The spatial disorientation problem in the United States Air Force.

Spatial disorientation (SD) in flight wastes hundreds of millions of dollars worth of defense capability annually and continues to kill air-crew. SD results primarily from inadequacies of human visual and vestibular sensory systems in the flying environment; but other factors, such as task saturation and distraction, precipitate it. The United States Air Force is conducting a three-pronged research and development effort to solve the SD problem. We are attempting 1) to elucidate further the mechanisms of visual and vestibular orientation and disorientation, 2) to develop ground-based and inflight training methods for demonstrating to pilots the potential for SD and the means of coping with it, and 3) to conceive and evaluate new ways to display flight control and performance information so that pilots can maintain accurate spatial orientation.

G-tolerance standards for aircrew training and selection.

G tolerance widely among individuals. It stands to reason that aircrew with higher G tolerance are less likely to experience symptoms of G stress in flight than are those with lower G tolerance, and that they can fly highly maneuverable aircraft with greater safety and effectiveness. To assure that aircrew with abnormally low G tolerance are not assigned to aircraft that operate in the high-G environment, a G-tolerance standard and the means to implement that standard are necessary. Since 1977, for human centrifuge operations, the USAF School of Aerospace Medicine has used an informal G-tolerance standard for selecting experimental subjects, evaluating medically disqualified aircrew, and ensuring efficacy of high-G training for aircrew. That standard consists of the subject's being able to sustain a rapidly applied +7-Gz load for 15 s, without totally losing peripheral vision or losing consciousness, while wearing a functioning anti-G suit, performing an anti-G straining maneuver, and sitting in a conventionally configured fighter aircraft seal. Inability to tolerate a 7-G, 15-s, rapid-onset G profile in a centrifuge is also the basis of internationally recognized (NATO, ASCC) definitions of low G tolerance. The rationale for choosing the 7-G, 15-s standard is discussed. Experience with use of this standard, and the equivalent standard of 8 G for 15 s when the F-16-configured seat is used, reveals that fewer than 1% of actively flying aircrew are unable to meet the standard. Eventually a formal, more stringent, G-tolerance standard may become a valuable component of the means of selecting and training aircrew for high-performance fighter aircraft.

High-G stress and orientational stress: physiologic effects of aerial maneuvering.

G stress can readily incapacitate pilots of modern fighter aircraft and result in mishaps due to G-induced loss of consciousness (GLC). The physiologic effects of high-G stress, including decreased head-level blood pressure due to hydrostatic pressure drop and decreased cardiac output due to inadequate venous return, result in the symptoms of visual loss and GLC. The body's primary natural defenses against the effects of G stress in flight, i.e., the neural tissue energy reserve and the cardiovascular baroceptor reflexes, determine the characteristic shape of the G-time tolerance curve, which is presented. Means of raising G tolerance fall into three categories: mechanical, physiological, and educational. Mechanical means include anti-G suits and valves, assisted positive-pressure breathing (APPB) systems, and special seats in which the seatback is reclined and/or the pilot's legs are elevated. Physiological means include frequent exposure to G stress, physical conditioning (weight training and moderate aerobic conditioning), selection of pilots for high natural tolerance, and performance of a vigorous and efficient anti-G straining maneuver. Educational means include briefings on methods of enhancing tolerance, and high-G training in a centrifuge to allow the pilot to perfect his anti-G straining maneuver. An improved anti-G valve, physical conditioning, high-G awareness briefings, and centrifuge training are now being applied in efforts to prevent GLC in current fighter aircraft. Future generations of even more maneuverable aircraft will probably necessitate the use of APPB, pilot selection, and high-G seats for protection of pilots from the effect of sustained high G forces.

Medical standards for experimental human use in acceleration stress research.

Acceleration research is necessary to ensure optimum protection for individuals flying high performance fighter aircraft. Human volunteers exposed to high sustained +Gz stress must be carefully screened to assure that no one is at increased risk for G-induced trauma. Rigorous medical standards for qualifying research subjects must be established and followed. Careful documentation of G-related symptoms and physiologic disturbances enhances the safety aspects of human experimentation. No severe or life-threatening incidents have occurred. A number of symptoms resulting from +Gz exposure have been documented with loss of consciousness being the most frequently documented symptom. The most frequent medical reason for disqualifying an individual volunteer from participation in the acceleration program was because of irregularities noted on spinal x-rays. The current medical standards and clinical diagnostic testing used to screen volunteer subjects at the USAF School of Aerospace Medicine are reviewed along with the symptoms which have resulted over a three year period of high sustained +Gz stress exposures.

High-G training for fighter aircrew.

From 8 Jan 85 through 12 Feb 86, 741 USAF fighter aircrew from Tactical Air Command underwent high-G training at the USAF School of Aerospace Medicine, the major objectives of the training being to increase their understanding of G stress and G protection and to raise their G tolerance. The didactics centered on discussion of the G-time tolerance curve and demonstration of an effective anti-G straining maneuver (AGSM). Exposure to G stress on the USAFSAM centrifuge allowed the trainees to determine their G tolerances and to perfect and practice their AGSM. The trainees' mean relaxed and straining G tolerances on the gradual-onset run (GOR) without anti-G suit inflation were 5.2 and 8.3 G, respectively; and 41% of the trainees reached the 9.0-G run limit. All but two of the trainees completed the 8-G, 15-s rapid-onset run (ROR) with anti-G suit inflated; 94% completed the 9-G, 15-s ROR; and 93% were able to tolerate the 9-G, 10-s ROR while looking back over their left shoulder. G-induced loss of consciousness (G-LOC) occurred in 9% of the trainees, most commonly on the GOR, less often on the 9-G RORs. Motion sickness was a significant problem in less than 1% of the trainees. Critiques provided by 382 trainees revealed 73% enthusiastic or positive assessments of the training, as opposed to only 2% negative or hostile responses. We conclude that high-G training is well-tolerated by fighter aircrew and is a highly appropriate method for minimizing the potential for aircraft mishaps due to G-LOC in flight.

Changes in clinical cardiologic measurements associated with high +Gz stress.

Because of reports of subendocardial hemorrhage and myofibrillar degeneration in animals exposed to sustained high G loads (greater than +6 Gz for 15 s or more), questions have been raised regarding the safety of exposing pilots and human subjects to the similar G-stress levels likely to be encountered in the new high-performance fighter aircraft. Noninvasive clinical cardiologic data, including ECGs, vectorcardiograms, systolic time intervals, and maximal treadmill stress tests, were obtained from two groups of subjects before and at several times after exposure to high-G stress. The group exposed to the greater G stress (three 40-s runs at 8 G and two 40-s runs at 10 G, all in one day) developed moderate cutaneous petechiasis and had other minor physical findings after the G stress, but showed few significant changes in cardiologic data: serum total CPK and LDH levels rose, and preejection period shortened at 48 h poststress. The group exposed to the lesser G stress (one 100-s variable-G maneuver peaking twice at 8 G for 3 s, once a week for 3 weeks) had no symptoms following the G stress, but the vectorcardiograms revealed transient T-loop angle changes, and preejection period measured at 1 week poststress was significantly decreased. Because the serum enzyme changes were noncardiac in origin, and because the few other changes were not in a direction indicative of cardiac damage, we conclude that the G stresses imposed in these studies were not significantly injurious.

Physiologic and anti-G suit performance data from YF-16 flight tests.

Biomedical data were collected during high-G portions of 11 YF-16 test flights. Test pilots monitored revealed increased respiratory rate and volume, decreased tidal volume, and increased heart rate at higher G levels, with one pilot exhibiting various cardiac arrhythmias. Anti-G suit inflation and pressurization lags varied inversely with G-onset rate, and suit pressurization slope was near the design value.

Transfer functions for arterial oxygen saturation during +Gz stress.

Discrete, finite, Fourier transforms of input +Gz stress and output arterial oxygen saturation (Sao2) were used to obtain transfer functions between G and Sao2 in 12 subjects. Ensemble averaging of transfer functions based on responses to variable G stress provided reasonable capability for predicting Sao2 responses to sustained 6-G stress. Prediction of responses to variable G stress from transfer functions based on responses to sustained G stress was unsuccessful, however. A synthetic transfer function with physiologic explicability and substantial predictive ability was obtained from an impulse response fabricated from simple mathematical functions.

Incidence of cardiac dysrhythmias occurring during centrifuge training.

High-G training has been reported to provoke dysrhythmias in many subjects. These reports have been based on small subject groups. Students attending aeromedical professional courses at the USAF School of Aerospace Medicine are offered the opportunity to participate in high-G centrifuge training on the Armstrong Laboratory centrifuge, during which ECG monitoring is routinely performed. This study documents the incidence of dysrhythmias in this large group of subjects. The pertinent information from the records of 1,180 training sessions from 1984-91 were transcribed to a database on a personal computer. Dysrhythmias were recorded in 552 (47%) of the training sessions. Ventricular ectopy occurred in 480 (41%) of the sessions, and supraventricular dysrhythmias appeared in 127 (11%). In 53 (4.5%) of the sessions, training either was or would have been terminated because of the dysrhythmia. Session-terminating dysrhythmias included: 26 ventricular tachycardias (2.2%), including 18 triplets (1.5%); 9 ventricular couplets (0.8%); 8 episodes of too-frequent ventricular premature beats (0.7%); 4 of supraventricular tachycardia (0.3%), including 2 with aberrant conduction (0.2%); 2 of aberrantly conducted beats (0.2%); and 4 of anomalous bradycardia (0.3%). Centrifuge training can provoke serious dysrhythmias in ostensibly healthy individuals, and ECG monitoring of aircrew undergoing such training is recommended for their safety. Because some of these dysrhythmias are disqualifying for aircrew duties, the need for a more lenient aeromedical disposition policy must be considered.

Visual field contraction during G stress at 13 degrees, 45 degrees, and 65 degrees seatback angles.

In support of the High Acceleration Cockpit program, two groups of six experienced subjects, operating a high-resolution visual field limit tracker, were exposed to gradual-onset (0.067 G/s) G stress to a 7-G maximum on the USAFSAM human centrifuge. Data obtained from one group described the G-induced vertical visual field contraction, and that from the other described horizontal visual field contraction--as they occurred in relaxed subjects in seats with 13 degrees, 45 degrees, and 65 degrees seatback angles. Curves of peripheral vision remaining vs. G level indicated a statistically significant difference in tolerance provided by the 65 degrees seat over that provided by the 13 degrees and 45 degrees seats in the 5- to 7-G range, and a significant difference in tolerance provided by the 45 degrees and 65 degrees seats over that provided by the 13 degrees seat in the 4- to 5-G range. Two-dimensional reconstructions of the superior half of mean binocular vision remaining at the various levels of G stress showed complete visual loss near 5 G in the 13 degrees seat, complete loss near 6 G in the 45 degrees seat, and substantial peripheral vision remaining at 7 G in the 65 degrees seat.

United States Air Force head-up display control and performance symbology evaluations.

The United States Air Force has conducted an extensive research effort to determine the most effective way to present aircraft control, performance, and navigation information on the head-up display (HUD). The primary objective of the research was to develop a standard HUD symbology set to be used as a primary flight reference for fighter-type aircraft during instrument meteorological conditions. This paper summarizes the research conducted by scientists in the Visual Orientation Laboratory at the Flight Motion Effects Branch of the Human Systems Center's Armstrong Laboratory. Five experiments that examined various control and performance symbology elements are reviewed. Suggested standardization guidelines based on experimental findings are discussed, including the following: the use of counter-pointers for airspeed and altitude indicators, vertical and horizontal asymmetry for climb/dive ladder configurations, a ghost horizon, analog vertical-velocity information, energy management symbology, and quickening for climb/dive markers.

A case of left hypoglossal neurapraxia following G exposure in a centrifuge.

Isolated hypoglossal (XIIth cranial nerve) neurapraxia is a rare clinical presentation. We present a case of an isolated hypoglossal neurapraxia following exposure to +7.2 Gz in a human centrifuge. Although a variety of cervical spine injuries have been reported as a result of G exposure in an aircraft, no cranial nerve injuries have been reported. A review of the literature of isolated hypoglossal neurapraxia is presented with discussion of the probable cause of this nerve injury.

Visual scene effects on the somatogravic illusion.

This study attempted to determine which visual scene cues are most effective in overcoming the somatogravic illusion (SGI), a form of spatial disorientation that occurs when a shift in the resultant gravitoinertial force vector created by a sustained linear acceleration is misinterpreted as a change in pitch or bank attitude. Nine subjects were exposed to a gravitoinertial force shift of -30 degrees in the pitch plane, both with their eyes closed and while viewing computer-generated visual scenes through a wide field-of-view head-mounted display. The scenes depicted acceleration over a shoreline by means of horizon, texture, perspective, and color cues that were presented both in isolation and in various combinations. None of the scenes significantly reduced the magnitude of the SGI relative to the eyes-closed (baseline) pitch illusion, even though the textured scenes produced some linear vection. It remains to be established whether low-cost head-mounted visual displays can reliably reduce the magnitude of the SGI and other spatially disorienting illusions.

Transfer functions for eye-level blood pressure during +Gz stress.

A description of the eye-level blood-pressure response to +Gz stress in relaxed humans was obtained as an empirical, ensemble-average, G-to-blood-pressure transfer function based on three subjects' responses to a total of 23 simulated aerial combat maneuvering runs on the USAFSAM human centrifuge. Three different analytic transfer functions were fitted to the empirical function for frequencies from 5 to 200 mHz, and predictive performance of the empirical and three analytic functions was examined. The double-zero, double-pole mathematical model most closely fit the empirical transfer function and displayed reasonable predictive ability.

G stress on A-10 pilots during JAWS II exercises.

Gz loads in A-10 aircraft were recorded on nine sorties during JAWS II exercises, and the pilots completed questionnaires on the effects of G stress in the A-10. Analysis of the recordings provided, among other statistics, the following means: maneuvering time, 28 min; peak +Gz load, +6.2; peak -Gz load, -0.1; G-time integral above 4 G, 85 G.s; G onsets greater than or equal to 6 G/s, 13. The questionnaires and follow-up interviews generated little evidence of G intolerance; the pilots did report significant task saturation, however. To compare the physiologic effects of G stress in the A-10 with those of G stress in the F-4, eight volunteer subjects were exposed to simulated A-10 and F-4 missions on the USAFSAM centrifuge. Although the two G-stress profiles were equally difficult and fatiguing, the subjects experienced less visual loss and had lower maximum heart rates during the A-10 profile.

Women's G tolerance.

G tolerances of 102 women and 139 men subjected to Standard Medical Evaluation (Medeval) G Profiles were compared. Unpaired t-tests revealed no significant difference between the women and men in either relaxed or straining G tolerance. Covariance analysis controlling for differences in tolerance due to age, height, weight, and activity status revealed the women to have marginally lower tolerance; the analysis also identified height as a factor having a strong negative influence on G tolerance, and weight as having a positive influence. When the women were matched only by height to the men in the comparison group, the women's mean G tolerances were significantly lower than the men's. On Standard Training G Profiles 88% of 24 women and 80% of 213 men completed the runs, but this difference was not significant. G tolerances of 47 women were measured on the Medeval Profiles both during and between menses, but no significant differences related to menstruation were found. No important differences between women and men in signs or symptoms of G stress were observed, except for two instances of urinary stress incontinence in women during the Training Profiles. We conclude that women should not categorically be excluded from aircrew duties for reasons of G intolerance.

A test of thumb and index finger control in overcoming a visual analogue of the giant hand illusion.

Anecdotal evidence suggests that a thumb and index finger grip might facilitate recovery from the manifestation of spatial disorientation known as the giant hand phenomenon. Sixteen pilots volunteered as subjects in an experiment that compared the effectiveness of the thumb and index finger versus the whole hand technique to overcome a visually-induced analogue of the giant hand phenomenon. Thumb and index finger control produced greater stability overall, but did not overcome the specific tracking bias induced by a background visual roll stimulus. Various hypotheses are discussed as to why the thumb and index finger technique was ineffective in the present instance.

The effects of acoustic orientation cues on instrument flight performance in a flight simulator.

An initial version of an acoustic orientation instrument (AOI), in which airspeed was displayed as sound frequency, vertical velocity as amplitude modulation rate, and bank angle as right-left lateralization, was evaluated in a T-40 (Link GAT-3) motion-based simulator. In this study, 15 pilots and 3 non-pilots were taught to use the AOI and flew simulated flight profiles under conditions of neither visual nor auditory instrumentation (NO INPUT), AOI signals only (AOI), T-40 simulator instrumentation only (VISUAL), and T-40 simulator instrumentation with AOI signals (BOTH). Bank control under AOI conditions was significantly better than under the NO INPUT condition for all flying tasks. Bank control under VISUAL conditions was significantly better than under the AOI condition only during turning and when performing certain complex secondary tasks. The pilots' ability to use the AOI to control vertical velocity and airspeed was less apparent. However, during straight-and-level flight, turns, and descents the AOI provided the pilots with sufficient information to maintain controlled flight. Factors of potential importance in using sound to convey aircraft attitude and motion information are discussed.

Classification problems of U.S. Air Force spatial disorientation accidents, 1989-91.

Spatial disorientation (SD) continues to contribute to a fairly constant proportion of military aircraft accidents. The U.S. Air Force (USAF) fielded a new accident investigation reporting form in July 1989, which for the first time specified Type I SD, Type II SD, and Type III SD as possible causes of aircraft accidents. Of a total of 91 major accidents that occurred over the 2-year period beginning in October 1989, SD was rated as contributing significantly to 13 (14%). Coding for SD on accident investigation reporting forms was not consistent, however. Individual flight surgeons differed in their approaches to coding accidents as SD-related; other differences were noted between flight surgeons and pilots, and additional procedural differences resulted in inconsistent reporting over time. There is a consensus that SD represents a major problem in military aviation, but a scientific approach to this important problem would be facilitated if agreement could be reached on definitional and semantic issues.

Rigid gas-permeable contact lens wear during + Gz acceleration.

This study was designed to determine how well rigid gas-permeable contact lenses maintain position on the cornea under high G forces, and the effect on visual acuity. One hyperopic and five myopic subjects were fitted with lenses made from Pasifocon C material (specific gravity = 1.07). Two lens diameters (8.8-9.4 mm and 9.6-10.0 mm) were compared upon the myopic subjects for centering on the cornea. Visual acuity was measured at + 1 Gz (baseline), +3 Gz, +4 Gz, +6 Gz, and +8 Gz in the straight-ahead, lateral, and up-gaze positions from three acuity charts mounted in the gondola. All lenses, as estimated from the videotape, decentered down the z axis 2-3 mm at high + Gz. Visual acuity was reduced at the higher +Gz levels to similar measurements in most subjects for both the contact lens and spectacle control rides. No lens displaced from the cornea or dislodged from the eye during any of the 25 centrifuge runs.