Warm weather runup

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Flying_Goat

Filing Flight Plan
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Flying_Goat
I was doing a runup yesterday with OAT around 100 (maybe a bit higher on the runup pad). I had a rough running left mag check but still had rising EGTs. Carb heat barely deflected RPM by about 50 RPM.

My thought is that outside air temp is resulting in richer mixture. This is producing a too-rich mixture in the cylinder causing rough running left mag maybe as a result of fouled plug. And the carb heat, already running rich due to heat, is not getting additional RPM drop when I apply carb heat.

How am I wrong in my thinking? Should OAT have much bearing on mag and carb checks? Should I be doing warm weather runup with a leaned mixture? Field is more or less sea level.
 
Check the density altitude. Humidity plays a role, too. Yes on leaning.
 
Clip4 said:
Was it windy and did you do a crosswind runup?
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Curious… I’ve never paid much attention to the direction the plane is pointed during any run up, regardless of the OAT and wind direction.

Does it really make any difference, or just another OWT that could be retired?
 
I always try to point the plane in the direction of the wind when doing a run-up...
 
Flying_Goat said:
My thought is that outside air temp is resulting in richer mixture.
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Do you aggressively lean on the ground? Then on the runup, enrichen only what's appropriate for the elevation or the density altitude?

If you don't currently lean after start up, pretty much ALL of the engine gurus say you should. So at idle for your engine (1000ish max for me) you lean until you see peak. This should keep the plugs from fouling AND if you are truly *aggressively* leaned, you should not be able to obtain full Takeoff power. So this will prevent you from actually taking off and damaging your engine because you are too lean at full power.

For the runup, enrichen a git so you'll get the normal drop in RPM when you do your Mag check. Then lean again if you're in a conga line waiting for departure. When you go to Takeoff, you first do the Mixture (Full Rich), check the Prop (if applicable) and then the Throttle for power. If the first few times you forget the procedure, you should be too lean to have Takeoff power. So no damage to the engine.
 
Thanks for the the replies. Yes, I'm leaning during ground ops, but maybe not enough. I'll usually go full rich at runup but will try runup in a leaned condition for hot weather. As far as wind direction, 9 times out of 10 it's just whatever direction I'm facing at the hold-short.
 
Rick A said:
Curious… I’ve never paid much attention to the direction the plane is pointed during any run up, regardless of the OAT and wind direction.

Does it really make any difference, or just another OWT that could be retired?
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Under the right conditions it can make a big difference.
 
Rick A said:
Curious… I’ve never paid much attention to the direction the plane is pointed during any run up, regardless of the OAT and wind direction.

Does it really make any difference, or just another OWT that could be retired?
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Never read of this as an OWT, but my experience it is a product of propeller diameter. The larger the prop diameter, the larger the wind angle and velocity and higher the temperature the larger the effect.
 
Rick A said:
Does it really make any difference, or just another OWT that could be retired?
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I think it is more of a comfort and noise issue in most GA planes.

And if you want to know if you hear funny noises during the runup, so adding gusts against the prop wash may just be a fake noise that makes you think something is up.
 
Well, OK. Thanks for clearing up my curiosity:

Noise, temperature of something, prop diameter/wind angle.

Finally got it! And I always thought it was an old wive’s tale related to having the wind magically assisting the prop wash to help keep the cylinders cool on a hot day run-up (which never made any sense to me).
 
I had always gone full rich for take-off. I trained in southern California at sea level and never thought much about it. Occasionally I experience the engine experiencing what I would call a stumble when taking off from a 900’ altitude strip in Georgia. I’d abort, taxi back, do another full run-up thinking maybe some carbon build up.

Sometime after buying 8 new plugs I figured out that between the 100 degree heat and high humidity, the density altitude was high enough that the mixture was too rich and it needed to be leaned before take-off.
 
Lean on startup and while you taxi to prevent fouling the plugs.
 
Ed Haywood said:
Lycoming says not to. Are they a guru?
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Yea, they are... But I think you are reading some old documents. Lycoming used to say that, but it really was the lawyers saying it because they didn't want people taking off lean and burning up there engines. Possibly even having and engine failure in flight and dying.

I'm sure there are multiple documents, but below is a link to one that I found. In it you'll find:
A. Start-up and Ground Operations
3. Once temperatures are stable, make the mixture Lean to Best Power.

The other "gurus" that speak on this tell you to lean as much as you can so that there is no way you can produce Takeoff Power. So there is no way you'll accidentally Takeoff without going full rich (or the appropriate mixture for our DA).
https://www.lycoming.com/sites/defa...es%20for%20Flight%20Training%20Operations.pdf
 
Ed Haywood said:
I follow the AEIO-360 Operators Manual, dated June 2007.
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Did you go to the Lycoming link I gave you? Dated 2011 I believe. So yes, you are reading out of date material. (And if that SI doesn't directly relate to you, my guess is you could find one that does.)

As for the lawyers... I guess you haven't been around large corporations that much, aviation or not. :D
Being serious though since it seems you haven't been exposed to this. Legal is usually involved in ALL public facing documents or sites. And their job is to reduce their companies exposure, so they will take the least risk possible position and try to think of every what-ifs there might be.
 
M20K said:
As for the lawyers... I guess you haven't been around large corporations that much, aviation or not. :D
Being serious though since it seems you haven't been exposed to this. Legal is usually involved in ALL public facing documents or sites. And their job is to reduce their companies exposure, so they will take the least risk possible position and try to think of every what-ifs there might be.
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Unfortunately “because of the lawyers” often becomes a default answer when there are other valid reasons for the way documents are written. But when it’s blamed on the lawyers, the documentation is often ignored.
 
Yes, hotter outside air is less dense (hence high DA on hot days), making the mixture richer than it would be on a standard day. This is also why your carb heat makes less of a difference.

Throttle to checklist run up RPM, lean mixture to peak RPM, restore RPM to checklist run up RPM, then do your run up with that. You may also want to take off with the mixture leaned for max RPM as well if the surface DA is 3,000 ft or above.
 
M20K said:
Did you go to the Lycoming link I gave you? Dated 2011 I believe.
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I did. You skipped over the applicability part. One plane, one engine, one operating environment.

M20K said:
Being serious though since it seems you haven't been exposed to this. Legal is usually involved in ALL public facing documents or sites.
Click to expand...

I've been exposed to the Internet enough to recognize self-sealing groupthink, where an unprovable assumption is manufactured to address every threat to a pet theory.
 
M20K said:
Yea, they are... But I think you are reading some old documents. Lycoming used to say that, but it really was the lawyers saying it because they didn't want people taking off lean and burning up there engines. Possibly even having and engine failure in flight and dying. ...
Click to expand...
If you are leaning after startup and during taxi, lean so aggressively that advancing the throttle for takeoff will cause a stumble or die. This point in your mixture control is easy to determine beforehand. You may even want to mark that position in some way depending on your mixture control type.
 
Per Lycoming operators manual:

If the magneto drop-off exceeds the maximum limit (usually 175 RPM) or the engine runs roughly, the fuel/air mixture may be too rich.
1. Slowly lean the mixture to the peak RPM.
2. Retard the throttle to the run-up setting again and repeat the magneto check.
3. If the mag check falls within limits while leaned, the ignition system is healthy, and the issue was simply an overly rich mixture for that density altitude.
 
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