Leaning on the ground....problems?

[Bear Cub]

Another one in the season of Bear Cub's "problems" postings.

Colleague of mine just e-mailed to ask if "leaning the mixture ON THE GROUND was a good or a bad thing" (or both - at different times)

Anybody want to open the debate?

I remember seeing pilots lean the mixture on some twins when taxying any distance - especially when I was out in Florida.

Their explanation was that, certainly on the PA44 (Seminole), in the Florida weather the engineers set the mixtures above 15:1 rather than the recommended 12:1. This kept the thing running when the power was at ground idle but almost guaranteed to foul the plugs and cause a "bad drop" during the mag checks, if a long taxy or departure delay was encountered.

I do remember seeing bad "mag drops" which went away when "burned off".

I also have a pilot friend who lives in Calgary, Canada -he told me a long time ago that he had to lean the mixture for take-off because of the high altitude he was at.

Thinking about it, I remember an air taxi pilot at Manchester leaning the mixture for the taxy in a PA31.

Comments...for or against?

Would especially appreciate if anybody would admit to being a PPL or Commercial flight test examiner - and on which side of the Atlantic (or Pacific).

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[Squawk 8888]

Leaning on the ground can be useful if your density altitude is high, especially if you're using 100LL in an engine that was designed for 80/87. Using the higher octane fuel can cause excessive fouling of the spark plugs and leaning definitely reduces it.

[Tip tank]

I too understand the merits of leaning whilst manouvering on the ground, but would be concerned about actually taking off with the mixture leaned out. It might lead to a "lean cut" at higher power settings if such terminology exists.

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...was that TO or FROM?

[AC-DC]

If you lean very aggressively you even won’t be able to taxi unless mixture is enriched. However, Lyco. does not recommend this method and says not to do so.

[Grandad Flyer]

I believe it is correct to lean slightly on the ground and whilst taxiing, and then go full rich for power checks and take off. Unless you are at a high altitude airport when leaning is absolutely essential. If you try flying full rich at a high altitude airport you may end up gliding to your destination. Been there, done that, not much fun. (Wasn't my fault, I was being checked out, by someone who claimed to be an instructor).
I have also experienced problems with plugs fouling up and worse, engineers reckoned all caused by people not leaning the mixture.
I hold FAA and CAA ATPLs and no-one has ever told me not to do it, including on flight tests.