A term used to describe a false sensation of bank when the aircraft is, in level flight. This illusion can occur in both VFR and IFR flight:
The aircraft is initially flying straight and level.
The pilot allows a wing to drop at a rate that is below that required to stimulate the fluid in the semicircular canals.
The pilot still believes that the aircraft is flying straight and level.
The pilot detects the error and returns the aircraft to straight and level.
The rate of roll is fast enough to stimulate the fluid in the semicircular canals.
The fast roll induces the pilot to believe that he is in a turn in the direction of the roll.
The false sensation of bank may persist for up to an hour, but this is unusual. To overcome the sensation flying must be referenced to the instruments. This can be draining especially if the sensation lasts for a long time. In some cases the pilot may align his body with the apparent vertical, rather than the normal axis of the aircraft. In a two pilot aircraft control should be always given to the other pilot.
Lean on Cloud
Clouds are not like the visual horizon that a pilot flies to. It is possible for the cloud to have slope. Pilots who believe that the cloud is level are liable to align the aircraft with the horizon given by the cloud and sky.
The worst possibility occurred over New York in 1965, where a B707 and L1049C collided. The pilot of one aircraft levelled his aircraft to the visual horizon given by a flat sheet of cloud, the other aircraft flying straight and level appeared to be turning in towards his aircraft. Turning to take evasive action caused the aircraft to collide.
Lean on Sun
A partial visual illusion because the aircraft is in cloud. Where an aircraft is flying close to the top of cloud it is possible to make out the position of the sun. The pilot interprets this brightness as the sun being vertically above the aircraft. The sun is rarely directly above and in the diagram below it is positioned to the left of the pilot.
In this case the aircraft is banked left to bring the aircraft into the perceived vertical position.
Somatogravic illusions occur when the Otoliths are stimulated by a linear acceleration. When standing still the perception is that gravity acts vertically down. In the Somatogravic Illusion any accelerating force can cloud this perception.
When short term linear acceleration is experienced then the pilot can easily distinguish between that and gravity. If the acceleration is long term, as an aircraft accelerating, the brain is unable to distinguish between the resultant acceleration and the acceleration due to gravity. The acceleration is combined with that of gravity to give a resultant force.
The Somatogravic Illusion in Yaw and Roll
During a prolonged visual turn the pilot knows that the aircraft is turning because of the visual clues given by the instruments and the visual horizon. If the visual clues are taken away, the pilot will still sense the turn because of the stimulation of the fluid in the semi-circular canals. As soon as a steady turn is achieved the fluid in the semi-circular canals reaches an equilibrium and the cupola returns to the normal position. The pilot loses all sense of a turn and the perception is that the aircraft is flying straight and level.
- F: Inertial Force of radial acceleration.
- G: Gravity.
- R: Resultant force.
In a flat turn, the opposite occurs, the pilot believes that the aircraft is banking in a turn.
Somatogravic Illusion in Pitch
Of greater importance is the somatogravic illusion in pitch.
Where the speed is linear and there is no acceleration the pilot will sense the forces below.
During a sustained acceleration, the pilot feels that the aircraft is in a nose-up attitude because of the resultant force. The illusion takes approximately one minute to develop fully.
- F: Inertial Force due to Acceleration/Deceleration.
- G: Gravity.
- R: Resultant force.
Even a brief acceleration, such as a catapult launch (5g for 2-3 seconds), can give rise to an apparent nose-up attitude of 5°, which may take a minute or more to die away.
Conversely, during a sustained deceleration such as applying airbrakes, the aircraft may appear to pitch down.
The somatogravic illusion can occur during take-off or on a missed approach and is a particular danger at night or in poor visibility. The natural response of any pilot is to counteract the pitch up sensation by pitching the aircraft nose down. This increases the acceleration because of the unloading of stick forces and the sensation becomes worse. The more the pitch down the greater the sensation of pitch up and hence the worsening of the illusion.
Note: If an aircraft is fitted with an air driven artificial horizon, as the aircraft accelerates, the indications given will support the somatogravic illusion ie a pitch up indication.
The diagrams below taken from an accident report show how the aircraft was bunted into the ground. The pilots sensed that the aircraft had pitched up and flipped onto its back.
A sensation of angular movement can be induced in a turning aircraft. The movement of the head in a turn when looking down at the instrument side panel can induce a tumbling sensation. Neither the movement, whether forward or backward, nor the rate, is consistent between individuals. Experiments have shown that a forward head movement in pitch made during a pull up from a dive produces a sensation of tumbling forward in pitch. The illusion occurs because of:
- A cross coupled stimulation of the semi circular canals, or
- A transient stimulation of the Otoliths.
The Oculogravic Illusion
The oculogravic illusion is regarded as a visual component of the somatogravic illusion. During acceleration the pilot can experience a pitch up sensation. This can be accompanied by the apparent upward movement of objects within the visual field. On deceleration, the visual field may appear to move downwards. The mechanism is not primarily because of eye movement, but the perception of how the brain has interpreted the sensation. If the external visual field is well defined the illusion is not a problem.
At night, when only a few stars or isolated lights are visible, or where external visual cues are largely inadequate, such as flight over water or desert, the oculogravic illusion can cause spatial disorientation.
The apparent movement and transient displacement of light sources in the external visual scene may be interpreted by the aviator as a change in aircraft attitude, Alternatively, the apparent movement of an isolated light may be misinterpreted as the light of another aircraft. In normal flight the lights of the runway appear to be below the aircraft.
As the aircraft accelerates the resultant force gives the illusory movement of the lights upwards. The pilot may assume this is a nose down change in attitude and counteract with a pitch up to what he believes is a safe attitude. The perception is in the opposite direction to that of the somatogravic illusion but is produced by the same change in the direction of the resultant force vector.
If there are no external visual cues, the somatogravic illusion is dominant.
Visual illusions can occur when there is a change in the magnitude of the vertical forces. These are termed Elevator Illusions, as they were first experienced in the high speed lifts built in America in the 1920's. In an updraught the gravity vector increases and there is a sensation of moving up. This is confirmed by an apparent up movement of the visual field. The converse happens in a downdraught.
False Perception of Angular Motion - Vertigo
Vertigo is defined as an illusorysense of turning. Unfortunately, the term has now become synonymous with spatial disorientation. Somatogravic and oculogravic refer to linear motion, for angular or rotating motion the terms somatogyral and oculogyral are used:
Somatogyral Illusion or Graveyard Spin
The semi-circular canals sense angular acceleration. During a prolonged turning manoeuvre at constant angular speed such as:
- A co-ordinated turn.
- A sustained roll.
- A spin.
The correct information is sensed for the first few seconds.
The initial sensation of a right turn will be lost after approximately 15-30 seconds. This depends upon:
- The speed of rotation.
- The axis of rotation.
- Cues from other sensory organs.
- The extent to which the pilot is familiar with the motion.
For a typical spin pilots will be unable to perceive rotation by purely vestibular means after 15-30 seconds. Spin direction can be determined from the blurred view of the outside world or by checking cockpit instruments.
Visual cues are usually adequate to allow the appropriate recovery action. Recovery produces an angular acceleration in the opposite direction to that on spin entry.
The sensation of turn in the recovery is in the opposite direction to that of the spin. This illusion occurs when the pilot is deciding whether the rotational component of the spin has ceased so that the recovery can be completed. The only reliable means of detection is reference to the visual references or instruments. If the rotation has been fast vision may have been degraded. It normally takes a pilot several seconds after the spin has ceased, for full visual acuity to be restored.
The presence of false sensations and impaired vision can cause problems in the spin recovery. The pilot may sense that the spin has stopped before the full recovery is complete. If the pilot tries to pull the aircraft out of the dive then the aircraft may be overstressed.
After recovering correctly, the pilot may make believe that the spin is now in the opposite direction. By attempting a recovery the aircraft may re-enter the original spin. This may result in a graveyard spin, the aircraft repeating the cycle several times.
Where impairment of visual acuity is caused by rotation, the semi circular canals may pass illusory signals to disorientate the pilot. These take the form of apparent motion and errors in the position of visual objects.
Not a problem in well defined visual conditions, but if external cues are poor, illusions canpersist for a few minutes. When the rotational movement has stopped a light that can be seen clearly will appear to rotate with the observer.
Illusions due to Cross-Coupled (Coriolis) Canal Stimulation
Complex motion will stimulate more than one semi-circular canal simultaneously. This can cause interactive sensations causing spatial disorientation. Interactive illusions involving the otolith organs may occur such as g excess illusion. Head movements made during an aircraft manoeuvre are the main cause of this type of illusion.
Head movements made during the initial part of a turn, do not give any false sensations, because the semi-circular canals sense the movement correctly. During this time, each canal senses the angular velocity correctly. The imposed rotation and the angular motion of the head are sensed correctly.
Any movement of the head after this initial period can result in the cross-coupling of the senses and the illusion occurs. Turning the head to change a radio frequency or Squawk is a common cause of Coriolis.
Saccadic Eye Movement
When the eyes are not tracking a moving target they move in a series of jerks called a saccade. This movement takes approximately ⅓ second. As a result of saccadic eye movements, it is not possible to make voluntary, smooth eye movements while scanning featureless areas.
A problem that occurs because of the saccadic movement of the eye. If a person stares at a single point of light such as a star then after about 5 to 10 seconds the star will appear to move. This can appear to the pilot as another aircraft in the sky.
Pressure Vertigo is caused by pressure changes within the middle ear normally when clearing the ears in the climb or in the descent. The vertigo sensed is intense, with blurring of vision and apparent movement of the visual field. The duration is short and will last no longer than 10-15 seconds.