Pilots’ spatial disorientation during flight

Spatial disorientation is the major causal factor in numerous aircraft accidents.

It occurs when pilots cannot perceive, understand or anticipate the state of their aircraft or the environment around them.

Spatial disorientation refers to the inability to determine position or relative motion occurring during periods of challenging visibility since vision is the dominant sense of orientation.

The body’s vestibular and proprioceptive systems collectively work to co-ordinate movement with balance, but can also create illusory non-visual sensations in the absence of strong visual cues, resulting in spatial disorientation.

Human senses are not naturally geared for the in-flight environment.

During flight, pilots may experience disorientation and loss of perspective, creating sensory illusions that range from false horizons to sensory conflict with cockpit instrument readings.

Illusions in aviation are caused when the brain cannot reconcile inputs from the vestibular and visual systems.

The visual system is the physiological basis of visual perception – the ability to detect and process light.

The system detects, transduces and interprets information concerning light within the visible range to construct images and build mental models of the surrounding environment.

The visual system is associated with the eye and functionally divided into the optical system including the cornea and lens and the neural system including the retina and visual cortex.

The vestibular system in humans is a sensory system that creates the sense of balance and spatial orientation for the purpose of co-ordinating movement with balance.

Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of the inner ear.

As movements consist of rotations and translations, the vestibular system comprises two components: the three semicircular canals which are interconnected tubes located in the innermost part of the inner ear. They are stimulated by angular accelerations in the pitch, yaw and roll axis and the otoliths, which indicate linear accelerations.

The vestibular system sends signals primarily to the neural structures that control eye movement.

They provide the anatomical basis of the vestibulo-ocular reflex that is required for clear vision.

Signals are also sent to the muscles that keep humans upright and in general control posture.

These provide the anatomical means required to enable pilots to maintain their desired positions during flight.

Illusions involving the semicircular and somatogyral canals of the vestibular system of the ear occur primarily under conditions of unreliable or unavailable external visual references and result in false sensations of rotation. These include the leans, the graveyard spin and spiral, and the Coriolis illusion.

The "leans" is the most common illusion during flight that causes pilots’ spatial disorientation.

Through stabilisation of the fluid in the semicircular canals, a pilot may perceive straight and level flight while actually in a banked turn.

This is caused by a quick return to level flight after a gradual, prolonged turn that the pilot failed to notice.

The phenomenon consists of a false perception of angular displacement about the roll axis and therefore becomes an illusion of bank.

This illusion is often associated with a vestibulospinal reflex that results in the pilot leaning in the direction of the falsely perceived vertical plane.

Changing a person's orientation will cause specific ducts to be stimulated due to these hair cells. When the head turns, the canals move. However, because of its inertia, the endolymph fluid tends to lag and thereby stimulates the hair cells. This stimulation results in awareness of angular acceleration in that plane. After approximately ten-20 seconds, the endolymph velocity matches that of the canal, which stops stimulation of the hair cells, and the person's awareness of rotation is reduced or stopped.

In addition, the canals cannot detect rotational acceleration of approximately two degrees per second or lower which is the brain’s detection threshold of the semicircular canals.

Therefore, a pilot may not notice a slow turn or a bank if entered gradually and maintained long enough.

If the pilot then exits the turn or bank and levels the wings, the endolymph fluid continues to move, re-stimulating the hair cells and producing the illusion that the plane is banking too much in the opposite direction, due to the input lag. As a response, the pilot often leans in the direction of the original turn to attempt to correct and regain their perception of the correct vertical position. The leans may also be caused by peripheral visual orientation cues that become misleading.

When the disorientation is unnoticeable and the pilot continues to lean, the aircraft may over bank in the wrong direction and cause rolling.

In all cases of spatial disorientation, pilots must rely on their cockpit instruments during control inputs to override false sensations.

The black-hole approach is another sensory illusion that can happen during a final approach at night with no stars or moonlight over water or unlit terrain to a lighted runway causing the horizon not to be discernible. During this approach, there is nothing to see between the aircraft and the intended runway, except a visual "black hole".

Pilots may confidently proceed with a visual approach instead of relying on cockpit instruments during night-time landings.

Pilots can experience glide path overestimation because of the lack of peripheral visual cues, especially, below the aircraft. With no peripheral visual cues allowing for an orientation relative to the earth, there can be an illusion of the pilot being upright and the runway being tilted and sloping. Consequently, the pilot initiates an aggressive descent and wrongly adjusts to an unsafe glide path below the desired three-degree glide path.

On January 1, 1978, a Boeing 747-237B aircraft operating Air India Flight 855 on a night flight from Mumbai to Dubai, crashed into the Arabian Sea, less than two minutes after take-off, killing all 213 passengers and crew on board. The crash investigation determined that the most probable cause was the captain becoming spatially disoriented after his cockpit altitude direction indicator toppled.