Airspeed in turbulence

[JammedStab]

Concerning the incident on the last page of this report....

http://www.bea.aero/ita/pdf/ita.005.en.pdf

Why is it better to select an airspeed than a mach number.

Conduct of flight in turbulent atmosphere
The pilot selected a Mach number, and not
a speed, in accordance with the instructions
in moderate turbulence, which led to speeds
above the recommended speed (308 kt
maximum) without, however, approaching
VMO(15). These actions were not sufficient in
face of the strength of the turbulence that was
in fact encountered.

[de facto]

In sever turbulence a speed nor a mach are to be set,just a turb N1.
For lighter turb or to get better reaction from the AT,selecting a speed rather than a mach nr is working quite well.

[Lightning Mate]

The aircraft gust envelope is plotted against EAS, that's why.

[Capt Claret]

At a guess I'd say IAS/Mach crossover (haven't read the report). On the Douglas/Boeing 717, turbulence penetration speed (above 10,000') is 275 kias/ M0.75 whichever is the less.

It's quite common to see folk preselect M0.75, which can be significantly faster than 275 kias, depending on the altitude. :ugh:

[mach 84]

Capt claret! You are spot on with your reply!
On a 777 the turb speed is m.82 or 280 whichever is less, if you are heavy it is
To be 15 kts above the yellow band, this causes sometimes confusion !??! ???
The crossover usually is between FL 340 and 350 then 280 becomes M.82
Amazingly during climb pilots observe this speeds quite strict,during descent
They tend to fly faster and do not care to much about turbulence, - strange, but
That's how it is!

[de facto]

The aircraft gust envelope is plotted against EAS, that's why.

I am aware of the gust envelope being in EAS however not really clear on the interface between the AT and that said envelope.
Would you be kind to explain or provide me an explanatory document?
Thanks a lot.