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Runway Visual Perspective
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Correctly judging the required angle of approach to a landing runway when using only direct visual reference, with no external slope indication such as VASI or PAPI and with no ILS GS display or non-precision approach plate to refer to, can be challenging when not carried out regularly. The difficulty may be greater at night because of the restriction of available visual cues to lit objects and runway lighting systems.
A form of Spatial Disorientation called Runway Visual Perspective illusion may induce a pilot to increase or decrease their approach angle by day or by night if the runway involved has a relatively abnormal (for that pilot) width or longitudinal slope or by day if the terrain below the final approach path has a persistent slope up or down which differs from any runway surface slope.
The Origins, Effects of and Potential Responses to False Perceptions
The problem arises because pilots learn to recognise a normal final approach by developing and recalling a mental image of the expected relationship between the length and the width of an average runway which has a final approach over flat terrain.
Relative differences in the slope of the runway and the approach terrain in various combinations can induce either inappropriate pitch up above the appropriate vertical profile or inappropriate pitch down below it:
- An approach over flat terrain to a runway which slopes upwards may produce the illusion of an approach angle which is excessive and lead to a response in which the aircraft is pitched downwards and / or power is reduced so as to regain what is perceived to be the normal approach angle. A risk of CFIT is created by such a response.
- An approach over flat terrain to a runway which has a downward slope towards the far end may produce the visual illusion of an approach which is too low. Such an illusion may lead to an inappropriate pitch-up response to increase the altitude. The outcome could be a low-altitude stall and possible Loss of Control, a missed approach, or a late attempt to recover the right vertical profile which leads to aircraft damage due to a mis-managed touchdown or a Runway Excursion after a late touchdown.
- An approach over terrain which slopes gently upwards towards a non sloping runway may produce the visual illusion of an approach which is too low and the possibility of a pitch up response to increase the altitude, which may again result in a low-altitude stall and possible Loss of Control, a missed approach, or a late attempt to recover the right vertical profile which leads to aircraft damage due to a mis-managed touchdown or a Runway Excursion after a late touchdown.
- An approach over terrain which slopes gently downwards towards a non sloping runway may produce the visual illusion of an approach which is too high and the response of a pitch down to decrease altitude which if CFIT is avoided may lead to a de-stabilised recovery and landing difficulties
Even where approach terrain is generally level and of similar elevation to the landing runway, visual illusions with similar effects to those discussed above can occur if the width or the length of the runway is unusual relative to the regular experiences of the pilot(s) involved. An approach to an abnormally narrow or unusually long runway may produce the visual illusion of an approach which is too high whereas an approach to an unusually wide or short runway may produce the visual illusion of an approach which is too low. The potential responses and their secondary consequences are as discussed for the relative slope approach terrain / runway slope differences above.
A Special Night-Only Case: The Black Hole Effect
One additional visual illusion which can be experienced on an approach at night (only) is known as the Black Hole Effect. This is most likely to be perceived during a visual reference only approach on a dark night with good in-flight visibility but with no stars or moon visible when over water or unlit terrain with a lit landing runway beyond which no horizon is visible. This absence of peripheral visual cues which normally help orient a pilot relative to the ground below has been known to cause the illusion of being upright with the runway then appearing to slope upwards as well as possibly being tilted. Another case, where there is a marked contrast between a completely unlit area prior to the runway and a significant area of urban lighting and / or visible rising terrain in line with and beyond it, has been known to produce the illusion of being too high on an approach. In both cases, inappropriate aircraft control responses have followed these circumstances.
However good the in flight visibility, it is important to periodically refer to relevant aircraft flight and engine instruments to confirm that an appropriate vertical profile is being flown if any of the precursors for the visual illusions noted above are present. For two crew aircraft, this would be an appropriate task for the PM
- Flying a Visual Approach
- Night Visual Approaches
- Visual Illusions
- Runway Excursion
- Stabilised Approach
- OGHFA Spatial Disorientation Situational Example
Flight Safety Foundation
- ALAR Briefing Note 5.3 - Visual Illusions
- ALAR Briefing Note 7.1 - Stabilized Approaches
- ALAR Briefing Note 7.3 - Visual References