Spatial Disorientation : Something Old & Something New

Spatial Disorientation : Something Old & Something New
by
Wg Cdr G Gomez
Cl Spl (Av Med)
IAM, IAF

Panelists:
Wg Cdr S Modak
Wg Cdr S Mishra
Wg Cdr P K Tyagi

INTRODUCTION
Since the first flights at Kittyhawk, Spatial Disorientation (SD) has been and remains the 'Numero-Uno' of all aeromedical stresses. This indeed is a matter of great concern, to all those related with aviation, both in the civil and military fields. SD has been plaguing aircrew and has been compromising flight safety. By far the most important consequence of SD is the orientation error accident, which claims lives of aircrew and passengers every year and is also responsible for loss of costly aircraft.(1)

HISTORICAL PERSPECTIVE
In 1926 Maj William C Ocker an aviator of the US Army Air Corps was undergoing his bi-annual medical examination. The flight surgeon at that time was Major David Meyers. Part of the examination included testing of the vestibular system by blindfolding the individual and rotating him on a revolving chair. While Ocker was being tested, he realized that he was experiencing bodily sensations that were similar to what he experienced in blind flying conditions. Together Ocker & Meyers studied the issue and came up with the reasoning for the strange body sensations in blind flying conditions, for this condition they coined the term -"Aviators Vertigo". (Till date the Americans still refer to SD by this term.). It was only in the year 1955 that two British workers, Clark and Graybiel coined and defined the term "Spatial Disorientation". (2)

DEFINITION
The most accepted definition today, is the one proposed by Dr AJ Benson (UK). He defines Spatial Disorientation as a term used to describe a variety of incidents occurring in flight, where the pilot fails to sense correctly the position, motion, or attitude of his aircraft or of himself within the fixed coordinate system provided by the surface of the Earth and the Gravitational vertical. In addition, errors in perception by the pilot of his position, motion or attitude with respect to his aircraft, or of his aircraft relative to other aircraft.(1)

ORIENTATION MECHANISMS
Man has the ability to perceive orientation in 3-D space and this depends on his interpretation of the continuous input of signals from many sensory receptors like the eye and the vestibular apparatus of the inne1r ear. Others are more generally distributed in the body and are found in the skin, the capsules of joints and supporting tissues. However accurate orientation percepts of the aircraft are dependent primarily on interpretation of visual cues from inside and outside the cockpit. 90% of all the inputs to the brain that give the sense of orientation are visually acquired. Vision is therefore by far the most important sensory modality sub-serving spatial orientation. The other senses play a supporting role in correct perception of aircraft orientation. The supporting role of the vestibular system can be seen in the 'vestibulo-ocular reflex.(1,3)

VISUAL & VESTIBULAR PHENOMENON
Visual Dominance : Certainly, when a pilot has a wide, clear view of the horizon, ambient vision adequately supplies virtually all orientation information, and potentially misleading linear or angular acceleratory motion cues do not result in spatial disorientation. When a pilot's vision is compromised by night or bad weather conditions, the same acceleratory motion cues can cause him or her to develop 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 his or her percept of spatial orientation to the exclusion of vestibular and non-vestibular 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 orientation percept. 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.(3)

Vestibular Suppression : is the active process \ ability of visually over-riding undesirable vestibular stimulation and sensations. eg. the figure skater who after much practice learns to abolish the post rotatory nystagmus resulting from figure skating spins. But even these individuals when deprived of vision develop the same nystagmus and falling we would expect. The ability to supress unwanted vestibular sensations is developed in flight by repeated exposure to both linear and angular accelerations. However this ability of the pilot to supress vestibular sensations is compromised when he is deprived of visual cues.(3)

Vestibular Opportunism : is the active process \ ability of visually over-riding undesirable vestibular stimulation and sensations. eg. the figure skater who after much practice learns to abolish the post rotatory nystagmus resulting from figure skating spins. But even these individuals when deprived of vision develop the same nystagmus and falling we would expect. The ability to supress unwanted vestibular sensations is developed in flight by repeated exposure to both linear and angular accelerations. However this ability of the pilot to supress vestibular sensations is compromised when he is deprived of visual cues.(3)

SD Inference: In-flight a pilot's ability to suppress unwanted vestibular sensations is compromised if deprived of visual cues. Vestibular information can break through a pilot's defences and can get incorporated in his orientational percept. Conflicts between focal vision and vestibular system resolve in favor of the latter. The reverse holds good if peripheral vision is in play.

In summary SD can be produced in flight for the following reasons :-

Angular and linear motion stimuli differ in intensity, duration and frequency from those to which a man is functionally adapted.

The aircraft operates and has to be controlled in six degrees of freedom and the aviator is not in contact with a fixed reference.

External visual cues can be atypical, deficient or difficult to interpret.

Visual cues from instruments utilize the focal rather than the ambient mode of vision and therefore place greater demands on central processing resources.

Demands of the flying tasks , particularly in high work load situations deplete resources for processing orientation cues.

Vestibular receptors may be stimulated atypically by changes in ambient pressure.

Vision can be degraded by vibration, High G, Glare etc

Degradation of central processing by drugs or toxic agents.

ILLUSIONS : CLASSIFICATION

Otolith related

Somatogravic & Oculogravic Illusion
Inversion illusion
Elevator illusion
G-excess illusion

Semi Circular Canals related

Leans
Somatogyral & Oculogyral illusion
Coriolis
Alternobaric vertigo
Gillingham illusion

Visual related

Black hole approach
Whiteout (atmospheric & blowing - snow)
Autokinesis
Vection (angular & linear)
Flicker vertigo
Dip illusion
Illusions during approach & landing
Lean on the sun
False Horizon
Ground-sky lights confusion
Coquet Effect

Centrally related

Break-off phenomenon
Giant Hand phenomenon
Fascination

Gillingham Illusion
Gillingham illusion is named after Dr Kent Gillingham as a tribute to his efforts towards the prevention of spatial disorientation and to making flying safe. It is basically a post roll misperception of attitude. A number of accidents have been attributed to this illusion. The common features in each accident were that the aircrafts were inverted and nose-diving just prior to the crash. Further each aircraft had made an abrupt roll or change of course/ heading just before the accident, and the pilot had unknowingly allowed the aircraft to become inverted. This illusion has been observed to occur in IMC conditions when the pilot banks his aircraft suddenly from one side to another in less than 09 seconds and when stabilizing he unknowingly applies pressure on stick to increase bank onto the side of the roll.

This illusion has similar mechanisms as the graveyard spin, the difference being that it is in the roll axis in contrast to the yaw axis of the graveyard spin. When the aircraft rolls from left to right over 3- 9 seconds, the cupula, which gave the input of roll initially, comes back to the resting stage due to this controlled roll and thereby the rolling sensation dampens. On termination of the roll, the pilot’s cupula, which was now at rest, starts turning in the opposite direction. This gives a feeling to the pilot that he has under banked. In an attempt to stabilize the aircraft in the intended roll plane, the pilot puts pressure on control stick and unknowingly over banks the aircraft and this leads to the accident. (4)

Dip Illusion
This occurs during formation flying at night when one aircraft is trailing another. To avoid the lead’s wake turbulence and yet to keep him in sight, the trailing pilot tends to keep his aircraft at a small but constant angle below the lead aircraft. If the pilot is, say 5 nautical miles apart, for every 1 degree below the lead, the pilot in trail flies 1.7 percent of the distance between the two aircrafts, lower. Thus if the trailing pilot is 2 degrees below the lead, his aircraft will descend to 350 m below the lead aircraft. To make matters worse when the aircraft in trail slows to establish separation, its pitch attitude increases by several degrees. If the pilot does not compensate for this additional angle and tries to maintain the lead aircraft image in the same relative position, the pilot then can double\triple the altitude difference. In the absence of ambient visual cues, the pilot does not notice the large loss of altitude and may inadvertently dip far below the intended flight path.(3)

Giant Hand Phenomenon
The giant hand phenomenon, described by Malcolm and Money, undoubtedly explains why many pilots have been rendered hopelessly confused and in effectual by spatial disorientation, even though they knew they were disoriented and should have been able to avoid losing control of the aircrafts. The pilot suffering from this effect of disorientation perceives falsely that the aircraft does not respond properly to his or her control inputs because every time the pilot tries to bring the aircraft to the desired attitude, it seems actively to resist his or her effort and 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, 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 giant had phenomenon is not rare : one report states that 15% of pilots responding to a questionnaire on spatial disorientation had experienced the giant hand. 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. (3)

Malcolm and Money (5) further in their analysis of the Giant Hand phenomenon incidents state that there are four pre-conditions for this effect to take place. These are :-

A state of anxiety or mental arousal seems to be have been prevalent for some minutes prior to the incident.
The control of the aircraft has involved a motor task of one or both hands.
Immediately prior to the event, the pilot has been distracted from the immediate task of controlling the attitude of the aircraft.
The resultant gravity vector has been rotated forward (as during deceleration) or the pilot felt that he had pitched forward as when diving or during some cross coupled head movements.

To prevail in this conflict between will and skill, the pilot must de-couple his or her 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 de coupling and thereby facilitate recovery from the giant hand.

Coquet (Cocquyt) Effect
This occurs if a pilot tries to maintain visual flight in an overshoot following a missed approach. He may not very well know the attitude of his aircraft and therefore be unable to interpret the position of lights or landmarks that he can see. In this the nose up attitude of the aircraft overshooting or taking off causes a light ahead to appear lower and the aircraft higher than it actually is. On descent with the nose down the reverse holds good.

The commonest situation arises when the pilot is flying with a slight pitch up attitude, at night with very low illumination levels on the ground and views an isolated light. The angular depression of the light with respect to the long axis of the aircraft is similar to that which would have occurred if the aircraft were in level flight but at a higher altitude. This illusion produces a fallacious overestimate of altitude. (6)

SD : TYPES
Type-I SD
(1,3) is one that is not recognized, the pilot is unaware of any error in his aircraft control and performance, in fact he is quite oblivious of the fact that SD is occurring. His control of the aircraft is therefore based upon a false percept whose final outcome is collision with the ground.

Type-II SD (1,3) is the recognized form and for which almost every pilot can relate an episode of. Herein the pilot is aware of conflicting inputs from his body orientation mechanisms and from what he is gleaning through the aircraft instruments. Most often the pilot is able to resolve the conflict in favour of believing his instruments and thereby recovering the aircraft.

Type-III SD (3) is the recognized, incapacitating form of this malady. The pilot knows that he is disoriented but because of incapacity is, unable to recover the aircraft. This can be brought about by a number of ways, such as vestibular-ocular disorganization which can be to such a degree that he is unable to read his instruments nor obtain a stable view of the outside world. Strong vestibular spinal reflexes to the shoulder and arm that he is unable to manipulate the controls can also incapacitate the pilot. Lastly, the pilot may be incapacitated by SD-Stress leading to intense fear so that he is unable to make a rational decision and he freezes on the controls.

SD : PREVENTIVE METHODOLOGIES
SD will occur to almost everyone who attempts to take to the aerial environment be it in a balloon or in the cockpit of a fast jet. This malady needs to be contained by a multi-pronged approach that includes strategies to indoctrinate and train aircrew, improve upon the instruments of flight in providing more natural and better orientation cues, identification of vertigo traps in the aircraft, developing systems to recover the aircraft automatically from a disabled pilot, avoidance of high risk environmental situations and introducing safe flight procedures. The basic concept of SD containment is to convert all Type I SD to Type II SD and prevent a Type II SD from becoming a Type III SD.

As on date SD indoctrination is the main thrust area in combating this stress. A large number of SD devices, trainers and simulators are available for this purpose. In aircrew indoctrination it is important to provide information and training on means and methods of avoiding SD, of overcoming it when it occurs and of reducing residual anxieties resulting from disorienting experiences. The main purpose of SD indoctrination is to generate awareness among aircrew about the fragility of the human orientation sense organs, the various manifestations of SD and combat skills against this stress.

SD indoctrination for aircrew in the IAF was first introduced in 1994 at the IAM. This training was imparted to 42 QFIs in the Training Command. Each course was of 4 days duration and included didactic lectures and practical experience of SD on a rotatory chair and the Human centrifuge. Gomez has reported that in these 42 QFIs, 58.6% were ill informed about SD, 48.3% did not know the orientation mechanisms and 44.8% considered the vestibular system as the main sense organ for orientation. Realizing the importance of this form of indoctrination, a year later SD training was merged with the High G training being imparted to fighter crew, as a regular course. (9)

An SD training film was also produced by IAM which is being extensively used for aircrew indoctrination at the squadron level. In India during the monsoon period the conditions are highly conducive for the production of SD. As a preventive measure in the IAF, all Squadron Medical Officers are required to take didactic lectures on SD for their squadron aircrew prior the outbreak of the monsoon. This helps to reiterate the problems of SD and increase awareness levels.

SD : EVALUATION & MANAGEMENT
One major factor in coping with Disorientation is the pilot’s ability to maintain composure and intellectual command of the aircraft despite distractions and disorienting inputs. Psychological disturbance is therefore one factor to be seriously considered in pilots whose presenting symptom is disorientation.

Impairment of higher mental function and reduced motor coordination that frequently accompany hyper arousal, can obviously be side effects of fatigue, tension due to personal problems, or poor health. Alcohol and various drugs are additional threats to the effective resolution of disorientation problems. To a surprising degree they can reduce visual control of eye movements in motion environments, while at the same time risking impairment of necessary intellectual control.

While probing for psychological factors, it is, however to bear in mind that individuals that experience strong vertiginous episodes as a result of some pathological condition are also greatly disturbed by the experience. The emotional disturbance might then lead to the conclusion by the doctor as well as the patient’s friends, that the whole episode is a sign of neurosis or an anxiety reaction. The same is true of the aviator who has had an exceptional disorientation episode. Whatever the actual cause of the disorientation, the emotional overlay that is likely to result from the episode must be dealt with.

In handling such cases it is important for the doctor to show that he is interested. This will ordinarily be accomplished in the process of taking a history of the incident and relevant background material. A thorough history is perhaps the most important step in the examination. This is to be followed up by a thorough ENT examination, which should include vestibular testing.

It is first necessary to establish clearly whether or not the occurrence of disorientation in a pilot due to a natural response to an unusual flight condition. The absence of similar reports from others in the same aircraft does not by itself constitute evidence of an abnormal reaction from the pilot. Crewmembers may have been equally disoriented without awareness of the fact because awareness sometimes depends upon checking the perceptual event against information from the instrument panel or from sudden VFR contact.

In attempting to relate disorientation to flight conditions, items in the checklists should be considered. When it appears that disorientation is attributable to normal reactions to either aircraft or flight conditions, then reassurance that the reaction was normal, possibly including discussions with other pilots may be sufficient to allay anxiety. If concern persists then a period of dual flying may serve to restore confidence but it may be necessary to seek the help of a specialist. There is some evidence that acquired fear of some aspect of flying in a previously confident aviator is amenable to treatment with a fairly high probability of success.

DISORIENTATION THREAT CHECK LIST
Pilots receive indoctrination on aspects of all these points in the course of their training but reminders are necessary. The following is a checklist for reviewing factors which constitute disorientation threats to the aviator in a helicopter or in a fixed wing aircraft. (10)

Flight environment

IFR in particular the transfer from external visual to instrument cues.
Night-ground/sky confusion. Isolated light sources enhance the probability of oculogravic, oculogyral and autokinetic illusions.
High Altitude-false horizontal reference. Dissociative sensations of detachment or remoteness from aircraft,from Earth or from reality ( Break-off Phenomenon). Break-off may occur in helicopter pilots at lower altitudes or on crossing encarpments.
Flight over featureless terrain-false perception of height.

Flight manoeuvers

Prolonged acceleration and deceleration in line of flight and catapult launches –somatogravic and oculogravic illusions.
Prolonged angular motion-sustained motion not sensed, somatogyral illusions on recovery; no sensation of bank during coordinated turn ,cross coupled and g-excess illusions if head movement is made while turning.
Sub-threshold changes in attitude-the leans induced on recovery.
Workload of flight manoeuvres-high arousal enhances disorientation and reduces the ability to resolve perceptual conflict.
Ascent or descent-pressure vertigo.
Cloud penetration-VFR/IFR transfer and attendant problems, especially when flying in formation or on breaking formation. In the lean-on-the-sun illusion a bright spot in the cloud may be interpreted as up. Depending on the heading of the aircraft relative to the bright spot, the false vertical reference may induce attitude errors in roll or pitch.

Aircraft factors

Inadequate instruments
Inoperative instruments
Visibility of instruments
Badly positioned displays and controls-head movements required to see and operate
High rates of angular and linear acceleration high manoeuverability
View from the cockpit-lack of visible aircraft structure enhances break-off and provides a poor visual frame of reference.

Aircrew factors

Flight experience
Training, experience and proficiency in instrument flight
Currency of flying practice
Physical health-upper respiratory tract infection and pressure vertigo
Mental health-high arousal and anxiety increase susceptibility to SD
Alcohol and drugs –impaired mental function. Alcohol and barbiturates even at low levels impair ability to supress nystagmus.
Fatigue or task overload.

In summary

Physiologic chain of events leading upto SD can be attacked in all its links.
Education and training; ground based physiological training. and in-flight training.
In the case of operational pilots: two stage approach to the problem viz-

Minimize the likelihood of SD by monitoring frequently and systematically the critical flight parameters displayed by instruments-bank, pitch, vertical velocity and altitude
When SD occurs make the instruments read right regardless of body sensations-in this apply the following procedures-

keep your head in the cockpit
concentrate on flying only the basic instruments
defer non essential tasks
bring aircraft to straight and level and hold for at least 60 sec or till disorienting sensations die down
use autopilot
declare emergency
do not mix external with instrument cues
hand over controls
control breathing do not panic
abandon aircraft if control cannot be regained.

THE FUTURE PERSPECTIVE
SUPERMANOEUVERABLE AIRCRAFT VS SPATIAL DISORIENTATION
One of the things that has changed the tactics of air-to-air combat in recent years is the all-aspect missile. These all-aspect missiles can be fired from any direction, and fighters so equipped need only to get their noses pointed in the general direction of the enemy. The fighter pilot who can get his nose pointed within the required field of view first, is the one most likely to survive. The name of the game is to be able to fire the first shot while still retaining enough speed to fly away to make another kill or to avoid being killed. The manned fighter aircraft quite likely will be around well into the twenty-first century both in an air-to-air and air-to-ground role. The emphasis is on technology that will allow fighters to survive and win in combat. In pursuit of this, there is great interest in a particular area of technology that goes under the generic title of "supermanoeuverability"(SM). (11)

What is Supermanoeuverability?
Credit for coining the word goes to Dr W. B. Herbst, who introduced the idea in 1980.He defined supermanoeuverability as the capability to execute manoeuvers with controlled side slip at angles of attack well beyond those for maximum lift. Today Doctor Herbst’s definition is termed post stall manoeuvering The term has been expanded to other concepts that can dramatically enlarge the flight envelope of an aircraft in terms of airspeed, turn rate, climb rate, acceleration, and so forth through more efficient wings, better performing engines, or more sophisticated flight control systems. Capabilities such as increased usable lift, dynamic lift overshoot, thrust vectoring and unsteady aerodynamic effects used in a synergetic fashion are all means of obtaining greatly enhanced. manoeuverability. Super manoeuverability can also be important in allowing an aircraft to avoid an enemy missile. With very high agility, the fighter would be able to outfly the missile and break lock with the missile’s guidance system. (11)

The NASA/Boeing X-36 Tailless Fighter Agility Research Aircraft has successfully completed its flight research program and demonstrated the feasibility of future tailless fighters achieving agility levels superior to today’s best military fighter aircraft. Rockwell and Messerschmitt-Bolkow-Blohm (MBB) X-31 now flying at Edwards AFB is the precursor of a new generation aircraft, which will embody radical new maneuvering capabilities. These capabilities will include multi-axis accelerations, and unprecedented angular velocities and accelerations around the three principal axes of the aircraft. Implicit in this new motion environment are new issues of pilot capability involving bio-dynamics, acceleration tolerance, performance, and above all spatial disorientation. (11)

Motion of current aircraft vs SM aircraft
A modern fighter aircraft such as the F-16, Mirage-2000, Su-30, Mig 29 and F-18 are very agile because they are built in accordance with what are known as relaxed stability criteria. Their agility is high since the aircraft are not entirely stable. Nevertheless, modern agile fighter aircraft continue to be flown as fighters have always been flown. They are maneuvered in roll, pitch, and yaw but the primary airframe accelerations are limited to the Z axis of the aircraft. Small values of load factors in the X and Y directions are present under certain conditions, but these are very low compared to the Gz accelerations currently attainable. Angular excursions of current aircraft about their principle axes are limited ordinarily to roll and pitch, with very limited yaw used only in sideslip manoeuvers. Pitch displacements do not usually exceed 20-degree angle of attack (AOA) at rates of perhaps 15 deg./second. Roll motion is unlimited and the angular velocities may exceed 360 deg./second. (11)

By contrast, SM aircraft are capable of significant excursions and accelerations in the X and Y directions as well as high sustained Gz load factors. These types of capabilities depend on the incorporation of new canard and wing geometry’s and the use of one- or two-dimensional vectored thrust. SM aircraft exploit manoeuvering capabilities obtained by flying at low airspeeds and at high angles of attack, a region generally referred to as the post stall regime. These aircraft will be fully controllable in this region of the flight envelope. Thrust vectoring contributes very rapid rates of pitch and yaw motions.

The major benefit conferred by post stall technology (PST) manoeuvers is the ability to rapidly point the nose of the aircraft in virtually any desired direction. Unconventional manoeuvering in roll, pitch, and yaw combined with the ability to yaw around the velocity vector provides unprecedented performance capabilities. PST manoeuvers can also be used defensively: by performing a high angle-of-attack barrel roll, a pilot can force his opponent to overshoot, thus placing his opponent in his gun sights. (11)

SM & SD
The new capabilities presented by PST will open up a slew of new problems which aviators will have to face when SM aircraft join the operational inventory. While technology can be used to produce supermanoeuverable fighters, it might be the physiological capabilities of the human pilot that could put the upper limit on manoeuverability. For example, the pilot will become disoriented when his aircraft moves against intuition and experience. It may take extensive training to get used to flying sideways, flying at attitudes well into the stall regime, or being able to point the nose up or down without climbing or diving. It is possible that cross coupled Coriolis and G-excess illusions can be induced in the absence of head movements by the pilot.(12)

Spatial Disorientation would become more serious as the body’s physiological orientation mechanisms are exposed to the rapid and unusual moves required by PST manoeuvers .With such unconventional manoeuvers happening very quickly, can the human orientation mechanisms maintain a proper orientation reference while under the stresses of close air combat? PST manoeuvers require a lot of concentration and attention to the target because of the complexity of the manoeuvers. In the fury of air combat, the workload and level of attention necessary may be something that pilots cannot handle or afford. It is conceivable that target fixation, task saturation, and channelized attention problems would become greater than they are now.o a great extent.(12)

REFERENCES :

Benson AJ. Spatial Disorientation-General Aspects. In : Ernsting J, Nicholson AN, Rainford DJ editors. Aviation Medicine, 3rd ed. Oxford : Butterworth & Heinmann,1999; 419-436.

Waite RE and Delucchi RM. Labyrinthine and Proprioceptive aspects of Aerospace Medicine. In : Randel HW Ed. Aerospace Medicine, 2nd ed. Baltimore : Williams & Wilkins Co..1971 : 254-267.

Gillingham KK and Previc FH. Spatial Orientation in Flight. In : Dehart RL Ed. Fundamentals of Aerospace Medicine, 3rd edition. Baltimore: Williams & Wilkins, 1996 : 309-398.

Ercoline WR et al. Post roll effects on attitude perception; “Gillingham Illusion”. ASEM 2000; 71 : 489-495.

Malcolm R and Money KE : Two specific kinds of Disorientation incidents : jet upset and Giant Hand. In : The Disorientation incident: AGARD CP-95, 1972, Part I : A 10-1 to A 10-4.

Benson AJ. Spatial Disorientation in Flight. In: Gillies JA Ed. A textbook of Aviation Physiology. 1st ed.Oxford : Pergamon Press, 1965 : 1086-1128.

Navathe PD and Singh B. Prevalence of SD in the IAF aircrew. ASEM 1994; 65 : 1082-1085.

Aggarwal A. Effect of Survey methods on the incidence of SD in fighter crew of IAF. Paper presented in the Annual Scientific Meeting ISAM 1999.

Gomez G. SD Training in the IAF. Paper presented in the Annual Scientific Meeting of ISAM 1996.

United States Naval Flight Surgeon’s Manual. Chapter 23 : Vestibular Function. 1991, 3rd edition.

Van Patten RE. Supermanoeuvrability and Superagility. Aeromedical & Training Digest 1993; 7 : 1-5.

Gomez G. Supermanoeuvrability aircraft versus Spatial Disorientation: A futuristic Reality. J Flight Safety 2000; 2 : 10-13.