Man’s desire to soar into the sky led to the departure from his natural habitat. This resulted in a mismatch between the orientation demands of the new environment and his innate ability to orient. Motion stimuli in aviation differ in magnitude, direction, frequency and in the degrees of freedom from that experienced on the ground. The human orientation sensory system, unable to cope with such dynamic, ever-changing scenario, results in a mismatch between human sensory limitation and dynamic motion of the flight. This mismatch is referred to as Spatial Disorientation and the physiological limitations of the sensory system result in pilots experiencing illusory sensations of position, attitude or motion in flight. Before understanding disorientation, here is a brief recapitulation of basics of orientation.
Orientation in humans is adapted for a stable terrestrial environment. This enables him to move around easily on any terrain. Man’s ability to sense or perceive orientation in three-dimensional space depends on his learned interpretation of the continuous input of signals from the sensory receptors. Some of these receptors are grouped together to form specialized sense organs like the eye and the vestibular apparatus (organ of balance) of the inner ear. Others are more generally distributed in the body and are found in the skin, the capsules of joints and supporting tissues. Together they form an ‘orientation triad’.
Interestingly, vision is responsible for almost 90% of all orientation cues in this ‘orientation triad’, while the vestibular system and the proprioceptive or kinaesthetic system (a.k.a ‘seat of the pants’ sensation) provide 5% inputs each.
Proprioceptive or Kinaesthetic Receptors. These are a multitude of sensory endings in the skin, the capsules of joints, muscles, ligaments and deeper supporting structures. They are stimulated mechanically, and thus are influenced by the forces acting on the body. This system in a combined manner provides orientation information regarding the various forces acting on the body and the positions of the various parts. This is the so called ‘seat of the pants‘ sensation. A word of caution here: a pilot must never ever trust this ‘seat of the pants’ sensation which is grossly inadequate and at variance with the actual attitude of the aircraft.
1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.
2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.
3. Human Performance & Limitations – JAA ATPL Theoretical Knowledge Manual. 2nd Edition. Jeppesen GmbH, Frankfurt 2001.
Acknowledgement. Image courtesy Wikimedia Commons