# A32NX cruise speed limit

I noticed as soon as I reach cruise altitude with the A32NX that the speed got immediately limited to about 270 ks IAS. In my CRZ preference I have 290.
The autothrottle properly handles this thing selecting itself 260 knots.
What am I missing here? During climbing the speed limit is above 310…

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Be happy to try to help you understand this. Above what is called the crossover altitude, the A320 (and all other similar airplanes) fly a constant Mach number rather than a constant airspeed. There will be a climb speed schedule the airplane is following, such as for the A32NX normally is 290KCAS/M0.78. The crossover altitude will be the altitude where 290 KCAS becomes equal to M0.78. The reason for the switchover to Mach is that the airspeed relative to the speed of sound becomes critical in determining what speeds you can safely fly.

Mach number is the ratio of your airspeed to the speed of sound. The speed of sound in air is a function of temperature, so as you climb at a constant airspeed with the speed of sound decreasing, that means your Mach is increasing. Once you get to the safe cruise Mach number, the airplane will switch over to maintaining that instead of a constant airspeed. You can tell by the number below the speed tape on the PFD, that the autopilot/autothrust is targeting the cruise Mach of 0.78, not an airspeed.

The maximum operating Mach number is 0.82 and is indicated by the bottom of the black and red barred line on the left side of your PFD As you can see, at your current altitude (actually temperature), Mach 0.82 is about 271-272 KCAS.

If you want to investigate the differences between KCAS, KTAS, and Mach number for different altitudes, I suggest using an online calculator like Airspeed Calculator (IAS/CAS/EAS/TAS/Mach) | AeroToolbox
(But don’t be too inquisitive, or you will find that MSFS does not do the airspeed conversions between Mach/KTAS and KCAS correctly!)

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I’m not expert at all of airliners and that’s one of my very first flights with the A320. I noticed it switched to mach indications and now everithing sounds much more clear (and makes a lot sense too)
Thank you again.

Sure. By the way, there is a Mach/Airspeed toggle switch next to the Mach/Airspeed selector. In the real airplane, the flightcrew can switch between Mach and airspeed at any time. But the switch is not simulated correctly yet. Currently, the airplane will switch to using Mach automatically, but can only be switched back to airspeed below around 26,000 feet.

I’m familiar with IAS, (CAS,EAS) TAS and GS concepts but not being used with transonic speed such I never considered temp factor.
I just read it’s really relevat. Form 20 degree to -50 you get as much frontal resistance with a difference of 20% speed (talking about transonic speed)
Basically temperature and air density plays conflicting roles. You get higher to have less air density and diminish your drag but then you get lower temperature that stops you due to sound speed approaching. Interesting topic really.

In the default A320, it allowed me to switch between Mach and knots at anytime. I wish that would get fixed on the mod because it’s a bit annoying.

not annoying to me because was the opportunity to learn something (just looking it in a positive way but I got your point)

I agree that it’s annoying . Looks to me like Asobo fixed the default airplane in WU3. We’re looking at it.