Boeing 717 FMS question
Just a quick question for those who are more intelligent about computers and performance engineering, etc than myself.
Currently on the 717, never flown anything else with a high-end glass cockpit before, just older stuff. Why does the FMS, when calculating an FMS speed for cruise, switch into Mach hold (instead of IAS) even if you are cruising at 12,000 feet? Obviously you would plan a crossover altitude when climbing to higher flight levels, but it doesn't make sense down so low. Is it just a programming thing, and if so, why? At those altitudes why would Mach number be more applicable than IAS? There are no charts in the book that would allow you to manually plan a low-altitude flight at say, .54 Mach, but that's what you get.
Again, only "modern" automation I've flown outside of turboprops, so this is probably a very basic programming question. Do other aircraft's systems do this as well (Airbus, Boeing, etc)? I assume the MD11 is the same?
Thank you for your time.