core lock

A core lock occurs when a turbine engine's rotating components seize against their stationary counterparts after shutdown, caused by uneven thermal contraction. As the engine cools, different materials and components contract at different rates due to their varying coefficients of thermal expansion. This differential cooling can close clearances designed to exist during hot operation, causing the rotor to bind against the stator and preventing free rotation. The condition is particularly dangerous because it can make engine restart attempts impossible and may cause mechanical damage if rotation is forced.

Core locks happen most readily in high-performance turbine engines, especially military jet engines and some industrial gas turbines. The risk increases when an engine experiences rapid shutdown following high-power operation, or when a flameout occurs while the core is still hot. Engines designed for sustained high-temperature operation accumulate larger thermal gradients between their outer cases (cooled by ambient air) and their inner rotating components (cooled only by internal airflow or conduction). A rotor made of nickel-based superalloy at 800 degrees Celsius contracting against a titanium compressor case at 200 degrees Celsius creates the conditions for seizure.

Prevention and design response

Engineers combat core lock through several design strategies. Compressor and turbine discs are often made from materials with similar thermal expansion coefficients to their surrounding casings. Running clearances are specified large enough to accommodate predicted contraction without bottoming out. Some engines incorporate shroud materials in the stator that have deliberately high thermal conductivity, allowing faster heat transfer to equalize temperatures throughout the engine. Bearing designs also receive attention, as some bearing materials contract more than others and can jam if clearance is inadequate.

Modern engines may include specific shutdown procedures intended to minimize core lock risk. Some designs allow controlled cooling through continued low-speed rotation after shutdown, or specify waiting periods before restart attempts. However, the condition remains a concern in engines that must respond to rapid power changes or that operate in high-altitude environments where external cooling is minimal. Ground crews learn to recognize core lock when an engine fails to rotate freely during starter engagement, and attempting to force rotation risks damage to compressor blades and seals.

The term has expanded beyond turbines to describe similar thermal seizure in other rotating machinery, though it remains most common in aerospace and power generation contexts. Engines with variable geometry stators and movable compressor bleeds face additional complexity, as these mechanisms must also account for thermal locking in their actuation systems.