Visual Illusions Awareness
Page 6
Flight path monitoring
Resist the tendency to pitch down and “duck under”; this is the greatest challenge during the visual segment of the approach, this includes:
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Pitching down into the approach light in an attempt to see the runway during a precision approach; or,
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Ducking under because of the impression of being too high when affected by visual illusions.
Maintain a combination of visual flying supported by monitoring of instruments (including the glide slope deviation during the visual segment of an ILS approach).
Monitor the VASI/PAPI, whenever available; this provides additional visual cues to resist the tendency to increase or decrease the rate of descent.
On runways equipped with an ALSF-II approach lighting system, be aware of the two rows of red lights aligned with the touchdown zone lights as an additional safeguard against “ducking under”.
The following provides a summary of the techniques available to counter visual illusions (and prevent from ducking under):
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Maintain instruments scan down to touchdown;
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Cross-check instrument indications against outside visual cues to confirm glide path;
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Use an ILS approach, whenever available; and,
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Use VASI / PAPI, if available, down to runway threshold (only when using a 3-bar VASI or a PAPI)
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Use available references and indications such as the ND extended runway centerline, the ILS-DME (or VOR-DME) distance and the altitude above airfield elevation to confirm the glide path (based on a typically 300 ft-per-nm approach gradient)
Crew coordination
The defined task sharing ensures a continued monitoring of visual and instrument references, throughout the transition to visual references and thereafter (i.e., during a visual approach or during the visual segment of an instrument approach).
In known or anticipated hazard conditions, the PNF should reinforce his / her monitoring of instrument references and of flight progress, for an effective cross-check and back-up of the PF.
Altitude and excessive-parameter-deviation callouts should be the same for instrument approaches and visual approaches, and should be continued during the visual segment (i.e., including glide slope deviation during an ILS approach or vertical speed deviation during a non-precision approach).
In case of a go-around, specific excessive-parameter-deviation callouts should be considered (as indicated in SOPs).
Typical Crew Actions and Results
The following crew actions and their consequences often are cited in the analysis of approach-and-landing incidents or accident resulting from visual illusions:
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Unconscious modification of the aircraft trajectory to keep a constant perception of visual references;
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Natural tendency to descend below the glide slope or the initial glide path (i.e., “ducking under”);
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Inability to arrest the rate of descent after descending below the intended glide path (i.e., late recognition of the flattening of runway and runway environment);
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Absence of reference to instruments to support the visual segment;
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Failure to detect the deterioration of visual references; and,
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Failure to monitor the instruments and the flight path, while both crewmembers are involved in the identification of visual references.
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