Many passengers do not understand Newton's first law:
Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.
Translation: Tie yourself to the aircraft.
The livestock is used to Trains and (ground) Buses which (usually) only accelerate in four directions. People can handle changes in velocity in those vehicles because the forces involved are similar to those experienced by a running footballer changing direction to avoid a tackle.
The problem is that some people are thick. They know that aeroplanes go up and down, but are unable to understand that gravity may be irrelevant.
Show them this footage of Lois (but lose the rocket and the external shots)
YouTube - Superman Returns - Superman saves an airliner