What is a "core lock" on an jet-engine?

Question

This question came up while reading the Wikipedia article on core lock and Pinnacle Flight 3701.

If my understanding is right then if for example inner parts of the jet engine heats up that so much that the metal will expand, then after that it cools down rapidly it can result in a lock of the ball bearing of the blades. This means the engine is not functional anymore?

The Wikipedia article says that a windmill and a APU-restart is not possible anymore - is this because the engine has been rendered permanently inoperable?

Could somebody explain exactly what a core lock is to me, possibly with pictures?

Answer 1

There is a good writeup here that explains the engine issues as well as the rest of the incident you mentioned.

Core lock happens when parts inside the engine cool off at different rates, causing interference between them. In the case of the CF34 engines on these Bombardier jets:

GE isolated the problem to contact between components of the high-pressure-turbine interstage seal - a static, pressurized honeycomb component - and the rotating seal teeth on the outer torque coupling.

_{Working on getting a good illustration of what they are talking about.}

This does not mean that the engine is "not functional." There is just much more resistance on the rotating components, but once the temperatures have time to equalize, the components will break free.

According to the NTSB, FDR data indicate that the engine cores were beginning to break free just before the impact.

During core lock, the resistance makes starting the engine more difficult. A windmill start does not provide very much turning force on the engine components, so the resistance from core lock makes that method of starting difficult. GE has done engine testing, and when the windmill restart fails, they are able to use bleed air from the other engine to restart the engine, which provides more turning force.

Answer 2

My experience shows that, on some short haul domestic type operations, some pilots practice shutting down the left wing-mounted engine quickly after landing to be sure the engine has stopped turning to allow passengers to disembark down the steps. The quick turnaround operation may mean another engine start within 30 minutes of shut down. The core (heavy turbine discs) is still hot whereas the lighter weight and exterior turbine cases have cooled and contracted onto the turbine airseals. This can lead to "seal squeal" if the engine is windmilling in the wind and attracts ground engineers' attention, or in severe cases may lock the core engine shaft. In most cases, allowing the engine to fully cool or, if possible, restarting the engine, allow 10 mins at idle to fully expand all parts equally and recut the turbine airseals, the engine can be deemed serviceable if all other parameters are normal. Continued use by pilots of this practice would mean for the engine to quickly lose performance, mostly seen as increased fuel consumption, through premature airseal deterioration.