No flying in The Pit
Be it by the sheer size of The Pit causing the massive vortex of air that pulls everything down, or by some magical force, nothing can fly in the main shaft of The Pit.
That rule doesn't extend to the layers, as many creatures such as bats and birds have been seen flying around different layers while steering clear from the biggest open area they could fly in. There were even people who tried to build airplanes and parachutes and jump there (it went really bad).
As it is easy to guess, thanks to this force it is also impossible to build anything extending into the shaft. The only exception to this rule is the presence of the Beasts from Above that adapted to be born with gas bladders which they fill with something unknown (either gas or magic) that lets them fly down the main shaft of The Pit and leave it. When the beast is killed, those bladders lose their ability to float.