Spatial disorientation (commonly referred to as Spatial-D) is the inability to determine one’s position, location, and motion relative to their environment. Spatial-D along with G-induced loss of consciousness (G-LOC) are two of the most common causes of fatality from human factors in military aviation. Spatial-D regularly affects pilots in all aircraft, whereas only military or acrobatic pilots in high performance aircraft have to worry about suffering a G-LOC. Another difference is that G-LOC can be prevented through the Anti-G Straining Maneuver, but there does not exist an anti-Spatial D maneuver. The mishap leading to the death of John F. Kennedy Jr was likely a result of unrecognized Spatial-D. I have personally lost a good friend as a consequence of this same phenomenon. My friend, Maj Luc “GAZA” Gruenther, was a highly respected military F-16 pilot who served in a leadership role in flight safety. This event reestablished for me the fact that any aviator at anytime can fall prey to this deadly human factor. The main contributing factor to the development of this condition is the fallibility of human anatomy and physiology in the flying environment. It is evident that the human body did not develop for the purpose of flight in 3-dimensional space. In order to fully understand this phenomenon, it is vital to appreciate the anatomy and physiology that allows a human being to orient oneself in space.
One’s capacity to know their body’s location and orientation in any setting is based on four functioning physiologic systems. Sensory information for orientation is provided by our visual system, vestibular system, proprioceptive system, and auditory system. We will briefly examine each of these systems independently below before looking at how each can break down and in some cases provide inaccurate information in flight.
Out of the above systems, vision is the most vital sense for determining orientation. Without vision, determining one’s position on the ground can even be difficult. The task of flying in the air without vision would be nearly impossible. Visual perception is often separated into two separate categories with two unique functions- focal vision & ambient vision. Focal vision is used for object recognition and occurs whenever light reflected from an object is focused on the fovea in the back of the retina. This process is dominated by photoreceptors known as cone cells. This type of vision uses only the central 30 degrees within the visual field. It is often said that focal vision answers the question of “What?” and relies on higher cognition for interpretation. Ambient vision, on the other hand, is described as answering the “Where?” question and is derived from stimulation of photoreceptors known as rods in the periphery of the retina, and therefore the corresponding peripheral visual fields. You may recall that rods function better than cones in darker environments, but also detect far less detail and color recognition. This type of vision is usually processed subconsciously, and therefore can provide vital information to one’s position relative to their environment while the central focal vision focuses on other unrelated objects.
Although not as vital as the visual sensory system, the role of the vestibular apparatus located in the inner ear is also highly important in determining spatial orientation. This system both cooperates with the visual system to assist with stabilization of an image on the retina when the head and/or body is in motion, and also acts independently to provide sensory information of the body’s position and motion in the absence of vision.
UNDERSTANDING THE PHYSIOLOGY OF THE VESTIBULAR APPARATUS
The anatomy of the vestibular apparatus resides within the bony labyrinth of the inner ear, located in the temporal bones of the skull. The cochlea, the vestibule, and the semicircular canals make up the bony labyrinth. The cochlea is the sensory organ for hearing. The vestibule is comprised of the otolith organs, the utricle and saccule, which sense linear acceleration. The semicircular canals, on the other hand, sense angular or rotational motion and accelerations of the head. Within the various vestibular end organs is a fluid called endolymph.
Although the actual way in which mechanical energy in the form of motion and acceleration translates into a neural input is quite complicated, it should suffice to say that relative motions and accelerations provoke either movement of the endolymph (fluid in the inner ear) and/or other moving parts within the vestibular apparatus. When these materials move, microscopic projections known as cilia from sensory receptors called hair cells physically bend. Depending on the direction the cilia bends will translate into either a cellular depolarization or hyperpolarization, thus initiating or terminating an action potential that travels down neural pathways to the brain for further interpretation. In this way, mechanical energy from physical motion and position are converted into a neural signal, which is then transformed by the brain into a spatial map of one’s orientation.
The third way in which humans sense their position and motion is through proprioceptors in connective tissue, joints, and the skin. Muscle spindles, Golgi tendon organs, cutaneous mechanoreceptors and other sensory receptors relay information about the relative position and interaction of the body’s extremities and skin with its environment. Lastly, the sense of hearing (acoustic energy transmitted to the cochlea from vibration of the eardrum through the three small bones known as the hammer, anvil and stirrup) has also demonstrated an ability to assist with spatial orientation thru perceiving the location of a sound’s source. When one is said to be “flying by the seat of their pants”, they are invoking sensory cues from these latter three systems rather than the dominant sense of vision.
In order to fully understand Spatial Disorientation (as experienced by the aviator), it is important to note several characteristics about these sensory systems and how they work together to determine the body’s relative motion and orientation. All of the above senses have specific minimum thresholds at which the particular sensation initiates a neural input perceived by the human mind. This means that below a certain signal intensity, inputs will not be perceived and therefore no action would occur. Fascinating studies have been conducted demonstrating that these thresholds and therefore one’s insensitivity to stimulus increases with inattention. It is easy to see how a pilot who is distracted or not vigilantly monitoring flight instruments is at increased risk for allowing important sensory information to go unnoticed. These senses also demonstrate both adaptation and habituation, which means the response to persistent or repetitive stimuli decreases or possibly ceases altogether over time. This phenomenon allows Spatial-D to be intentionally reproduced in an aircraft, simulator, or spinning Barany chair in order to demonstrate how the mind can be easily fooled by persistent angular motion such as experienced during a prolonged banked turn.
Watch the below video as the US Navy student pilot points his thumbs in the direction of perceived rotation. After a certain amount of time under constant rotational acceleration, the fluid in the semicircular canals reaches an equilibrium and without the aid of vision to overrule his senses, he is unable to determine that he is still spinning.
NAVY PILOT BARANY CHAIR TRAINING
The interaction of the above senses need be considered. The vestibular sense seems to be suppressed in favor of visual dominance and enhanced by new, novel stimulatory sensations. This means that when vision is intact and unobstructed, the mind will favor vision’s spatial information if the vestibular system provides conflicting data. If a pilot enters the weather, however, the mind quickly reverses the suppressive effect on the vestibular senses as spatial orientation becomes derived from these inputs. This can sometimes occur with disastrous results as the vestibular system is prone to very specific and reproducible errors when linear or angular accelerations are maintained for significant periods of time. Even on the ground at 1 G and 0 knots, the various senses that provide humans with their spatial orientation can be prone to error. We know that humans require at least 2 of the senses described above to be operating correctly to maintain an accurate spatial and postural equilibrium. The Romberg Test examines the functioning of these systems by having the patient stand and close their eyes. If one’s proprioceptive or vestibular systems have been damaged due to a stroke or other injury, balance will not be able to be maintained without visual input.
It can now be understood how human biology can predispose aircrew to an erroneous perception of their location and motion relative to the Earth, aka Spatial Disorientation. There are variety of specific types of Spatial Disorientation that are known to affect aircrew. These will be covered in detail elsewhere. Spatial-D is broadly divided into three categories:
Type 1 – Unrecognized
Type 2 – Recognized
Type 3 – Capacitating
It is thought that more than half of the mishaps caused by Spatial-D are a consequence of Type 1 SD and that a majority of the remainder are Type 2. Type 3 SD may be due to various causes, such as a nystagmus (rhythmic beating of the eye) preventing the pilot from physically being able to see flight controls, dominant reflexes that hijack the controls, a cognitive inability to solve the disorientation, or incapacitating fear. These examples of Type 3 Spatial-D are fortunately quite rare. This is a good thing as most cases of Type 3 Spatial-D result in controlled flight into terrain (CFIT) with survival only in the event of ejection. The following video demonstrates a Type 1 SD case followed by a re-creation of a fatal Type 3 SD case, both in the F-16 Fighting Falcon. Note that in the first case, the instructor pilot’s (IP) recommendations to “get on the round dials” possibly saves his wingman’s life as the disoriented pilot immediately corrects the attitude of his aircraft. This is likely due to the fact that his IP’s advice prompted him to transition from visually assessing his environment outside the cockpit in favor of trusting his flight instruments inside.
In summary, Spatial Disorientation remains a constant threat to general, military, and commercial aviation pilots alike. Globally, Spatial-D is thought to be the leading cause of mishaps in flight. This article provides a foundation for the anatomy, physiology and pathology for Spatial Disorientation and will be referred to in future articles on the topic. A list of types of Spatial D and how the above sensory systems make a pilot prone to this deadly syndrome can be found elsewhere.
1. Fundamentals of Aerospace Medicine (Ed. 4) by Jeffrey R. Davis (Editor), Robert Johnson (Editor), Jan Stepanek (Editor), Jennifer A. Fogarty (Editor).