Mach number characteristics

[akafrank07]

"Mach number is used as a speed reference at high altitudes, usually above 26000 ft, because the MN becomes the aircraft's limiting speed in preference to IAS. That is, up to approximately 26000 ft, an aircraft will climb at a constant IAS against an increasing MN, where the MN speed reaches the aircraft's MN limiting speed. Then above 26000 ft the aircraft is flown at a constant MN with a decreasing IAS for an increase in altitude."

From the above i take it that the mach number limiting speed is the mach number speed that you reach when you change from IAS to MN?

I would i also to right in saying when you are climbing at a constant IAS to 26000 ft your TAS would always have to be increasing due to ram air affects and also the density becoming less meaning you would have to fly faster to keep the indicated airspeed constant?

Cheers

[bravobravo74]

Not necessarily. It would seem to me that when changing from IAS to Mach in the climb you would typically be below MMO (the maximum operating Mach number) and the cruise Mach number which would itself be equal to or below MMO depending on the FMC cost index.

You're correct - climbing at a constant IAS yields an increasing TAS due to a decreasing air density. It's not so much that you 'have to fly faster' in the sense that there's no extra effort; the aircraft accelerates in relation to the air mass under whilst the drag remains constant.

[selfin]

the aircraft accelerates in relation to the air mass under whilst the drag remains constant

After Mcrit there will be an additional drag component associated with shockwaves. Since all cruise Mach numbers are above Mcrit this wave drag will be encountered at some point during the latter portion of the climb. For something like the 737NG, if memory serves, Mdiv is within the 0.6-0.7 Mach range.

Mcrit is by no means limiting for transonic aircraft.

...TAS would always have to be increasing...

It will always be monotonically increasing except when a discontinuity in temperature with altitude is encountered. You'd need roughly a 30 K temperature difference per 1000 m to observe that exception therefore it will be quite rare.

The pressure altitude at which a given CAS corresponds to a given Mach number, called the crossover altitude, does not vary in the presence of a temperature deviation from ISA.

Here's a post from Old Smokey which might interest you. http://www.pprune.org/tech-log/225989-what-737s-critical-mach-no.html