A-320 speed brake versus engine anti ice

[gmb]

hi,
the fcom state on the SOP,Descent


SPEEDBRAKES........AS RQRD
In OPEN DES : Use speedbrakes to increase the rate of descent. The pilot may use up to half speedbrakes to maintain the required rate of descent, when engine anti-ice is used.

why is the reason of this statement,I mean, to use of half speedbrake when using engine anti ice?

thanks

[BusBoy]

....because when you have Eng Anti Ice ON the existing Idle Thrust is increased as the bleed air is taken off. More thrust = reduced descent rate on Open Descent, so use 1/2 speedbrake to counter effect of Eng AI being ON

Very general and simplistic, I'm sure more detailed answers to follow!