Spatial Disorientation - every which way but up ....

Although the human body is an extremely complex device, its “design envelope” is fairly limited. A narrow temperature range is essential, and the system performs best in a pedestrian-speed, lg environment. Anything more advanced than walking about in a warm climate needs assistance. Although they are not conscious of the fact, humans constantly monitor their position in relation to the world by means of several sensors which pass positional data to the brain. This organ then builds up its picture of body orientation and motion in relation to the surroundings, and maintains postural stability. The primary sensor is vision, which is aided by the vestibular system of three-axis fluid gyros in the inner ear. Receptors in the muscles, tendons, skin, and joints also contribute data to the overall perception built up by the brain. When the main positional sensor of vision is lost, the brain must rely on the secondary sensors which, unfortunately for aviators, were evolved to cope only with the ambulatory lg environment of early homo sapiens. When the brain receives a conflicting secondary input that says the body is tilting or rotating, and the primary sensor of vision says it is not, or vice versa, the orientation perception becomes confused. This conflict of information is one cause of spatial disorientation (SD). Other causes can be corruption of vision by optical illusions.

Vestibular perceptions
The vestibular system is basically a three-axis rate gyro system, consisting of three 1mm-diameter semi-circular tubes called canals, about 20mm long and meeting at the vestibule. These fixed tubes contain a fluid, endolymph, which, because of inertia, stays still as the head and tubes move.  In a larger-diameter section of each tube is a membrane, the cupula, which is of the same density as the fluid, but contains hair sensors to detect relative movement between the cupula and the fluid. Nerve impulses generated by the hairs indicate to the brain the direction of apparent head movement. If the tertiary sensors do not indicate head movement in relation to the rest of the body e.g. nodding or twisting, then the brain assumes that the whole body has therefore moved. In the vestibule are the otolith organs, similar hair cells arranged fore and aft to sense linear acceleration.

The vestibular system provides highly accurate data in the pedestrian environment. In flight, however, it can be subjected to motions outside its normal performance envelope. The middle-ear sensors have a threshold below which they cannot detect movement and this threshold can be “overlooked” if other sensors provide stronger signals. The sensors are also more accurate in detecting a change in status rather than measuring an absolute level. In flight, the danger of SD arises when vision of the outside world is cut off. Thus blind flying without reference to instruments is impossible. Nobody from this planet can fly by the seat of his pants without losing control—a guaranteed and absolute fact. Simulator and airborne experiments have shown that it takes between 60 and 90 seconds to lose control from straight and level flight, and even less from balanced and skidded turns. The times allow the aircraft to attain an attitude from which recovery would have taken a minimum of l0,000ft. The vestibular system’s primary function is to maintain equilibrium. Part of its function is to control involuntary eye movement, to maintain smooth target tracking as the head moves. Thus, if the head moves 100 left and 50 up, the vestibular system signals via the brain that the eyes must move 100 right and 50 up to maintain track of the point of interest, and to avoid a smearing of the perceived image as the head moves. So, if the vestibular system itself becomes over- stimulated, it can affect vision, and the surroundings appear to continue spinning after the body stops revolving: phenomena enjoyed by children. SD arises when the brain attempts to relate motion stimuli in the airborne environment to its “library” of ground-deduced stimuli examples.

Vestibular illusions
Somatogravic illusion (SI) has been identified as the cause of several accidents, from the early days of night flying to a Royal Air Force Tornado fatal accident last year. The classic SI-induced crash has also claimed several US Navy aircraft during night catapult launches from aircraft carriers. Because, by definition, the pilot is not aware of the illusion, the accidents are usually fatal.  An SI occurs when the otolith orgam sense a sustained linear acceleration. According to the brain’s library, the only sustained acceleration is caused by gravity. As the gravity vector is overlaid by the acceleration vector, the brain produces a resultant force at an intermediate angle which, with eyeballs-in acceleration, is deduced to be a pitch-up. Without visual cues to correct this impression, the pilot will push the nose down in an attempt to regain the correct climb angle. This causes further acceleration, another pitch-up sensation, and another push forward, and the cycle continues until impact with the ground.

A forward acceleration of only 0.lg can lead to an illusion of a 6° pitch-up. A carrier- catapult shot provides a sustained acceleration of 6g. In the Tornado accident, the navigator recognised the situation and twice called for the pilot to pull up, but the accident data recorder showed a steady push until impact on a hillside. Oculogravic illusion (01) is closely linked with SI. As the vestibular system believes the SI, it compensates by automatically moving the eyes in the opposite direction. So a pilot experiencing an SI of pitch-up might also notice a corresponding upward movement of his field of view as the eyes are moved down to compensate for the illusion on deceleration. The effect is the opposite—his vision signals a pitch-down. This is not a problem when good external visual references are apparent, but at night, in cloud, or when viewing a single light at night, with no depth of field, it is deadly. It can be seen that the effects of SI and 01 in the pitch-up illusion should cancel each other out, but when there are no visual cues the SI effect is dominant. Without visual cues or single-point cues, the 0I effect becomes prevalent.

Somatogyral illusion
Somatogyral and oculogyral illusions are similar in cause to their near cousins, the oculogravic and somatogravic illusions outlined above, but are a result of angular motion stimuli rather than linear stimuli. During a prolonged turning manoeuvre at a constant angular speed, the middle ear’s semicircular canals can only detect the start and end of the angular motion. They do not detect the steady state in between. When the steady state is achieved, the time taken for the sensation of turning to drop below the threshold level is between 10 and 20 seconds. The exact time depends on the speed and axis of rotation plus the fidelity of other sensory inputs—with vision as the primary perception source. However, on recovery from the angular motion, the fluid in the canals is accelerated from its steady state as the canals themselves stop moving with respect to the fluid. If this exceeds the threshold level, the action is perceived as the start of rotation in the opposite direction to the original motion. The effect of this illusion—on spin recovery, for instance—can be to make the pilot believe that he has recovered but has then entered a spin in the opposite direction.

Similarly, on stopping a roll, the effect can give the impression of entering a roll in the opposite direction. As the vestibular system drives the direction of subconscious vision, the above condition can lead to an oculogyral illusion. In this, the outside world can appear to gyrate. This illusion is strongest when a single light source is the only visual cue. An illusion similar in effect to both somatogravic and somatogyral illusion is alterenobaric or pressure vertigo. The mechanics of this are unclear, but it is caused by pressure changes in the middle ear affecting the state of the vestibular system. The illusion of vertigo is usually short, but less intense vertigo can last for several minutes.

Flicker vertigo
This phenomenon is caused by a flickering visual stimulus, such as an anti-collision beacon reflecting from cloud, or the shadow of a helicopter’s blades impinging on its cockpit. The usual effect is irritation and annoyance to aircrew, but it can cause the illusion that the aircraft is moving in the opposite direction to that of the shadow. Flicker can also produce mild nausea and epilepsy in those susceptible, a very low percentage of people in general and aircrew in particular.  

Visual illusions
The primary positional sensor is vision, and the visual cues provided to a pilot are the appearance of the external world or the information derived from aircraft instruments. In the external sense, disorientation is not usually a problem if there is a clear view. This can also be misleading, however, if a straight edge of cloud is used as a false horizon or errors of scale are derived from natural cues. The latter was considered a major contributory factor in the fatal crash of a Harrier some years ago. The pilot was flying over rough, snow-covered terrain, and is thought to have misjudged his ground clearance by 7eference to fir trees, which were about 6ft tall instead of the expected 50ft-6Oft of mature trees in that area. The aircraft impacted just under a ridge line during a crossover turn.

Flight over featureless terrain is fraught with danger if visual cues are implicitly relied on. Another visual illusion is “leaning on the Sun”. This happens when an aircraft is approaching the top of a layer of cloud from within. The brightening of the cloud is perceived as “up”, which is “where the Sun is”, and the aircraft is banked to conform with this new vertical. With a few equatorial exceptions the Sun is usually at an angle, however, so the perception that the Sun equals””• rarely correct.

Fascination
Sometimes known as fixation, this is the concentration of the senses on one particular problem, or instrument, or on one outside reference to the exclusion of all else. This usually, but not always, occurs under high workload. In instrument meteorological conditions a pilot might fix his attention on one instrument, and this breakdown of scan can prove fatal. Several attacking pilots have killed themselves by fixating on the target. Hitting the target becomes such a pre- occupation that the ground’s proximity is not noticed until it is too late.  A night approach to a “black hole” airfield or helicopter pad is also a prime time to experience an illusion. Here, the lack of peripheral cues tends to produce an under- shoot. Similarly, the same peripheral cues can cause an undershoot when trying to land on an unusually wide runway, or an over- shoot when landing on a narrow runway.

A problem which has arisen over the last few years is the wearing of night vision goggles (NVG), which provide a flat or 2D representation of the outside world. This makes errors of depth perception more likely, especially when the wearer is operating in an unfamiliar area, where vegetation and man-made artifacts might be considerably different in size from those with which he is familiar. Headup displays have done much to improve the fast-jet pilot’s lot, but when they fail the pilot is left with the standby instruments. In the USA HUDS are not the primary instruments, as they are in Europe. Thus, US fighters usually have a large and central attitude reference. European standby instruments are usually small, and tucked away off the forward line of vision. This means that the pilot must move his head to refer to them (not good for avoiding SD), and that he must cone his attention more, using the focal visual system instead of the ambient visual system.

Other optical illusions include mistaking lights on the earth’s surface for stars, and vice versa. This has led to pilots rolling inverted to put the star-like cluster of a fishing fleet’s lights “above”, and a USAF F16 pilot taking violent avoiding action on a converging 747. When the 747 kept formation through the break, the Eagle pilot realised he had taken action to avoid the lights of Okinawa, some miles away.  

Break-off phenomena
An RAE fast-jet pilot reported that he was at high level when he suddenly had the feeling that he was outside the cockpit, sitting on the wing, and watching himself fly the aircraft. This is an extreme example of the break-off phenomenon, which is not a psychiatric disorder.  Break-off is usually, but not always, experienced by single-seat pilots operating at high level but with low workload—perhaps on a long-range transit flight with little to do. The lack of a marked horizon and the deep blue sky above are other factors. According to the RAF Institute of Aviation Medicine (JAM), about two-thirds of the pilots who experience break-off are not particularly bothered by it. Some enjoy the sensation of remoteness from the world, citing it as one of the pleasures of flying, but the other third find the experience disagreeable.

Some say that they experienced a feeling that the aircraft was balanced precariously, “on a knife edge” or “on a pin head”, and could easily “fall out of the sky”. These pilots can be treated by reassurance, but only if they make their fears known. The JAM points out that break-off is normal, but if the subject is afraid of the phenomenon it could possibly develop into a phobia. The break-off sensation can be interrupted by redirection of attention to something as simple as a cockpit check or a radio call. Without a stimulus the mind wanders.

From Flight International - 18 March 1980