In the context of aviation and aerospace, "Active Clearance Control" (ACC) refers to a technology used to manage and optimize the clearance between turbine blades and the surrounding casing in a gas turbine engine. This clearance is critical, as it affects the engine's efficiency, performance, and overall durability.

In traditional gas turbine engines, the clearance between the turbine blades and the casing is fixed and predetermined during the design phase. However, during engine operation, the clearance can change due to various factors such as:

1. Thermal expansion: The turbine blades and casing expand and contract at different rates due to temperature changes, affecting the clearance.
2. Vibration: The turbine blades can vibrate, causing the clearance to fluctuate.
3. Wear and tear: The turbine blades and casing can wear out over time, altering the clearance.

Active Clearance Control is a system that uses sensors, actuators, and control algorithms to dynamically adjust the clearance between the turbine blades and the casing in real-time. This is typically achieved by:

1. Measuring the blade tip clearance using sensors, such as capacitive or optical probes.
2. Analyzing the measurement data to determine the optimal clearance.
3. Adjusting the casing or blade position using actuators, such as air jets or electric motors, to achieve the desired clearance.

The benefits of Active Clearance Control include:

1. Improved engine efficiency: By maintaining optimal clearance, the engine can achieve better performance and fuel efficiency.
2. Increased power output: ACC can enable the engine to operate at higher power levels without compromising reliability.
3. Reduced emissions: By optimizing the clearance, the engine can produce fewer emissions and operate more cleanly.
4. Extended engine life: ACC can help reduce the wear and tear on the turbine blades and casing, leading to a longer engine lifespan.

Active Clearance Control is a complex technology that requires sophisticated sensors, actuators, and control systems. It is typically used in advanced gas turbine engines, such as those found in commercial aircraft, military jets, and industrial power generation applications.