"xerj" wrote in message
...
[...] what I don't understand immediately is why at altitude the MP
required is only 18.2 rather than 20.9.

Anyone have a simple explanation?

Su at the higher altitude, lower air density allows the necessary air to
go through the throttle at a lower MP.

Or: at the higher altitude, lower air temperature provides greater air
density at a given MP, allowing for more power at a lower MP.

Or...

You didn't say you wanted a *correct* explanation. I have no idea if either
of the above theories are correct. Personally, I'm leaning toward the
second (the first seems a little fishy to me). Does the power setting table
you're looking at assume standard temperature at both altitudes?

Pete