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Spatial Disorientation – Visual Illusions

Various visual illusions are enumerated here. Though there are many an illusions, only the common visual illusions have been briefly discussed.

 

 

 

 

Shape constancy.   The pilot uses the shape of various objects, e.g., the runway to get a perspective of his position in space above the runway. If the runway slopes upwards or downwards, this perspective gets warped. For example, when landing on an up-sloping runway, feeling that he is high on approach, the pilot tends to come in low approach. The down-sloping runway leads to a higher than normal approach.

Size constancy.  Size constancy is extremely important in judging distances. During landing, the size of the runway determines the landing approach slope angle flown by the pilot. In case of variation in the width or the length of the runway, a pilot may err in making his approach.  Say, during combat, in a rapidly changing scenario, distances between aircraft are gauged by the size of the retinal image of other aircraft. Consider a pilot used to flying a large aircraft like a SU-30. He is used to seeing an aircraft of a particular size at a distance of 3 Km. This pilot, during dissimilar combat with a MiG-21, is likely to come in extremely close, expecting the MiG to look the same size as a Su-30.

Visual Auto kinesis.   The apparent wandering of an object or a light when viewed against a visually unstructured background or dark background is called auto kinesis. This is a result of the pursuit tracking movement of the eyes especially if there is little to focus upon. A bright star may be seen as moving in a circle or moving linearly. During night formation flying, when only one wandering light of the lead aircraft is seen, other pilots may have trouble distinguishing the real movements of the aircraft. The pilot should avoid staring at solitary lights for more than a few seconds and establish a reliable reference to some structure in the aircraft, such as the canopy bow.

Linear and Angular Vection.   If a nearby large structure moves forward, there is an illusion that one is slipping backwards. The most familiar situation occurs when one is in a train and an adjacent train moves forward. A false impression is created that your train has started moving in the opposite direction. A person may also perceive a rotational sensation when an image rotates in the surrounding background e.g. in a static simulator or while watching a film especially on big screen like an IMAX.

Black-hole Approach.   The black hole illusion is produced during night landings, when there are no references except for the runway lights. This situation may be worsened when the lights of city on an up-sloping terrain at the end of the runway make the approach look high and the horizon is not distinct. A natural tendency to lower the aircraft nose may cause a crash short of the runway.

False Horizon or Sloping Cloud Deck. A sloping cloud deck may cause the pilot to adjust the aircraft attitude to what is perceived as the real horizon. There is a strong tendency to accept the level appearance of the clouds as the true horizon, especially if the horizon is indistinct. An unperceived angle or bank will lead to loss of altitude, if it is not corrected. This is particularly hazardous when flying near mountainous terrain.

Lean on the Sun Illusion. Terrestrial creatures are accustomed to seeing the brighter part of the horizon above and the darker ground below. This may be reversed, especially when flying in weather or at high altitudes above clouds. In such circumstances, basing ones decisions on such an assumption may result in an accident.

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Reference

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 www.dibujosbiblicos.net and Wikimedia Commons.

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