The somatogravic illusion is a vestibular illusion which is prevalent during high accelerations/deccelerations when a pilot has no clear visual reference (Wilson, 1995 8).
The word Somatogravic is derived from somato meaning ‘of the body’ and gravic meaning ‘pertaining to the gravitational force’ and is a strong pitching sensation (either up or down) when the body is exposed to either high acceleration or decelerations (Kern, 1998 1). This illusion is due to the interaction of unnatural accelerations (such as those experienced in an aircraft) on our Otolith Organs, specifically our utricle (Massey University, 2011 2).
Normal Operation of the Balance System
Our vestibular system uses the Otolith Organs (the sacculus and the utricle) to detect accelerations. Our sacculus detects accelerations in the vertical plane and our utricle detects accelerations in the horizontal place. Our Otolith Organs are designed to help us sense tilt (i.e. if our head is upright). If we tilt our head backwards, the hairs in our utricle bend backwards (due to the acceleration force of gravity) and so sense this as a tilt (vice versa for a forward head tilt) (Purves et al, 2001 4).
If we accelerate rapidly in the horizontal plane this has a very similar effect on the hairs in the utricle—they bend backwards. As far as our brain is concerned this is the same sensation as the movement of a head tilt (Nordian, n.d. 3). This concept is shown in the figure below. Note how the hair (or macula) movement for head tilt backward and acceleration forward are exactly the same.
|Forces acting on head (image embedded from Neuroscience (2nd Ed.) on 14 August 2012)|
Consequences for Aviation
In aviation we are faced with the combination of rapid acceleration and reduced/no visual cues (i.e. IMC and/or night flying). As we no longer have the benefit of our visual system to resolve the ambiguity, our brain uses the signals it is receiving and interprets them as a ‘tilt’. The net result is a tilt back (i.e. pitching up) sensation under acceleration, and a tilting forward (i.e. pitching down) sensation under deceleration (Wilson, 1995 8).
Typically this occurs during the missed approach or go-around segment of a flight at night or in IMC. Speed is slow, power is rapidly applied and the aircraft then accelerates rapidly (U.S. Navy, n.d. 7). As no visual cues exist, this generates a strong ‘tilt back’ sensation which the pilot interprets (incorrectly) as a rapid pitching up sensation. Despite this perception the aircraft may still actually be in a level attitude or only a slight climb (Tait, 2003 6). This is the somatogravic illusion. The pilot will then push forward on the control column to control this (imaginary) climb thinking they are lowering the aircraft nose back to level flight, when in actual fact they are lowering the nose into a dive. As the aircraft nose lowers, the aircraft continues to accelerate, generating additional pitch up sensations, causing the pilot to lower the nose even further. Tragically, this illusion normal ends with the pilot commanding the aircraft into a high speed steep dive and contact with the ground quickly ensues (Schappert, n.d. 5). This effect is shown below.
|Perceived and Actual Flight Paths due to the Somatogravic Illusion (image embedded from MzeroA.com on 14 August 2012)|
An example of these fatal consequences can be clearly seen in Gulf Air Flight 072, an A320 which crashed into the Gulf of Bahrain. During an overshoot on a dark night, the pilot’s were overcome with somatogravic illusion and the aircraft entered a 15 degree dive impacting the water shortly thereafter with no survivors (Nordian, n.d. 3).
Whilst not common (as high horizontal deceleration forces are rare in aviation), the reverse also applies. If a strong deceleration force is present and no visual cues exist, the pilot may experience a false pitch down sensation forcing them to raise the nose, possibly to the point where the aircraft stalls (Nordian, n.d. 3).