This Briefing Note (BN) describes the human vestibular system and the illusions it can create in a pilot. It is intended to help flight crew avoid the traps associated with vestibular illusions and to increase flight safety through better awareness of their causes.
Included in this BN is an overview of:
- How the vestibular system works
- Major influences on the vestibular system:
- perception of motion and orientation
- gaze stabilization
- postural/balance systems
- Spatial disorientation related to the vestibular system
- Motion sickness
- Best practices on how to:
- recognize when illusions are occurring
- know which sources of information to trust (i.e., instruments versus sensations)
- take appropriate and safe action
It is not possible to quantify the role of vestibular illusions in accidents and incidents. However, the types of accidents and incidents that can result from vestibular illusions are known and include:
Vestibular system: anatomy and major influences
Role of the vestibular system
Humans sense position and motion in three-dimensional space through the interaction of a variety of body proprioceptors, including muscles, tendons, joints, vision, touch, pressure, hearing, and the vestibular system. Feedback from these systems is interpreted by the brain as position and motion data. The vestibular system enables a person to determine body orientation, sense the direction and speed of movement and maintain balance. When there is limited visual input, as is common in many flight situations, the vestibular sense becomes important for gathering information. However, the vestibular system is designed to work on the ground in a 1G environment and therefore during some flight maneuvers can provide flight crews with erroneous or disorienting information.
The vestibular system is believed to play a role in the onset of motion sickness and simulator sickness. Other sensations and illusions are generated during turns and maneuvers involving linear or angular acceleration. Such illusions are so compelling they can be extremely dangerous. Reacting to them in the wrong way or by reflex can lead to disaster.
Many pilots experience unusual sensations or illusions at one point or another in their flying careers but are afraid to talk about them for fear of losing medical clearance. In most cases, these are well-known sensations caused by external factors and are not a problem. Such illusions are the product of an otherwise well-functioning vestibular system that is not naturally adapted for flight.
Function of the vestibular system
In order to maintain control of the body (balance) during everyday tasks, the brain must combine signals from:
- The vestibular receptors in the inner ear, which measure rotation and translation of the head in space
- The eyes
- Stretch receptors in the muscle tissue that inform the brain on the current position of the arms and legs relative to the body.
The brain then uses this information to:
- Know the body’s position in space (spatial orientation)
- Stabilize the gaze of the eyes during rapid movement of the head (as when walking or running)
- Maintain proper posture and balance of the body.
The vestibular system’s primary function is to detect rotational and translational movements of the head and generate a corresponding response signal. These signals contribute to perceptions of motion and orientation, the effective coordination of eye movements, posture and balance (Figure 1).
Figure 1 - Inputs and Outcomes
Illusions can be characterized as:
- Vestibular (false “feeling” illusion)
- Visual (false “seeing” illusion); discussed in the Visual BN.
In many real-life cases, accidents occurred due to a combination of vestibular illusions and poor visibility. When the body is subjected to certain forces that cause a vestibular illusion, vision is often the only thing that can contradict these false perceptions (e.g., seeing the horizon through the window). However, in darkness or other poor visibility conditions, it is much easier to be deceived by an illusion and to ignore information from instruments. The illusions may be false, but they are very compelling.
The inner ear has a hearing (auditory) component, the cochlea, and a balance (vestibular) component, the vestibular apparatus. The vestibular apparatus consists of three semicircular canals and a utricle and saccule (Figure 2). The anterior, posterier, and horizontal semicircular canals are sensitive to angular accelerations of the head. Positioned at 90 degrees to one another, the three semicircular canals detect changes referred to in aviation as pitch (nose up/down), roll (rotation about the longitudinal axis), and yaw (nose right/left). The utricle and saccule sense dynamic changes in linear motion and acceleration of the head. Such linear accelerations are experienced, for example, when an aircraft is picking up speed on the runway for takeoff.
Figure 2 - The inner ear
The semicircular canals rising out of the utricular sac are filled with viscous endolymph fluid and are characterized by high potassium content and low sodium content. The internal diameter of each canal is very small (approximately 0.3 mm, 0.01 in).
Both the semicircular canals and the otolith organs rely on a common type of receptor cell, the hair cell. The activity of the sensory cells is determined by the bending of the hair. Each hair cell contains approximately 50-70 small cilia and one large cilium arranged along one surface of the hair cell. Each cilium membrane contains several hundred mechanically sensitive channels for conducting positive sodium ions.
Perception of motion and orientation
Figure 3 shows the structure of the inner ear. The three semicircular canals have swellings called ampullae, and within each ampulla is a sense organ, called the crista. In the cristae, the hair cells are embedded in a gelatinous mass, called the cupula, which extends across the ampulla and is considered a “watertight swing door.”
Semicircular canals only encode dynamic changes in head movement. When the head begins to rotate, experiencing angular acceleration, the semicircular canal in the plane of the acceleration rotates with the head while the endolymph within the canal remains stationary. The viscosity and inertial force generated by the endolymph act against the cupula, forcing it to bow in the direction opposite to that of the rotation. This deflection bends the cilia of the hair cells and generates the efferent nerve signal. However, if the head continues to rotate at continued constant angular rotation (i.e., zero acceleration), the endolymph will "catch up" with the canal and the cupula will return to a vertical position, creating the sensation that the turn has ceased. Therefore, a prolonged constant-rate turn results in the false sensation of not turning at all. With the cessation of angular rotation, the moving fluid pushes against the cupula. The cupula is deflected in the opposite direction, which creates the sensation of a turn in the opposite direction.
Figure 3 - The Crista and the Macula of the inner ear
Each of the otolith organs contains a small sensory area known as the macula that is approximately 2mm (0.08 in) in diameter. Each macula contains several thousand vestibular hair cells. The cilia that emerge from these hair cells are covered by a gelatinous mass called the otolithic membrane that contains small masses of calcium carbonate crystals, called otoliths. As the head or body moves, the movement of the membrane against the sensory hairs registers gravity. This force causes the cilia to bend. When experiencing constant velocity, the otoliths reach a state of equilibrium, and a person no longer perceives motion. The utricle’s macula is located in the horizontal plane so as to be sensitive primarily to horizontal linear accelerations, and the saccule’s macula is positioned vertically to be maximally sensitive to vertically directed linear accelerations, including gravity.
Under normal resting conditions, the afferent nerve fibers leaving the hair cells transmit continuous nerve impulses at a rate of approximately 100 impulses per second. When the cilia are bent in one direction, the impulse rate may increase to several hundred impulses per second. When the cilia are bent in the opposite direction, the impulse rate decreases, often stopping completely. The hair cell uses this bending, or lack of it, to create an electrical signal that the nervous system can understand and use.
Eye movement coordination: the vestibular-occular reflex
The vestibular-occular reflex is involved in the stabilization of eye movements during natural movement of the head when a person walks, runs or is exposed to vibration. The vestibular system exercises control over the eye muscles to stabilize an image of an object on the retina as the head moves. If the eyes moved directly with the head, the image of an object fixed in space would be degraded. The vestibular-occular reflex has an angular velocity approximately equal to but in the opposite direction of the movement of the head, which helps to stabilize the image on the retina.
When angular movement of the head is prolonged, the vestibular nystagmus is generated. The eyes, after their initial compensatory movements, quickly flicker in the direction of the turn and then start compensatory movements.
Posture and balance
The vestibulospinal reflex allows input from the vestibular organs to be used for posture and stability in a gravity environment. The projections from the vestibular system travel to muscles for coordinated movements that help to maintain posture. Changes in linear acceleration, angular acceleration and gravity are detected by the vestibular system and the proprioceptive receptors and then compared with visual information. A major role of the saccule and utricle is to keep the body vertically oriented with respect to gravity. If the head and body start to tilt, the vestibular system will automatically compensate with the correct postural adjustments (e.g., head-righting reflex).
Postural stability is maintained through vestibular reflexes acting on the neck and limbs. The vestibular apparatus signals the angular movement and attitude of the head with respect to the gravitational vertical. These reflexes are key to successful motion synchronization.
In general, vestibular illusions occur under conditions in which a pilot is unable to see a clear horizontal reference. The risk is increased at night, in clouds or in bad weather. A number of vestibular-related spatial disorientation illusions have been well-described in the literature. Among the more common are:
- Somatogyral illusions - caused by angular accelerations or decelerations sensed by the semicircular canals. These include:
- The leans
- Graveyard spin and spiral
- The Coriolis illusion
- Somatogravic illusions - (pitch-up illusions), are caused by changes in linear accelerations and decelerations, or a change in gravity (G) forces. These illusions include:
- The inversion illusion
- The head-up illusion
- The head-down illusion
The most common vestibular illusion is the leans and the most dangerous is the Coriolis illusion.
The leans corresponds to a false sensation of roll attitude. Several situations can lead to the leans, but the most common is a recovery from a coordinated turn to level flight when flying by instruments. Spatial disorientation can occur when movement is below the sensory threshold for the semicircular canal (0.2-8.0 degrees per second), especially during slow rotational movement. A pilot will feel as if the aircraft is in a wings-level attitude while, in fact, it is banked. If recovery from the turn is made abruptly, the semicircular canal in the plane of the rotation is stimulated. Thus, the pilot may feel that the aircraft is flying one wing low when the attitude display indicates the wings are level. These perceptions may lead a pilot to align his or her body with the apparent vertical. Alternatively, a pilot may roll the aircraft into an incorrect attitude to neutralise the false sensation of bank. The leans disappear as soon as the pilot has a strong visual reference to the horizon or ground.
The graveyard spiral is a high-speed, tight, descending turn. Since any rate of roll of less than two degrees per second is not perceived, the wing can drop and the aircraft may begin a turn without the pilot realizing it. As the aircraft spirals downward and its rate of descent accelerates, the pilot senses the descent but not the turn. With the bank angle having gradually increased, any control input only tightens the turn and increases the descent rate.
The graveyard spin occurs when a pilot enters a spin and initially has a sensation of spinning in the same direction as the aircraft since the flow of the endolymph bends the hair cells accordingly. If the pilot applies opposite rudder and stops the spin, the endolymph will abruptly flow in the opposite direction. That will bend the hair cells in the opposite direction, which gives the pilot the illusion of a spin when in reality the aircraft is flying straight and level. If the pilot applies the rudder to correct this perceived spin, the pilot will unknowingly re-enter the original spin. If the pilot believes the body sensations instead of trusting the instruments, the spin will continue.
The Coriolis illusion generally occurs when a pilot is in a turn and bends the head downward or backward (e.g., to look at a chart or the overhead panel). This angular motion of the head and of the aircraft on two different planes can cause problems. The turn activates one semicircular canal and the head movement activates another. The simultaneous stimulation of two semicircular canals produces an almost unbearable sensation that the aircraft is rolling, pitching and yawing all at the same time and can be compared with the sensation of rolling down a hillside. This specific spinning sensation is called vertigo. It can quickly disorient a pilot and cause a loss of aircraft control.
The inversion illusion occurs when an abrupt change from climb to straight-and-level flight causes excessive stimulation of the gravity and linear acceleration sensory organs. This combination of accelerations produces an illusion that the aircraft is inverted or tumbling backwards. A common response to this illusion is to lower the nose of the aircraft.
The head-up illusion involves a sudden forward linear acceleration during level flight where the pilot perceives that the nose of the aircraft is pitching up.
The head-down illusion involves a sudden linear deceleration (e.g., air braking, lowering flaps, decreasing engine power) during level flight where the pilot perceives that the nose of the aircraft is pitching down.
Motion sickness in flight is termed airsickness. Air sickness is a normal response of some healthy individuals when exposed to a flight environment characterised by unfamiliar motion and orientation cues.
Causes: Motion sickness arises from conflicting or mismatched sensory input (e.g., visual, vestibular and proprioceptive pathways).
Symptoms: Vertigo, nausea, vomiting, cold sweating, skin pallor, difficulty concentrating, blurred vision.
Predisposing factors to airsickness:
- Fatigue, alcohol, drugs, medications, stress, illnesses, anxiety, fear and insecurity can increase individual susceptibility to motion sickness.
- Airsickness is uncommon among experienced pilots, but it does occur with some frequency among student pilots.
- Low mental workload during exposure to an unfamiliar motion has been implicated as a predisposing factor for airsickness. A pilot who concentrates on the mental tasks required to fly an aircraft will be less likely to become airsick.
Preventing and coping with airsickness:
- Repeated exposure to the flight environment decreases an individual’s susceptibility to airsickness.
- Pilots who are susceptible to airsickness should not take anti-motion sickness medications. Research has shown that most anti-motion sickness medications cause a temporary deterioration of navigational skills.
- A pilot who starts to feel airsick should avoid unnecessary head movements, open air vents, loosen clothing, use supplemental oxygen and keep the eyes focused on a point outside the aircraft.
Operational and human factors aspects
Vestibular illusions are most likely to contribute to accidents during a go-around. This is because when flaps and gear are retracted and full thrust is applied at landing weight, the aircraft accelerates quite quickly, in fact much faster than it would during a normal takeoff at maximum takeoff weight from the runway. If the pitch-up illusion is experienced, pilots can be led to believe that they are actually at a much greater angle than they really are and will feel as if the aircraft might stall. The instinctive human reaction to this is to push the nose down, ignoring indications from instruments. The reactions of many people when in a state of confusion tend to be quite illogical. Therefore, while pilots think they are reducing pitch to a “normal” climb angle, the aircraft may actually be level or in a nose-down attitude. By the time they realize what has happened at a low altitude, it may be too late to recover.
Similarly, the nose-low illusion due to deceleration just after touchdown can cause the pilot to pull up, resulting in excessive pitch and a tail strike.
Factors contributing to vestibular illusions
Linear acceleration. As explained previously and illustrated in Figure 1, forward acceleration shifts the gravito-inertial resultant vector (GIA vector) away from the vertical centerline of the torso resulting in a misperception of attitude. There is a time lag associated with this illusion, so visual cues are very important. Apart from changing the angle of the GIA vector, linear acceleration also increases its magnitude, which further increases the illusion of climbing because the pilot experiences the G-excess effect (FG >1 G).
Angle of bank. The angle of bank increases the resultant GIA force vector. It does so even more so when the vertical velocity is increasing, which contributes to the G-excess effect (Guedry et al., 1972) and the increased pitch sensation. There is a time lag in both the onset and offset of the effect.
Rate of climb. The climb rate, especially after a go-around, adds a vertical acceleration component that further increases the magnitude of the GIA and the effects described above.
True pitch changes. Angular acceleration due to changes in pitch can affect a vestibular illusion when the angular acceleration acts in the same direction as the illusion. A strong linear acceleration can block the effects of this angular displacement if the two forces oppose each other (McGrath, 1990).
Despite the strong physical forces acting on the body that can cause illusions, it is still possible to maintain control and disregard false sensations if the crew observes and monitors reliable sources of information such as the instruments. Monitoring the instruments, however, can be difficult due to a variety of factors:
Direct crew factors
- Tunneling of attention, where the crew fixates on things that should take second priority
- Automatic behaviors that are performed without awareness or intent
- Confusion to a point that crewmembers are uncertain how to perform a task.
Background, environment and situation factors
Many other secondary factors can affect the prevalence of vestibular illusions, or an incorrect response to illusions. For more information see the BN on Situational Awareness.
Best practices, prevention strategies and lines of defense
Detecting vestibular illusions
This section provides insight on how to recognize the typical conditions and symptoms related to vestibular illusions, courses of action to prevent them and how to mitigate the consequences of illusions already occurring. It must be remembered that both pilots can experience illusions simultaneously, thereby creating a particularly dangerous condition.
- Takeoff and go-around
- Acceleration/deceleration in flight (e.g., sudden deployment of spoilers)
- Prolonged banked turns
- Darkness, poor visibility or no external visual reference cues (e.g., no visible horizon)
- Feeling of excessive pitch (either upward or downward)
- Apparent contradiction between artificial horizon and pilot-perceived angle
- Feeling that the aircraft is straight and level when it is in a prolonged turn
- Temptation to push the nose down
- Runway lights or other ground lights that appear to move during times of darkness
Actions in response to vestibular illusions
- Understand the mechanisms causing the illusion
- Anticipate when the illusion might affect you (e.g., go-around, acceleration, turn etc.)
- Be particularly vigilant in darkness or when the true horizon is not visible
- Avoid disorientation by cross-checking primary instruments regularly
- Always monitor the PFD, especially:
- Pitch Attitude (artificial horizon)
- Bank Angle (artificial horizon)
- Rate of climb
- Variation in airspeed
- Give visual information a higher priority than vestibular information, both because the input from the eyes overrides input from the vestibular system when in conflict and because visual information from instruments is much more reliable than sensations
- Be prepared to recognise and acknowledge illusions when they occur. Vestibular illusions are a normal side effect of flying and do not constitute any form of illness.
- The vestibular system has primary responsibility for equilibrium/balance and plays a major role in the subjective sensation of motion and spatial orientation.
- Vestibular inputs to the nervous system help control eye movements and stabilize the eyes during head movements.
- Vestibular inputs to the nervous system help control muscle activity and body position to adjust posture.
- Vestibular illusions may appear in the absence of visual references. Illusions are primarily caused by:
- Sensory threshold. Slow and gradual motion below perception thresholds will not be detected by the vestibular system. This threshold of sensitivity is approximately 2 degrees per second.
- Sensory adaptation. If sustained acceleration (10 - 20 seconds) takes place in one direction, the fluid in the appropriate canal also remains continually displaced. As a result, the hair cells will eventually return to the vertical position and the brain will perceive that the acceleration has stopped.
- Simultaneous sensory stimulations. The angular motion of the head and of the aircraft in two different planes can stimulate two of the three semicircular canals simultaneously which can cause illusions.
- Motion sickness arises from conflicting or mismatched sensory input from visual, vestibular and proprioceptive pathways.
- Acceleration causes pitch-up illusion.
- Deceleration causes pitch-down illusion.
- Banking and angular acceleration increase the effects of vestibular illusions.
- The pitch-up/pitch-down illusion may sometimes be accompanied by visual illusion.
- Returning to a wings-level position after a prolonged bank can feel like a bank in the opposite direction.
- Sudden tilting of the head during a turn can cause total disorientation and loss of control.
- Always monitor the PFD for: airspeed, rate of climb, angle of climb and bank attitude.
- Do not respond to sensations by pushing nose down when instruments contradict this action.
Other associated OGHFA materials
The following BNs and Visuals complement the above information:
- Aviation medicine, J.Ernesting and P. King, Butterworths, 1988
- LIVING ALOFT: Human Requirements for Extended Spaceflight
- Aero Medicine - spatial disorientation
- The Inner Ear: The Vestibular Apparatus
- Inadequate Visual References in Flight Pose Threat of Spatial Disorientation
- Visual Scene Effects on the Somatogravic Illusion, Previc F.H., Varner D.C. and Gillingham K.K., 1992 Aviation Space and Environmental Magazine
- Visual Influence of the Magnitude of Somatogravic Illusion, Evoked on Advances Spatial Disorientation Demonstrator, Tokumaru O, Kaida K, Ashida H, Mizumoto C, Totsuno J., 1998 Aviation Space and Environmental Magazine