What illusion can be felt when approaching sloping terrain down to the runway threshold?

The feeling of being too low can occur when approaching sloping terrain down to the runway threshold due to the visual perception of the ground. As pilots descend towards a runway that's situated on a slope, the downward slope can create an optical illusion that makes them feel as though they are lower than they actually are. This phenomenon is linked to how the brain interprets the relationship between perceived altitude and the visual cues provided by the surrounding terrain.

When the terrain below is sloping downwards, the pilot may perceive a closer proximity to the ground than is accurate, creating a heightened sense of urgency to descend more slowly or to initiate a go-around, even when maintaining a safe altitude. This can lead to misjudging their position and ultimately affect their approach and landing decisions.

Pilots must remain vigilant and rely on their instruments to counteract this illusion, ensuring they maintain the correct altitude and approach path, regardless of the visual cues presented by sloping terrain.