Originally Posted by Sigwx

It was all air conditioning and thus ultimately pressurisation related. One of the right hand cabin air compressors failed, no problem in itself per se, still one for the right hand air conditioning pack still working, a go item with a few restrictions but in the whole not at all limiting. Then in flight up at 40,000ft the left hand air conditioning pack failed. Procedure followed and descent initiated to get to a level in order to reset the pack. The reset failed. You now have only one pack with one source of air to pressurise the aircraft, you are about to set off over the Atlantic and you’ve had a gradually cascading set of related failures on a critical system. There is no chance I’d personally be taking that aircraft over the pond given that set of circumstances. So that’s the expanded reason.