No, I was actually referring to the Red line. That also decreased with altitude.
Been a bit of a long day so I may be missing something here, but why does the torque limit reduce with altitude?
Although the engine is turning at the same speed (Ng), due to the air being less dense, it is producing less torque, and therefore SHP. I thought that there would be less "strain" on the engine for the same amount of work being done.
Is it something to do with air pressure across the engine?
Maybe I'm looking at it from the wrong angle and I'm more than happy to be re-educated.