Spatial disorientation (SD) is a term used to describe various incidents that occur in flight where pilots fail to correctly perceive the position, motion, or attitude of the aircraft or themselves in a fixed coordinate system provided by the Earth’s surface and vertical gravity [1]. SD is a hazard in aviation, whether in commercial or military aviation, as evidenced by a questionnaire survey conducted among aviators. The survey entitled “Spatial disorientation in military vehicles: causes [2],” consequences and cures, shows that SD experiences are generally comparable among United States Air Force pilots, British military pilots and the Hellenic Air Force, with causal factors such as distraction, task saturation and poor crew coordination being common. Incidents of SD of such severity as to affect flight safety were reported by the majority of pilots in this survey. Factors such as intense vestibular stimulation, impaired vision, and illusory sensation of rotation during manoeuvres were identified as causes of SD during roll and high G transitions.

SD is a critical issue in aviation, both in civilian and military operations, as it can lead to fatal consequences. SD occurs when a pilot’s perception of the aircraft’s position, motion and altitude relative to the earth or surrounding environment is inaccurate or distorted. This phenomenon is particularly dangerous during flight operations, as it can cause pilots to lose control of the aircraft, which can result in accidents and loss of life. SD can arise from various factors, including visual illusions, vestibular system disorders, and cognitive overload. In civil aviation, SD has been associated with many accidents, resulting in significant financial losses and fatalities. Similarly, in military aviation, SD is a serious threat to pilots involved in high-performance manoeuvres, combat operations and night missions [3].

Based on the data obtained, the incidence of incidents/accidents due to SD in military aviation ranges from 2.5% to 30.8% in various types of aircraft [4]. In a study of Polish military aircrews, the rate was calculated to be around 8%. Although SD training has become part of the educational programme at the Military Institute of Aviation Medicine (Wojskowy Instytut Medycyny Lotniczej - WIML) in Warsaw, Poland, there has been no significant reduction in the incidence of SD. This suggests the need to re-evaluate the effectiveness of existing SD training. Bowles & Verna LLP [5] states that although SD is the cause of only 5 per cent of all aviation accidents, 90 per cent of these accidents are fatal. This highlights the severity of the consequences when pilots become disorientated.

SD is a common physiological problem among combat aviators, with approximately 80% of participants experiencing SD events in flight. There were significant differences in the prevalence of SD among different types of pilots, with fighter pilots having the highest prevalence at 87.0%, followed by trainer pilots at 79.8%, transport aircraft pilots at 70.6%, and helicopter pilots at 66.7%. Pilots are sensitive to SD in clouds and in low visibility conditions, with more than 70% of pilots experiencing visual illusions such as horizon loss, tilt, false horizon, Coriolis illusion, and confusion when entering adverse meteorological weather conditions [6]. The incidence of SD is more common in combat aviators due to the high G-forces generated during acrobatic combat manoeuvres, which can stimulate vestibular illusions. Given the high prevalence of SD in fighter aviators that can result in vestibular illusions, this literature review will address vestibular illusions in fighter aviation including their implications and management.

Therefore, it is important to continuously evaluate the effectiveness of existing SD training and seek new methods to reduce the risk of SD in military aviation. This literature review will discuss vestibular illusions in fighter aircraft flight, including their implications and management, with the aim of increasing understanding and awareness of SD issues among military pilots.

1. Human orientation

Humans are naturally designed to maintain orientation while on the ground in a two-dimensional environment. Flight incorporates a three-dimensional environment and can cause sensory conflict, making orientation difficult or even impossible to maintain, SD is achieved through three main sensory sources: visual, vestibular and proprioceptive [7].

The human eye provides visual and spatial orientation, responsible for providing about 80% of the sensory input required to maintain orientation. The vestibular system in the inner ear accounts for 15%. Proprioceptive sensory input from receptors located in the skin, muscles, tendons and joints accounts for 5% of the sensory information used to determine orientation. The complex coordination between these sensory inputs is then translated and interpreted by the brain. Misinterpretation or inaccuracy of these three sources of information can lead to a ‘sensory mismatch’, resulting in a variety of visual or vestibular illusions [8].

In a book written by Davis et al. [8] in 2008, entitled ‘Fundamentals of aerospace medicine’ the physiology of orientation in humans involves three main sensory systems, namely visual, vestibular, and proprioceptive.

1) Visual orientation

Vision is the most important sensory modality in determining spatial orientation, especially in moving vehicles such as aircraft. Without vision, flight as we know it would be impossible, whereas this is not always the case in the absence of the vestibular system or other sensory systems that provide orientation information. There are two separate visual orientation systems that have two distinct functions: object recognition and spatial orientation. Knowledge of these systems is essential to help understand visual illusions in flight and appreciate the difficulties inherent in using flight tools for spatial orientation. Visual and vestibular orientation information are integrated at a very basic neuronic level. Therefore, SD often cannot be corrected through higher-level neuronal processing.

2) Vestibular function

The role of vestibular function in spatial orientation is not as obvious as vision, but it is crucial for three main reasons. Firstly, the vestibular system provides the structural and functional substrate for reflexes responsible for stabilising vision when head and body movements would result in blurring of retinal images. Second, the vestibular system provides orientation information against which skilful motor activities and reflexes are automatically executed. Thirdly, the vestibular system provides reasonably accurate perception of movement and position in the absence of vision, as long as the stimulation pattern is within naturally occurring limits.

3) Proprioception non-vestibular

Besides vision and vestibular, there is also the contribution of the non-vestibular proprioceptive system in human orientation. These include muscle, tendon, joint receptors, as well as skin mechanoreceptors. Muscle and tendon receptors respond to changes in muscle length and tension. The brain uses information from these muscle and tendon receptors to generate myotatic reflexes (or muscle stretch reflexes). Joint receptors provide information about joint position and changes in position. Skin mechanoreceptors provide information about touch and pressure on the skin.

These sensory systems work together to provide an understanding of spatial orientation. Vision is the dominant modality in flight, but information from the vestibular and proprioceptive systems is also important, especially in situations where vision is limited or not available at all. The accuracy of orientation perception influenced by these systems plays an important role in flight safety.

2. Spatial disorientation

SD is a term used to describe various incidents that occur in aviation where pilots fail to correctly sense the position, motion, or attitude of the aircraft or themselves in the fixed coordinate system provided by the Earth’s surface and vertical gravity. In aviation, orientation implies a sense of location in relation to the surrounding environment. The ability to orientate is an essential requirement for all free-living organisms, and a number of sophisticated physiological mechanisms have evolved to achieve this. One of the more advanced aspects of orientation relates to the position of the body relative to the surrounding environment and the force of gravity. SD is not a disease, but rather a physiological thing that any pilot can experience, whether they have many or few flying hours.

Pilots experiencing SD may fail to accurately recognise the position, motion or attitude of the aircraft, or even themselves, in the coordinate system given by the Earth’s surface and the vertical of gravity. In addition, errors in perception by pilots of their position, motion or attitude of the aircraft, or of their own aircraft in relation to other aircraft, may also fall under the broader definition of SD in aviation. This includes what is referred to as ‘geographic disorientation’, i.e. an error in the position of the aircraft with respect to its intended location on a map. Conventionally, this type of disorientation is usually excluded from true SD as it involves navigational errors rather than aircraft attitude errors. Nevertheless, errors in location can bring an aircraft into close proximity to high altitude or other obstacles in the same way that altitude errors are considered a form of SD. Errors in location can also result in distraction leading to errors in aircraft attitude that the pilot may not realise.

The concept of situational awareness (SA) has gained importance since the early 1980s, although there is still uncertainty about what the definition of the term should include. For some, SA has become synonymous with an aviator’s awareness of the spatial orientation of the aircraft. However, more commonly, the term is used more broadly to encompass not only awareness of spatial orientation but also other aspects of the current state of the aircraft and the external environment. It also includes an element of anticipation of future events. A pilot who has an incorrect perception of the aircraft’s orientation also experiences loss of SA, but loss of SA can occur for many different reasons in the absence of SD.

In some countries, especially in the United States, the term ‘vertigo’ or ‘aviator vertigo’ is used as a synonym for SD. Vertigo has the specific meaning of a false sensation of spinning, and use of the term should be restricted to that particular sensory experience. A pilot experiencing vertigo may also experience SD, but there are many instances where pilots experience SD but not vertigo.

3. Why humans experience spatial disorientation

Humans can experience SD for several reasons, namely: Firstly, SD can result from conflicts between information received by sensory systems such as vision, vestibular (balance) and proprioceptive (body position). When information from these three systems is misaligned or conflicting, the brain can have difficulty in accurately interpreting the body’s orientation to the surrounding environment. To achieve proper orientation, the body relies on accurate perception and cognitive integration of the three systems. If visual, vestibular and proprioceptive stimuli vary in magnitude, direction and frequency, the resulting effect can be SD [8].

Secondly, environments that lack visual information or conditions with low visibility can trigger SD. In such situations, the visual system cannot provide adequate information about orientation, so the brain must rely on other sensory systems that are prone to illusions and distractions [8].

Thirdly, extreme flight manoeuvres or manoeuvres with high acceleration can cause unusual stimulation to the vestibular system, leading to incorrect perception of body movement and orientation. This is often experienced by fighter pilots performing acrobatic manoeuvres with high g-force loads [8].

Fourth, other factors such as fatigue, stress, and certain medications can affect the function of the sensory and cognitive systems, thereby increasing the risk of SD [8]. Therefore, it is important for pilots and aviators to maintain good health and alertness when flying an aircraft.

4. Types of spatial disorientation

Here are some types of SD:

1) Type I

Unrecognised SD occurs when the pilot experiences a mismatch between natural and synthetic orientation perception (information from cockpit instruments), but the pilot is unaware that the aircraft is in an abnormal condition. In this condition, the pilot does not perceive any difference between his orientation perception and the actual situation.

Unrecognised SD can be divided into three types: (1) Insensate SD, which occurs when external stimuli are too weak to be detected by human receptors, but can still cause incorrect judgements about position and motion. (2) Unperceived SD, where sensory information has reached the primary cortex, but is not further processed or realised by the human brain. (3) Perceived SD, where external stimuli are correctly detected by human receptors, and sensory information is passed on to the primary and association cortex, but is still incorrectly interpreted.

2) Type II

Type II SD is more common than Type I. In this form of disorientation, the pilot becomes aware that there is a problem. Although the pilot may or may not be aware that the problem is SD, in this form of disorientation, they are aware that something is not right, that their sensory system is providing information that does not match the information available from the instruments, or that something does not make sense. The conflict between their own perception and the perception provided by the instruments or the outside visual world alerts them to a problem, which they must then address. If this is successfully addressed, SD accidents are less likely to occur. Pilots may have learnt a valuable lesson about SD and how to recover from it.

3) Type III

In SD Type III, the pilot experiences the most extreme form of disorientation stress. Pilots may be aware of the disorientation, but are mentally and physically overwhelmed to the point of being unable to recover from the situation. They may daydream at the controls, or make control inputs that tend to exacerbate the situation rather than effect recovery from it. The pilot may fight the aircraft into the ground, never once achieving controlled flight. Such forms of disorientation are the result of a disruption to normal cognitive processes, which may be caused by the overwhelming nature of the situation, especially if other factors such as fatigue and high workload are also present.

5. Types of spatial disorientation

There are many SD, in this study, the researcher focuses on the types related to military aviation. In this study, the author limits the discussion to four main types of illusions often experienced by military combat pilots, namely: somatogravic illusion, vestibular illusion, visual illusion, and somatosensory illusion [9]. These four types of illusions were chosen because: (1) The somatogravic illusion was chosen because it is particularly relevant to combat aviation which involves high-speed and high-velocity manoeuvres, such as take-offs, landings and U-turns. (2) Vestibular illusions were chosen because the vestibular system plays an important role in maintaining a pilot’s spatial orientation, especially when performing extreme manoeuvres and flying in low visibility conditions. (3) Visual illusion was chosen because combat flights are often conducted under conditions of limited visibility, such as night or bad weather, increasing the risk of visual illusion. (4) Somatosensory illusion was chosen because combat flight involves a lot of somatosensory stimuli, such as pressure, vibration, and acceleration, which can lead to incorrect interpretation of spatial orientation.

1) Somatogravic illusion

Somatogravic illusion is a type of spatial orientation illusion caused by the linear acceleration experienced by the pilot. It occurs when the vestibular and somatosensory systems provide incorrect information about the direction of gravitational acceleration. In combat aviation, somatogravic illusion often occurs in manoeuvres such as takeoffs, landings and U-turns. Examples of somatogravic illusions are the pitch-up illusion during takeoff and the pitch-down illusion during landing. In such situations, pilots may experience a false perception of the aircraft’s pitch-up or pitch-down angle, which can endanger the flight if not immediately recognised and corrected [10].

2) Vestibular illusion

Vestibular illusion is a type of SD that occurs when the pilot experiences a misperception of body movement or position due to a disturbance of the balance system in the inner ear. For example, vestibular illusion, meliputi somatogravic illusion, somatogyral illusion, leans, coriolis illusion, and G-excess illusion [10].

(1) The somatogravic illusion

The somatogravic illusion is a type of vestibular illusion that occurs when pilots feel the sensation of pitching up while the aircraft is accelerating, such as during take-off. This illusion occurs because the brain perceives linear acceleration as a pitching up event. As a result, the pilot may push the aircraft controls forward to compensate for the perceived pitching up sensation, which may eventually cause the aircraft to pitch down and risk flying into the ground. An example of the somatogravic illusion is when the pilot experiences a strong pitching up sensation during takeoff in low light or bad weather conditions, where insufficient visual information is received to confirm the actual flight path. This may cause the pilot to misinterpret the sensation of linear acceleration as pitching up, and ultimately perform an incorrect manoeuvre that is potentially dangerous.

(2) The somatogyral illusion

The somatogyral illusion, also known as ‘graveyard spin or spiral’, occurs when the vestibular system registers an initial angular acceleration upon entry into a spin or spiral turn. Once the turn has stabilised, the semicircular canals may not be stimulated, giving the false impression that no turn has occurred. When the turn stops, the canalise may register a change in angular velocity, creating the illusion of a turn in the opposite direction. An example of a somatogyral illusion is when a pilot experiences the sensation of no longer turning during a spiral turn or spin due to poor visual cues. After stopping the turn and returning to straight flight, the pilot may mistakenly perceive a turn in the opposite direction, potentially leading to a dangerous manoeuvre.

(3) The leans

Leans are a common form of SD in which the pilot experiences a false sensation of spin, leading them to lean to one side to counter the perceived spin. This illusion often occurs when the aircraft enters a gentle turn without affecting the semicircular canals causing the pilot to believe they are still straight and level when in fact the aircraft is turning. Upon realising the turn, the pilot may return to straight and level flight at a speed that affects the semicircular canals, thus causing a turn that appears to be in the opposite direction, leading to the sensation of leaning in the direction of the initial turn. An example of lean is when the pilot, focused on the task in the cockpit, fails to notice the slight wing drop that initiates a gentle turn. The pilot may feel straight and level due to lack of canal activation, leading to a false sense of orientation. When the pilot looks up and corrects the turn, the canals register the apparent turn in the opposite direction, thus causing the pilot to lean in the direction of the initial turn to neutralise the perceived roll.

(4) The Coriolis illusion

Coriolis illusion, also known as cross-coupled stimulation, occurs when a pilot moves his or her head out of the plane of rotation while turning, thereby causing opposing signals to the brain from the semicircular canal. This can cause severe falling sensations and feelings of nausea. An example of the Coriolis illusion is when a pilot performs a coordinated turn during the approach to landing. If the pilot moves his head, such as looking backwards in the direction of the turn or downwards into the cockpit, this can result in cross-stimulation of the canal, leading to a false sensation of falling and potentially disorientation (Fig. 1) [10].

(5) The G-excess illusion

The G-excess illusion is a dangerous phenomenon that can occur when a pilot enters a turn at a G rate greater than the normal +1 Gz and then looks back into the turn. This illusion can make the pilot feel that the initial bank angle is reduced, resulting in a noticeable underbank during the turn. As a result, the pilot may inadvertently apply more bank, leading to a significant overbank phenomenon, potentially leading to a dramatic loss of altitude or a stall. An example of the G-excess illusion is when the pilot, during a high G turn, looks back at the turn and misinterprets the bank angle, leading to overbanking and potentially dangerous flight conditions.

The visual system is also subject to misinterpretation. Given that the visual system is the dominant system for normal orientation, visual illusions can be very powerful. Visual illusions can occur even in perfect weather, and in many cases, the illusions that occur depend on expectations of what the pilot ‘should’ see. Common visual illusions include [11]: (1) Sloping cloud banks and false horizons; (2) illusions relating to runway size, shape and slope; (3) autokinesis; (4) the blackhole approach.

Somatosensory illusion is a type of spatial orientation illusion caused by the incorrect interpretation of somatosensory stimuli (such as pressure, vibration, and acceleration) on the pilot’s body. This illusion occurs due to a mismatch between somatosensory information and visual or vestibular information. In combat aviation, somatosensory illusion can occur in high-speed manoeuvres or manoeuvres involving sudden acceleration. Examples of somatosensory illusions are the inversion illusion, where the pilot feels upside down when the aircraft is actually in a normal position, and the linear translation illusion, where the pilot feels like he is moving in a certain direction when he is not. Somatosensory illusions can be very confusing and misleading, requiring a good understanding of spatial orientation and in-flight experience to overcome them [12].

1. Mitigating vestibular illusion in military aviators

The management of vestibular illusion in military aviators involves training and awareness of the types of vestibular illusions that may occur during flight [6]. Airmen are trained to recognise the symptoms of vestibular illusion and use specific techniques such as minimising head movement, being patient until the spinning sensation disappears, and relying on flight instruments to maintain balance and stability of the aircraft [10].

In addition, military aviators can also use advanced training devices that simulate vestibular illusion situations that may occur during flight. Thus, airmen can be trained to better cope with and identify vestibular illusions, thereby reducing the risk of accidents caused by vestibular disorders. In military aviator training, vestibular illusion simulator training has been integrated into IR (Instrument Rating) training as a mandatory part. This training helps military aviators to recognise and experience vestibular illusions first-hand, thus improving overall flight safety. This training is usually implemented at the initial or final stages of IR training [13].

There are several ways to mitigate vestibular illusion in military aviators. Here is an explanation.

1) Promotif action

In the promotive stage, education on vestibular illusions is provided, such as lectures, video presentations, and medical counselling on the types of illusions, risk situations, and their impact on aviation. Share experiences from senior pilots who have experienced vestibular illusion to increase the alertness of new pilots. It is necessary to ensure excellent pilot health conditions, namely: (1) Periodic medical examinations to detect disturbances in the vestibular, visual, and proprioceptive systems. (2) Maintaining physical fitness through exercise and adequate rest. (3) Avoiding the consumption of drugs or substances that can interfere with vestibular function. (4) Controlling risk factors such as stress, fatigue, and a history of illness that can affect body orientation.

2) Preventif action

In the preventive stage, training is conducted in the vestibular illusion simulator, which helps pilots experience illusory sensations in a controlled manner such as rotation, head movement, and changes in acceleration. Then, flight training with manoeuvres that can trigger vestibular illusion, such as sharp turns, acrobatic manoeuvres, and sudden changes in acceleration. Then, training using Virtual Reality (VR) technology to simulate vestibular illusion situations immersively and safely.

Education can then be provided in the form of lectures, video presentations, and general medical advice. Some important aspects that need to be provided in education include: (1) Explanation of physiological limitations that can cause disorientation, such as disorders of the vestibular system. (2) Understanding of the aerodynamic behaviour of the aircraft that can also contribute to the occurrence of disorientation. (3) Introduction to situations and manoeuvres that are at high risk of disorientation, such as take-off in foggy conditions, go-around procedures, and abortion manoeuvres at low altitude. (4) The importance of relying on aircraft instruments as the main reference for spatial orientation and not relying on body sensations alone.

VR training then provides an effective and immersive learning experience, allowing pilots to train in a risk-free environment and improve skills in dealing with complex manoeuvres and situations. One study conducted at Embry-Riddle Aeronautical University used the Virtual Reality Aviation Illusion Trainer (VRAIT) software programme and the Force Dynamics 401cr Motion Simulator to assess participants’ knowledge and self-efficacy in dealing with flight illusions. The results show that VR training can improve pilots’ confidence and expertise in recognising and managing visual and vestibular illusions [7].

In addition, practical training is also very important to prepare pilots to deal with and overcome SD. Some of the types of training that can be provided include:

(1) Training in the disorientation simulator, where: (a) This simulator is designed to help pilots experience disorientation sensations in a controlled manner, such as rotation in the vertical axis and performing head movements under these conditions. (b) The aim is to demonstrate the limitations of the vestibular sensors in detecting movement, especially rotation. (c) However, this training should also emphasise that disorientation is not always accompanied by confusing sensations.

(2) Flight training, where: (a) It involves actual flight with manoeuvres that may cause disorientation, such as flying blindfolded to demonstrate the mismatch between sensation and actual aircraft attitude. (b) It focuses on high-risk situations of unrecognised disorientation, such as go-arounds, take-offs in foggy conditions, and low-altitude abort manoeuvres. (c) It focuses on low-altitude abort manoeuvres.

(3) Training in conventional flight simulators, where: modern flight simulators, which can be used to create various scenarios that test a pilot’s ability to prioritise tasks and maintain an accurate flight path under increased workloads.

2. In-flight management

If military aviators experience vestibular illusion while flying, actions to take include: (1) Rely on flight instruments as the primary reference and ignore misleading body sensations. (2) Minimise unnecessary head movements to reduce vestibular overstimulation. (3) Be patient and allow time for the vestibular system to adapt, the sensation of spin or disorientation will usually disappear after a while. (4) If possible, ask another pilot or the machinist to verify the orientation of the aircraft. (5) If required, implement recovery procedures such as returning the aircraft to straight and level flight before another manoeuvre. (6) Share the vestibular illusion experience after the flight to increase awareness and knowledge for other pilots [2].

SD is a term used to describe various incidents that occur in aviation where pilots fail to correctly sense the position, motion, or attitude of the aircraft or themselves in a fixed coordinate system. SD is a significant problem in aviation especially military. SD can be very dangerous as it can lead to fatal accidents. There are three types of SD: type I (unrecognised), type II (recognised), and type III (incapacitated). SD illusions refer to perceptual errors that occur in pilots or other individuals in interpreting their sensory information regarding position, orientation, or movement in three-dimensional space. There are two divisions of SD illusions: visual illusion and vestibular illusion. Due to the challenging conditions, the chance of vestibular illusion in military aviators is very high. The impact of vestibular illusion on military aviators can result in hazards during flight that can threaten safety. Therefore, it is very important to pay attention to the treatment that can be done. One way that can be done is to impose training on military aviators such as simulating vestibular illusion situations that may occur. The application of simulation using technology is a good solution in handling vestibular illusion in military aviators.

No potential conflict of interest relevant to this article was reported.

None.

None.

Conceptualization: all authors. Data curation: all authors. Analysis and interpretation: all authors. Writing the original draft: ADR. Critical revision of the article: RW. Final approval of the article: RW. Overall responsibility: all authors.