A320 managed vs selected speed
 

Hello all,

I have a general question about scenarios where you may experience light/moderate turbulence in the cruise.

in my first few hundred hours on the bus, I have observed people going from managed to selected speeds when it gets a bit choppy, to either select 0.76, or even select the current Mach the managed mode was already maintaining.

my question is, by going to a selected Mach, will the autothrust actually hold the speed better compared to that of a managed mach.

the FCOM suggests to me that unless you force it out of Mach mode, (by pressing SPD on FCU), alt soft mode will stay engaged (providing in alt crz and after 2 mins), allowing for minor deviations in Mach by allowing 50ft variation from the CRZ FL.

this makes me think that, unless you’re actually going to SPEED mode, it doesn’t actually matter if the mach is managed or selected, if you’re in mach (fma) and alt crz, you’ll still have alt soft mode engaged.

if this is the case, why do I see people on the line selecting the same mach? maybe I’m missing something?


The only time I think I really press the SPD button on the FCU is when ATC ask in the descent for x kts now when I’m still on a mach. Even then it seems to just want to go back to mach instantly anyway!

thanks all

Depends on the software update of your aircraft, the most recent updates of the FBW software selecting speed will result in a more responsive auto thrust mode that is designed to maintain the selected Mach. This was a development on the A350 brought to other fleets. It’s a bit like teh difference between ALT and ALT CRZ.

thanks for the response. I don’t supposed you know the name of this software update?

Will be interested if you have a source for that. Cheers.

Say you're flying at CI100 and the airplane is flying at .80 and you just want to slow it down a bit. Selected mach works really well. Maybe choose .79 or .78.

Selected speed works well if it's very turbulent and even selecting the middle of Vls and Vmo you get nervous how close it's getting to either end (usually a heavy plane close to max FL). The a/thr will be much more responsive in selected speed.

Be careful though with selected speed, the thrust will come right off if you get too fast. On the other side of a wave when you start loosing airspeed the thrust will take ages to roll back up from idle. A few times in really bad mountain waves I have just turned the auto thrust off and that was much easier, while descending into thicker air to lower Vls and have a bigger window, even if the turbulence is worse down there.