Disturbances to sensory systems or the inaccurate perception of a given situation may produce several dangerous illusions in flight
Driven by a variety of sources and interpretations, pilots can experience illusions in flight, which break down into four main categories: vestibular, visual, landing, and atmospheric
Together, prevention and coping skills help mitigate dangerous outcomes from occurring
In-Flight Illusion Sources:
Generally, pilots determine the flight attitude of an airplane by reference to the natural horizon
When the sensory system doesn't agree with where you believe to be in space, spatial disorientation has occurred
When the natural horizon is obscured, attitude can sometimes be maintained by reference to the surface below
If neither horizon nor surface references exist, the airplane's attitude is determined by artificial means - an attitude indicator or other flight instruments
However, during periods of low visibility, the supporting senses sometimes conflict with what is seen
Vestibular System Illusions:
Vestibular system illusions are related to the inner ear
If entering a turn too slowly to stimulate the motion sensing system in the inner ear, an abrupt correction of a banked attitude can create the illusion of banking in the opposite direction [Figure 1]
Occurs when the pilot allows a breakdown in the instrument scan
The disoriented pilot will roll the aircraft back into its original attitude, or if level flight is maintained, will feel compelled to lean in the perceived vertical plane until this illusion subsides
Instrument Flying Handbook, Angular Acceleration
Coriolis illusion:
The Coriolis illusion occurs when a pilot has been in a turn long enough for the fluid in the ear canal to move at the same speed as the canal, which is then followed by an abrupt head movement
A movement of the head in a different plane, such as looking at something in a different part of the flight deck or grabbing a chart, may set the fluid moving and create the illusion of turning or accelerating on an entirely different axis
The disoriented pilot may maneuver the aircraft into a dangerous attitude in an attempt to correct the aircraft's perceived attitude
Pilots must develop an instrument cross-check or scan that involves minimal head movement
Always avoid abrupt maneuvers with your head, especially at night or in instrument conditions, while making prolonged constant-rate turns
Graveyard Spin:
Proper recovery from spin stops stimulating motion system
An abrupt stop can stimulate a spin in the opposite direction [Figure 2]
Pilot corrections for this illusion could return the aircraft into the original spin
Graveyard Spiral:
As in other illusions, a pilot in a prolonged coordinated, constant rate turn will have the illusion of not turning [Figure 2]
An observed loss of altitude during a coordinated constant-rate turn that has ceased stimulating the motion sensing system can create the illusion of being in a descent with the wings level
During the recovery to level flight, the pilot will experience the sensation of turning in the opposite direction (leans)
The pilot may return the aircraft to its original turn and, in doing so, continue to lose altitude
Instruments will likely indicate a descent at this point, causing the pilot to try to correct for the illusion of a level descent
Pilot pulls back on yoke, tightening the spiral and increasing loss in altitude
Instrument Flying Handbook, Graveyard Spiral
Somatogravic Illusion:
A rapid acceleration, such as experienced during takeoff, stimulates the otolith organs in the same way as tilting the head backward
This action creates the 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
Inversion Illusion:
An abrupt change from climb to straight and level will make the pilot feel like he is tumbling backward
The disoriented pilot will push the nose forward (low) and possibly intensify the illusion
It is as though you're in a recliner, and you feel like your looking up and graity is pushing you into the chair, but in reality you're descending, and the aircraft accelerating downward is pushing you into the seat
Elevator Illusion:
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
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
Visual/Night Illusions:
Of the senses, vision is the most important for safe flight
However, various terrain features and atmospheric conditions can create optical illusions
These illusions are primarily associated with landing
Since pilots must transition from reliance on instruments to visual cues outside the flight deck for landing at the end of an instrument approach, it is imperative they be aware of the potential problems associated with these illusions and take appropriate corrective action
False Horizon:
Dark nights tend to eliminate reference to a visual horizon
Sloping cloud formations, an obscured horizon, a dark scene spread with ground lights and stars, and specific geometric patterns of ground light can make the illusion of not being aligned with the horizon
Geometric patterns of ground light can create illusions of not being aligned correctly with the actual horizon
The disoriented pilot will align with an incorrect horizon and hence a dangerous attitude
As a result, pilots need to rely less on outside references at night and more on flight and navigation instruments
Autokinesis:
Caused by staring at a single point of light against a dark background for more than a few seconds
After a few moments, the light appears to move on its own
The disoriented pilot will lose control of the aircraft in attempting to align it with the light
To prevent this illusion, focus the eyes on objects at varying distances and avoid fixating on one target
Be sure to maintain a typical scan pattern
Vertigo:
A feeling of dizziness and disorientation caused by doubt in visual interpretation
Distractions and problems can result from a flickering light in the cockpit, anti-collision light, strobe lights, or other aircraft lights and can cause flicker vertigo
Often experienced from a lack of a well-defined horizon
Also experienced leaving a well-lit area (a runway) into darkness
Possible physical reactions include nausea, dizziness, grogginess, unconsciousness, headaches, or confusion
Black-hole Approach:
When landing at night from over water or non-lighted terrain, the runway lights are the only source of light
Without peripheral visual cues to help, pilots will have trouble orientating themselves relative to Earth (horizon)
The runway can seem out of position (down-sloping or up-sloping) and, in the worse case, results in landing short of the runway
If navigation aids (NAVAIDs) are unavailable, pilots should pay careful attention to using the flight instruments to assist in maintaining orientation and a normal approach
Night landings are further complicated by the difficulty of judging distance and the possibility of confusing approach and runway lights:
Bright runway and approach lighting systems, mainly where few lights illuminate the surrounding terrain, may create the illusion of less distance to the runway leading to a higher-than-normal approach
When flying over terrain with only a few lights, it will make the runway recede or appear farther away, leading to a lower-than-normal approach
If the runway has a city in the distance on higher terrain, the tendency will be to fly a lower-than-normal approach
A good review of the airfield layout and boundaries before initiating any approach will help the pilot maintain a safe approach angle
For example, when a double row of approach lights joins the boundary lights of the runway, there can be confusion where the approach lights terminate, and runway lights begin
Under certain conditions, approach lights can make the aircraft seem higher in a turn to final than when its wings are level
The pilot should execute a go-around if at any time he or she is unsure of their position or altitude
Landing Illusions:
Various surface features and atmospheric conditions encountered in landing can create illusions of incorrect height above and distance from the runway threshold
Anticipating landing error illusions can prevent them during approaches, aerial visual inspection of unfamiliar airports before landing, using electronic glide slope or VASI systems when available, and maintaining optimum proficiency in landing procedures
Runway Width:
[Figure 3]
Instrument Flying Handbook, Runway Width Illusion
A narrower-than-usual runway can create an illusion the aircraft is at a higher altitude than it actually is, especially when runway length-to-width relationships are comparable
The pilot who does not recognize this illusion will fly a lower approach, with the risk of striking objects along the approach path or landing short
A wider-than-usual runway can have the opposite effect, with the risk of leveling out high and landing hard or overshooting the runway
Instrument Flying Handbook, Runway Width Illusion
Runway Slope:
[Figure 4]
Instrument Flying Handbook, Runway Slope Illusion
An up-sloping runway, up-sloping terrain, or both, can create an illusion the aircraft is at a higher altitude than it actually is
The pilot who does not recognize this illusion will fly a lower approach
Down-sloping runways and down-sloping approach terrain can cause pilots to fly higher approaches
Flying a higher approach can impact landing performance, eating into runway available and devolving into higher approach speeds as pilots attempt to lose excess altitude.
Instrument Flying Handbook, Runway Slope Illusion
Featureless Terrain:
Lack of horizon or surface reference is common on overwater flights, at night, or in low visibility conditions
An absence of surrounding ground features, such as overwater approach, darkened area, or terrain made featureless by snow, can create an illusion the aircraft is at a higher altitude than it is
This illusion is sometimes referred to as the "black hole approach" (explained above), causing pilots to fly a lower approach than is desired
Ground Lighting:
Light along a straight path such as a road can be mistaken for a runway
Bright runway and approach lighting systems, especially where few lights illuminate the surrounding terrain, may create the illusion of less distance to the runway, causing pilots to fly a higher approach
Conversely, the pilot overflying terrain which has few lights to provide height cues may make a lower than normal approach
Atmospheric Illusions:
Illusions concerning weather and the appearance it creates regarding terrain
Surface references or the natural horizon may at times become obscured by smoke, fog, smog, haze, dust, ice particles, or other phenomena, although visibility may be above Visual Flight Rule (VFR) minimums
This is especially true at airports located adjacent to large bodies of water or sparsely populated areas, where few if any, surface references are available
Other contributors to disorientation are reflections from outside lights, sunlight shining through clouds, and light beams from the airplane's anti-collision rotating beacon
Water Refraction:
Rain on the windscreen can create an illusion of being at a higher altitude due to the horizon appearing lower than it is
This can result in flying a lower approach than is desired
Haze:
Haze can create an illusion of being farther from the runway
As a result, the pilot will tend to be low on the approach
Extremely clear air (clear, bright conditions of a high altitude airport) can give the pilot the illusion of being closer to the runway
As a result, the pilot will tend to fly high on approach, which may result in an overshoot or go around
The diffusion of light due to water particles on the windshield can adversely affect depth perception
The lights and terrain features normally used to gauge height during landing become less effective for the pilot
Fog:
Penetration of fog can create an illusion of pitching up
May cause for a steepened approach quite abruptly
Illusion Prevention:
Various complex motions and forces and certain visual scenes encountered in flight can create illusions of motion and position
Spatial disorientation from these illusions can be prevented only by visual reference to reliable, fixed points on the ground or flight instruments
Anticipate the possibility of visual illusions during approaches to unfamiliar airports, particularly at night or in adverse weather conditions
Consult airport diagrams and the Chart Supplement U.S. for information on runway slope, terrain, and lighting
Make frequent reference to the altimeter, especially during all approaches, day and night
If possible, conduct aerial visual inspection of unfamiliar airports before landing
Use Visual Approach Slope Indicator (VASI) or Precision Approach Path Indicator (PAPI) systems for a visual reference, or an electronic glide slope, whenever they are available
Utilize the Visual Descent Point (VDP) found on many non-precision instrument approach procedure charts
Recognize that the chances of being involved in an approach accident increase when some emergency or other activity distracts from usual procedures
The following fundamental steps to assist in preventing spatial disorientation::
Before flying with less than 3 miles visibility, obtain training and maintain proficiency in airplane control by reference to instruments
When flying at night or in reduced visibility, use the flight instruments
Maintain night currency if intending to fly at night. Include cross-country and local operations at different airports
Study and become familiar with unique geographical conditions
Check weather forecasts before departure, enroute, and at the destination. Be alert for weather deterioration
Do not attempt visual flight when there is a possibility of getting trapped in deteriorating weather
Rely on instrument indications unless the natural horizon or surface reference is clearly visible
Coping with Spatial Disorientation:
The sensations that lead to illusions during instrument flight conditions are normal perceptions experienced by pilots
In fact, its never a question of if you will experience these illusions, but when
Pilots cannot completely prevent undesirable sensations, but through training and awareness, pilots can ignore or suppress them by developing absolute reliance on the flight instruments
As pilots gain proficiency in instrument flying, they become less susceptible to these illusions and their effects
To prevent illusions and their potentially disastrous consequences, pilots must:
Understand the causes of these illusions and remain always alert for them. Take the opportunity to understand and then experience spatial disorientation illusions in a device such as a Barany chair, a Vertigon, or a Virtual Reality Spatial Disorientation Demonstrator
Always obtain and understand preflight weather briefings
Before flying in marginal visibility (less than 3 miles) or where a visible horizon is not evident such as flight over open water during the night, obtain training and maintain proficiency in airplane control by reference to instruments
Do not continue flight into adverse weather conditions or dusk or darkness unless proficient in the use of flight instruments. If intending to fly at night, maintain night-flight currency and proficiency. Include cross-country and local operations at various airfields
Ensure that when using outside visual references, they are reliable, fixed points on the Earth's surface
Avoid sudden head movement, particularly during takeoffs, turns, and approaches to landing
Be physically tuned for flight into reduced visibility by following the "IMSAFE" checklist. That is, ensure proper rest, adequate diet, and, if flying at night, allow for night adaptation. Remember that illness, medication, alcohol, fatigue, sleep loss, and mild hypoxia are likely to increase susceptibility to spatial disorientation
Most importantly, become proficient in the use of flight instruments and rely upon them. Trust the instruments and disregard your sensory perceptions
Illusions in Flight Case Studies:
NTSB Identification: WPR11FA256 The National Transportation Safety Board determines the probable cause(s) of this accident to be: the non-instrument-rated pilot's decision to conduct a visual flight rules flight over mountainous terrain into a region covered by clouds, which likely resulted in spatial disorientation and subsequent loss of airplane control
NTSB Identification: CEN13FA135 The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's inadvertent controlled descent into terrain due to spatial disorientation. Contributing to the accident was a lack of visual reference due to night conditions
Conclusion:
Illusions rank among the most common factors cited as contributing to fatal aircraft accidents
The degree of disorientation may vary considerably with individual pilots, as do the conditions which induce the problem
Various complex motions and forces and certain visual scenes encountered in flight can create illusions of motion and position
Spatial disorientation from these illusions can be prevented only by visual reference to reliable, fixed points on the ground or flight instruments
The acronym "ICEFLAGS" is a tool to remember the different types of vestibular and visual/night illusions
