For a non-turbocharged piston airplane, does cruise altitude really
matter? I made a complicated spreadsheet which produces a fairly simple
answer: Unless you climb way above your critical altitude, trip time
and fuel burn don't vary much at all. Over a 500nm trip, it only takes
about 4% longer to fly at 3000MSL instead of 7000 (approximately critical
altitude) and 7% longer if you go clear up to 13000. For my plane that's
a difference of about 7-15 minutes out of 3+ hours.

How high does your critical altitude have to be (due to turbocharging
or jet/turbine) before it really starts to matter what your cruising
altitude is?

--
Ben Jackson

http://www.ben.com/