So why exactly isn't the water level parallel to the horizon when flying a turn? Centrifugal forces? Hydrogen bonds? Funny I accidently found this video as this question already arose a couple of years ago when I flew on a normal airplane.,(reply>), @Sebastian Dierks yeah it is the centripetal force. (physically the centrifugal force isn't existing as a force for itself, its only the actio - reactio to the centripetal force) if youre turning you have the centre of this "circle". this point is horizontal seen above your plane (imagine it a bit like a swing carousel). So in turn you have the centripetal force and the uplift force. school physics -&gt; parallelogram of forces -&gt; resulting force1 (this may be the chain in our imagination)  now actio - reactio the "centrifugal force horizontal the gravity vertikal -&gt; parallelogram of forces -&gt; resulting force2. this resulting force2 is parallel to resulting force1 (cause we're in stable flight but thats by now unimportant) but into the other direction. resulting force2 is literally resulting gravity so now we have why the fluid isnt horizontal (sry for writing as much but i want to show correctly and understandable (im not native speaker))