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Illusions in Flight

Introduction:

  • The flight attitude of an airplane is generally determined 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 must be determined by artificial means - an attitude indicator or other flight instruments
  • Sight, supported by other senses such as the inner ear and muscle sense, is used to maintain spatial orientation
  • However, during period of low visibility, the supporting senses sometimes conflict with what is seen
  • Many different illusions can be experienced in flight which break down into four main categories:
Instrument Flying Handbook. Figure 1-6, Angular Acceleration
Figure 1: Instrument Flying Handbook, Angular Acceleration

Vestibular System Illusions:

  • The Leans:

    • An abrupt correction of a banked attitude, which has been entered too slowly to stimulate the motion sensing system in the inner ear, 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
  • 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
    • It is important that pilots 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 and can stimulate spin in 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 because it is in a turn, 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. Figure 1-7, Graveyard Spiral
Figure 2: 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 backwards
    • 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:

    • Abrupt change from climb to straight and level will make the pilot feel like he is tumbling backwards
    • The disoriented pilot will push nose low and possibly intensify the illusion
  • 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 certain geometric patterns of ground light can make 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 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 normal scan pattern
  • Vertigo:
    • 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
    • Occurs when the landing is made at night from over water or non-lighted terrain where 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 a visual glide slope indicator is available, it should be used
    • If navigation aids (NAVAIDs) are unavailable, careful attention should be given 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, especially 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
    • If at any time the pilot is unsure of his or her position or attitude, a go-around should be executed

Landing Illusions:

  • Various surface features and atmospheric conditions encountered in landing can create illusions of incorrect height above and distance from the runway threshold
  • Landing errors from these illusions can be prevented by anticipating 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]
    • 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. Figure 1-9a, Runway Width Illusion
    Figure 3: Instrument Flying Handbook, Runway Width Illusion
  • Runway Slope:

    [Figure 4]
    • 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 have the opposite effect
  • Instrument Flying Handbook. Figure 1-9b, Runway Slope Illusion
    Figure 4: Instrument Flying Handbook, Runway Slope Illusion
  • Featureless Terrain:

    • Lack of horizon or surface reference is common on over water flights, at night, or in low visibility conditions
    • An absence of surrounding ground features, as in an over-water approach, or darkened areas, 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 a 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 have a tendency to be low on the approach
    • Extremely clear air (clear bright conditions of a high attitude airport) can give the pilot the illusion of being closer to the runway
      • As a result, the pilot will have a tendency to be high 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:

  1. 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 to flight instruments
  2. Anticipate the possibility of visual illusions during approaches to unfamiliar airports, particularly at night or in adverse weather conditions
  3. Consult airport diagrams and the Airport/Facility Directory (A/FD) for information on runway slope, terrain, and lighting
  4. Make frequent reference to the altimeter, especially during all approaches, day and night
  5. If possible, conduct aerial visual inspection of unfamiliar airports before landing
  6. 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
  7. Utilize the Visual Descent Point (VDP) found on many non-precision instrument approach procedure charts
  8. Recognize that the chances of being involved in an approach accident increase when some emergency or other activity distracts from usual procedures
  9. The following are certain basic steps which should assist materially in preventing spatial disorientation:
    1. Before flying with less than 3 miles visibility, obtain training and maintain proficiency in airplane control by reference to instruments
    2. When flying at night or in reduced visibility, use the flight instruments
    3. Maintain night currency if intending to fly at night. Include cross-country and local operations at different airports
    4. Study and become familiar with unique geographical conditions in areas in which the flight is intended
    5. Check weather forecasts before departure, enroute, and at destination. Be alert for weather deterioration
    6. Do not attempt visual flight when there is a possibility of getting trapped in deteriorating weather
    7. 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
  • These undesirable sensations cannot be completely prevented, 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:
    1. Understand the causes of these illusions and remain constantly 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
    2. Always obtain and understand preflight weather briefings
    3. 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
    4. Do not continue flight into adverse weather conditions or into 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
    5. Ensure that when outside visual references are used, they are reliable, fixed points on the Earth's surface
    6. Avoid sudden head movement, particularly during takeoffs, turns, and approaches to landing
    7. 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
    8. Most importantly, become proficient in the use of flight instruments and rely upon them. Trust the instruments and disregard your sensory perceptions

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 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 to flight instruments

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