Introduction

Aviation-associated spatial disorientation, as described by Benson, occurs when “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 other words, spatial orientation is the natural ability to maintain body orientation and/or posture in relation to one's environment while at rest and during motion. Humans are naturally designed to maintain orientation while on the ground in a two-dimensional environment.  Aviation incorporates a three-dimensional environment and can lead to sensory conflicts, making orientation difficult or even impossible to maintain. Spatial disorientation is a phenomenon that is well known to aviators, but it remains unclearly defined and continues to be one of largest causes of aviation mishaps.

Spatial disorientation is achieved through three major sensory sources: visual, vestibular, and proprioceptive. To achieve appropriate orientation the body relies on accurate perception and cognitive integration of all three systems. If visual, vestibular, and proprioceptive stimuli vary in magnitude, direction and frequency the resulting effect can be spatial disorientation.

The human eye provides visual and spatial orientation, which is responsible for providing about 80% of the sensory inputs needed to maintain orientation. The vestibular system within the inner ear contributes 15%. Proprioceptive sensory inputs from receptors located in the skin, muscle, tendons, and joints account for 5% of the sensory information used to establish orientation.[1] Complex coordination between these sensory inputs is then translated and interpreted by the brain.[2] Misinterpretation or inaccuracy of these three sources of information can lead to “sensory mismatch,” resulting in a variety of visual or vestibular illusions.

Issues of Concern

The Naval Aerospace Medicine Institute (NAMI) Curriculum classifies spatial disorientation (SD) as follows:

Type 1: Unrecognized

In this type, the corrective control input is not made because the need is unknown. Example: The Leans

Type 2: Recognized

In this type, the pilot recognizes the perceptual confusion. Proper control inputs are still possible. Example: Graveyard Spin

Type 3: Incapacitating

In this type, the plot is incapacitated/debilitated to such a severe degree that he is unable to recover the aircraft. Even if he is aware of their disorientation, he is unable to correct it secondary to extreme sensory system conflicts. Example: Coriolis Effect

Factors impacting the incidence of pilot spatial disorientation.

• Physical factors such as weather, time of night, mission duration, or mission type.
• Physiological factors such as illness, self-medication, alcohol, or fatigue.
• Other factors include pilot experience, mission preparation, etc.

Conditions that make spatial disorientation more likely include instrument flight conditions, night vision goggle flights, unaided night flight, and excessive-G flight. But spatial disorientation can occur during any type of flight.

The following are sensory system physiology, pertinent anatomy, and some of the more common aviation-related types of illusions:

Visual Illusions

During the flight, visual reference is the largest contributor to accurate spatial orientation. Both central (foveal) and peripheral (ambient) vision act synergistically to establish visual orientation.

Central vision is used in object identification. It is consciously controlled, requires active attention to focus, and can be easily distracted. Central vision interprets great detail, allows clear object recognition, and contains the majority of retinal cones leading to enhanced color perception. Peripheral vision is often subconscious, does not require focused attention, and is not distractible. Peripheral vision is used to gather general environmental information and identify motion.

By using visual references, the pilot is able to gather information about distance, speed, and depth. Several tools are used to establish a visual reference, including binocular vision, monocular vision, motion parallax, and retinal image size. Binocular vision utilizes the differences between retinal images to establish object location and movement and is effective for objects or terrain up to 200m away. Monocular vision allows for an expansion of the visual field at the cost of limited depth perception. The motion parallax employs monocular vision and relates to the relative velocity of objects as they move across the retina. One interprets faster-moving objects to be closer than slower-moving objects within the visual field. Retinal image size acts to make larger objects appear closer and smaller images look more distant. Additionally, visual reference can be established by differences in texture, detail, shadows, illumination, object clarity, and the size of objects as compared with known features or objects. Additionally, a pilot is able to maintain a reference to the horizon based largely on the visual reference. Without a visual horizon or a surface reference, aviators must use and trust their flight instruments over visual and sensory inputs.[3]

Visual illusions can be encountered during everyday life as well as within the aviation environment. These visual illusions are often a result of loss or deception of focal or ambient visual cues.

Common Visual Illusions

Aerial perspective illusion: a pilot will naturally develop a mental catalog of what a normal runway looks like. He/she builds an image of the expected length, width, and slope of the average runway based on his/her experience. This baseline allows them to adjust the glide path on final approach.

Runway Size and Shape Constancy Illusions

A wide runway may lead to an illusion of being closer to the ground than expected, causing the pilot to erroneously feel like he/she should correct by increasing altitude. This results in a high approach (increased risk of a stall or missed approach) to the runway.

A narrow runway may lead to an illusion of being further from the ground, causing the pilot to erroneously feel like he/she should correct by decreasing altitude. This results in a low approach to the runway (increased risk of landing short of the runway or a stall on flare).

Landing to an up-sloping runway may lead to an illusion of a high-altitude approach. The pilot may be inclined to correct by decreasing altitude. This results in a low approach to the runway (increased risk of landing short of the runway or a stall on flare).

Landing to a down-sloping runway may lead to an illusion of a low- altitude approach. The pilot may be inclined to correct by increasing altitude which will result in a high approach (increased risk of a stall or missed approach) to the runway.                  

Terrain Size and Constancy Illusions

Landing on a runway with up-sloping terrain leading to the final approach may create the illusion of being at a higher elevation than expected. The pilot may be inclined to correct by decreasing the altitude, leading to a low-altitude approach (increased risk of landing short of the runway or a stall on flare).

Landing to a runway with down-sloping terrain leading to the final approach may create the illusion of being at a lower elevation than expected. The pilot may be inclined to correct by increasing the altitude, leading to a high-altitude approach (increased risk of a stall or missed approach).

Ambien Illusions

Vection Illusion: An illusion where someone feels like his or her body is moving when no movement is taking place. The brain perceives peripheral motion without sufficient other cues of that motion. Sitting in a car at a stoplight and the car next to you inching forward can trigger the perception of one's car moving backward.

False Horizons Illusion: An illusion that occurs when a pilot orients the aircraft to a false horizon. This can occur during night flying, flying over featureless terrain, flying through clouds, and flying near ground lights that are difficult to distinguish from the night sky.

White-out/ Brown-out: As an aircraft nears the ground in snowy (white-out) or dusty (brown-out) conditions, the rotor downwash may blow up the ground cover. This can lead to a loss of visual reference among pilot and crew as they approach the ground. This loss of visual reference can cause spatial disorientation and loss of situational awareness.

Black-hole Illusion: This visual illusion can occur during night landings or in dark conditions when the horizon is not visible and the terrain is unlit. This creates the perception of a “black hole” between the aircraft and the runway which can lead to glide path overestimation and the erroneous initiation of an aggressive descent.[4]

Waterfall Effect Illusion: As a rotary wing aircraft hovers over a body of water, the rotors will draw water up and it will fall down over the windscreen. This can cause the visual illusion that the aircraft is rising.

Vestibular Illusions: The vestibular sensory systems make up 15% of the inputs used to maintain spatial orientation and contribute to a sense of balance. The vestibular system is part of the membranous labyrinth within the inner ear and is embedded deep within the temporal bone. The vestibular apparatus is connected to the auditory portion of the labyrinth and is comprised of the semicircular canals and the otolith organs.

Semicircular Canals

The three semicircular canals - horizontal, anterior, and posterior - are oriented perpendicularly (90-degree angle) to one another. The canal positioning allows for detection of angular acceleration in all three planes of movement: coronal, sagittal, and transverse axes. Acceleration is detected within the three canals when the liquid endolymph within the canals moves, causing movement of the cilia attached to the hair cells located within the cupula of the semicircular canal. It is the deflection of the cilia that causes activation of the hair cells.[5]

While flying straight and level, there is no movement of the endolymph, and therefore there is no movement of the sensory hair cell cilia. This is interpreted by the body as an environment without angular acceleration, i.e., there is no turn. The hair cells act as accelerometers and under a constant velocity remain de-activated.

During flight, as the pilot's head or the aircraft moves, the semicircular canal, and therefore, the sensory hair cell moves. However, inertia will act on the endolymph to keep in briefly stationary. The stationary fluid will pull on the moving hair cell cilia and lead to the accurate perception of a turn. If this turn is maintained for 10 to 20 seconds, the endolymph will catch up with the semicircular canal, and the hair cells will go back to a neutral upright orientation. When this happens, the aircraft may still be in a turn, but the pilot's brain may mistakenly interpret that the turn has stopped because there is no detected angular acceleration in the vestibular system. As the pilot returns to straight and level flight, the same phenomenon will occur. The semicircular canals and hair cells will move with the roll-out, but inertia will act on the endolymph to keep it momentarily stationary. This will erroneously signal to the brain that the pilot is now turning in the opposite direction when in reality the pilot is rolling out of the original turn.

Hair cells within the vestibular system can detect acceleration change, however, they are unable to distinguish between rest and a constant velocity. The miscommunication described above within the angular acceleration sensory system is the basis of semicircular related vestibular illusions. These illusions usually are triggered when the brain interprets a false sensation of rotation in the absence of reliable external visual cues.[1]

Somatogyral Vestibular Illusions: The false sensation of turning (or lack of turning) due to the inherent problems associated with semicircular canal function.

The Leans Illusion: Caused by an abrupt return to level flight after a prolonged unnoticed turn. As the pilot moves to level flight, he will feel that he/she is turning in the opposite direction.

The Graveyard Spin Illusion: Caused when a pilot intentionally or unintentionally enters a spin. Initially, the semicircular canals will register the angular motion, but in a prolonged spin, that sensation will progressively decrease. When the pilot applies the appropriate rudder to stop the spin, the pilot may feel like they are spinning in the opposite direction. This causes conflict between what the pilot feels and what is seen on the flight instruments. If the pilot believes his or her body, they will continue to original spin and lose altitude.

The Graveyard Spiral Illusion: Caused when a pilot enters a prolonged turn. When the pilot returns to level flight, it may feel like he/she is actually turning in the opposite direction. If the pilot believes the sensation from their body, they will re-enter the original turn. If the illusion remains unrecognized by the pilot, the pilot will continue the original turn and lose altitude.

The Coriolis Illusion: Occurs when the pilot tilts his/her head forward/backward while in a turn. This will cause simultaneous stimulation of two semicircular canals and leads to a sensation of aircraft roll, pitch, and yaw all at once which can be extremely disorienting during flight.

Otolith Organs

The otoliths organs are made up of the saccule and utricle and are positioned at a right angle from each other. The utricle detects linear acceleration in the horizontal plane. The saccule detects gravity changes in the vertical plane. They are located within the inner ear, near the vestibules of the semicircular canals. The otolith organs are covered in hair filaments, which extend up into a gelatinous material called the cupula. There are small calcium particles called otoconia that overlay the cupula. With movement, the otoconia will pull the cupula in relation to the hair filaments. This will convey the sensation of linear acceleration and gravitational pull. Illusions related to the otolith organs usually occur in conditions of degraded external visual cues and references.

Somatogravic Vestibular Illusions: A false pitch change sensation resulting from a linear acceleration

The Inversion Illusion: Caused by a steep ascent during flight which triggers the sensation of forward acceleration, followed by a sudden return to level flight. This may give the illusion that the pilot is in inverted flight.

The Heads-Up Illusion: Caused by sudden, forward, linear acceleration that is perceived as a nose-up flight attitude which causes the pilot to want to correct the aircraft by pitching the nose down. This is a significant risk for high-performance aircraft at take-off; for example during a catapult take-off from an aircraft carrier.

The Heads-Down Illusion: Caused by sudden linear deceleration with the perceived illusion of a nose down attitude. The pilot’s natural response will be to pitch the aircraft nose up. This can lead to a stall on final approach to landing.

Proprioceptive Receptors

Proprioceptive sensory inputs give us a reference to posture and the relative position of our body in relation to our environment. They play a small role in maintaining a sense of spatial orientation. A sensory mismatch between the environment and proprioceptive receptors alone will not result in spatial disorientation. However, they can add to disorientation if other visual and vestibular systems are involved as well.

Somatosensory Illusion: Can occur when a pilot undergoes G-forces. The pilot’s proprioceptive receptors receive conflicting inputs from gravitational forces vs. G-forces. It is commonly known as the "seat of the pants" illusion. The proprioceptive system is unreliable and can be easily overridden by the other sensory systems.

Clinical Significance

Military aviation began in the early 1900s, and the capabilities and limitations of the platform continue to be tested and expanded. In the 1950s, U.S. Naval aviation began to track their mishap rates. Data released by the U.S. Navy shows that during the first half of the 1950s, naval aviation class A mishap rates were greater than 50 mishaps per 100,000 flight hours. With the implementation of several safety features, protocols, and improved technology the mishap rate since the 1990s has been less than five mishaps per 100,000 flight hours.

However, spatial disorientation continues to be the most common cause of human-related aircraft accidents. Analysis of Naval Aviation mishap causes rank spatial disorientation is one of the leading cause of Class A mishaps. On review of data from the NAMI Safety Center, in Class A mishaps, including spatial disorientation as a causal factor, from 2000 through 2017 the rate of crew fatality was 38% (other resources site much higher fatality rates). Some reports cite spatial disorientation as contributing to 25% to 33% of all aircraft mishaps.[6] It is also evident that the rate of mishaps related to disorientation has not decreased proportionally with other mishap causes. Spatial disorientation continues to be a leading cause of aviation mishaps and fatalities and affects all types of aviation including military, commercial, and general aviation.

Spatial disorientation is a phenomenon that has been described since this beginning of aviation; however, the complex mechanisms of the process have remained indefinable. Loss of spatial orientation is almost always a result of a breakdown in the basic operator control loop. There is a distraction which causes a breakdown of the conscious center's ability to process sensory input information accurately. This can be secondary to a breakdown in the cockpit scan, task saturation which exceeds the brain's limit of tasks that can be processed, or another outside source. This breakdown is caused by a visual, vestibular, proprioceptive, or cognitive discrepancy which can lead to a perception difference between the actual and perceived aircraft orientation. However, just having a mismatch does not necessarily mean that there will be an illusion. If the crew is aware of a sensory mismatch, they can consciously or unconsciously disregard that sensory input and, instead, rely on their flight instruments. Mishaps and fatalities occur when the crew is unable to overcome the internal sensory inputs or when they are unaware that any perceptual confusions exist. 

Spatial Disorientation Mitigation

Personal experience of spatial disorientation is one of the best ways to quickly recognize when it is happening during flight. Tools like the Barany chair or Virtual Reality Spatial Disorientation Demonstrator simulate sensory illusions and give a pilot first-hand experience before they occur. Before a crew leaves for a flight, they should brief terrain, features and know the size and shape of the major objects they will be flying over. Tools that can be used in the cockpit during flight include maintaining an effective instrument scan. Continuous cross-check of instruments will help determine the aircraft's true orientation even if the pilot is experiencing an illusion. Additionally, maintained crew coordination and communication between the crew and the tower will improve situational awareness and share task loading to make spatial disorientation less likely. Finally, a pilot who begins to feel the effects of spatial orientation should trust the instruments over the conflicting signals coming from their body. If two pilots are in the aircraft, transfer control of the aircraft to the pilot who is not experiencing sensory illusions.[1]

More research, training, and technology is needed to better define and understand aviation-related spatial disorientation, to measure spatial disorientation events, and to prepare aircrew to successfully mitigate its effects.