Originally Posted by
TheBusFlyer
Thank you for your detailed explanations tdracer and Fullwings.
Please correct me if I am mistaken, but based on the discussion—particularly your input, tdracer —I realize I may have been confusing the computation of target N1 (performed by the TMCS) with the computation of maximum rated N1 thrust (performed by the EEC).
As I understand it,
in both modes the EEC does not compute the target N1, and
a given Thrust Lever Angle (TLA) corresponds to a specific N1 value. The distinction lies in how that reference is scheduled:
- Normal mode: lower N1s are scheduled relative to the dynamically calculated maximum N1 rated thrust, which is continuously updated.
- Alternate mode: lower N1s are scheduled relative to a max N1 based on the corner point temperature for that altitude (so constant for a given altitude?)
This means that in Normal mode, since the maximum N1 rated thrust is recalculated as conditions change, a given TLA can correspond to different N1 values at different times. For example (random values), if the TMCS requires 57% N1 and positions the levers at 25°, the EEC will deliver it. However, as the max rated thrust value is updated, that same 25° TLA may no longer correspond exactly to 57% N1. This implies that the TMCS must have precise knowledge of the current maximum rated thrust in order to correctly position the thrust levers.
In Alternate mode, for a given altitude, a given TLA will always command the same N1 value as max N1 value does not change.
Is it correct?
Yes, it sounds like you understand.
The nice thing in 'Normal' mode is that if you set a specific power setting (e.g. Max Climb, or a specific derated climb), the thrust lever position will stay constant as you climb. Same thing with Max Con.
In Alternate mode, you'd need to be adjusting the lever position as you climbed (of course, the autothrottle will do that automatically if it's active).