Full rich for take off?

[Kurt Garbin]

I have a 62' -250 that I have upgraded to from a Cherokee 180. Per the transition, with instruction, I have been using basically the same rule of thumb during warm up. Lean back to stumble...slowly inriching to peak. Then 5 half turns inriching mixture. I have used this technique without any issues. After reading the poh ( again) and many an article, I continue to have a different thought of "full rich". I spoke to my mechanic, and questioned his warm up to take off pattern on his Cessna 414. We live at 7000', and he leans for warm up and taxi. As he starts his roll, he also goes to the panel with full rich on both engines.
I have never been told, or shown the actual correct process for the 250 at this altitude.
I would love to hear the process any of you use, especially anyone who lives at altitude.

Thank you!!

Kurt

[N3322G]

Kurt,

Hi from Snowmass -

Our POH for the Twin has performance charts that suggest fuel flow at various altitudes so that is a good place to start - assuming you have them in the 250 POH. Our panel also has a small chart on the panel that can help.

I'll share what we do on our fuel injected Twin, don't know if yours has a carburetor and whether that makes a difference.

We have digital fuel flow on the panel now and it is one of the gauges we check during initial take-off roll. If we don't see the right #'s we abort.

When we flatlanders fly into the mountains, as we have been doing for a lot of years now, I'm hypersensitive to mixture. Density altitude is such a significant factor, during run-up, after governor check and mag check, I'll run to max and lean/enrich until I hear max power. Unless something doesn't sound right I don't touch mixture on runway.

The 160 hp engines are cute so to get more climb power, I leave the props at full 2700 rpm on Twins until we reach the desired altitude vs near sea level operations, we pull them back at 1000' AGL.

Your operations may vary - also suggest taking http://comanchetraining.webs.com/

Hope this helps.

[LeWayne Garrison]

This is the technique I have used for years at 4700 feet. Lean to rpm drop on run up. Richen to peak rpm and then enrichen maybe half a turn for take-off. The cut-off altitude for this has varied among my different instructors but I settled on around 3000-3500 feet. Below that I use full rich or very near it. By the way my airplane is a carbureted 250.

[Kurt Garbin]

[Kurt Garbin]

[LeWayne Garrison]

I always use 2000 RPM for run up. It is fast enough to judge mag drop and adjust the mixture, but is (slightly) less likely to pick up FOD and damage the prop. Although there probably isn't enough difference in what the prop will ingest between max RPM and 2000 RPM, it allows me to adjust the mixture by using the RPM rather than judging smoothness.

[Kurt Garbin]

[LeWayne Garrison]

Kurt
If you're ever down south, I'm in Pueblo. We have 4 Comanche 250's in our hangar and although we don't claim to be experts, we have seen a lot with these airplanes. If you're around Pueblo sometime or just looking for a place to fly give me a call at 719-250-0269 and I'll try and meet you at the airport.

Good luck with your Comanche.

LeWayne

[Tom Deml]

Kurt,
I'm a flatlander with a carb 250, but fly regularly to KBJC, Rocky Mtn Metro at 5200 MSL. And I've taken a mountain flying course and spent two days in the fourteeners with an instructor. I do my runup to around 1700-1800 rpm and I lean with a bit patience until I see the RPM start to drop. I then richen mixture carefully to peak RPM, then 3 additonal turns rich.....equals 540 degrees of rich (3 half turns).
My engine has never stumbled and produced adequate power at whatever elevation using this method. I use this method whenever the density altitude is 5000 or higher (I round up from 4500 density altitude).

[Kurt Garbin]

[Kurt Garbin]

[Michael Kutner]

Kurt,

In regards to idle mixture, one thing you might just double check is to see how your mechanic adjusted the idle mixture in his plane and your plane.

Did he adjust the idle mixture at 7000 feet?

It is my belief that the mixture adjustment should be made at sea level. But my guess is that there is a chance that he adjusted his idle mixture and your idle mixture at the airport elevation of 7000 feet.

The biggest dis-advantage if the idle mixture is adjusted to 7000 feet would be that the engine would be very difficult to start at sea level because it would be too lean to start. Conversely, the mixture should never be too rich for take off. But I would double check with your mechanic to see if this is correct.

Mike