Definition
A turbine engine design feature that controls the clearance between the tips of the turbine blades and the surrounding engine case by regulating the temperature of the case. Cooling air, typically bled from the fan or compressor, is directed onto the outside of the turbine case to shrink it slightly and hold the gap between the case and the blade tips at the optimum size during cruise.
Plain English
A system that keeps the gap between the spinning turbine blade tips and the engine casing as small as possible while the engine is running. It does this by spraying cooler air on the outside of the casing to control how much the casing expands from the heat.
Context Anchor
Seen in turbine engine systems, engine maintenance descriptions, and performance discussions.
Derivation
Active means the system adjusts itself in real time, rather than relying on a fixed built-in tolerance. Clearance refers to the small gap between two parts. Together, the term describes a system that actively manages that gap while the engine runs.
Why Pilots Care
Reduces fuel burn, improves thrust, and prevents blade damage from contact with the casing.
Grounding Statement
As engine parts heat up and cool down, metal expands and shrinks, so the system manages the gap while the engine is operating.
Intuition Check
“Clearance” does not mean permission from air traffic control here. It means the small physical space between moving blade tips and the engine case.
Example Sentence 1
Modern high-bypass turbofans use active clearance control to improve fuel efficiency during cruise.
Example Sentence 2
During the borescope inspection, the technician confirmed the active clearance control valves were operating correctly.