Originally Posted by
plhought
99% of the turbine-powered external air carts are the same as any aircraft APU or engine. Air is bled off just like an engine. In the common Garret GTCP series, the lubrication system is a pretty simple two bearing system with little risk of oil contamination, especially because of the design of the annular compressor. The hot bleed air is really no different than bleed air off the aircraft's power plant.
Yeah, that was kind of my take on it. Again coming from a Herc perspective, there doesn't seem to me to be much meaningful difference between using the GTC installed in the aircraft to run the packs, or to use a ground cart. In either case the compressor is a pretty much identical unit. Even on the later Hercs which have an APU instead of the earlier GTC, the APU is merely a higher capacity version of the same Garrett gas turbine engine.