HELICOPTER SOMATOGRAVIC ILLUSIONS
Somatogravic and Somatogyral illusions 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.
The vestibular organs are part of the human body’s mechanism for achieving posture and stability. Changes in linear acceleration, angular acceleration and vertical acceleration (gravity) which occur as a result of flight control inputs, made to accomplish a change in the flight path, are detected by the vestibular system and may create either or both of these illusions.
Either illusion may arise in one of three ways
- In the presence of an imperceptibly slow change in motion – in the case of roll at a rate of less than approximately 2° per second
- After sustained – 10 to 20 seconds – acceleration in any one direction, it may be perceived that the acceleration has stopped.
- When sensory stimulation from the angular motion of the head and of the aircraft occur simultaneously in two different planes – for example interrupting looking ahead to look up to an overhead panel whilst the aircraft is also in angular motion.
Both somatogravic and somatogyral illusions are most likely occur in conditions of marginal external visual reference or in visual ‘dark night’ conditions.
The CFIT/LOC Risk
Whilst there are many situations in which these illusions can occur, one of the most likely, and certainly the most dangerous, is when the positive changes in acceleration, which accompany the initiation of a go around or the transition to initial climb after take off, are occurring. In both cases, the consequences can rapidly lead to CFIT if the condition is not recognised or to LOC if the situation is recognised but the complexities of recovery are mishandled.
Flap and gear retraction and changes in thrust whilst seeking to achieve and maintain a specific climbing flight path involve considerable changes in acceleration which, in turn, are conducive to a somatogravic illusion. As the inputs to establish sustained climb take effect, a perception of excessive pitch-up may occur. This can lead to a fear that the stalling angle of attack may be approaching. The instinctive reaction to this is to push the nose down in the belief that a reduction in pitch to a more “normal” climb angle is being achieved. If the aircraft is banked, either intentionally or unknowingly, at the same time, the perception of angular acceleration may be underestimated because of a somatogyral illusion, leading to an instinctive tendency to increase the angle of bank.
Sometimes, the strength of these illusions may be so intense that even a conscious cross reference to the flight instruments, which do not validate the perception, may be insufficient to lead to a corrective input to the flight controls by the affected individual. However, in a multi-crew aircraft, it is uncommon for both pilots to be similarly affected at the same time and effective monitoring becomes critical if the risk of an unintended excursion is to be avoided.
- Be aware of this risk when operating in conditions where there is insufficient visual reference to assure safe flight entirely by reference to external visual cues.
- Control the aircraft flight path by reference to the flight instruments whenever external visual cues are less than that required to fly by such external reference.
- Anticipate the possibility of a pitch up illusion when acceleration occurs and a pitch down illusion when deceleration occurs. Remember than banking and angular acceleration increase the effects of any illusions.
- At the first sign of any irrational flight control inputs by the other pilot, take control, especially if near the ground or where a sustained climb is essential for terrain safety.
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 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.