Editing Spatial disorientation (section)

== The vestibular system and sensory illusions ==
{{main|Sensory illusions in aviation}}
[[File:Blausen 0329 EarAnatomy InternalEar.png|thumb|right|upright=1.5|Inner ear]]
The vestibular system detects linear and angular (rotational) acceleration using specialized organs in the inner ear. Linear accelerations are detected by the [[otolith organs]], while angular accelerations are detected by the [[semicircular canals]].

=== Misleading sensations ===
Without a visual reference or cues, such as a visible horizon, humans will rely on non-visual senses to establish their sense of motion and equilibrium. During the abnormal acceleratory environment of flight, the [[Vestibular system|vestibular]] and [[proprioceptive system]]s can be misled, resulting in spatial disorientation. When an aircraft is maneuvering, inertial forces can be created by changes in vehicle speed (linear acceleration) and/or changes in direction (rotational acceleration and [[centrifugal force]]), resulting in perceptual misjudgment of the vertical, as the combined forces of gravity and inertia do not align with what the vestibular system assumes is the vertical direction of gravity (towards the center of the Earth).

Under ideal conditions, visual cues will provide sufficient information to override illusory vestibular inputs, but at night or in poor weather, visual inputs can be overwhelmed by these illusory nonvisual sensations, resulting in spatial disorientation. Low visibility flight conditions include night,<ref name=AC60-4/> over water or other monotonous/featureless terrain that blends into the sky,<ref name=AC60-4/> [[Whiteout (weather)|white-out weather]],<ref name=AC60-4/> or inadvertent entry into [[instrument meteorological conditions]] after flying into fog or clouds.

[[File:Load factor and the g-force in turn.svg|thumb|left|Lift (L) and weight/gravity (w) forces acting on an aircraft making a banked or coordinated turn]]
For example, in an aircraft that is making a [[Banked turn#Banked turn in aeronautics|coordinated (banked) turn]], no matter how steep, occupants will have little or no sensation of being tilted in the air unless the horizon is visible, as the combined forces of lift and gravity are felt as pressing the occupant into the seat without a lateral force sliding them to either side.<ref>{{cite news |url=https://japantoday.com/category/national/ana-pilots-unaware-for-20-seconds-that-plane-was-almost-turning-upside-down |title=ANA pilots unaware for 17 seconds that plane was almost turning upside down |date=August 31, 2012 |work=Japan Today |access-date=11 February 2021 |quote=The aircraft tipped more than 130 degrees to the left at one point, but the darkness outside meant many of those on board did not realize the craft had almost flipped over.}}</ref> Similarly, it is possible to gradually climb or descend without a noticeable change in pressure against the seat. In some aircraft, it is possible to execute a loop without pulling negative g-forces so that, without visual reference, the pilot could be upside down without being aware of it.{{Citation needed|date=March 2011}} A gradual change in any direction of movement may not be strong enough to activate the vestibular system, so the pilot may not realize that the aircraft is accelerating, decelerating, or banking.

[[File:BASIC_Flight_instruments_Improved.svg|thumb|right|Standard set of [[flight instruments]], including [[attitude indicator]] (top center) and [[turn and slip indicator]] (bottom left)]]
Gyroscopic [[flight instruments]] such as the [[attitude indicator]] (artificial horizon) and the [[turn and slip indicator]] are designed to provide information to counteract misleading sensations from the non-visual senses.

===Otoliths and somatogravic illusions===
Two otolith organs, the saccule and utricle, are located in each ear and are set at right angles to each other. The utricle detects changes in linear acceleration in the horizontal plane, while the saccule detects linear accelerations in the vertical plane; humans have evolved to assume the vertical acceleration is caused by gravity. However, the saccule and utricle can provide misleading sensory perception when gravity is not limited to the vertical plane, or when vehicle speeds and accelerations result in inertial forces comparable to the force of gravity, as the otoliths only detect acceleration, and cannot distinguish inertial forces from the force of gravity.<ref name=FAA-spatial/> Some examples of this include the inertial forces experienced during a vertical take-off in a helicopter or following the sudden opening of a parachute after a free fall.{{cn|date=June 2022}}

Illusions caused by the otolith organs are called somatogravic illusions and include the Inversion, Head-Up, and Head-Down Illusions. The Inversion Illusion results from a steep ascent followed by a sudden return to level flight; the resulting relative increase in forward speed produces an illusion the aircraft is inverted.<ref name=FAA-spatial/> The Head-Up and Head-Down illusions are similar, involving sudden linear acceleration (Head-Up) or deceleration (Head-Down), leading to a misperception the nose of the aircraft is pitching up (Head-Up) or down (Head-Down); the aviator could be fooled into pitching the nose down (Head-Up) or up (Head-Down) in response, leading to a crash or a stall, respectively.<ref name=FAA-spatial/>

Typically, the Head-Up illusion occurs during take-off, as a strong linear acceleration is used to generate lift over the wing and flaps. Without a visual reference, the pilot may assume from the vestibular system the nose has pitched up and command a dive; if this occurs during take-off, the aircraft may not have sufficient altitude to recover before crashing into the ground.<ref name=ATSB-SD2007/>{{rp|7}}

===Semicircular canals and somatogyral illusions===
[[Image:innernvestib.jpg|thumb|right|[[Inner ear]] with semicircular canals shown, likening them to the roll, pitch and yaw axis of an aircraft]]
In addition, the inner ear contains rotational [[accelerometer]]s, known as the semicircular canals, which provide information to the lower brain on rotational accelerations in the [[flight dynamics|pitch, roll and yaw axes]]. Changes in angular velocity are detected from the relative motion between the fluid in the canals and the canal itself, which is fixed to the head; because of inertia, the fluid in the canals tends to lag when the head moves, signaling a rotational acceleration. However, semicircular canal output ceases after prolonged rotation (beyond {{val|15|-|20|u=s}}) as the fluid has now been entrained into motion through friction, matching the motion of the head. If the rotation is then stopped, the perceived motion signal from the inner ear indicates the aviator is now turning in the opposite direction from actual travel, as the fluid continues to move while the canal has stopped.<ref name=FAA-spatial/> In addition, the inertia of the fluid means the detection threshold of rotational acceleration is limited to approximately 2°/sec{{sup|2}}; angular accelerations below this value cannot be detected.<ref name=ATSB-SD2007/>{{rp|5}} Specific common somatogyral illusions induced by the semicircular canals are the Leans, Graveyard Spin, Graveyard Spiral, and Coriolis.

{{main|The leans}}
If the aircraft enters an unnoticed, prolonged turn gradually, then suddenly returns to level flight, [[the leans]] may result. The gradual turn sets the fluid into the semicircular canals into motion, and rotational acceleration of two degrees per second (or less) cannot be detected. Once the aircraft suddenly returns to level flight, the continued fluid motion gives the sensation the aircraft is banking in the opposite direction of the turn that just ended; the aviator may attempt to correct the misperception of the vertical by banking into the original turn.<ref name=FAA-spatial/> The leans is considered the most common form of spatial disorientation.<ref name=ATSB-SD2007/>{{rp|9}}

{{main|Graveyard spiral|Graveyard spin}}
[[File:FAA PHAK 2008 Fig 16-5 Graveyard spiral.png|thumb|right|Graveyard spiral and graveyard spin]]
The [[graveyard spiral]] and [[graveyard spin]] are both caused by the acclimation of the semicircular canals to prolonged rotation; after a banked turn (in the case of the graveyard spiral) or spin (for the graveyard spin) of approximately 20 seconds, the fluid in the semicircular canals has been entrained into motion by friction, and the vestibular system no longer perceives a rotational acceleration. If the aviator then ends the turn or spin and returns to level flight, the continued motion of the fluid will cause a sensation the aircraft is turning or spinning in the opposite direction, and the pilot may re-enter the original turn or spin inadvertently; the aviator may not recognize the illusion before the aircraft loses too much altitude, resulting in a collision with terrain<ref name=FAA-spatial/> or the [[g-force]]s on the aircraft may exceed the structural strength of the [[airframe]], resulting in catastrophic failure. One of the most infamous mishaps in aviation history involving the graveyard spiral is [[John F. Kennedy Jr. plane crash|the crash involving John F. Kennedy Jr.]] in 1999.<ref name="auto">{{Cite web |url=https://goflightmedicine.com/jfk-jr-piper-saratoga-mishap/ |title=JFK Jr Piper Saratoga Mishap |author=Jedick, Rocky 'Apollo' |publisher=Go Flight Medicine |date=April 15, 2014}}</ref>

Once an aircraft enters conditions under which the pilot cannot see a distinct visual horizon, the drift in the inner ear continues uncorrected.  Errors in the perceived rate of turn about any axis can build up at a rate of 0.2 to 0.3 degrees per second.{{citation needed|date=July 2016}}  If the pilot is not proficient in the use of gyroscopic flight instruments, these errors will build up to a point that control of the aircraft is lost, usually in a steep, diving turn known as a [[graveyard spiral]].  During the entire time, leading up to and well into the maneuver, the pilot remains unaware of the turning, believing that the aircraft is maintaining straight flight.<ref>{{cite book |url=https://books.google.com/books?id=wjVPDwAAQBAJ&pg=PA125 |title=VPNavy! USN, USMC, USCG and NATS Patrol Aircraft Lost or Damaged During World War II |author=Campbell, Douglas E. |edition=2018 |date=2018 |publisher=Syneca Research |isbn=978-1-387-49193-3}}</ref>{{rp|125}}

In a 1954 study (180 – Degree Turn Experiment), the University of Illinois Institute of Aviation found that 19 out of 20 non-instrument-rated subject pilots went into a graveyard spiral soon after entering simulated instrument conditions.  The 20th pilot also lost control of his aircraft, but in another maneuver. The average time between onset of instrument conditions and loss of control was 178 seconds.<ref>{{cite book| publisher=University of Illinois |first1=Leslie |last1=Aulls Bryan |last2=Stonecipher |first2=Jesse W. |last3=Aron |first3=Karl |year=1954 |asin=B0007EXGMI |title=180-degree turn experiment |url=https://archive.org/details/180degreeturnexp11brya/mode/2up|ol=207786M |lccn=a54009717 |oclc=4736008}}</ref>

{{main|Coriolis effect (perception)}}
Spatial disorientation can also affect [[instrument flight rules|instrument-rated]] pilots in certain conditions. A powerful tumbling sensation ([[Vertigo (medical)|vertigo]]) can result if the pilot moves his or her head too much during instrument flight. This is called the [[Coriolis effect (perception)|Coriolis illusion]]. Because the semicircular canals are set in three different axes of rotation, if the aviator suddenly moves their head during a rotational acceleration, one canal may abruptly start to detect an angular acceleration while another ceases, resulting in a tumbling sensation.<ref name=ATSB-SD2007/>{{rp|9}}