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. There are two kinds of air starts: neumatic air (continuous airflow) and bottled air, where it looks like a big propane storage tank that is pressurized in advance. Either way, the process must be taken seriously as the crew is working with highly pressurized air hoses. They force air through the engine manually, while the enine sprays fuel into the air/fuel mix. Once ignited, that engine can supply enough power to start the other engine and power the entire a/c. While this airstart occurs, there is nothing supplying air conditioning to the a/c, so it needs to be done quickly in hot temps.

Here is the at-the-gate process:


AS unit is running at idle next to a/c, personel at controls and on headset in push tractor, communication through hand signals amongst ground crew
Door of a/c is closed
(bridge cannot be pulled yet because ground power from bridge is still connected)
All clear is given to flight crew that ground crew is "off" the a/c
Captain gives go ahead to give engine 2 (starboard) air...really loud sound from AS unit as ground crew runs up air pressure on AS unit
Air is applied to #2 and the flight crew starts that engine
Engine runs to power level
Captain gives okay to pull ground power
GPU pulled and bridge comes off
Captain gives okay to disconnect air
Ground crew releases pressure in AS unit and disconnects it from a/c
(this is critical since the connection point is directly under the a/c between the two engines)
Aircraft is pushed out from gate
Once tractor is disconnected, the second engine is started. This could happen earlier, but the #2 engine will need to run up to almost 70% power to "cross-bleed" enough air over to the #1 engine.



Hope this helps!

fellow airplane geek