Accelerations due to changing speed and/or direction of flight may produce very considerable physiological effects upon the occupants of an aircraft depending on the following factors:
rate of acceleration (change in speed),
direction of acceleration (in relation to current direction),
personal protection such as seat harnesses.
Speed itself in straight and level flight has no effect on the human body.
Accelerations of relatively short duration (a few seconds) are associated with situations such as flying in turbulence, rough landings and heavy braking. Provided a suitable restraining system is used there should be little risk of injury from short-term accelerations and rapid decelerations. Shoulder harnesses provide flight crew with greater security (than passenger lap belts), especially in a crash situation, and such harnesses also make it less likely that an incapacitated pilot’s body might interfere with the operation of flight controls.
Long-duration accelerations may result in sensory illusions. The fluid-filled semicircular canals of the vestibular system detect motion in three dimensions. The normal mode of stimulation for these organs (i.e. to register movement) is an abrupt, short-duration acceleration followed immediately by a short deceleration. If the movement (acceleration) is below the threshold of detection, then the brain will not be conscious of any change in speed or direction. An undetected acceleration can result in speed and/or direction changing substantially without either being noticed. There are two main categories (or types) of common spatial disorientation “illusions” that humans are susceptible to in flight:
somatogravic – experiencing linear acceleration and deceleration as climbing and descending.
somatogyral – not detecting movement, and experiencing movement in a different (mostly opposite) direction to that actually being flown. Most commonly results in the actual angle-of-bank being different to that being “felt”, or, the direction of turn being opposite to that “felt”.