Final answer:
The operation of APU bleed air in parallel with main engine bleed air is prohibited according to the technical order specified. The APU provides pneumatic and electrical power on an aircraft, and using its bleed air along with main engine bleed may cause safety and operational issues.
Step-by-step explanation:
The question refers to a specific procedure in the operation of an Auxiliary Power Unit (APU) on an aircraft. According to the given reference (APU) (TO 1E-8C-1) (pg. 1C-11 PDF 89), the operation of APU bleed air in parallel with main engine bleed air is prohibited.
The APU is a device on an aircraft that provides energy for functions other than propulsion, such as generating pneumatic pressure for air conditioning and electrical power.
Bleed air is compressed air taken from the compressor stage of a gas turbine engine, which can be used for a variety of purposes.
However, given that bleed air from the APU and the main engine may not be compatible or may interfere with each other's operation when used simultaneously, restrictions like this are put in place for safety and efficiency reasons. Additionally, the specific restrictions can vary based on the aircraft make and model, so always consulting the relevant technical orders (TO) and manuals is critical.