Spatial Disorientation in Flight

Definitions

Spatial disorientation is a state characterized by an erroneous orientational percept, i.e., an erroneous sense of one’s position and motion relative to the plane of the earth’s surface. Geographic disorientation, or “being lost,” is a state characterized by an erroneous locational percept. These definitions together encompass all the possible positions and velocities, both translational and rotational, along and about three orthogonal earth -referenced axes.

Orientation information includes those parameters that an individual on or near the earth’s surface with eyes open can reasonably be expected to process accurately on a sunny day. Lateral tilt, forward-backward tilt, angular position about a vertical axis, and their corresponding first derivatives with respect to time are the angular positions and motions included; height above ground, forward-backward velocity, sideways velocity, and up-down velocity are the linear position and motions included. Absent from this collection of orientation information parameters are the location coordinates, the linear position dimensions in the horizontal plane. In flight, orientation information is described in terms of flight instrument-based parameters (Fig. 35). Angular position is bank, pitch, and heading; and the corresponding angular velocities are roll rate, pitch rate, and turn rate (or yaw rate). The linear position parameter is altitude, and the linear velocity parameters are airspeed (or groundspeed), slip/skid rate, and vertical velocity. Inflight navigation information is composed of linear position dimensions in the horizontal plane, such as latitude and longitude or bearing and distance from a navigation reference point.

United States Air Force Manual 51-37, Instrument Flying,37 categorizes flight instruments into three functional groups: control, performance, and navigation. In the control category are the parameters of aircraft attitude (i.e., pitch and bank} and engine power or thrust. In the performance category are airspeed, altitude, vertical velocity, heading, turn rate, slip/skid rate, angle of attack, acceleration (G loading), and flight path (velocity vector). The navigation category includes course, bearing, range, latitude/longitude, time, and similar parameters useful for determining location on the earth’s surface. This categorization of flight instrument parameters allows us to construct a useful operational definition of spatial disorientation: it is an erroneous sense of the magnitude or direction of any of the control and performance flight parameters. Geographic disorientation, in contrast, is: an erroneous sense of any of the navigation parameters. The practical utility of these operational definitions is that they can establish a common understanding of what is meant by spatial disorientation among all parties investigating an aircraft mishap, whether they be pilots, flight surgeons, aerospace physiologists, or representatives of some other discipline. If the answer to the question, “Did the pilot not realize the aircraft’s actual pitch attitude and vertical velocity (and/or other control or performance parameters)?” is “Yes,” then it is obvious that the pilot was spatially disoriented, and the contribution of the disorientation to the sequence of events leading to the mishap is clarified.

Sometimes aircrew tend to be imprecise when they discuss spatial disorientation, preferring to say that they “lost situational awareness” rather than “became disoriented,” as though having experienced spatial disorientation stigmatizes them. Situational awareness involves a correct appreciation of a host of conditions, including the tactical environment, location, weather, weapons capability, administrative constraints, etc., as well as spatial orientation. Thus, if the situation about which a pilot lacks awareness is the aircraft’s position and motion relative to the plane of the earth’s surface, the pilot has spatial disorientation, as well as a loss of situational awareness, generally.

Types of Spatial Disorientation

We distinguish three types of spatial disorientation in flight: Type I (unrecognized), Type II (recognized), and Type III (incapacitating). In Type I disorientation, no conscious perception of any of the manifestations of disorientation is present; i.e., the pilot experiences no disparity between natural and synthetic orientational percepts, has no suspicion that a flight instrument (e.g., attitude indicator) has malfunctioned, and does not feel that the aircraft is responding incorrectly to control inputs. In unrecognized spatial disorientation the pilot is oblivious to his or her disorientation, and controls the aircraft completely in accord with and in response to a false orientational percept. To distinguish Type I disorientation from the others, and to emphasize its insidiousness, some pilots and aerospace physiologists call Type I spatial disorientation “misorientation”.

In Type II disorientation, the pilot consciously perceives some manifestation of disorientation. Pilots may experience a conflict between what they feel the aircraft is doing and what the flight instruments show that it is doing. Or the pilot may not experience a genuine conflict, but may merely conclude that the flight instruments are incorrect. The pilot also may feel that the aircraft is attempting to assume a pitch or bank attitude counter to the intended one. Type II disorientation is the kind to which pilots are referring when they use the term “vertigo,” as in “I had a bad case of vertigo on final approach.” Although Type II spatial disorientation is labeled “recognized,” this does not mean that pilots necessarily realize they are disoriented: they may only realize that there is a problem in controlling the aircraft, not knowing that the source of the problem is spatial disorientation.

With Type III spatial disorientation, the pilot experiences an overwhelming– i.e., incapacitating–physiologic response to physical or emotional stimuli associated with the disorientation event. For example, the pilot may suffer from “vestibulo-ocular disorganization” due to the presence of vestibular nystagmus, so that the flight instruments cannot be read and a stable view of the outside world cannot be obtained. Or, control of the aircraft may be impeded by strong vestibulospinal reflexes affecting the shoulder and arm muscles. The pilot may even be so incapacitated by fear that rational decisions may be thwarted–e.g., the pilot may freeze on the controls. The important feature of Type III disorientation is that the pilot is disoriented and most likely knows it, but can’t do anything about it.

An orientational percept is a sense of one’s linear and angular position and motion relative to the plane of the earth’s surface. It can be primary (i.e., natural), meaning that it is based on ambient visual, vestibular, or other sensations that normally contribute to spatial orientation in our natural environment; or it can be secondary (i.e., synthetic), meaning that it is intellectually constructed from focal visual, verbal, or other symbolic data, such as that presented by flight instruments. While the former type of orientational percept is essentially irrational and involves largely preconscious mental processing, the latter type is rational and entirely conscious. A locational percept, to be distinguished from an orientational percept, is a sense of one’s position in (as opposed to relative to) the plane of the earth’s surface. An accurate locational percept is achieved by reading a map or knowing the latitude and longitude of one’s location.

Examples of Disorientation

The last of four F-15 Eagle fighter aircraft took off on a daytime sortie in bad weather, intending to follow the other three in a radar in-trail departure. Because of a navigational error committed by the pilot shortly after takeoff, he was unable to find the other aircraft on his radar. Frustrated, the pilot elected to intercept the other aircraft where he knew they would be in the arc of the standard instrument departure, so he made a beeline for that point, presumably scanning his radar diligently for the blips he knew should be appearing at any time. Meanwhile, after ascending to 4000 ft (1200 m) above ground level, he entered a descent of approximately 2500 ft/min (13 m/sec) as a result of an unrecognized 3° nose-low attitude. After receiving requested position information from another member of the flight, the pilot either suddenly realized he was in danger of colliding with the other aircraft or he suddenly found them on radar, because he then made a steeply banked turn, either to avoid a perceived threat of collision or to join up with the rest of the flight. Unfortunately, he had by this time descended far below the other aircraft and was going too fast to avoid the ground, which became visible under the overcast just before the aircraft crashed. This mishap resulted from an episode of unrecognized, or Type I, disorientation. The specific illusion responsible appears to have been the somatogravic illusion, which was created by the forward acceleration of this high-performance aircraft during takeoff and climb-out. The pilot’s preoccupation with the radar task compromised his instrument scan to the point where the false vestibular cues gained access to his perceptual processing. Having unknowingly accepted an inaccurate orientational percept, he controlled the aircraft accordingly until it was too late to recover .

Examples of recognized, or Type II, spatial disorientation are easier to obtain than are examples of Type I because most experienced pilots have anecdotes to tell about how they “got vertigo” and fought it off. Some pilots were not so fortunate, however. One F -15 Eagle pilot, after climbing his aircraft in formation with another F-15 at night, began to experience difficulty in maintaining spatial orientation and aircraft control upon leveling off in clouds at 27,000 ft (8,200 m). “Talk about practice bleeding,” he commented to the lead pilot. Having decided to go to another area because of the weather, the two pilots began a descending right turn. At this point, the pilot on the wing told the lead pilot, “Jim flying upside down.” Shortly afterward, the wingman considered separating from the formation, saying, “I’m going lost wingman.” Then he said, “No, I’ve got you,” and finally, “No, I’m going lost wingman.” The hapless wingman then caused his aircraft to descend in a wide spiral, and crashed into the desert less than a minute later, even though the lead pilot advised the wingman several times during the descent to level out. In this mishap, the pilot probably suffered an inversion illusion upon leveling off in the weather, and entered a graveyard spiral after leaving the formation. Although he knew he was disoriented, or at least recognized the possibility, he still was unable to control the aircraft effectively. That pilots can realize being disoriented, see accurate orientation information displayed on the attitude indicator, and still fly into the ground always strains the credulity of nonaviators. Pilots who have had spatial disorientation, who have experienced fighting oneself for control of an aircraft, are less skeptical.

The pilot of an F -15 Eagle, engaged in vigorous air combat tactics training with two other F -15s on a clear day, initiated a hard left turn at 17,000 ft (5,200 m) above ground level. For reasons that have not been established with certainty, his aircraft began to roll to the left at a rate estimated at 150 to 180°/sec. He transmitted, “Out-of-control autoroll,” as he descended through 15,000 ft (4,600 m). The pilot made at least one successful attempt to stop the roll, as evidenced by the momentary cessation of the roll at 8,000 ft (2,400 m); then the aircraft began to roll again to the left. Forty seconds elapsed between the time that the rolling began and the time that the pilot ejected–but too late. Regardless of whether the rolling was caused by a mechanical malfunction or was an autoroll induced by the pilot, the likely result of this extreme motion was vestibulo-ocular disorganization, which not only prevented the pilot from reading his instruments but also kept him from orienting with the natural horizon. Thus, Type III disorientation probably prevented him from taking appropriate corrective action to stop the roll and keep it stopped; if not that, it certainly compromised his ability to assess accurately the level to which his situation had deteriorated.

Statistics

Because the fraction of aircraft mishaps caused by or contributed to by spatial disorientation has doubled over the four decades between 1950 and 1990, one might conclude that continuing efforts to educate pilots about spatial disorientation and the hazard it represents have been to no avail. Fortunately, the total number of major mishaps and the number of major mishaps per million flying hours have dropped considerably over the same period (at least in the United States), so it appears that such flying safety education efforts actually have been effective.

A number of statistical studies of spatial disorientation mishaps in the United States Air Force provide an appreciation of the magnitude of the problem in military aviation. In 1956, Nut tall and Sanford38 reported that, in one major air command during the period of 1954 to 1956, spatial disorientation was responsible for 4% of all major aircraft mishaps and 14% of all fatal aircraft mishaps. In 1969, Moser39 reported a study of aircraft mishaps in another major air command during the four-year period from 1964 through 1967: He found that spatial disorientation was a significant factor in 9% of major mishaps and 26% of fatal mishaps. In 1971, Barnum and Bonner40 reviewed the Air Force mishap data from 1958 through 1968 and found that in 281 (6%) of the 4679 major mishaps, spatial disorientation was a causative factor; fatalities occurred in 211 of those 281 accidents, accounting for 15% of the 1462 fatal mishaps. A comment by Barnum and Bonner summarizes some interesting data about the “average pilot” involved in a spatial disorientation mishap: “He will be around 30 years of age, have 10 years in the cockpit, and have 1500 hours of first pilot/instructor-pilot time. He will be a fighter pilot and will have flown approximately 25 times in the three months prior to his accident.” In an independent 1973 study, Kellogg41 found the relative incidence of spatial disorientation mishaps in the years 1968 through 1972 to range from 4.8% to 6.2% and confirmed the high proportion of fatalities in mishaps resulting from spatial disorientation. The major (Class A) Air Force mishaps over the ten-year period from 1980 through 1989 were reviewed by Freeman (personal communication, 1990). He found that 81 (13%) of the 633 major mishaps during that period, and 115 (14%) of the 795 fatalities, were due to spatial disorientation. If we consider only the mishaps caused by operator error, disorientation accounted for approximately one-fourth of these (81 out of 356). If we only consider the Air Force’s front-line fighter/attack aircraft, the F-15 and F-16, nearly one-third (26 of 86) of the losses of these aircraft resulted from spatial disorientation. The cost of the Air Force aircraft destroyed each year in disorientation mishaps until the decade of the 1980s was on the order of $20 million per year. From 1980 through 1989, over $500 million dollars worth of Air Force resources were lost as a result of spatial disorientation. Currently, the average annual dollar cost of spatial disorientation to the Air Force is on the order of $100 million; but occasional losses of particularly expensive aircraft result in much higher figures in some years.

Regarding the fractions of the disorientation-related mishaps for which the various types of spatial disorientation are responsible, the conventional wisdom is that more than half of the mishaps involve Type I disorientation, most of the remainder involve Type II, and very few involve Type III. The same wisdom suggests that the source of the disorientation is visual illusions in about half of the mishaps, and vestibular/somatosensory illusions in the other half, with combined visual and vestibular illusions accounting for at least some of the mishaps. An analysis of Air Force aircraft mishaps in 1988, in which spatial disorientation was suspected by the investigating flight surgeon, revealed that all 8 involved Type I; 2 apparently resulted from visual illusions, 3 from vestibular illusions, and 3 from mixed visual and vestibular illusions.42

The recent experience of the United States Navy with spatial disorientation is also instructive.43 During the years 1980 through 1989, 112 Class A flight mishaps involved spatial disorientation as a definite, probable, or possible causal factor. Of the 40 mishaps in the “definite” category, 20 occurred in daytime and 20 happened at night; 17 occurred during flight over land, and 23 resulted during flight over water. Thirty-two aircraft, including 15 fighter/attack aircraft, 6 training aircraft, and 11 helicopters, were destroyed; and 38 lives were lost in the 13 fatal mishaps out of the 40 Class A mishaps. The mean experience for the Navy pilots involved in spatial disorientation mishaps was 1488 hours (median: 1152 hours), nearly the same as that for Air Force pilots. Surprisingly, the incidence of spatial disorientation-related mishaps for the Air Force, Navy, and Army has been remarkably similar over the years, even though the flying missions of the several military services are somewhat different. 44,45.

One problem with the mishap statistics related above is that they are conservative, representing only those mishaps in which disorientation was stated to be a possible or probable factor by the Safety Investigation Board. In actuality, many mishaps resulting from spatial disorientation were not identified as such because other factors–such as distraction, task saturation, and poor crew coordination–initiated the chain of events resulting in the mishap; these factors were considered more relevant or more amenable to correction than the disorientation that followed and ultimately caused the pilot to fly the aircraft into the ground or water. In the Air Force from 1980 through 1989, 263 mishaps and 425 fatalities, at a cost of over two billion dollars, resulted from “loss of situational awareness” (Freeman, J .E.; personal communication, 1990). It is apparent that the great majority of those mishaps would not have happened if the pilots had at all times correctly assessed their pitch/bank attitude, vertical velocity, and altitude–i.e., if they had not been spatially disoriented. Thus we can infer that spatial disorientation causes considerably more aircraft mishaps than the disorientation-specific incidence statistics would lead us to believe, probably two or three times as many.

Although statistics indicating the relative frequency of spatial disorientation mishaps in air-carrier operations are not readily available, it would be a serious mistake to conclude that there have been no air-carrier mishaps caused by spatial disorientation. Fourteen such mishaps occurring between 1950 and 1969 were reportedly due to somatogravic and visual illusions that resulted in the so-called “dark-night takeoff accident.”31 In addition, 26 commercial airliners were involved in jet-upset incidents or accidents during the same period.33 Spatial disorientation also is a problem in general (nonmilitary, nonair-carrier) aviation. Kirkham and colleagues 46 reported in 1978 that although spatial disorientation was a cause or factor in only 2.5% of all general aviation aircraft accidents in the United States, it was the third most common cause of fatal general aviation accidents. Of the 4012 fatal general aviation mishaps occurring in the years 1970 through 1975, 627 (15.6%) involved spatial disorientation as a cause or factor. Notably, 90% of general aviation mishaps in which disorientation was a cause or factor were fatal.

Part of the process of learning how to fly solely by reference to flight instruments, as opposed to flying by visual reference to the outside world, involves acquiring an ability to select and process information and to deselect unreliable information cues. Visual dominance and vestibular suppression are concepts of how this ability is manifested.