In general term, vertigo could mean dizziness, unsteadiness or light-headedness. This term has been commonly used in medical field as a major symptom of balance disorder. From its etymology, vertigo comes from the Latin "vertere" which means to turn and -igo which is a condition of turning about.
Although vertigo could mean differently in different professions, vertigo or spatial disorientation, in the aviation world, is a condition wherein which an aircraft pilot's sense of direction contradicts or does not agree with reality. It is a condition wherein which the sufferer is unable to determine the true position of the body. According to its most widely used definition, one that has been accepted by a large number of countries, Spatial Disorientation (SD) refers to the pilot's: “failure to sense correctly the position, motion or attitude of his aircraft or of him/herself within the fixed coordinate system provided by the surface of the earth and the gravitational vertical". Spatial Disorientation is different from geographic disorientation as the latter is the loss of awareness of location in relation to a particular place in the Earth's surface.
By nature, human beings are able to maintain spatial orientation on ground. It is our natural ability to maintain our body's orientation and/or posture in relation to the surrounding environment (physical space) at rest and during motion. However, the flight environment is unfamiliar to the human body under normal circumstances. It can be harsh and tricky and it creates sensory conflicts and illusions that make spatial orientation difficult, and, in some cases, even impossible to achieve. This is where Spatial Disorientation takes place.
While vertigo can literally mean dizziness, it is the human's failure to picture the position relative to the horizon that makes it a truly dangerous problem. This is basically the reason why spatial disorientation is one of the major concerns and issues in the aviation world. Further to my readings, "statistics show that between 5% and 10% of all general aviation accidents can be attributed to spatial disorientation, 90% of which are fatal."
Moreso, Spatial Disorientation has been considered a phenomenon and is one of the significant hazards in the industry. In fact, several major accidents involving pilot's spatial disorientation in large commercial airlines have caused attention to the international aviation safety community.
Some flight incidents/reports concerning Spatial Disorientation
This phenomenon was extensively reported in the press in 1999, after John F. Kennedy, Jr.'s plane went down during a night flight over water near Martha's Vineyard. Subsequent investigation pointed to spatial disorientation as a probable cause of the accident.
It is believed that world-famous singer, Jim Reeves, was suffering from spatial disorientation when his Beechcraft aircraft crashed in the Brentwood area of Nashville, Tennessee during a violent thunderstorm on Friday 31st July 1964, claiming the lives of both Reeves and his pianist Dean Manuel.
An accident near Nassenwil/Zurich, Switzerland on 10 January 2000, about 1756 local time, Crossair Flight 498, a Saab 340B carrying three crewmembers, seven passengers, took off from Unique Zurich Airport (ZRH), Switzerland in night instrument meteorological conditions.
Accident on 23 August 2000 about 1930 local time, Gulf Air Flight 72, an Airbus A320 carrying eight crewmembers and 135 passengers crashed into the Arabian Gulf near Muharraq, Bahrain in dark night visual meteorological conditions.
On 3 January 2004, about 0445, Flash Airlines Flight 604, a Boeing 737-300 carrying six crewmembers and 142 passengers, crashed after departing Sharm el-Sheikh International Airport (SSH) in a dark night visual meteorological condition.
(Accident reports based from the case studies of William J. Bramble, Jr. Of the US National Transportation Safety Board on his case studies and countermeasures on Spatial Disorientation in Large Commercial Airplanes)
Reason behind spatial disorientation
Clinical/medical explanation of the phenomenon lies with the inner ear in the human body. Its functions are to give the brain information about rotation, altitude and linear motion of the head. It uses complex mechanism of canals and organs of equilibrium. Also, "good spatial orientation on the ground relies on the use of visual, vestibular (organs of equilibrium located in the inner ear), and proprioceptive (receptors located in the skin, muscles, tendons, and joints) sensory information. Changes in linear acceleration, angular acceleration, and gravity are detected by the vestibular system and the proprioceptive receptors, and then compared in the brain with visual information."
This is the reason why spatial orientation is hard to achieve in flight because of these numerous sensory stimuli responsible for our normal body's state. Also these stimuli vary in magnitude, direction and frequency. Any disagreements and conflicts between these stimuli will result in confusion and sensory mismatch that can produce illusions and eventually lead to spatial disorientation. Not having a good visual of the outside can add up to the feeling of confusion and disillusion leading to SD.
There are many illusions pilots can experience which lead to spatial disorientation. The ultimate fix? To know about them, how and why they occur, when they occur, and how to recover from these tendencies
The leans occur when a quick correction of a banked attitude happens to slowly. The sensory in your inner ear send your brain the sensation of banking in the opposite direction. However the disoriented will just over bank in the wrong direction possibly rolling the aircraft.
The Coriolis Illusion Is caused by making a quick head movement during a constant rate turn that has ceased stimulating the inner ear. The most common occurrence of this is. A pilot making a timed 180 degree turn for one minute, dropping a pen, approach plate etc. Thus stirring up the fluid in the inner ear, when the pilot comes back up he will feel as though he is straight and level, although his instruments show he is still turning. It is best to avoid abrubt head movements while under IFR.
Graveyard Spin The pilot recovering from a spin that was stopped the fluid in the inner ear can create the illusion that he or she has entered a spin in the other direction although they just re-enter the orignal spin
Graveyard Spiral While turning you notice you have lost altitude. Your sensory systems makes you feel as though you are in level flight so you pitch back. Although you are actually in a turn, the abrupt pitching back tightens the turn and you loose more altitude and increase the loss of altitude, the process then repeats itself.
Somatogravic Illusion This is caused usually during take-off. The rapid acceleration pushes the pilot back in his or her seat, giving them the sensation of a nose up attitude to correct the pilot noses the plane over towards the earth. NOTE: A rapid deceleration has the opposite effect.
Inversion Illusion A quick change from a climb to level flight makes the pilot feel as though he or she is tumbling backwards. The natural tendency is to nose the aircraft over, which actually intensifies the illusion.
Elevator Illusion On a turbulent day and updraft could cause extreme vertical acceleration the pilot then proceeds to nose over the aircraft, this illusion also has the opposite effect with downdrafts. Although intense this illusion presents the least of troubles, considering it usually happens at higher altitudes.
False Horizon A false horizon can be caused by city lights, clouds, stars, darkness. It causes the pilot to believe that it is the horizon. The pilot then places the aircraft in this dangerous attitude.
Autokinesis at night, when a light is stared at for a long period of time it begins to “move” While flying at night pilots should not stare at stationary lights for long periods of time to avoid this sensation