Leaning for high density altitude takeoff

[cptndavis]

Hi Folks:
A quick question for all who have input on the subject. Is it proper procedure to lean for maximum power on takeoff roll during high density altitude operations? Can leaning be accomplished during run-up? I assume it must be done during the takeoff roll since leaning at run-up power does not approximate best power. I am a little nervous about leaning my brand new airplane on the takeoff roll though, don't want to run it off the end of the runway while fiddling around with the mixture. Any suggestions on good technique??

Thanks,

Mike

[bike21]

Welcome to JC Mike!

I have flown out of Colorado for many years including all of the really fun high mountain airports. I have never leaned mixture during the takeoff roll. IMO it is a much to critical time to be leaning. Proper lean technique takes time and shouldn't be rushed.

Generally speaking, leaning in the run up area with about 75% power always does the trick. A full power lean isn't usually necessary. There are only a handful of situations where you might really need that extra bit of power. For instance, very short field, terrain and a really warm day at high altitudes.

Hope this helps!

[MidlifeFlyer]

I do it every time I fly.

Yes, you can do it during run-up, but recognize that it is an approximation. Here's the procedure I teach (this is assuming a normally aspirated piston). There are minor variations depending on the make and model:

When you are ready to do the run-up:

1. Enrichen the mixture (you should have leaned it for taxi, so you need to enrichen it for the run-up power demand). You don't really need to go back to full rich at this point, but there's no harm in doing so until you learn about where to set it)

2. Go to run-up power.

3. Lean. You will initially see a rise in RPM as you reach best power and then a drop. When you see the drop, enrichen back to peak.

4. Enrichen more. On airplanes with a mixture vernier control, 3-4 twists will do it. Without a vernier, about 1/4-1/2 tends to take care of it. The enrichment is for 2 reasons: to approximate the additional requirements for takeoff and for engine cooling.

In most airplanes, this will be exactly what you need for takeoff (an a lot of instructors around here stop here) but bear in mind that this is an =approximation= that needs to be cross-checked, at least until you have learned that the run-up technique works for your airplane.

The cross-check should be done at full power. If the brakes will hold, you can do it while still at the runup area or before beginning the takeoff roll. But I usually do that final check on the takeoff roll (and what you do depends a lot on the airplane - some will have fuel-flow targets). I have never had to move the mixture more than a 1/2 twist - takes only a second.

Finally, remember that this isn't brain surgery. Look at your POH - that instruction to lean "above 3000') is a pretty good indication of how much leeway is built in.

[Duck_Twacy]

During my week in CO last week, we leaned after start and during the run up, both of which were checklist items.

MidlifeFlyer's technique is exactly what we used for the week - three twists of the mixture knob - but on my checkride my examiner showed me one additional trick.

After setting the mixture as described above, he would fine-tune by advancing to [nearly] full throttle. If there was a miss or cough prior to developing power, he would throttle back to run-up power, lean the mixture, rinse, and repeat until the engine smoothly advanced with the throttle.

[cptndavis]

Thanks for all the great advice. My airplane will roll foward at any RPM above 2000 so I will cross check on takeoff. Also is there a rule of thumb for mixture position for engine start?? Does it matter?

I will be flying across the country this spring and I am excited about getting some experience over the mountains. I plan to do a mountain flying training course when I am in Colorado!

[bike21]

A good rule of thumb (depending on mixture control knob) is about 1" or a couple of fingers width lean after start. Just back far enough where it certainly isn't running anywhere near full rich.

Have fun on your big trip! Flying in the mountains out here is tons of fun!

[joness0154]

John Deakin writes a great column on avweb called Pelican's Perch. There's some great information on engines there, including how to lean for taxi, runup, takeoff, climb, cruise, etc.

Heres the one for taxi/takeoff/climb. If you aren't a member its free to sign up.

http://www.avweb.com/news/pelican/182179-1.html

I suggest everyone read his articles on leaning, how throttle/mixture/prop work, etc. I knew the basics but learned alot more after reading his articles.

[cptndavis]

Excellent advice from Deakin. His articles are first rate. I am going to take his course as soon as I can commit the time.

http://www.advancedpilot.com/

[aloft]

The Deakin articles are good, so is Sparky Imeson's coverage on the subject in his Mountain Flying Bible.

Cessna even says that a full-throttle static runup for leaning is prudent if conditions warrant.