Final answer:
Bleed air from an APU is used to start the main engines, and for cabin pressurization and conditioning on the ground or sometimes in flight. It reaches the ECS for pressurization, air distribution, and the pneumatic system for engine starting.
Step-by-step explanation:
The question 'Bleed air from the APU can reach:' relates to the function of the Auxiliary Power Unit (APU) in aircraft systems engineering. The APU is a device on a plane that provides energy for various functions while the plane's engines are not running. The bleed air from the APU is typically used to start the main engines and to provide cabin pressurization and air conditioning when the aircraft is on the ground or, occasionally, during flight if the main engines cannot supply sufficient bleed air.
Bleed air from the APU can reach the environmental control system (ECS) for cabin pressurization, air distribution systems, and also the pneumatic system for engine starting. Essentially, the APU's bleed air is crucial for maintaining comfortable and safe conditions inside the aircraft cabin and for assisting in the engine start-up sequence before the aircraft takes off.