TP 13312

•	Foreword
•	Section 1
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•	Neurology
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Orientation and Disorientation

• Vision
• The Vestubular System
• Figure 10 - View of the Right Labyrinth
• Proprioception
• Visual Illusions
• Autokinesis
• Vectional Illusions
• Vestibular Illusions
• Figure 11
• The Leans
• Figure 12 - The Action of the Cupula During Prolonged Rotation
• Prolonged Turns
• The Graveyard Spiral
• The Coreolis Phenomenon
• Types of Illusions

To the earth bound individual, orientation means being aware of one’s body position relative to the earth. Gravity acts towards the centre of the earth and is recognized as down. The aviator however lives in a different world, a world in which proprioceptive senses may give rise to false information. At the top of a loop for example, where centrifugal force replaces gravity, down appears to be up and up appears to be down! 

Disorientation, in the pilot’s sense, (sometimes described as “vertigo”) is to be unable to locate oneself in space and can be one of the most terrifying and lethal of experiences. It has been invoked as a contributory cause in 12% of general aviation accidents and in 15–20% of military accidents. We orient ourselves by vision, the vestibular system and by proprioceptive nerve data. The mental images of orientation that we derive from these impulses are learned from birth and relate to our terrestrial habitat. So strong are these sensations that it is possible to produce nausea by placing us in an environment where what we see is different to what we feel. This is the theoretical basis of motion sickness and will be described later.

Vision

Vision is the strongest orienting sense, and the one to which we turn when other senses fail. It is functionally divided into two parts. One, employing central foveal vision and sharp focus, is concerned with object recognition and is used together with learned conditioned reflexes in instrument flight. The other, ambient orientation, is peripheral, less acute and is directly connected to vestibular function. That the two parts of vision are independent can be observed in a driver who reads a map and follows the road at the same time. Although we can orient ourselves and function normally when the vestibular apparatus is absent or ablated, without vision orientation is much more difficult. However vision can also give rise to illusions both of location and of movement. How strong an impression it can give is amply demonstrated by experiencing an IMAX© movie where the camera appears to plummet the viewer through a ride on a roller coaster!

The Vestibular System

The vestibular system has three functions. It acts to stabilize vision via the oculo-vestibular reflexes, to orient the body in relation to movement in the environment and to give a perception of motion. These functions are performed by two 1.5 cm. structures imbedded in the petrous bones of the skull. Each vestibule (see Fig. 10) consists of three bony, semi-circular canals lined by tubules containing endolymph. Each canal lies in a separate plane of space: one is horizontal, one vertical and one lateral. The canals sense angular accelerations in the planes of yaw, pitch and roll respectively. They are connected at each end to the utricle, a dilated central area in which are the ampullae. In the ampullae delicate hair cells topped by a gelatinous cupola project into the endolymph and move with it like river bottom plants in a current.

The utricle is connected to the saccule and in the floor of these chambers are the macula (sacculi). The macula in the utricle lies in the horizontal plane and that in the saccule lies in the vertical plane. The maculae consist of hair cells projecting into the endolymph and covered by a gelatinous membrane containing tiny calcium carbonate crystals. They are referred to as otoliths and act as linear accelerometers.

The vestibular apparatus has connections to the visual cortex, to the innervation of the extra-ocular muscles and to the vestibular nuclei in the cerebrum. Try holding your hand up in front of your face and then moving it from side to side. The movement does not have to be fast before focused vision of the fingers is lost. Holding the hand still however and moving the head from side to side allows sharp focus to be maintained at much greater rates. Occulovestibular reflexes make this possible.

Figure 10 - View of the Right Labyrinth

Proprioception

Proprioception is of only secondary importance to vision on the ground, but is much less reliable in the air. While flying, centripetal and centrifugal forces compete with gravity and proprioception may be confused. Although proprioception enables the pilot to stabilize his body in the cockpit and gives valuable clues to changing directions and attitudes in visual flying conditions, in instrument conditions “flying by the seat of the pants” can rapidly become lethal. In one experiment private pilots untrained in isntrument flying were placed in a simulator and taken from visual flying conditions into dense cloud where they were required to make a 180° turn. All crashed within 178 seconds!

Visual Illusions

These may be foveal or vectional, that is concerned with central vision or orientation vision. The former type is often associated with landing approaches and is most common where visual clues are reduced or unfamiliar. Apilot approaching an unfamiliar runway with a minor uphill slope, for example, may feel that he is too high and may fall below the normal glide slope. If the fields runs down hill, he may land long. Pilots inexperienced in the Arctic may miscalculate their height on final approach because the trees they use for unconscious reference are shorter than trees in the South. Landing in “whiteout” conditions, where the ground and horizon are obscured, or landing on a smooth, reflective lake makes judgement of height extremely difficult. Particular problems may occur at night when approaching a lighted runway in an otherwise featureless area if in the distance there is a well lit town at a higher elevation. The eye, in the absence of other clues, tends to place the two lighted areas on the same elevation which may lead to premature ground contact.

Autokinesis

There is a special problem at night with small light sources, such as stars or distant ground lights. When watched intently they will appear to move and may be mistaken for other aircraft. This movement of stationary objects is known as autokinesis and has been responsible for accidents. Where the light source is bright or large this illusion is uncommon.

Vectional Illusions

The commonest type of vectional (movement) illusion is that experienced sitting in a car at a traffic light when the adjoining car creeps forward. This causes a sensation of backward movement and often reflex braking. In the rotational plane similar illusions occur. In a darknened chamber where light from a rotating source is reflected on the walls, the movement of the light on the walls is soon replaced by a sensation of body rotation, the walls appearing fixed. Other problems are confusion between ground lights and the stars when flying over prairie areas or pilots orienting the aircraft to sloping cloud decks or to the Northern Lights rather than to the true horizon. 

Vestibular Illusions

These may arise from the otoliths, the semicircular canals or from a combination of the two. They are among the most serious of the illusions and the most likely to cause lethal accidents. 

At rest or in constant motion gravity is the only force acting on the otolithic membrane. We are used to interpreting gravity as a force pointing to the centre of the earth and, when our plane of movement is changed, falsely interpret sensations according to this precept. A pilot accelerating down the runway and rotating to lift-off is exposed to an acceleration which pushes him back in the seat, together with the force of gravity acting downwards. (Fig. 11).

The resultant is interpreted as a single force acting upwards and backwards. Because the brain interprets the force of gravity as being vertical, the sensation is of pitch-up and the pilot may instinctively make a forward stick movement for control. This can complicate the situation because causing negative G stimulates an oculo-vestibular reflex movement of the eyes which gives rise to the sensation that the instrument panel is moving upwards, heightening the illusion. This is the known as the oculogravic illusion. With deceleration, such as that experienced on descent when the flaps are deployed, a pitch down sensation may be felt. These sensations are normal and of no great importance if the pilot is experienced or visual flight is maintained. At night however, particularly taking off from a lighted runway into a dark featureless area, and accident can occur due to inappropriate control movements performed in the transition from visual to instrument flight. Even an experienced pilot can take as long as 7 seconds to adjust.

Figure 11

The Leans

A common form of disorientation is a sensation of incorrect rotation (or absence of rotation) caused by the semicircular canals. The cupola in its neutral position is upright. When the head rotates, the bony canals move but there is inertia in the endolymph. The cupola is therefore deflected leading to a sensation of rotation. Our sensitivity to rotation however is not perfect and can be diminished by any form of distraction. Rotation in the vertical axis of 1-3 per second may not be perceived. If a pilot, flying straight and level, gradually drops the left wing by 15 degrees whilst otherwise occupied and suddenly becomes aware from the instruments of this attitude and corrects it at a much faster rate, only the correction will be sensed. The pilot then feels as though the aircraft has rolled 15 degrees towards the right and will lean towards the left to maintain balance. This is called “the leans” and is extremely common.

Figure 12 - The Action of the Cupula During 

Prolonged Turns

A different problem arises with prolonged turns at a constant rate, such as those encountered in a holding pattern at a busy airport. On entering a turn the cupola is deflected by the inertia of the endolymph (see Fig. 12). As the turn continues the endolymph will begin to move until it is in equilibrium with the bony canal and at this point the cupola will return to its central position. (Depending on the steepness of the turn this may occur in 10-30 seconds). When the turn is terminated the bony canal will cease to rotate immediately but the endolymph, due to inertia, will continue to swirl thus moving the cupola in the opposite direction. This gives rise to the impression that a turn in the opposite direction has been entered and the tendency will be to correct this and so to reenter the original turn.

The Graveyard Spiral

Although this is a minor distraction under most circumstances, in instrument meterological conditions it can be extremely serious and lead to a “graveyard spiral”. Here the inexperienced pilot, having inadvertently entered a steep descending turn under instrument conditions, makes the correct stick movements to control the aircraft but experiencing the sensation of entering a turn in the other direction may re-enter the spiral. As the aircraft is also descending, pulling back on the stick to stop the loss of altitude, although giving rise to a comforting feeling of gravitational pull in the seat, actually steepens the spiral, ultimately driving the aircraft into the ground.

The Coreolis Phenomenon

The most extreme form of vestibular disorientation is due to the Coreolis phenomenon. This is thought to be caused when two different semi-circular canals are stimulated at the same time. As an example, a pilot taking off from an airport in instrument conditions, banks towards the left while climbing. So far there is stimulation of the otolith and of one canal. In order to reach a switch or see a gauge however the pilot turns the head quickly downwards and towards the right. Two different canals have now been stimulated and, as all are connected, a movement of endolymph takes place in the third canal. The result is a sensation of tumbling which may be extreme and worsened by visual problems due to oculo-vestibular reflexes. Even if control of the aircraft can be maintained under these very trying circumstances, the pilot may still be subject to the leans or other abnormal sensations until able to obtain a visual reference. 

Types of Illusions

Distinction is sometimes made between two different type of disorientation. Type I is unrecognized and Type II recognized. Obviously a Type I illusion is more likely to lead to an accident or incident. Illusions are also divided into oculo-gyral (somatogyral) or oculogravic (somato-gravic). An oculogyral illusion is defined as the apparent movement of an object in the visual field resulting from stimulation of the semi-circular canals by angular acceleration. An oculo-gravic illusion is the false perception of tilt induced by stimulation of the otolith by linear accelerations. The terms somato-gyral and somatogravic refer to the resulting body sensations.