The Head-Up Illusion: do you remember it?

The Head-Up Illusion is the feeling of a too high aircraft nose-up pitch, following a sudden forward linear acceleration. The illusion generally occurs in conditions of poor visual cues, such as night operations or instrument meteorological conditions (IMC).

flydubay3

Photo (C) RICK SCHLAMP

I’m not saying this is the cause of the Flydubai flight 981 accident. I am just reviewing the issue because I was reminded of it by the accident scenario: a missed approach and go-around at night with associated poor weather conditions.(See: Flydubai accident Interim Report)

The Head-Up Illusion is the feeling of a too high aircraft nose-up pitch, following a sudden forward linear acceleration. The illusion generally occurs in conditions of poor visual cues, such as night operations or instrument meteorological conditions (IMC).In the absence of external visual references and visual monitoring of instruments, can cause the pilot flying (PF) to push the control column or the side-stick forward to pitch the nose of the aircraft down reducing the aircraft pitch towards inappropriate values. A night take-off from a well-lit airport into a totally dark sky (black hole) can also lead to this illusion, and could result in a crash.

This is a vestibular illusion and belongs to the somatogravic illusions group. The illusion is due to the interaction of unnatural acceleration and its resultant inertia with the otolith organs deep inside our inner ear.

Just let’s remember that two otolith organs, the saccule and utricle, are located in each ear and are set at right angles to each other. The utricle detects changes in linear acceleration in the horizontal plane while the saccule detects gravity changes in the vertical plane. However, the inertial forces resulting from linear accelerations cannot be distinguished from the force of gravity, therefore, they can produce a sensation of change in both horizontal and vertical axes. The combination of these two vectors produces a third resultant vector.

somatogravic

Image modified from Roy DeHart, Fundamentals of Aerospace Medicine, Williams and Wilkins.

The inertial forces resulting from a forward linear acceleration produce a backward displacement of utricle(horizontal)  but also stimulates the saccule (vertical). Therefore, the false sensation is that you are being simultaneously pushed back and down. In the absence of visual references, this produces the illusion that the nose is pitching up.

Another somatogravic illusion that is likely under these conditions is the Inversion Illusion.

inversion illusion

Image modified from Roy DeHart, Fundamentals of Aerospace Medicine, Williams and Wilkins.

The Inversion Illusion involves a steep ascent (forward linear acceleration) in a high-performance aircraft, followed by a sudden return to level flight. When the pilot levels off, the aircraft’s speed is relatively higher. This combination of accelerations produces an illusion that the aircraft is in inverted flight. The pilot’s response to this illusion is to lower the nose of the aircraft.

These are some recent major accidents that have been related with somatogravic illusions and spatial disorientation:

  • Gulf Air Airbus A320 into the Arabian Gulf during an attempted go-around in night visual meteorological conditions (VMC) on Aug. 23, 2000
  • Flash Airlines 737-600 descended into the Red Sea minutes after takeoff on a moonless night from Sharm el Sheikh International Airport in Egypt on January 3, 2004.
  • Armavia Airlines A320 during a missed approach to the Sochi (Russia) airport at night with weather conditions that, while VMC on May 3, 2006 (See: Armavia A320 crash during go-around at night in poor meteorological conditions)
  • Kenya Airways Boeing 737-800 crashed into a swamp after spatial disorientation and loss of control occurred during climb-out on a dark night on departure from Douala Airport in Cameroon, on May 5, 2007
  • Ethiopian Airlines Boeing  737-800 crashed into the Mediterranean Sea shortly after takeoff on a dark and stormy night from Beirut International Airport in Lebanon on January 25, 2010.
  • Afriqiyah Airways Airbus A330-202 during a non-precision approach to Tripoli, Libya, on May 12, 2010.
  • Tatarstan Airlines Boeing 737-500 during a go-around at Kazan, Russia on  17 November 2013.(See: Tatarstan B735 crash during go-around at night. Learning from the recent past)

The somatogravic illusions are little known by the flight crews and they can’t be recreated in existing standard simulators so it is difficult to train pilots to recognize it.

 The head-up illusion and other types of spatial disorientation occur when the pilot’s sensory and perceptual capabilities are exceeded which produces the pilot to experiment false sensations about the aircraft’s motion, position or attitude. Spatial disorientation can happen to any pilot at any time, no matter his/her experience, knowledge and expertise. Often is associated with degraded visual conditions, fatigue, stress and startle, distraction, high workload, highly demanding cognitive tasks and some medications secondary effects.

FURTHER READING

  1. Flydubai accident Interim Report
  2. Armavia A320 crash during go-around at night in poor meteorological conditions
  3. Tatarstan B735 crash during go-around at night. Learning from the recent past
  4. Going around with all engines operating
  5. Speaking of going around
  6. Loss of flight crew airplane state awareness 

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minime2By Laura Victoria Duque Arrubla, a medical doctor with postgraduate studies in Aviation Medicine, Human Factors and Aviation Safety. In the aviation field since 1988, Human Factors instructor since 1994. Follow me on facebook Living Safely with Human Error and twitter@dralaurita. Human Factors information almost every day 

 

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