This article was written by Dougal Watson. Copyright: Dougal Watson © 1992 - .
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'Flying by the seat of our pants' often refers to daring, sometimes foolhardy, demonstrations of aviation prowess. 'Seat of the pants' implies that something was only barely achieved, as in a baseball batter sliding under a baseman's outstretched arms only just to touch base safely. The baseball runner literally makes it by the seat of his pants. However in aviation the phrase tends to take on a slightly different meaning. Flying by the 'seat of our pants' goes beyond the idea of having 'just made it' to give the impression that the flight was also achieved with limited input from instruments or, for that matter, the senses of sight and balance.
Our senses of sight, hearing, touch, balance, smell, and taste are usually taken for granted in everyday life. We can walk upright, smell the air and listen to sounds around us without giving a second thought to the complex processes at play within our bodies providing us with the various sensations. It is also extremely unusual for our every day terrestrial lives to push our senses to their limits. Our sensory mechanisms have spent millennia evolving to their present level of sophistication well able to cope with most likely terrestrial experiences. Once we leave 'terra firma' and enter the sky our senses take on a whole new importance and some of their limitations may well become apparent. This article describes the senses that we use in flying and outlines some of the limitations our senses may show during flight. Future articles will discuss the interesting, often bizarre, illusions that result when our senses are unable to cope with the flying environment.
Vision is the main sense that we use during flight. Most of our spatial orientation during flight, even when 'on instruments' in IMC, comes from our visual system. If there is any disagreement between vision and the other senses the visual orientation information tends to override the other senses. In this manner vision can also be thought of as being the dominant sense during flight. This visual dominance is a potential cause for illusions and disorientation during flight.
The sense of vision relies on our eyes, nerves from our eyes (Optic nerves), and areas within our brain (Figure 1). Light passes through the clear front cover of the eye, called the cornea, to the lens. The lens focuses the light that then travels to the back of the eye and falls on the retina where it stimulates the special visual cells. Activation of the visual sense cells sends impulses via the optic nerve to the brain. The brain processes this information to produce an image of the surrounding environment. The visual information is used in conscious and subconscious decision making. Some of the information from the eyes bypasses areas of consciousness within the brain and travels directly to regions involved in balance and protective reflexes. Bypassing consciousness in this way saves time. An example of this type of reflex is seen when the eyes flick rapidly closed to prevent an insect or piece of debris entering them even before you are consciously aware of the threat. Similar subconscious reflexes also operate in maintaining balance and orientation. It is often useful to consider our sense of vision as comprising a 'focal' component and an 'ambient' component. Focal vision involves recognition and identification of objects and tends to answer the question 'What?' Focal vision usually involves fine detail and is a function that we are largely aware of at a conscious level. Ambient vision is primarily concerned with spatial localization and orientation and tends to answer the question of 'Where?' Ambient vision does not require fine detail and is usually not recognized at a conscious level. Instrument flight is difficult because it relies on focal vision as the main source of orientation information and does not usually employ the ambient visual system that is especially adapted for orientation purposes.
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Our visual mechanisms are well adapted for everyday terrestrial living but during aviation pursuits a variety of limitations may become apparent. Visual dominance has the potential for incorrectly overriding other sensory information causing disorientation in flight. A number of visual illusions can also occur during the approach and landing phases of flight, especially at night. Visual autokinesis is an illusion where, at night, a small solitary light or group of lights can appear to move when actually they are stationary. Most of us have probably experienced this phenomenon during episodes of star gazing. How many have stopped to think that autokinesis may cause us to mistake a stationary star near the horizon for the lights of another aircraft in flight? Limitations of our visual apparatus also occur during periods of high vibration, high centrifugal acceleration (pulling 'G' during aerobatics), mild oxygen starvation during night flying, and multiple rolls or spins where the movement and fixation of the eyes can't keep up with the rate of rotation.
Aircraft accidents have occurred because of illusions caused by limitations of our sense of vision. Knowledge of these illusions is an important step in the prevention of future accidents. Sensory information of balance and position is provided by the 'vestibular system'. Tiny structures deep within our ears (Figure 2) called 'semicircular canals' and 'otolith organs' sense changes in head position. The semicircular canals and otolith organs provide position and movement information to assist with maintenance of balance and fixation of the eyes on objects. This connection between the inner ear and the eyes helps us keep our eyes on an object while we move our head. These connections also have the potential to cause illusions such as the 'vection illusion' that occurs when you are stopped at traffic lights and the next door car edges forward. A neighbouring car's forward movement, if viewed in your peripheral (ambient) vision, is interpreted by the brain as rearward movement of your car. So strong is the illusion that you are momentarily convinced that you will roll back into the car behind you.
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The semicircular canals are bone lined, fluid filled half circular tubes within the depths of each ear. Each of the three canals in each ear is aligned along a different axis of rotation and contains a small tuft of sensory hairs protruding into the fluid from the canal wall. When there is no rotational acceleration in the plane of a particular canal the fluid is stationary relative to the canal's bony walls. Because no fluid moves past them the sensory hairs stand erect and indicate to the brain that no rotational acceleration is occurring (Figure 3-1). When the head is rotated the inertia of the fluid within the semicircular canals causes it to lag a little behind the bony walls. This relative motion between the fluid and the canal walls leads to bending of the sensory hairs and the transmission of rotational information to the brain (Figure 3-2). If the rotation continues at a constant rate the fluid catches up with the rest of the canal, the sensory hairs stand upright, and an illusion of no rotation occurs (Figure 3-3). Should a constant rotation suddenly cease the fluid will continue to rotate for a short while bending the hairs in the opposite direction and giving the brain the impression of rotation in the opposite direction (Figure 3-4).
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If the rate of change of rotation is sufficiently small the semicircular canals will not register any change in rotation at all. It is therefore possible gradually to build up to a rapid rates of rotation and yet be completely unaware of it unless there is visual information available to override and correct the vestibular system. Limitations in the sensation of rotation by our semicircular canals may result in 'the leans' a common but perplexing illusion during instrument flight or may lead to the aptly named 'graveyard spin'. Another illusion involving the semicircular canals is the 'Coriolis illusion' that results in unexpected dizziness or vertigo when the head is moved during an aircraft turn. These and other semicircular canal illusions will be detailed in later articles.
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The otolith organs provide the brain with information about position of the head and straight line acceleration. They contain sensory hairs with small crystals attached to the free ends (Figure 4). The action of gravity on these otolith crystals provides information concerning head position. Moving an otolith organ from the horizontal causes gravity to pull on the crystals and bend the sensory hairs (Figure 5). This bend in the sensory hairs is usually interpreted by the brain as a change in the orientation of the head.
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When linear acceleration occurs in the plane of an otolith organ the crystals lag a little due to their inertia. This lag causes the sensory hairs to bend and information concerning the acceleration to travel to the brain (Figure 6-1). After a period at constant speed the crystals 'catch up', the hairs stand erect, and no acceleration is sensed (Figure 6-2). When you slow from a constant speed the crystals tend to 'shoot ahead', due to their inertia. This causes bending of the sensory hairs and the sensation of deceleration (Figure 6-3).
In certain aviation situations, most notably take off into a dark night, an illusion may develop due to the otolith organs sensing both head position and straight line acceleration. In the absence of overriding visual information linear acceleration may be incorrectly interpreted as indicating a change in head position. An aircraft's acceleration after rotation may cause the illusion of further nose up pitch (Note similarities between Figures 5-3 and 6-1). This false sensation can lead to the pilot making inappropriate stick forward correction and flying into the ground. Otolith illusions will be discussed in depth in future articles.
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Distributed throughout every muscle, tendon, joint, and area of skin within our body are a variety of sense organs and nerves that provide self position information. During flight these position sensors usually provide information which supports the prime visual and vestibular orientation senses. These senses produce the feeling of extra weight during a steep turn. Another example is found during inverted flight when skin receptors in the shoulder harness region would sense extra pressure and those in the 'seat of the pants' area would sense a reduction in pressure. These sensory functions play an important but secondary role in orientation during flight.
While hearing is important in orientation on the ground it is of limited value during flight. The high noise levels in most aircraft cockpits makes localization of particular sounds difficult. Hearing does play a part in monitoring airspeed, attitude, and engine performance. An increase in intensity or pitch of the background airflow noise may stimulate a pilot to check for increased airspeed. A change in engine or propeller noise may similarly prompt a close check of engine instruments. While sound is able to generate illusions of movement these are not of great importance during flight.
Most of our spatial orientation information during flight is provided by the visual and vestibular senses. Hearing as well as muscle, skin, and joint sense provides additional orientation to supplement the visual and vestibular information. Our senses have evolved to cope with our usual terrestrial domain. During flight the innate limitations of our orientation senses may be exposed and potentially dangerous illusions and loss of orientation may result. The resultant disorientation may or may not be recognized and may cause aircraft accidents. Many pilots have crashed and died due to disorientation in flight. Education and understanding is the main method that we have available to us for avoiding potentially fatal spatial disorientation accidents during flight.
This article was originally published as:
'Visual dominance can lead to illusions and disorientation.'
in the CAA Aviation Bulletin, Number 2, March 1992.