Your ears are not just for hearing. They also contain a natural apparatus that tells you how your body is placed in space and what is happening to it.
Your vestibular system has two main components: the semi-circular canals, which detect rotation; and the otolith organs, which detect linear acceleration.
Each ear has three semi-circular canals, like little curved spirit levels (but with no bubble). Each canal is aligned to a primary axis of motion; pitch (up and down), roll (rotation) and yaw (left and right). When your head moves, the fluid in the canals presses against tiny hairs, which transmit the sensation of movement to your brain. Most of the time you don't notice this.
The otolith organs are, essentially, tiny rectangular bones, one set oriented vertically, and one horizontally. As in the semicircular canals, tiny hairs detect inertia and send signals to the brain. Think of the otoliths as a bar of soap resting on a brush.
Your vestibular system helps your body balance, regulate its posture, and see clearly when moving. But it, and the proprioceptive signals that come from the skin and muscles, (the 'seat of the pants') are mechanisms designed for earthbound life, but can be easily fooled, especially in situations of flying.
A modern passenger plane can have you all over the sky avoiding bad weather, and you’d never know. All you experience is some occasional turbulence and a safe flight to your destination.
Referenced and adapted from the CASA Flight Safety On-line Magazine March 2014.