Jay Honeck wrote:

You know, I've heard that since Day One of ownership, too, but ya just
gotta wonder if it's not yet another "old wive's tale", like so many of
these "tried and true" things. How does the engine know the
difference between ground runs at, say 2000 RPM (run-up speed on our
plane) and an extended descent? Is *that* "bad" for the engine, too?

I've wondered that myself. The only big difference that I can think of is that
the circulation of cooling air will be different. As far as long descents is
concerned, I've seen statements that this is harmful due to excessive cooling
and possible plug fouling. Never seen a claim that oil circulation suffers, though.

Doesn't running it for 30 minutes on the ground circulate the oil, and
prevent corrosion? Can't you get oil temps up to 150 or better (I
know, 180 is optimal, but...) with a ground run? Isn't that better
than letting it sit and rot till spring?

As several others have pointed out, circulation of the oil isn't the problem
with the cam lobes. Those are lubricated by splash from the crank. I've never
seen any claims that this differs much from splash while in flight, though.

What I've read is that you really can't get the oil temperature high enough by
running it on the ground. The claim is that ground running adds water and acids
to the oil (flying also does this) and never gets the oil hot enough to vaporize
these. Crankcase ventilation through the breather is also very poor on the
ground; as a result, vapors that do form never leave the crankcase. As another
poster suggested, some form of positive crankcase ventilation would probably
help that.

If the only real issue proves to be the buildup of pollutants in the oil from
ground running, then changing the oil after every other run would take care of
the problem; however, given a choice between this and simply flying the plane
for a few hours, I'd be in the air.

George Patterson
Coffee is only a way of stealing time that should by rights belong to
your slightly older self.