Originally Posted by JC5280
The air start unit is commonly used when the APU in inop. However, you can have a perfectly good running APU that is not supplying electrical which could require an air start as well. Think of it like a jump start for your car.
If it's not supplying electrical power, and you require an air start, what exactly is the APU doing??? It's either broken, supplying electrical but no pneumatics, supplying pneumatic but no electrical, or supplying electrical and pneumatic (a perfectly good APU)