What is the yellow pipe attached to the plane at the gate?

When the plane is parked at the gate, why do they put in the yellow pipe onto the bottom (usually) of the plane?|||the yellow pipe provides air conditioning until the engines are started|||Provides Air Conditioning until the planes engines are run up.|||It is the duct for pre-conditioned air (PCA) that is directed from the pre-conditioned air units (that are part of the gate facilities) to keep passengers and crew comfortable on the aircraft while it is parked on the ramp with the engines and APU shut down.





The aircraft can provide their own air conditioning with the auxillary power unit (APU) powering the onboard A/C but APU use on the ramp is becoming prohibited/discouraged at more and more airports these days. Of course, the aircraft can also provide their own A/C when the engines are running.








EDIT: gotta love the thumbs down trolls!