Originally Posted by
FullWings
I think the amber line (maximum) is still displayed on the N1 dial, the difference is that the EEC will allow you to exceed it. I assume this is because you don’t want thrust limited to a lower than necessary value if the rated thrust calculation has gone wrong.
In Normal mode, the amber line (N1 or EPR Max) is provided by the EEC. In Alternate mode, it comes from the FMC since EEC is now in an 'unrated' mode (if the FMC has detected unreliable air data, the amber line will blank entirely).
One semi-common problem is Ice Crystal Icing (ICI). ICI has a tendency to ice up heated temperature probes (aircraft TAT and/or engine inlet T12) - causing the sensor to read ~0 deg C. At high altitudes, that can be well above cornerpoint temperature resulting in a thrust cutback on all engines. On EPR engines, the procedure is to manually select 'Hard' Alternate mode and set thrust manually in order to maintain altitude/airspeed. N1 engines are not susceptible to ICI T12 corruption because the inlet probe is unheated, however it may be necessary to disconnect the A/T and set thrust manually.
Alternate mode is always N1 control, even on EPR engines (since unreliable EPR is a primary reason for going to Alternate mode). If the EEC can still validate EPR (e.g. the failure was on the other engine) EPR may still be displayed, but the EEC is controlling to N1.