retroyou nostal(G) 2002 - 2003 : Spatial Disorientation in FlightDynamics

Spatial Disorientation in FlightDynamics

The lack of adequate orientation cues and conflicts between competing sensory modalities are only a partial explanation of disorientation mishaps, however. Why so many disoriented pilots, even those who know they are disoriented, are unable to recover their aircraft has mystified aircraft accident investigators for decades. One possibility is that the psychologic stress of disorientation results in a disintegration of higher-order learned behavior, including flying skills. Another is a complex psychomotor effect of disorientation that causes the pilot to feel the aircraft itself is misbehaving.

Visual Dominance

It is naive to assume that a certain pattern of physical stimuli always elicits a particular veridical or illusory perceptual response. Certainly, when a pilot has a wide, clear view of the horizon, ambient vision supplies virtually all necessary orientation information, and potentially misleading linear or angular acceleratory motion cues do not result in spatial disorientation (unless, of course, they are so violent as to cause vestibulo-ocular disorganization). When a pilot's vision is compromised by night or bad weather conditions, the same acceleratory motion cues can cause spatial disorientation, but the pilot usually avoids it by referring to the aircraft instruments for orientation information. If the pilot is unskilled at interpreting the instruments, if the instruments fail, or, as frequently happens, if the pilot neglects to look at the instruments, those misleading motion cues inevitably cause disorientation. Such is the character of visual dominance, the phenomenon in which one incorporates visual orientation information into a percept of spatial orientation to the exclusion of vestibular and nonvestibular proprioceptive, tactile, and other sensory cues. Visual dominance falls into two categories: the congenital type, in which ambient vision provides dominant orientation cues through natural neural connections and functions, and the acquired type, in which orientation cues are gleaned through focal vision and are integrated as a result of training and experience into an orientational percept. The functioning of the proficient instrument pilot illustrates acquired visual dominance: such an individual has learned to decode with foveal vision the information on the attitude indicator and other flight instruments and to reconstruct that information into a concept of what the aircraft is doing and where it is going, which is then used in controlling the aircraft. This complex skill must be developed through training and maintained through practice, and its fragility is one of the factors that make spatial disorientation such a hazard.

Vestibular Suppression

The term vestibular suppression often is used to denote the active process of visually overriding undesirable vestibular sensations or reflexes of vestibular origin. An example of this aspect of visual dominance is seen in well-trained figure skaters who, with much practice, learn to abolish the postrotatory dizziness, nystagmus, and postural instability that normally result from the high decelerations associated with suddenly stopping rapid spins on the ice. 16 But even these individuals, when deprived of vision by eye closure or darkness, have the very dizziness, nystagmus, and falling that we would expect to result from the acceleratory stimuli produced.17 In flight, the ability to suppress unwanted vestibular sensations and reflexes is developed with repeated exposure to the linear and angular accelerations of flight. As is the case with the figure skaters, however, the pilot's ability to prevent vestibular sensations and reflexes is compromised when visual orientation cues are disrupted by night, weather, and inadequate flight instrument displays.


Opportunism on the part of the primary (ambient visual and vestibular) orientation-information processing systems refers to the propensity of those systems to fill an orientation-information void swiftly and surely with natural orientation information. When a pilot flying in instrument weather looks away from the artificial horizon for a mere few seconds, this is usually long enough for erroneous ambient visual or vestibular information to break through and become incorporated into the pilot's orientational percept. In fact, conflicts between focal visual and ambient visual or vestibular sources of orientation information tend to resolve themselves very quickly in favor of the latter without providing the pilot an opportunity to evaluate the information. It is logical that any orientation information reaching the vestibular nuclei--whether vestibular, other proprioceptive, or ambient visual--should have an advantage in competing with focal visual cues for expression as the pilot's sole orientational percept, because the vestibular nuclei are primary terminals in the pathways for reflex orientational responses and are the initial level of integration for any eventual conscious spatial orientation percept. In other words, although acquired visual dominance can be maintained by diligent attention to synthetic orientation cues, the challenge to this dominance presented by the processing of natural orientation cues through primitive neural channels is very potent and ever present.

Disintegration of Flying Skill

The disintegration of flying skill perhaps begins with the pilot's realization that spatial orientation and control over the motion of the aircraft have been compromised. Under such circumstances, the pilot pays more heed to whatever orientation information is naturally available, monitoring it more and more vigorously. Whether the brain stem reticular activating system or the vestibular efferent system, or both, are responsible for the resulting heightened arousal and enhanced vestibular information flow can only be surmised; the net effect, however, is that more erroneous vestibular information is processed and incorporated into the pilot's orientational percept. This, of course, only makes matters worse. A positive-feedback situation is thus encountered, and the vicious circle can now be broken only with a precisely directed and very determined effort by the pilot. Unfortunately, complex cognitive and motor skills tend to be degraded under conditions of psychologic stress such as occur during Type II or Type III spatial disorientation. First, there is a coning of attention; pilots who have survived severe disorientation have reported that they were concentrating on one particular flight instrument instead of scanning and interpreting the whole group of them in the usual manner. Pilots also have reported that they were unaware of radio transmissions to them while they were trying to recover from disorientation. Second, there is the tendency to revert to more primitive behavior, even reflex action, under conditions of severe psychologic stress. The highly developed, relatively newly acquired skill of instrument flying can give way to primal protective responses during disorientation stress, making appropriate recovery action unlikely. Third, it is suggested that disoriented pilots may become totally immobilized--frozen to the aircraft controls by fear or panic--as the disintegration process reaches its final state.

Giant Hand

The giant hand phenomenon described by Malcolm and Money33 undoubtedly explains why many pilots have been rendered hopelessly confused and ineffectual by spatial disorientation, even though they knew they were disoriented and should have been able to avoid losing control of their aircraft. The pilot suffering from this effect of disorientation perceives falsely that the aircraft is not responding properly to control inputs, because every attempt to bring the aircraft to the desired attitude, seemingly is resisted by its tendency to fly back to another, more stable attitude. A pilot experiencing disorientation about the roll axis (e.g., the leans or graveyard spiral) may feel a force--like a giant hand--trying to push one wing down and hold it there (Fig. 36), whereas the pilot with pitch-axis disorientation (e.g., the classic somatogravic illusion) may feel the airplane subjected to a similar force trying to hold the nose down. The phenomenon is not rare: one report states that 15% of pilots responding to a questionnaire on spatial disorientation had experienced the giant hand.47 Pilots who are unaware of the existence of this phenomenon and experience it for the first time can be very surprised and confused by it, and may not be able to discern the exact nature of their problem. A pilot's radio transmission that the aircraft controls are malfunctioning should not, therefore, be taken as conclusive evidence that a control malfunction caused a mishap: spatial disorientation could have been the real cause.

Figure 36

Figure 36. The giant hand phenomenon. This pilot, who is disoriented with respect to roll attitude (bank angle), feels the aircraft is resisting the attempt to bring it to the desired attitude according to the flight instruments, as though a giant hand is holding it in the desired attitude according to the erroneous sense of bank angle.

What mechanism could possibly explain the giant hand? To try to understand this phenomenon, we must first recognize that an individual's perception of orientation results not only in the conscious awareness of position and motion but also in a preconscious percept needed for the proper performance of voluntary motor activity and reflex actions. A conscious orientational percept can be considered rational, in that one can subject it to intellectual scrutiny, weigh the evidence for its veracity, conclude that it is inaccurate, and to some extent modify the percept to fit facts obtained from other than the primary orientation senses. In contrast, a preconscious orientational percept must be considered irrational, in that it consists only of an integration of data relayed to the brain stem and cerebellum by the primary orientation senses and is not amenable to modification by reason. So what happens when pilots know they have become disoriented and try to control their aircraft by reference to a conscious, rational percept of orientation that is at variance with a preconscious, irrational one? Because only the data comprising one's preconscious orientational percept are available for the performance of orientational reflexes (e.g., postural reflexes) and skilled voluntary motor activity (e.g., walking, bicycling, flying), it is to be expected that the actual outcome of these types of actions will deviate from the rationally intended outcome whenever the orientational data on which they depend are different from the rationally perceived orientation. The disoriented pilot who consciously commands a roll to recover aircraft control may experience a great deal of difficulty in executing the command, because the informational substrate in reference to which the body functions indicates that such a move is counterproductive or even dangerous. Or the pilot may discover that the roll, once accomplished, must be reaccomplished repeatedly, because of the automatic tendency to return the aircraft to its original flight attitude in response to the preconsciously perceived orientational threat resulting from his or her conscious efforts and actions to regain control. Thus, the preconscious orientational percept influences Sherrington's "final common pathway" for both reflex and voluntary motor activity, and the manifestation of this influence on the act of flying during an episode of spatial disorientation is the giant hand phenomenon. To prevail in this conflict between will and skill, the pilot must decouple voluntary acts from automatic flying behavior. It has been suggested that using the thumb and forefinger to move the control stick, rather than using the whole hand, can effect the necessary decoupling and thereby facilitate recovery from the giant hand.47

The salient features of the dynamics of spatial orientation and disorientation are diagrammed in Figure 37 to ease the student's burden of assimilating the rather abstract concepts discussed above. In particular, the relations between the conscious and preconscious orientational percepts, visual dominance, vestibular suppression, opportunism, giant hand, disintegration of flying skill, and other aspects of orientation information flow are presented.

Conditions Conducive to Disorientation

From knowledge of the physical bases of the various illusions of flight, the reader can readily infer many of the specific environmental factors conducive to spatial disorientation. Certain visual phenomena produce characteristic visual illusions such as false horizons and vection. Prolonged turning at a constant rate, as in a holding pattern or procedure turn, can precipitate somatogyral illusions or the leans. Relatively sustained linear accelerations, such as occur on takeoff, can produce somatogravic illusions, and head movements during G-pulling turns can elicit G-excess illusions.

Figure 37

Figure 37. Flow of orientation information in flight. The primary information-flow loop involves stimulation of the visual, vestibular, and other orientation senses by visual scenes and linear and angular accelerations; processing of this primary orientation information by brain stem, cerebellum, and cerebral centers; incorporating the solution into a data base for reflexive and skilled voluntary motor activity (preconscious orientational percept); and effecting control inputs, which produce aircraft motions that result in additional orientational stimuli. A secondary path of information flow involves the processing of symbolic data from flight instruments into derived orientation information by higher cerebral centers. Subloop a provides for feedback from various components of the nervous system, and includes efferent system influences on sensory end-organs. The phenomena of visual dominance, vestibular suppression, and vestibular opportunism occur in conjunction within the functioning of this loop. Subloop b generates conscious perception of orientation, both from the body's naturally obtained solution of the orientation problem and from orientation information derived from flight instrument data. Voluntary control commands arise in response to conscious orientational percepts; and the psychic stress resulting from conflicting orientation information or from apparently aberrantly responding effectors can influence the manner in which orientation information is processed, leading ultimately to disintegration of flying skill. Subloop c incorporates feedback from muscles, tendons, and joints involved in making control inputs, and provides a basis for the giant hand phenomenon.

But what are the regimes of flight and activities of the pilot that seem most likely to allow these potential illusions to manifest themselves? Certainly, instrument weather and night flying are primary factors. Especially likely to produce disorientation, however, is the practice of switching back and forth between the instrument flying mode and the visual, or contact, flying mode; pilots are far less likely to become disoriented if they get on the instruments as soon as out-of-cockpit vision is compromised and stay on the instruments until continuous contact flying is again assured. In fact, any event or practice requiring the pilot to break an instrument cross-check is conducive to disorientation. In this regard, avionics control switches and displays in some aircraft are located where pilots must interrupt their instrument cross-checks for more than just a few seconds to interact with them, and are thus known as "vertigo traps." Some of these vertigo traps require substantial movements of the pilot's head during the time of cross-check interruption, thereby providing both a reason and an opportunity for spatial disorientation to strike.

Formation flying in adverse weather conditions is probably the most likely of all situations to produce disorientation; indeed, some experienced pilots get disoriented every time they fly wing or trail in weather. A pilot who has little if any opportunity to scan the flight instruments while flying formation on the lead aircraft in weather is essentially isolated from any source of accurate orientation information, and misleading vestibular and ambient visual cues arrive unchallenged into the pilot's sensorium,

Of utmost importance to a pilot in preventing spatial disorientation is competency and currency in instrument flying. A noninstrument-rated pilot who penetrates instrument weather is virtually assured of developing spatial disorientation within a matter of seconds, just as the most competent instrument pilot would develop it while flying in weather without runctioning flight instruments. Regarding instrument flying skill, one must "use it or lose it," as they say. For that reason, it is inadvisable and usually illegal for one to act as a pilot in. command of an aircraft in instrument weather without a certain amount of recent instrument flying experience.

Even the most capable instrument pilot is susceptible to spatial disorientation when attention is diverted away from the flight instruments and' the primary task of flying the airplane is neglected. This can happen when other duties, such as navigation, communication, operating weapons, responding to malfunctions, and managing inflight emergencies, place excessive demands on the pilot's attention and lead to "task saturation." In fact, virtually all aircraft mishaps involving Type I spatial disorientation occur as a result of the pilot's failure to prioritize several competing tasks properly. "First, fly the airplane; then do other things as time allows," is always good advice for pilots, especially for those faced with a high mental workload. Not to prioritize in this manner can result in disorientation and disaster .

Finally, conditions affecting the pilot's physical or mental health must be considered capable of rendering the pilot more susceptible to spatial disorientation. The unhealthy effect of alcohol ingestion on neural information processing is one obvious example; however, the less well-known ability of alcohol to produce vestibular nystagmus (positional alcohol nystagmus) for many hours after its more overt effects have disappeared is probably of equal significance. Use of other drugs, such as barbiturates, amphetamines, and especially the illegal "recreational" drugs (marijuana, cocaine, etc.) certainly could contribute to the development of disorientation and precipitate aircraft mishaps. Likewise, physical and mental fatigue, as well as acute or chronic emotional stress, can rob pilots of the ability to concentrate on the instrument cross-check and can, therefore, have deleterious effects on their resistance to spatial disorientation.