Here it leads up to and comes out in the main air lock.
5 seconds sound clip from the Alien (1979) movie soundboard.
You can hear this line at 01:08:44 in the Blu-ray version of the movie.
Quote context
[...]
- No question, it's... it's using the air ducts to move around.
- No. I mean, I don't think so.
- Well, this air shaft may work to our advantage.
- Here it leads up to and comes out in the main air lock.
- This son of a bitch is huge! I mean, it's like a man. It's big!
- Come on, Ash. Science department should be able to help us.
[...]