A Question About Altitude/Speed

[Farid]

Why at aircraft like airbuses at higher altitude
the speed limited get increased and tighted?

thankyou in advance

[Microburst2002]

Stall speed (EAS) remains constant with altitude, but due to compressibility effects in the pitot, stall IAS increases with altitude (with mach number).

also, the maximum IAS is approximately constant with altitude, if you only consider dynamic pressure induced problems. however, when mach numbers are high (at high altitude and high speed), then compressibility effects such as shockwaves will ocurr well before the IAS at which you have aerodynamic problems. therfore the maximun IAS decreases with altitude as the IAS corresponding to maximum mach number decreases.

So yo have a gap that narrows and narrows with altitude. When both ends meet, you are in the coffin corner

[stormyweathers]

Over lower altitudes, stall speed does not vary with altitude. This is because at these lower altitudes, Mach number at Vs is less than 0.4M, too low for compressibility effects to be present. At higher altitudes the CL max (max coefficient of lift) decreases due to compressibility. Thats because the airmass ahead of the wing leading edge must deflect through a greater angle and loses kinetic energy, seperation occurs at a lower angle of attack.
If the CLmax decreases, the IAS must increase for lift to be maintained. Thats why you see the lower airspeed band creep up as you increase altitude.