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While observing taxiing A321 planes, I noticed that they had an open hatch-like opening on the bottom right of door 4R. I marked it here on a picture I found online.

enter image description here

I could not locate it in the Airbus A321 Aircraft Characteristics manual. What is it for? Why was it open? Does it stay open during flight?

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marked as duplicate by ymb1, Ron Beyer, Ralph J, Manu H, Community Sep 1 at 20:04

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