Description

Visual illusions occur when the pilot's eye is deceived into making a faulty assessment of aircraft position or orientation in relation to the external environment.

An analysis of 76 approach and landing accidents and serious incidents conducted for the Flight Safety Foundation (FSF), including controlled-flight-into-terrain (Controlled Flight Into Terrain (CFIT)) accidents, worldwide between 1984 and 1997, found that twenty-one percent involved flight crew disorientation or visual illusions, and that poor visibility was a circumstantial factor in 59 percent of the accidents and incidents.

Types

The following are some common types of visual illusions. This list is by no means comprehensive.

• False horizon: When flying at night or in poor visibility, a pilot can mistake a cloud layer, a set of streetlights, a shoreline, or other features for the horizon.
• Autokinesis: When flying at night, if a pilot stares at a stationary light in the distance for long enough, the light can seem to move. This can lead to errors in judging aircraft speed, attitude, and position.
• Black hole effect: When approaching an airport surrounded by dark terrain with few other lights in the area, pilots can get an illusion that they are higher than they really are. This can lead to a dangerously low approach.

Effects

Flight Safety Foundation (FSF) Approach-and-landing Accident Reduction (ALAR) Briefing Note 5.3 — Visual Illusions points out that:

"Visual illusions result from the absence of visual references or the alteration of visual references, which modify the pilot’s perception of his or her position (in terms of height, distance and/or intercept angle) relative to the runway threshold." The briefing note continues.

"Visual illusions are most critical when transitioning from instrument meteorological conditions (Instrument Meteorological Conditions (IMC)) and instrument references to visual meteorological conditions (Visual Meteorological Conditions (VMC)) and visual references.

"Visual illusions affect the flight crew’s situational awareness, particularly while on base leg and during the final approach.

"Visual illusions usually induce crew inputs (corrections) that cause the aircraft to deviate from the vertical flight path or horizontal flight path.

"Visual illusions can affect the decision process of when and how rapidly to descend from the minimum descent altitude/height (Minimum Descent Altitude/Height)."

Defences

Pre-flight and pre-descent briefing on conditions in the airport environment and the nature of the runway itself.

Typical Scenarios

• The pilot of an aircraft approaching a very long runway gains the impression that the aircraft is high on the approach and descends dangerously close to the ground;
• The pilot of an aircraft approaching a runway with a pronounced up-slope gains the impression that the aircraft is high on the approach and descends dangerously close to the ground;
• The pilot sees a row of street lights and mistakenly lines up on them instead of the runway;
• The pilot makes a low approach to a runway and misjudges is the aircraft height, resulting in touchdown in the undershoot.

Contributory Factors

The Briefing Note lists the following factors which affect the flight crew’s ability to perceive accurately the environment, resulting in visual illusions:

• Airport environment:
  • Ground texture and features;
  • Off-airport light patterns, such as brightly lighted parking lots or streets;
  • “Black-hole effect” along the final approach flight path; and/or,
  • Uphill-sloping terrain or downhill-sloping terrain in the airport vicinity.
• Runway environment:
  • Runway dimensions;
  • Runway slope (uphill gradient or downhill gradient);
  • Terrain drop-off at the approach end of the runway;
  • Approach lighting and runway lighting; and/or,
  • Runway condition.
• Weather conditions:
  • Ceiling;
  • Visibility; and/or,
  • Obstructions to vision.

Solutions

• Flying an instrument approaches in preference to a visual approach;
• Use of instruments and electronic aids to navigation to supplement visual appreciation;
• Use of approach lights and Visual Approach Slope Indicator Systems to aid correct orientation.

Related Articles

• Spatial Disorientation

Further Reading

Flight Safety Foundation

• ALAR Briefing Note 5.3 — Visual Illusions