What is this yellow tube (lower right quadrant of the picture) which is blowing air into the airplane while it's at the terminal? You can see the open hatch on the bottom of the jet through which it's connected.

Mysterious yellow tube


1 Answer 1


Most likely it’s a compressed air line which connects into the jet’s bleed air system. This can be used to crank the engines as opposed to using bleed APU air to do it.

Another use (if you’re talking about the line under the jetway) is to provide cool air for cabin air conditioning without needing to run the APU to do so.

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    $\begingroup$ The tube looks quite thin and flimsy, like it's made of a light fabric. Do you think there is a compressed air line inside it? $\endgroup$ Mar 4, 2019 at 12:57
  • $\begingroup$ I think you're confused, the yellow pipe with black ribs under the jetway on the right side of the picture, is pre-cooled air feed into the mix manifold so the plane doesn't need to run the air conditioners, which in turn need the APU to be on (which causes local polution). There another pipe which is orange on the left side of the picture which joins the aircraft just under the door, which looks like a compressed air line. $\endgroup$
    – james
    Mar 5, 2019 at 9:34
  • $\begingroup$ Conditioned air. Start air has a smaller diameter. $\endgroup$
    – Anilv
    Mar 5, 2019 at 9:47
  • $\begingroup$ Well there’s two different items here. There the large tube under the jetway then a smaller one running under the aircraft and connected to it by an access port on the belly. I can’t twll which one the OP is indicating. $\endgroup$ Mar 5, 2019 at 12:18

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