Originally Posted by
Dan64456
I understand believe it or not... Mainly concerned with IAS in this case, because of the overspeed situation mentioned above... So I guess I have to constantly 'reset' (set to .80, and back to .81 again) when I'm in a climb or descent since it doesn't seem to 'update' the bug on the speed tape.
Heres another question - you said it depends on altitude. Well FL350 is not always equal to true altitude... Or does it cancel this out because of the pressure involved is the same?
When do you switch to IAS to Mach? I heard that the magic number is FL260, but what determines this? (I know that for compressibility reasons you make the switch when at high altitude, but how do you know when?)
Talking only about the switchover from mach to indicated airspeed; it depends on the climb and descent speed schedule you are flying. On the 747-400 at heavy weights you may plan on 350 kias until Mach .85, and your switchover would be around FL 270. If you plan a speed schedule of 270 kias until Mach .83, your switchover will be FL 370.
Joe