"Peter Duniho" wrote in message ...
"Robert Moore" wrote in message
. 6...
(Malcolm Teas) wrote

Think about density altitude. At -20 C and sea level, the density alt
is just under -10000 feet according to the my trust E6B. At -40 C and
9000 feet pressure alt, the density alt is -10000 feet.


Better get a new E6B. :-)

He got the wrong numbers, but it IS true that the density altitude winds up
about the same for both calculations (around -5000'). So maybe the answer
IS something that has to do with density altitude.

I think it's more likely a limitation on the engine (to keep MP within
limits), rather than some altimeter thing. But it seems logical that
density altitude is the problem, in this particular example (that may or may
not apply to other aircraft, of course).

Pete

I suspect it is related to the maximum power output from the engine
(not MP, because that is not related to density altitude). Engine
power increases roughly 2% per 1000' decrease in density altitude. At
-5000' you will be developing 110% of the rated horsepower. That is a
nice round number, and could very well be the reason for the
limitation.