My training manual says:
HPT CLEARANCE CONTROL SYSTEM
The high pressure turbine (HPT) clearance control system, uses high pressure compressor (HPC) bleed air to obtain maximum steady-state HPT performance and to minimize exhaust gas temperature (EGT) transient overshoot during rapid change of engine speed.
The system consists of:
● HPT Clearance Control Valve
● HPT Clearance Control Timer (on 56-3C1 and optional on 56-3B-2 engines)
● HPTCC Timer Lockout Solenoid Valve (on 56-3C1 and optional on 56-3B-2 engines)
● HPT Schroud Manifold Air Tubes
HPT Clearance Control Valve
The valve consists of two pistons operating two butterfly valves for the 5th and 9th stage bleeds. Valve operation is hydraulic using fuel from the main engine control.
After engine start and with the engine at ground idle power setting, the airflow to the HPT shroud is from the HPC 9th stage bleed. When the thrust lever is advanced or retarded to change the core engine speed (N2), the airflow is regulated to maintain optimum HPT shroud to blade tip clearance.
The airflow selection for various steady-state power settings is:
● Idle RPM....................9th Stage
● Take Off RPM............ 9th Stage
● Climbe RPM.............. 9th and 5th Stage
● Cruise RPM............... 5th Stage
HPT Clearance Control Timer
The High Pressure Turbine Clearance Control (HPTCC) timer controls the operation of the HPTCC valve during the first 182 seconds of engine operation if N2 RPM is 94% or greater during take off condition. The HPTCC solenoid valve prevents timer operation after take off through an air sensing relay.
The timer will be activated if:
● Aircraft on GRD and
● N2 RPM >94%
The normal schedule will be interrupted for 180 sec. The following cooling air schedule will be performed:
● 0 - 8 sec........... no air
● 8 - 152 sec........... 5th stage
● 152 - 182 sec........... 9th and 5th stage
● >182 sec............9th stage ( normal schedule )
With the shutdown of the engine ( RPM < 5% N2 ) the timer will reset automatically.
HPTCC Timer Lockout Solenoid Valve
Depending on the AIR/GRD switching the lockout solenoid valve activates the timer.
● On GRD............deenergized, the timer is activated.
● In AIR................energized, the timer is deactivated.