The Eyes & Vision

Empty Field Myopia:

• Empty Field Myopia, sometimes called Empty Space Myopia, is a condition in which the eyes, having nothing specific within the available visual field upon which to focus, focus automatically at a range of the order of a few feet ahead.
• This condition typically occurs when flying above the clouds or in a haze layer that provides no specific features to focus on outside the aircraft.
• Under these conditions, the pilot's eyes relax and focus on a comfortable distance, which may range from 10 to 30 feet.
• The pilot is looking without seeing.
• Pilots will struggle to focus on distant objects.
• To combat this, focus the eye on infinity (something well outside the cockpit) and maintain a consistent scan.
• You must refocus the eye each time you look in and then back out.

Parallax Error:

• A change in viewing position causes the apparent displacement of an object, known as parallax error.
• This error occurs to some extent when viewing an analog instrument from any direction other than directly head-on.
• Regarding reading instruments, digital displays correct for this error.
  • Despite digital instruments, there remain backup analog instruments that may be even farther away (i.e., more parallax error) on the dashboard.
• Parallax error also affects the way a pilot views the outside world in flight, most pronounced during turns in aircraft with side-by-side seating (the pilot is seated on either side of the longitudinal axis).
  • Assuming the pilot is sitting in the left seat and entering a left turn, the nose appears to rise.
  • If, however, the pilot is in the same left seat and performs a turn to the right, the nose seems to fall.
  • This error results in either an instinctual push down on the stick with a turn to the left (altitude loss) or a pull-back on a turn to the right (altitude gain).
  • To overcome this error, the pilot must first be aware of the illusion to avoid improper inputs while maintaining an instrument scan to backup inputs.

Blind Zones:

• The day blind spot is on the optic nerve, where there are no light receptors.
• The night blind spot is due to the concentration of cones in an area surrounding the fovea on the retina.
  • Because there are no rods here, directly looking at an object will cause it to disappear; therefore, you must offset where you are looking.

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, they must be aware of the potential problems associated with these illusions and take appropriate corrective action.

• False Horizon:

  • Low-light 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 create the illusion of not being aligned with the horizon.
  • Geometric patterns of ground light can create illusions that the light is not 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 as they attempt to align it with the light.
  • To prevent this illusion, focus your eyes on objects at varying distances and avoid fixating on a single 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 due to 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 orienting themselves relative to Earth (horizon).
  • The runway can appear to be out of position (down-sloping or up-sloping) and, in the worst case, result in landing short of the runway.
  • Utilize visual glide-slope indicators, if available.
  • 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.
  • Difficulty judging distance and distinguishing approach from runway lights further complicates night landings:
    • Bright runway and approach lighting systems, particularly in areas with limited lighting, may create the illusion of a shorter distance to the runway, leading to a higher-than-normal approach angle.
    • When flying over terrain with only a few lights, the runway will appear to recede or appear farther away, leading to a lower-than-normal approach.
    • If the runway is near a city on higher terrain in the distance, 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 meets the boundary lights of the runway, there can be confusion about where the approach lights end and the 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 they are unsure of their position or altitude at any time.
  • The black-hole illusion is not just a problem for approaches, but also for departures.
  • See also: AOPA Air Safety Institute - Safety Quiz: IFR Into a Black Hole.