From the RFM;
ENGINE SHUTDOWN PROCEDURE
CAUTION
AVOID RAPID ENGAGEMENT OF ROTOR BRAKE IF HELICOPTER IS ON ICE OR OTHER SLIPPERY OR LOOSE SURFACE TO PREVENT ROTATION OF HELICOPTER.
Apply rotor brake between 36% and 30% rotor RPM.
As you can see Bell do not expand on the basic instructions, but as noted in the previous posts if you hear or feel chattering of the drive train release the brake to reduce any possible damage. The term "Avoid Rapid Engagement" may also be a good guideline to follow irrespective of the surface conditions.
It seems that a lot of engineers (mechanics) hate rotor brakes but a good guideline has always been;
"I you have one fitted and you need to use it, then use it. If you don't need to use it, then don't use it".
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