When I read about engines going to idle due to overspeed, it got my attention.
As I noted before, the FADEC EEC will protect rotor speed redlines, even in alternate mode. Only if there are faults that are affecting the EECs ability to control the engine is there any risk of a rotor speed exceedance exceedance - and if the EEC can't control the engine it doesn't much matter if it's Normal or Alternate mode operation. It's also worth noting that the EEC will not protect EGT redline except during autostart (at least on Boeing installations) so there is no risk of a cutback due to EGT. Selecting alternate mode in order to get additional thrust in an emergency will adversely affect engine life, but the risk of a cutback to idle due to an overspeed isn't a meaningful concern.
Cosmo, dispatch is allowed in Alternate mode per the MEL. Would you really accept an airplane where all the engine controls were "failed"?
