it has been a very long time since I flew a 172 at 5000' field elevation BUT if I remember right, you leaned it for peak RPM and then turned it 2 turns (I think is was 2) in (Rich). If you didn't richen from peak you were running on the ragged edge of being lean and could develop detonation and other power robbing symptoms.... Not a good thing on take-off (or in any other regime of flight for that matter!)
 

bobbysamd

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Leaning for takeoff

I remember the same method. Full power, slowly lean for peak, and enrichen it enough to assure smooth operation.

Lean for taxi, too, to prevent plug fouling.

My instructor taught me that at shutdown you should bring the power up, I don't remember to which RPM, lean the mixture for a few moments, and then shut down the engine. This method supposedly burns the carbon off the plugs. I had other instructors and people tell me just to kill the engine with mixture and not worry about carbon.

In flight, lean to peak EGT if you have it and richen the engine enough to assure smooth operation. If you don't have an EGT, lean until just when the engine runs rough and richen for smooth operation. Of course, POH recommendations trump any of these suggestions or other old wives' tales you hear about leaning.
 
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Vik

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I'm not as experienced as some of you, but at my school we do not do what the Cessna 152, 172, 172RG manuals want for leaning during cruise.

Cessna wants you to lean to peak and lean two turns. They wrote this because they are in the business of selling aircraft and so lower fuel consumption may be advertised.

We always lean to peak EGT or peak RPM for fixed pitch props and then enrich two turns or 50 degrees on the EGT.
 

Dan CFI/CFII

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The guys above had it right on.

I am based at an airport that hasn't seen a DALT less than 9,000 feet for months...

Run the engine up with the mixture rich (some people do full power, I just do it at the runup RPM--it'll be pretty close). Then slowly lean the mixture to the peak. This would be the peak RPM, or the peak EGT (they happen at the same time--but the EGT lags a bit). Once it is there, enrichen the mixture just a bit to give you a cushion against detonation.

If you go from a high airport to a low, you will need to enrichent he mixture on descent--I use a turn per thousand feet in Cessnas, and I TLAR it on the way down (That Looks About Right).

Dan
 

Timebuilder

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I wouldn't try a takeoff at peak rpm or EGT, as the exhaust valves are being slowly destroyed, and you don't have full power. As mentioned above, risk of detonation is HIGH.

I alwas lean for the taxi, as I will have less "cleanup" to do at the runup area.

No leaning before shutdown, as I will clean the carbon during the "leaned" taxi after the next startup.

I'm glad you asked the question. You just may have increased your lifespan. :)
 

BigFlyr

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Re: Leaning for takeoff

bobbysamd said:
I remember the same method. Full power, slowly lean for peak, and enrichen it enough to assure smooth operation.
Bobby... good technique for cruise but not for takeoff. On takeoff from anywhere, by the way, to get max power lean for max RPM, not max EGT, during a full power runup. This will not harm the engine since as you gian altitude this setting will become richer in the realatively less dense air as you climb. Besides that, your average piston popper out there doesn't even have an EGT guage since it is not required.
 

Timebuilder

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My only concern there would be the fact that in a high density altitude environment, that climb would be a very slow climb, and it would be some time until that relative "richening" would happen.

I'd still add a little more fuel after finding peak RPM.
 
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BigFlyr

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Yeah sure... but you want to avoid richening that would cause and RPM drop (less power) unless you have power to spare. Also, keep in mind that once you're at a safe altitude, say 1000 ft AGL you'll want to power back a bit anyway. The best advise would be to read the manual for your particular airplane or better yet, your particular engine manual and do what it says instead of guessing... Hmmm was it peak EGT or peak RPM then richen to 75 degrees rich of peak? Which peak EGT or RPM and which temp, EGT or CHT? :confused:
 
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