The question says it all. While reading I came across this that we can under no circumstances use APU bleed air for supplying wing anti ice? Any reason?
On multiple PPRUNE threads they incorrectly say:
The APU bleed is much cooler than the engine bleed system, and would be ineffective for WAI.
But the reason is:
The APU bleed temperature is unregulated and thus could damage the wing slats if used for anti-ice.
Highlight shows regulation of engine bleed temperature.