Illusions occur when conditions modify the pilot’s perception of the environment relative to his or her expectations possibly resulting in spatial disorientation or landing errors.
Visual Landing at night presents greater risk because of fewer references and because of illusions or SD.
Factors Affecting Visual Illusions.
Terrain drop off at the approach end of runway.
Approach lighting and runway lighting.
Obstruction to vision.
Visual Illusions (VI).
VI most critical when transitioning from IMC and instrument reference to VMC and visual reference.
VI affect situational awareness (SA) particularly while on base leg and during final approach.
VI usually induce crew inputs that cause the ac to deviate from vertical or horizontal flight path.
VI can affect the decision process of when and how rapidly to descend from MDA/H.
- Uphill slope of Runway or Helipad gives illusion of being too high.
- Downhill slope of Runway or Helipad gives illusion of being too low.
© A wide or short runway creates an impression of being
- A narrow or long RW creates an impression of too high.
- Approach, RW including touch down zone lighting affect depth perception depending on light intensity, day or night conditions and weather conditions.
- Bright lights create impression of being closer to the RW.
- Low intensity Lts –farther away from the RW.
- Nonstandard lts modify the pilots perception.
- If RW lighting partially visible-on base leg or circling approach, RW may appear farther away.
- Flying in haze creates the impression that RW is farther away inducing a tendency to shallow glide path and land long.
- In light rain, the RW may appear indistinct because of the rain halo effect increasing risk of misperception of vertical/horizontal deviation.
- Heavy rain affects depth perception and distance perception.
- Rain on wind shield creates refraction effect that causes crew to believe the ac too high.
- Day light, rain reduces intensity of lights resulting in impression of being further away.
- Night time rain increases the apparent brilliance of the ALS making RW appear close.
- When breaking out at both min ceiling and visibility minimum, the slant Vis may not be sufficient for crew to see farther bars of VASI/PAPI thus reducing visual clues.
- In cross wind, RW light and environment will appear at an angle to ac heading, resist the tendency to align with center line.
- A wet RW, reflects very little light, affecting depth perceptions, impression of ac farther away, resulting late flare and hard landing.
CONSEQUENCES OF VI
- Unconscious modification of ac trajectory to maintain a constant perception of visual reference.
- Natural tendency to descend below glide slope or path, inability to judge the proper flare point because of restricted visual references (hard landings before reaching touch down point).
- Inadequate reference to instruments to support visual segments.
- Failure to detect the deterioration of visual references.
- Failure to monitor the instruments and the flight path because both pilots are involved in the identification of visual references.
Guard Against Adverse Effects of Visual Illusions
- Flight Crew should be aware of:-
- Weather factors.
- Surrounding Terrain and obstacles.
- Assess the airport environment, airport and RW hazards.
- Adhere to defined PF/PNF task sharing after transition to visual flying.
- PF to monitor outside visual reference while referring to instrument references to support and monitor flt path.
- Monitoring by PNF of head down references while PF Flies.