Groundgripper,
Thanks for contributing. All the overspeed protection built in to the engine relies on it being controlled or even shut down by managing the fuel flow as far as I know. First thing of course, I don't think we actually know that the IPT did overspeed before breaking up. As you say if the IP disc de-coupled from the shaft no overspeed would be detected, in fact quite the opposite! Overspeed resulting from decoupling would be unstable and as far as I know there are no sensors on the engine which would detect an overspeed of a disc loose on its shaft. Even if there were, what mechanism would there be to stop the disc overspeeding given that there would still be hot gas flow on it (the HP compressor would still be feeding the combustion chamber) and no load from the compressor. The only way to stop the catastrophic overspeed would be a mechanical system to directly brake the disc before it totally disintegrated. Not easy, but given the risks that seem to be present, I think the issue may need to be addressed.