Low pressure turbine clearance control valve:
At take-off and low altitude the valve is in its normal closed position allowing cooling airflow to the core compartment. When an altitude of 19,000 feet +5000 feet is reached, the altitude sensor switches to supply compressor discharge pressure to the signal port of the valve, causing the valve piston to move to the open position, thus allowing cooling airflow to the low pressure turbine cooling manifold.
During descent, at approximately 15,000 feet +1500 feet, the altitude sensor switches back and cuts off the compressor discharge signal pressure to the valve and the positioning spring in the valve returns the piston to its normal closed position. Operation can be monitored by the electrical position indicator switch and a disagree flightdeck light.