Our sense of orientation is probably one of the greatest abilities of the human body. The world's ability to manipulate and lie to this ability is probably one of its greatest weaknesses.
Any pilot has at some point in their training learned about visual and vestibular illusions. While these illusions are highly stressed during instrument training, private pilot applicants are also required to demonstrate proficiency under the hood. While this training is limited in nature, it is still required due to its importance.
Most instructors begin, or should, introduce students to how pilots and all humans alike maintain their reference in space before attempting to explain how pilots lose their sense of orientation.
Simply put, spatial orientation is our natural ability to maintain our body's orientation and position in relation to the surrounding environment. Specifically for flying, spatial orientation can be defined as an awareness of the aircraft's position and oneself in relation to a specific reference point - while in flight the reference point is the horizon in most instances.
When a healthy person is on the ground, the human body is able to maintain spatial orientation very well.
However, while in flight, the same bodily systems that maintain orientation on the ground can be fooled, leading to incorrect, misleading, and conflicting sensations - otherwise known as spatial disorientation.
When a person is spatially disorientated, they will be unable to determine and maintain the correct orientation in the space surrounding them. Pilots experiencing spatial disorientation while in flight will be unable to correctly determine their orientation regarding the airplane's position, attitude, or movement in space. Spatial disorientation can be very difficult and sometimes impossible to overcome. Today, statistics show that between 5 - 10% of all general aviation accidents can be attributed to spatial disorientation - with 90% being fatal.
Spatial orientation, and spatial disorientation, while on the ground or in-flight, is a perceptual phenomenon.
A person's ability to perceive or sense their orientation in a three-dimensional space is based upon that person's ability to interpret the continuous and changing signals delivered to the brain from many sensory receptors located across the body.
A persons' orientation is maintained with three different perceptual "senses" :
In most cases, all three areas must match and agree with what the other areas are sensing to maintain spatial orientation.
If the information provided by these sense organs conflicts with another sense organ, a sensory mismatch occurs, which may produce illusions and eventually spatial disorientation.
The eyes play the most prominent role in determining a person's spatial orientation and, subsequently, the aircraft.
This is because the eyes can provide information about the aircraft relative to the environment for both aircrews and their passengers. The eyes can also give pilots information about the aircraft's position over the earth's surface while providing attitude information regarding pitch, bank, and yaw.
Probably the greatest ability of the visual sense is that the information provided by the eyes can easily override any false sensations determined by the other sense organs, thus preventing a sensory mismatch and potential illusions. This is why occupants of aircraft that are experiencing motion sickness - a symptom of sensory mismatch - are told to look outside to help ease their uneasiness.
With the visual information passed from the eyes to the brain, pilots generally have minimal difficulty maintaining orientation while in visual meteorological conditions (VMC) where visual cues are prevalent. The task of determining the aircraft's orientation is simply an extension of the interpretative skills acquired during childhood.
However, during flight in IMC, or even at night, visual cues are removed and false sensations can easily cause a pilot to become disorientated extremely quickly.
Pilots must learn to interpret the aircraft instruments to avoid becoming spatially disorientated.
The problem is that the visual information provided by the instruments is symbolic. This visual information lacks familiarity, and therefore, the strength of these visual cues is diminished.
Proper and effective training must be received to acquire the necessary interpretive skills required to determine the aircraft's orientation solely by the aircraft's instruments. The phrase "Trust Your Instruments" could not be more stressed, and any student pilot that has gone through hood or IFR training no doubt has heard this phrase multiple times throughout their training. It should be stated that even with proper training it is still possible for a pilot to become spatially disorientated. Especially in the case of inadvertent flight into IMC.
However, pilots that do undergo proper training and remain proficient in their abilities can and regularly do perform flight with sole reference to the instruments without a reduction in safety.
If you were to stand up and close your eyes, you can still maintain your balance and remain standing. You are also able to walk, run, twist, turn, etc.
This ability to maintain orientation or balance without any visual cues is mainly provided by the vestibular system, with help from the somatosensory system which is described just below.
Located in the ears are two primary organs:
The cochlea is the organ responsible for hearing. While sound plays a very limited role, it still can aid, or hinder, a pilot's ability to determine orientation, especially in aircraft speed and pitch.
The vestibular apparatus can be described as the organ of equilibrium. This large organ is specially designed and extremely sensitive to movement. You can think of the vestibular apparatus as your own internal gyroscope. While it functions differently, the idea is very similar.
At only the size of a pea, the vestibular apparatus can sense angular accelerations as low as .05 degrees per second and linear accelerations of less than .01g.
The vestibular apparatus consists of 3 semi-circular canals - one for each axis (Lateral, Longitudinal, and Vertical). You can think in terms of pitch, bank, and yaw as that is simply the term for moving about each respective axis.
Each semi-circular canal contains a fluid called endolymph. Within the center of each semi-circular canal is a gelatinous structure that sits on top of sensory hairs called the cupola. These sensory hairs connect directly to the vestibular nerve which sends information to the brain.
The sensation of movement occurs when the sensory hairs are stimulated by a deflection in the cupola.
When the human body moves, the entire vestibular apparatus moves with it. However, the endolymph fluid within the semicircular canals tends to remain in place.
The tendency of the endolymph fluid to remain in place due to inertia causes a force to be applied to the cupola. This force will continue to be applied until the endolymph fluid has been accelerated to match the same speed of the entire vestibular apparatus. A process that can take upwards of 15 - 20 seconds while in flight.
However, normal head movements on the ground result in accelerations and decelerations that take less than a second to occur. This allows the fluid to accelerate and decelerate in very small time frames allowing very accurate sensations regarding a person's movement. This is mainly due to the sensory hairs being capable of responding to new stimuli upwards of 10 times a second.
An easy example of the vestibular systems movement is the simple motion of turning your head to the right.
While looking foreword the endolymph fluid in the ear is in its static, non-moving, position. When you turn your head the vestibular apparatus moves, and the endolymph fluid lags behind. This causes the fluid to rotate around the ear and deflect the cupola. This deflection is then detected by the sensory hairs and interpreted as movement by the brain. When you stop turning your head the fluid returns back to rest and the sensation of movement ceases.
While this system appears to be full-proof at first glance, pilots should be aware that it is not.
The vestibular system is capable of providing rapid and correct information to the pilot regarding rapid, transient angular, and linear movements. However, the vestibular system is also the most prone to providing inaccurate information and is the most likely to cause a pilot to experience an illusion. These illusions are more commonly called "Vestibular Illusions" and are simply the product of subjecting the vestibular apparatus, an organ formed on the ground, to unfamiliar forces that can occur in the flying environment.
The postural sensory system goes by many terms, such as the somatosensory system, proprioceptive stimuli, and kinesthesis, for example.
The somatosensory system consists of various receptors located in the skin, muscles, tendons, joints, ligaments, and deep supporting structures located around the body. Each receptor is stimulated mechanically by forces acting on the body, with most forces occurring from linear accelerations - such as a change of power or the force of gravity. Somatosensory receptors in the skin are especially sensitive to touch and pressure.
The somatosensory system is designed to adapt rapidly. They are the most capable at detecting a change in force rather than a steady-state condition. This ability allows the receptors in the body that make up the somatosensory system capable of complimenting the dynamic sensory ability of the vestibular system.
In all humans, the somatosensory system plays a crucial role in a person's ability to maintain spatial relationships and movements of one part of the body compared to the rest.
This ability is easily demonstrated.
Simply close your eyes, hold one arm fully outstretched in a random direction. Then, with your eyes still closed, move the other arm in an attempt to clap your hands together - without moving the outstretched arm. A healthy individual is capable of clasping both hands together without any visual reference.
This ability to accurately determine the position of one limb in respect to other parts of the body is called the proprioceptive or kinaesthetic sense. Mainly guided by receptors in the joints, with receptors in the muscles and tendons of the muscles also contributing.