|Quoting CuriousOne (Reply 2):|
Especially the big difference on the A380 between max cruise Mach number (0.89) and max Mach number (0.96)
The A380 and M=0.96 was one test flight before certification, and it should never be repeated.
The purpose was to demonstrate that there was a sufficient safety margin well above max cruise Mach number, which could be reached in case of some incidense without destructive flutter.
I remember that the captain on that test flight told that it was a very delicate thing to do. First of all it had to be done in a rather steep dive since the engines are not designed to work very well at such high speed. And as he said: "We want to make sure that we do not get anywhere near M=0.97".
Various "maximum Mach numbers" do not tell anything about the quality of the plane. It is only something which the designers agree upon after having evaluated thousands of other compromises.
Minimum Mach number is totally irrelevant since it is totally dependent upon altitude density and weight. And of course actual high lift device configuration. Depending on the circumstances it can on the same plane in principle be anything from around M=0.20 and up to max cruise Mach number. In the real world, however, things like max certified takeoff weight, max certified altitude, or simply inability to reach max altitude because of heavy weight, will limit the latter to a somewhat lower value.
There are also somewhat complicated operating rules which dictate that the pilots observe a margin - airliners shall always keep a certain margin to "coffin corner". As opposed to the Lockheed U-2 spy plane which was designed to fly as high as possible where min and max Mach number matched, and the slightest disturbance by turbulence would make the plane fall out of the sky.
Various minimum speeds are always expressed as IAS (indicated air speed) most often expressed in knots (KIAS). IAS (as opposed to TAS
- true air speed) is the figure at any altitude where the air reacts on the plane the same way as it does at the same figure of TAS
at sea level. If you are going 250 knots IAS at 35,000 feet, then you are actually going more like 500 TAS
. Mach numbers are even more different since speed of sound expressed as TAS
decreases with decreased temperature at altitude.