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# IAS to MACH change Alititude

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Hi All Change from IAS to Mach @ around 25500 ft. Is there a formula using assumed Amb temp. (from lapse rate) and TAS. Using an altitude switch to change and would like to get it more realistic.

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Hi,Below is a code I used for my 737; however it was for a changeover at FL260, so I changed it to 25500 ft.1. first part for the change over code is for your Autopilot.2. second part needs to have for example: (L:2550fl, bool) in the mouse click for the ISA/MACH Changeover Switch.I'm not sure if this is what you are looking for, but It works fine for me and I hope this helps.Regards,Benny(A:Indicated Altitude, feet) 25500 > if{ 1 (>L:2550fl, bool) } els{ 0 (>L:2550fl, bool) }(L:2550fl, bool) (A:Airspeed select indicated or true, knots) 100 > && if{ 1 (>L:Mach-IAS select, bool) }Click to display IAS or Mach(L:N1 switch, bool) ! (L:2550fl, bool) ! && if{ (L:Mach-IAS select, bool) ! (>L:Mach-IAS select, bool) 1 (A:Autopilot mach hold, bool) 0 (>K:AP_PANEL_MACH_OFF) 1 (A:Autopilot airspeed hold, bool) 0 (>K:AP_PANEL_SPEED_OFF) }

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Thanks for that Benny, I learned today that mach = TAS @ Amb. temp -35 c that's close to what I need to get appox altitude to change over. Paul

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Hey Paul,Just take a look at this thread. I'm curious that you are making a change IAS-MACH at a given altitude...If this is for speed window in autopilot, AFAIK it usually changes at a fixed relationship between both variables. For example, in 757 it will show Mach when IAS equals 0.40 Mach and above; going back to IAS when IAS is <= 399. Tom

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Hello Tom From what I think Happens (never used RW mach before) 737 climbs to Altitude = Amb temp -35 where mach and TAS meet using IAS and then to stop overspeeding the mach climb max using IAS, The Speed hold and speed window chages to mach. Then all speeds above are mach refs. If anyone knows how this all works please comment. Paul PS I don,t see a link in your post.

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