NOW - I understand. It wasn't clear (to me) before that the TR doesn't actually 'slow down' perceptibly does it? The flt idle speed (or whatever the term given) is sufficient to maintain the Nf such that the reduction gearbox output to TR retains its TRE. Cool - now I understand thanks JJ Got there in the end.
Thank you all and well done.
So unlike other helicopters where if you incur a high speed shaft failure and enter an auto, and if you have time shut off the fuel valve before touchdown, in the A350/Arriel design you must keep the ENG on to maintain TRE. Is that correct?
Keeping it in context and as was said before, high speed shaft failures (or what I call the short shaft) are very rare, but I have thoroughly enjoyed the discussion nonetheless. Thanks.