Thirty years ago spatial disorientation was a leading cause of military aircraft accidents and it may still be.
In the civilian world of transporting people and things from Point A to Point B, it takes a much smaller toll because very few of us intentionally pull more than a G and a half or ever venture beyond a standard rate turn. But in the world of instrument flight, it is a threat.
The cure, for an instrument pilot, is to stay on instruments. But sometimes that is easier said than done. The cure, in that case, is knowledge. If you know about what can happen, you will be better prepared to deal with it when it does.
I was in the right seat of an Air Force Boeing 707, while one of the sharpest pilots I've ever known was in the left seat. We were both instructor pilots and the night was for proficiency in receiver air refueling. It was a moonless night over the ocean with no external references other than the lights on the tanker. He was doing a great job until the first turn. "Take the airplane," he said. "Take it now." I took the aircraft and he moved his head from the tanker to his attitude indicator. He said he got the leans all of a sudden. He never experienced them before, but he knew what the problem was before it happened.
Everything here is from the references shown below, with a few comments in an alternate color.
Figure: Somatosensory System, from AFM 51-37, Figure 7-3.
[U.S. Army Aeromedical Training Manual, ¶9-1.] Spatial disorientation is an individual’s inability to determine his or her position, attitude, and motion relative to the surface of the earth or significant objects; for example, trees, poles, or buildings during hover. When it occurs, pilots are unable to see, believe, interpret, or prove the information derived from their flight instruments. Instead, they rely on the false information that their senses provide.
Spatial disorientation is not unusual, a properly trained pilot may have a moment of disorientation but "pulls out of it" once recognized.
[U.S. Army Aeromedical Training Manual, ¶9-2.] A sensory illusion is a false perception of reality caused by the conflict of orientation information from one or more mechanisms of equilibrium. Sensory illusions are a major cause of spatial disorientation.
We used to get this all the time while air refueling. Our eyes were on the wings of the tanker, which became our local horizon. A cloud deck below or the actual earth's horizon could disagree with the tanker's attitude and we could often find ourselves with momentary disorientation. Every now and then it got so bad we had to exchange aircraft control between pilots.
You can get it in every day flight by things like a sloping cloud deck or even a long, steady turn. The key, of course, is to check your attitude and performance instruments.
[U.S. Army Aeromedical Training Manual, ¶9-3.] Vertigo is a spinning sensation usually caused by a peripheral vestibular abnormality in the middle ear. Aircrew members often misuse the term vertigo, applying it generically to all forms of spatial disorientation or dizziness.
This Army manual seems to assume there is a copilot present to gain control of the aircraft. There have been cases where the copilot was unwilling to assume command for fear of angering the pilot, such as: Korean Air 8509 or where the rest of the crew was probably not engaged actively in backing up the captain because the culture of their airline was that the captain could never be challenged, as in: Pan American World Airways 816.
[U.S. Army Aeromedical Training Manual, ¶9-4.] A disoriented aviator does not perceive any indication of spatial disorientation. In other words, he does not think anything is wrong. What he sees—or thinks he sees—is corroborated by his other senses. Type I disorientation is the most dangerous type of disorientation. The pilot—unaware of a problem—fails to recognize or correct the disorientation, usually resulting in a fatal aircraft mishap:
[U.S. Army Aeromedical Training Manual, ¶9-5.] In Type II spatial disorientation, the pilot perceives a problem (resulting from spatial disorientation). The pilot, however, may fail to recognize it as spatial disorientation:
[U.S. Army Aeromedical Training Manual, ¶9-6.] In Type III spatial disorientation, the pilot experiences such an overwhelming sensation of movement that he or she cannot orient himself or herself by using visual cues or the aircraft instruments. Type III spatial disorientation is not fatal if the copilot can gain control of the aircraft.
The important lesson here is we tend to believe our vestibular and propriocepti systems naturally, but it is our visual system that is most reliable. We have to train ourselves to overrule the vestibular and proprioceptive systems in favor of the visual.
Figure: The Three Equilibrium Systems, from U.S. Army Aeromedical Training Manual, Figure 9-1.
[U.S. Army Aeromedical Training Manual, ¶9-7.] Three sensory systems—the visual, vestibular, and proprioceptive systems—are especially important in maintaining equilibrium and balance. Figure 9-1 shows these systems. Normally, the combined functioning of these senses maintains equilibrium and prevents spatial disorientation. During flight, the visual system is the most reliable. In the absence of the visual system, the vestibular and proprioceptive systems are unreliable in flight.
[U.S. Army Aeromedical Training Manual, ¶9-8.] Of the three sensory systems, the visual system is the most important in maintaining equilibrium and orientation. To some extent, the eyes can help determine the speed and direction of flight by comparing the position of the aircraft relative to some fixed point of reference. Eighty percent of our orientation information comes from the visual system.
Figure: The Vestibular System, from U.S. Army Aeromedical Training Manual, Figure 9-2.
[U.S. Army Aeromedical Training Manual, ¶9-11.] The inner ear contains the vestibular system, which contains the motion- and gravity detecting sense organs. This system is located in the temporal bone on each side of the head. Each vestibular apparatus consists of two distinct structures: the semicircular canals and the vestibule proper, which contain the otolith organs. [The figure] depicts the vestibular system. Both the semicircular canals and the otolith organs sense changes in aircraft attitude. The semicircular canals of the inner ear sense changes in angular acceleration and deceleration.
Figure: The Otolith Organs, from U.S. Army Aeromedical Training Manual, Figure 9-3.
[U.S. Army Aeromedical Training Manual, ¶9-12.] The otolith organs are small sacs located in the vestibule. Sensory hairs project from each macula into the otolithic membrane, an overlaying gelatinous membrane that contains chalk-like crystals, called otoliths. The otolith organs, shown in [the figure], respond to gravity and linear accelerations/decelerations. Changes in the position of the head, relative to the gravitational force, cause the otolithic membrane to shift position on the macula. The sensory hairs bend, signaling a change in the head position.
[U.S. Army Aeromedical Training Manual, ¶9-13.] When the head is upright, a "resting" frequency of nerve impulses is generated by the hair cells.
[U.S. Army Aeromedical Training Manual, ¶9-14.] When the head is tilted, the "resting" frequency is altered. The brain is informed of the new position.
[U.S. Army Aeromedical Training Manual, ¶9-15.] Linear accelerations/decelerations also stimulate the otolith organs. The body cannot physically distinguish between the inertial forces resulting from linear accelerations and the force of gravity. A forward acceleration results in backward displacement of the otolithic membranes. When an adequate visual reference is not available, aircrew members may experience an illusion of backward tilt.
Figure: Reaction of the Semicircular Canals to Changes in Angular Acceleration, from U.S. Army Aeromedical Training Manual, Figure 9-7.
[U.S. Army Aeromedical Training Manual, ¶9-16.] The semicircular canals of the inner ear sense changes in angular acceleration. The canals will react to any changes in roll, pitch, or yaw attitude.
[U.S. Army Aeromedical Training Manual, ¶9-17.] The semicircular canals are situated in three planes, perpendicular to each other. They are filled with a fluid called endolymph. The inertial torque resulting from angular acceleration in the plane of the canal puts this fluid into motion. The motion of the fluid bends the cupula, a gelatinous structure located in the ampulla of the canal. This, in turn, moves the hairs of the hair cells situated beneath the cupula. This movement stimulates the vestibular nerve. These nerve impulses are then transmitted to the brain, where they are interpreted as rotation of the head.
Figure: Position of Hair Cells During No Acceleration, from U.S. Army Aeromedical Training Manual, Figure 9-9.
[U.S. Army Aeromedical Training Manual, ¶9-18.] When no acceleration takes place, the hair cells are upright. The body senses that no turn has occurred.
Figure: Sensation During a Clockwise Turn, from U.S. Army Aeromedical Training Manual, Figure 9-10.
[U.S. Army Aeromedical Training Manual, ¶9-19.] When a semicircular canal is put into motion during clockwise acceleration, the fluid within the semicircular canal lags behind the accelerated canal walls. This lag creates a relative counterclockwise movement of the fluid within the canal. The canal wall and the cupula move in the opposite direction from the motion of the fluid. The brain interprets the movement of the hairs to be a turn in the same direction as the canal wall. The body correctly senses that a clockwise turn is being made.
Figure: Sensation During a Prolonged Clockwise Turn, from U.S. Army Aeromedical Training Manual, Figure 9-11.
[U.S. Army Aeromedical Training Manual, ¶9-20.] If the clockwise turn then continues at a constant rate for several seconds or longer, the motion of the fluid in the canals catches up with the canal walls. The hairs are no longer bent, and the brain receives the false impression that turning has stopped. The position of the hair cells and the resulting false sensation during a prolonged, constant clockwise turn is shown in [the figure]. A prolonged constant turn in either direction will result in the false sensation of no turn.
Figure: Sensation During Slowing or Stopping of a Clockwise Turn, from U.S. Army Aeromedical Training Manual, Figure 9-12.
[U.S. Army Aeromedical Training Manual, ¶9-21.] When the clockwise rotation of the aircraft slows or stops, the fluid in the canal moves briefly in a clockwise direction. This sends a signal to the brain that is falsely interpreted as body movement in the opposite direction. In an attempt to correct the falsely perceived counterclockwise turn, the pilot may turn the aircraft in the original clockwise direction. [The figure] shows the position of the hair cells—and the resulting false sensation when a clockwise turn is suddenly slowed or stopped.
[U.S. Army Aeromedical Training Manual, ¶9-22.] This system reacts to the sensation resulting from pressures on joints, muscles, and skin and from slight changes in the position of internal organs. It is closely associated with the vestibular system and, to a lesser degree, the visual system. Forces act upon the seated pilot in flight. With training and experience, the pilot can easily distinguish the most distinct movements of the aircraft by the pressures of the aircraft seat against the body. The recognition of these movements has led to the term "seat-of-the-pants" flying.
The book I've used for reference here, Basic Flight Physiology, is quite good. Don't let the illustrations of the light aircraft throw you. The effects are more pronounced in faster aircraft.
Photo: The leans, Reinhart, Figure 8-9
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[Reinhart, pp. 139 - 140] Leans. The most common illusion, this occurs when the pilot senses a bank angle when the aircraft is actually in level flight. If she maintains the level attitude of the aircraft (as she should), she will still feel compelled to align her body with the perceived vertical. In doing so, she actually leans in the opposite direction of the perceived turn (Fig. 8-9). The leans can easily occur if the pilot’s attention is removed from the instruments for even a short period of time. For example, if during this distraction the aircraft begins to turn slowly to the right, undetected by the pilot, the canals will respond accordingly. After a period of time in the turn, the brain “forgets” that it is in a turn. When the pilot’s attention is again directed back to the aircraft and instruments and she returns to level flight, the pilot will have a false sense that the aircraft is banking to the left. If she reacts to her senses rather than to the instruments, she will roll the aircraft to the right.
Photo: Coriolis illusion, Reinhart, Figure 8-8
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[Reinhart, pp. 138 - 139]
Photo: Oculogravic illusion, Reinhart, Figure 8-10
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[Reinhart, p. 140] When an aircraft accelerates or decelerates in level flight, the otolith organs sense a nose-high attitude relative to gravity (Fig. 8-10). If that sensation is acted upon by the pilot without cross-checking instruments, he might pitch the aircraft down. Deceleration causes a similar sensation of a nose-low attitude.
[Instrument Flying Handbook, pgs. 1-3 and 1-4] As in other illusions, a pilot in a prolonged coordinated, constant-rate turn, will have the illusion of not turning. During the recovery to level flight, the pilot will experience the sensation of turning in the opposite direction. The disoriented pilot may return the aircraft to its original turn. Because an aircraft tends to lose altitude in turns unless the pilot compensates for the loss in lift, the pilot may notice a loss of altitude. The absence of any sensation of turning creates the illusion of being in a level descent. The pilot may pull back on the controls in an attempt to climb or stop the descent. This action tightens the spiral and increases the loss of altitude; hence, this illusion is referred to as a graveyard spiral. At some point, this could lead to a loss of control by the pilot.
[Instrument Flying Handbook, pgs. 1-3 and 1-4] A rapid acceleration, such as experienced during takeoff, stimulates the otolith organs in the same way as tilting the head backwards. This action creates the somatogravic illusion of being in a nose-up attitude, especially in situations without good visual references. The disoriented pilot may push the aircraft into a nose-low or dive attitude. A rapid deceleration by quick reduction of the throttle(s) can have the opposite effect, with the disoriented pilot pulling the aircraft into a nose-up or stall attitude.
I believe pilots who spend a lot of time in the "smooth mode" (trying very hard to keep the airplane experience un-airplane like) are especially susceptible to this illusion. Especially if very tired and distracted, the powerful acceleration given by many modern aircraft can cause senses to tumble. There has been a lot of historical research verifying that this is true, but not enough thought given to survival strategies. That is starting to change. See: Somatogravic Illusion Mitigation Strategies, below.
[Instrument Flying Handbook, pgs. 1-3 and 1-4] An abrupt change from climb to straight-and-level flight can stimulate the otolith organs enough to create the illusion of tumbling backwards, or inversion illusion. The disoriented pilot may push the aircraft abruptly into a nose-low attitude, possibly intensifying this illusion.
[Instrument Flying Handbook, pgs. 1-3 and 1-4] An abrupt upward vertical acceleration, as can occur in an updraft, can stimulate the otolith organs to create the illusion of being in a climb. This is called elevator illusion. The disoriented pilot may push the aircraft into a nose-low attitude. An abrupt downward vertical acceleration, usually in a downdraft, has the opposite effect, with the disoriented pilot pulling the aircraft into a nose-up attitude.
[Instrument Flying Handbook, pgs. 1-3 and 1-4] A sloping cloud formation, an obscured horizon, an aurora borealis, a dark scene spread with ground lights and stars, and certain geometric patterns of ground lights can provide inaccurate visual information, or false horizon, for aligning the aircraft correctly with the actual horizon. The disoriented pilot may place the aircraft in a dangerous attitude.
Photo: Autokinesis, Reinhart, Figure 8-12
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[Reinhart, p. 146] Autokinesis is the perception of false movement when a static source of light is looked at by the pilot for a period of time (minutes) in the dark (Fig. 8-12). This moving reference point (an illusion) could lead the pilot to visually follow it. It is felt that the cause is the brain’s and eyes’ attempts to find some other point of reference in an otherwise featureless visual field. Prevention is a combination of realizing the eyes must focus on other objects at varying distances, not fixating on one target, and basic scanning.
Somatogravic illusions -- the sense that you are pitched up in a climb because of acceleration forces, or pitched down in a descent because of deceleration forces -- have been cited on at least seven large tranport aircraft accidents since January 1, 2000, and several before then. The problem is we sometimes lose aircraft for unknown reasons and the lack of tangible evidence places the crash in the "cause unknown" category. For our purposes, we need to assume the somatogravic illusion in many of these accidents to help us come up with survival / mitigation strategies.
Photo: Somatogravic illusion, www.skybrary.aero.
Click photo for a larger image
Ludlow, pp. 4 - 5]
Ludlow, pp. 13 - 14]
Ludlow, pp. 14 - 17]
Ludlow, pp. 18]
22 July 1973 — Pan American World Airways 816 (Boeing 707-321B) — Taking off on a moonless night from the island of Paprette, the horizon was not visible. The captain appeared to enter a descending left turn righ after takeoff, right into the ocean. Of 79 people on board, only one survived.
23 August 2000 — Gulf Air GF072 (Airbus A320-212) — The captain appeared to suffer from spatial disorientation after going around from an approach that had to be balked because he was too fast and too high. During the go around, at about 1,000 feet, the captain pitched down 15° and impacted the sea doing 280 knots. The first officer did not call out these deviations. All 143 persons on board were killed.
3 January 2004 — Flash Airlines 604 (Boeing 737-3Q8) — The captain experienced vertigo right after takeoff and disegngaged the autopilot because the instrument indications did not agree with his senses, he expressed a degree of confusion when the first officer announced he was in a right turn. The first officer did not make any assertive moves to take control of the aircraft or forcefully correct the captain's actions. They crashed into the sea in a descending right turn, killing all 148 persons on board.
3 May 2006 — Armavia 967 (Airbus A320-211) — The captain "made nose down control inputs due to the loss of pitch and roll awareness" while aborting an instrument approach. The copilot failed to monitor the approach properly and neither reacted correctly to EGPWS warnings. All 113 people on board were killed.
16 October 2013 — Lao 301 (ATR 72-212A) — The crew flew an instrument approach with the altitude select feature of their autopilot set to the MDA set to the nearest 100 feet, but low. The crew disconnected the autopilot and initiated the missed approach but the go around vertical mode immediately switched to altitude hold. The pilot began a right turn (which was not part of the missed approach procedure) and gegan losing altitude. The captain pitched up to 33°, and then down again until impact with the river. All 49 persons on board were killed. Somatogravi illusions were listed as a possible cause.
17 November 2013 — Tatarstan U9363 (Boeing 737-53A — The captain didn't understand that the Take Off / Go Around function of his aircraft would disengage the autopilot), the crew advanced the power, retracted the flaps and gear, but made no pitch inputs of their own. The aircraft pitched up to 25° at which time the captain pitched down aggressively to about 75° nose low, reaching about 5,000 fpm before impacting the terrain, killing all 50 persons on board. The captain's pitch down was thought to be as a result of spatial orientation.
Army Field Manual 3-04.301, Aeromedical Training for Flight Personnel, 29 September 2000z
Air Force Manual 51-37, Instrument Flying, 1 December 1976
FAA-H-8083-15, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2001.
Ludlow, Simon. Reducing the Threat of the Somatogravic Illusion, Flight Safety Foundation, 2016.
Reinhart, Richard, M.D., "Basic Flight Physiology," Third Edition, McGraw Hill, 2008.
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