Core lock is a jet engine failure that can happen on aircraft in flight, after an engine stops working for a period. When a jet engine has a flameout, the cooling air no longer circulates normally. If an aircraft is moving slowly, the sizes of some engine parts may change quicker than others, such that they no longer fit together as designed. If this happens, the engine may seize. On an internal combustion engine that is not subject to extreme flight conditions, this may not be a problem. However, aircraft jet engines are built to exacting standards; core lock makes it difficult for pilots to windmill restart or perform an APU-assisted engine restart.
Core lock was cited as one of the contributing causes to the Pinnacle Airlines Flight 3701 accident.
"the pilots' failure to achieve and maintain the target airspeed in the double engine failure checklist, which caused the engine cores to stop rotating and resulted in the core lock engine condition. Contributing to this accident was 1) the engine core lock condition, which prevented at least one engine from being restarted, and 2) the airplane flight manuals that did not communicate to pilots the importance of maintaining a minimum airspeed to keep the engine cores rotating..."