B777 EEC Alternate Mode thrust computation (N1)
Hi everyone,
I’m having some difficulty understanding the difference in thrust computation on the 777 when the EEC is operating in Alternate mode.
According to the FCOM, in Normal mode the EEC sets thrust by controlling N1 based on thrust lever position, while in Alternate mode it schedules N1 as a function of thrust lever position. I’m struggling to grasp the practical difference between these two descriptions.
From my understanding, the TMCS positions the thrust levers via the ASM (Autothrottle Servo Motor), and those lever positions then correspond to a commanded N1. If that’s the case, what exactly changes in the computation between Normal and Alternate modes? If the TMCS has already positioned the thrust levers, wouldn’t that mean it has effectively computed the target N1, leaving the EEC to simply match it?
I do understand the aspects regarding the possible overboost and overspeed protection in Alternate mode. My confusion is really centered on the difference in how N1 is computed between the two modes.
Thanks in advance for any clarification.