Boeing 717 FMS question

[hikoushi]

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.

[aviatorhi]

Paraphrasing here, but I recall seeing the following in an MD-11 manual...

For low cruise altitudes, cruise speed may be displayed in MACH on the FMA even though there aircraft is in the normal IAS range.

Sounds like a programming "thing".

[hikoushi]

Thought so. Seems like it chooses a cruise Mach for the FMS ECON speed based on cost index (let's say for example, arbitrarily, .612 Mach at 17,000 feet) and a climb IAS that typically crosses over to the cruise Mach a couple of thousand feet below the cruise flight level (let's say ECON climb would be something like 294KIAS / .612 on the PERF page).

So, climbing through 10,000 it accelerates from 250 to 294 knots, maintains 294 knots until around 15,000 feet where Mach crossover to the selected ECON Mach number occurs. 294 PITCH changes to .612 PITCH on the FMA, and the airplane zooms up slightly to maintain the Mach number, slowing to around let's say 280-285 knots in the couple of thousand feet prior to levelling at 17,000 feet where it enters cruise flight with the FMA showing .612 THRUST.

Makes sense why it would be programmed that way up at high altitude where Vmo exceeds Mmo (normal Mach crossover technique), but it just doesn't make sense to my dense mind WHY they would have chosen to program it that way regardless of cruise level. Sometimes it seems like it can give an uncomfortable ride in those last couple of thousand feet, especially if there happens to be an increasing headwind in that altitude band, whereupon the plane starts skyrocketing up to chase the ever-decreasing airspeed target and then immediately pushes over to level off. Usually just level it in VS if it starts doing that.

Sounds like lazy programming to me, but hey, I'm not that smart!:ok: