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.

Normal Mode = Control N1

-The EEC controls N1 based on thrust lever angle (TLA).
-But it’s not a direct mapping , it also references other parameters (ambient pressure, temperature, Mach, bleed air, etc.) to calculate:
-If the thrust lever is here , the required N1 should be this value.
-The result: N1 is actively controlled to match the commanded thrust (accurate and adjusted for conditions).

Alternate Mode = Schedule N1
-If required data is not available, the EEC switches to scheduling N1 directly from a lookup table between TLA and N1.
Meaning:
-For this thrust lever angle , apply this scheduled N1 value.
-No adjustment for real performance conditions , less accurate.
-That’s why it’s called “schedule”: it’s just using a fixed mapping, not dynamic control.


-Control (Normal mode): EEC uses multiple parameters to control N1 precisely according to thrust lever position.
-Schedule (Alternate mode): EEC uses a fixed schedule (TLA → N1 table), less accurate and more basic.

Thank you very much Noknoipobin for your reply. So in normal mode, from the TLA, EEC computes the target N1 using several parameters. It’s not a direct mapping like in Alternate mode.

Nevertheless, how does the thrust management control system know without computing any N1 where to position the thrust levers ?

From FCOM

The thrust management function operates the autothrottle in response to flight crew mode control panel inputs or to FMC commands.

-calculates reference thrust limits and thrust settings, or follows FMC thrust settings

-commands thrust levers

then EEC controls engines to achieve THR
EEC doesn’t compute N1, just controlling or scheduling N1 as above mentioned.

TMS/FMC “ I need 95%”
ECC. “. Yes sir “

It also allows you to possibly overboost the engines in Alternate Mode, which is why when you are following a NNC that puts the EEC into alternate, it asks you to retard the TLs before doing it.

My question might sound stupid…but you wrote -If the thrust lever is here , the required N1 should be this value.

If TMS/FMC are already doing the maths for the target N1 to be achieved, why is EEC trying to figure out what is the N1 value ? Why can’t TMS/FMC just send the target directly to the EEC?

From what I understand, TMS/FMC are doing the maths to figure out what’s the target N1. Behind they position the thrust levers. Given their position, EEC also does the maths using different parameters + TLA to figure out the required N1…sounds a bit redundant. Also implies that TMS/FMC know the EEC parameters (Mach, pressure etc) as well to position the thrust levers correctly.

AFAIK the FMC doesn’t send anything to the EEC, it just computes the appropriate N1 for the mode that it is in and displays/demands this. If, say, the TAT is in error (this happens in ICI) then the wrong values for the ambient conditions will be calculated which could lead to a significant under/over N1 command for what actually exists outside. The engines and the FMC get environmental variables but they are not from the same sources, hence the possibility for disparity and alternate mode.

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?

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).

Well I thought I had it…but I don’t understand how a specific power setting (let’s say max CLB) can keep the thrust lever position at the same TLA throughout the climb. As the max N1 rated thrust will keep changing, won’t the lower N1s (eg.CLB N1) position on the levers also change?

Example (random values):

• Sea level, ISA
  • Max rated N1 = 104%
  • Max CLB rating = 94%

If I push the levers full forward, I get 104% N1.

• FL300, ISA
  • Max rated N1 (if computed) = 96%.

If I push the levers full forward, I get 96% N1.

I am aware that max CLB rating will reduce as altitude increases. But let’s suppose I want to keep climbing at 94%. Won’t the thrust levers move fwd at FL300 compared to their position at sea level?
Keeping the thrust levers at the same TLA during climb would imply that the decrease in max rated thrust as altitude increases is perfectly proportional to the decrease in max climb rated thrust.

The only reason I can see is this one: in normal mode, even though there’s no “CLB detent”, a single lever angle happens to remain valid for CLB thrust as conditions change, because the EEC silently rescales the map (TLA/N1) underneath.

Thanks

In Normal mode, Max Climb is obtained at a set TLA. The FADEC calculates the Max TO (or outside the takeoff envelope, Max Con) - which is also a set TLA (roughly 2 degrees from the forward stop - from there to the forward stop all you'll get is the 'headroom').
N1 increases linearly between the Max Climb TLA and the Max TO/Con TLA. Below the Max Climb TLA, the TLA/N1 relationship follows a shape that roughly gives linear thrust (the thrust/N1 relationship is very non-linear at lower N1s).
So if you set Max Climb - by definition the TLA won't change as you climb since that's what the FADEC controls to.
I don't recall what's done on the GE90, but on some engines above 30k, Max Climb and Max Con merge - so you end up with a big flat above the Climb TLA setting.

Thanks tdracer for your time and knowledge. Looks like you made the system !

Do you confirm that except max rated thrust, it’s the TMCS (or getting inputs from the FMCF) that’s doing the maths to compute all other thrust settings (CLB, CRZ…etc.) ?

That's basically correct - although I'm reasonably sure the FMC does the rating calculations and passes that on to the TMC.