Originally Posted by
tdracer
Simple answer:
For N1 rated engines (GE), in Normal mode the FADEC calculates the max N1 rating for the ambient conditions (temp, Mach/airspeed, altitude). Full forward thrust lever position will give you that max rated thrust (plus a few tens of a percent "headroom"). You can't have a significant overboost in Normal mode.
In Alternate Mode, the FADEC doesn't calculate the max rating (typically because reliable Mach/airspeed is unavailable - the FADEC requires two sources of Mach/airspeed that agree (within a small tolerance) - on the 777 that means both ADIRU and SAARU as the GE FADEC doesn't have an independent source of total pressure). Instead, the FADEC calculates a max N1 based on the corner point temperature for that altitude - that becomes the full forward thrust lever position, and that's used to schedule the N1 for lower thrust lever positions. Depending on the actual temperature, you can command a significant overboost relative to the actual max N1 for those conditions.
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?