Final answer:
Low-pressure compressed air for aircraft engine starting can come from an Auxiliary Power Unit (APU), Ground Power Units (GPUs), another aircraft's APU, or the aircraft's internal compressors. This air is used to spin the air turbine starters, allowing the aircraft engines to start.
Step-by-step explanation:
Possible sources of low-pressure compressed air used for starting jet transport aircraft equipped with air turbine starters include:
Air is produced by an Auxiliary Power Unit (APU), which is a small jet engine located at the tail of the aircraft that can provide power when the main engines are not running.
Ground Power Units (GPUs) also known as air start units, are mobile units typically used at airports to supply compressed air and electric power to aircraft when the APU is not available or when maintenance works are conducted on the APU.
Another aircraft's APU can be used to provide the necessary compressed air, a process frequently known as cross-bleeding.
Internal compressors located within the jet transport aircraft can pressurize the air using the aircraft's electrical systems, although this option may be less common than the use of an APU or GPU.