Human Factors · HF

Runway and Terrain Slope Illusions on Approach

FAA Ref: FAA-S-ACS-6C Area I Task H (Human Factors)

Scenario & Physics Summary

An upsloping runway or terrain makes the aircraft look higher than it is, so an unaware pilot flies a lower approach. A downsloping runway or terrain has the opposite effect (Pilot's Handbook of Aeronautical Knowledge).

Interactive 3D Visual Plates & Scenario Media

Runway and Terrain Slope Illusions on Approach Plate 1

Plate 1: Runway and Terrain Slope Illusions on Approach

Runway and Terrain Slope Illusions on Approach Plate 2

Plate 2: Runway and Terrain Slope Illusions on Approach

Runway and Terrain Slope Illusions on Approach Plate 3

Plate 3: Runway and Terrain Slope Illusions on Approach

Runway and Terrain Slope Illusions on Approach Plate 4

Plate 4: Runway and Terrain Slope Illusions on Approach

Aerodynamic & Operational Physics

An upsloping runway, upsloping terrain, or both can create the illusion that the aircraft is at a higher altitude than it actually is. The pilot who does not recognize it will fly a lower approach.
A downsloping runway or approach terrain can create the illusion that the aircraft is lower than it actually is, leading to a higher approach.
A narrower-than-usual runway can create an illusion that the aircraft is higher than it is; a wider-than-usual runway, lower than it is.
The handbook recommends consulting airport diagrams and data for approaches to unfamiliar airports, particularly at night or in adverse weather.

FAA Exam Gotcha Alert

Exam Trap: Learn the direction of each illusion, not just the name. Upslope looks high and leads to a low approach; downslope looks low and leads to a high approach.

Frequently Asked Questions & Exam Explanations

What does an upsloping runway do to the perceived approach?

It can create the illusion that the aircraft is at a higher altitude than it actually is, and a pilot who does not recognize the illusion will fly a lower approach.

How can a pilot reduce the risk of slope illusions?

Consult airport diagrams and data before approaches to unfamiliar airports, particularly at night or in adverse weather, and use fixed, reliable visual references and instruments to check the approach path.

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