Active clearance control provides a useful compromise by shrinking the casings by the correct amount to match engine running conditions. This is achieved by blowing cool air onto the outside of the casings. This cools the casings, causing them to contract, thereby reducing tip clearances.
Not quite KW.
The casing does not shrink (except during cooldown). The casing is still going to expand. The TCC system just controls the rate at which it expands until the turbine disc expansion catches up. That is where the timers come in.
BTW during high power maintenance engine running the TCC systems are deactivated. (stops us kack-handed throttle bashers trashing the motor dontchaknow).
