Somatogravic Illusion – Pilot Knowledge Bank

Somatogravic and Somatogyral illusions are recurring factors in aviation accidents and are the two most common forms of vestibular or ‘false sensation’ illusion which may be encountered when no clear horizon is present and flying wholly or partly by visual external reference is attempted.

In essence “SI” is a false sensation that the body has tilted.  If SI is not recognised, a pilot may choose to “correct” for this false sensation and unknowingly put themselves into a loss of control situation.

Humans were not designed to fly in three dimensions, and the limitations of our sensory systems cause our balance organs to be unable to differentiate between acceleration and a climb or descent.

The sensory organ responsible for presenting acceleration data to the brain are the semicircular canals, of Otoliths, embedded within the ear. They contain tiny hair like structures, which are biased to the vertical, and which react to acceleration by bending rearwards.

This occurs in three canals at right angles to each other, and therefore present data on lateral movement as well. This may cause an associated phenomenon called somatogyral illusion.

In a car, or in visual flying conditions, the brain uses the other senses to correctly assess the vertical and horizontal, but on dark featureless nights or in instrument meteorological conditions (IMC), the brain can be tricked into intercepting acceleration as a tilt back or climb.

RELEVANCE TO PILOTS

The danger to aviation occurs when a pilot, sensing an acceleration as a steeper than commanded climb, pushes forward on the controls. It is most likely to occur in the Take Off and Go Around phases. The simulator is not helpful in training to combat SI as they are usually limited to 1G and do not accurately replicate the illusions.

Rather insidiously, the illusion is self reinforcing because the very act of checking forward on the controls lowers the nose and the aircraft accelerates, further exacerbating the illusion.

There have been numerous accidents around the world, attributed to SI. The Ethiopian 737-800 and Gulf Air accidents are two recent examples.

HOW CAN WE COMBAT SOMATOGRAVIC ILLUSION?

  1. Regular practice in IFR flying techniques is critical
  2. The importance of an effective “scan” cannot be underestimated. Especially the ADI (Attitude Direction Indicator) and VSI (Vertical Speed Indicator)
  3. Be aware that you may be presented with misleading information and sensations
  4. Do not revert to instinct in stressful situations
  5. Make the instruments “look correct” for the phase of flight
  6. Finally, believe your instruments and, if in doubt, cross check with the other crew member instruments.

Accident and Incident Examples

  • A320 Bahrain 2000: On 23 August 2000, a Gulf Air Airbus A320 flew at speed into the sea during an intended dark night go around at Bahrain and all 143 occupants were killed. It was subsequently concluded that, although a number of factors created the scenario in which the accident could occur, the most plausible explanation for both the descent and the failure to recover from it was the focus on the airspeed indication at the expense of the ADI and the effect of somatogravic illusion on the recently promoted Captain which went unchallenged by his low-experience First Officer.

  • A330 Tripoli 2010: On 12 May 2010, an Afriqiyah Airways ethiopean Airbus A330 making a daylight go around from a non precision approach at Tripoli, which had been discontinued after visual reference was not obtained at MDA, did not sustain the initially established IMC climb and, following flight crew control inputs attributed to the effects of somatogravic illusion and poor CRM, descended rapidly into the ground with a high vertical and forward speed, The aircraft was destroyed by impact forces and the consequent fire and all but one of the 104 occupants were killed.
  • A310 vicinity Quebec 2008: On 5 March 2008, an Air Transat A310-300 was unintentionally mishandled by the flight crew during, and shortly after, departure from Quebec and effective control of the aircraft was temporarily lost. Whilst it was concluded that the origin of the initial difficulties in control were a result of confusion which began on the take off roll and led to a take off at excessive speed followed by subsequent mismanagement and overload, the inappropriate steep descent that followed was attributed to the effect of somatogravic illusion in respect of aircraft attitude control in conjunction with a singular focus on airspeed.