Originally Posted by
Dave Therhino
They are trying to keep the fan shaft (really the LP compressor) out of a speed/flow condition that, with the recently implemented different stator vane schedule, puts an LP compressor blade stage into resonance. I suspect the reason for the anti-ice restriction at high altitude is that anti-ice puts a heavy bleed load on the engine, changes the flow/speed relationship within the compressor, and gets them into resonance as well. If they allowed anti-ice bleed at high altitude they'd have to further reduce the fan speed limit I suspect. High altitude climb is where fan speeds are typically highest, and reducing the allowed fan speed there is a heavy performance penalty.
I would not look too keenly on a pilot -engine control action which leads to an engine failure in a single flight. There must be more to this.