Originally Posted by
tdracer
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).
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