Cognition in Aviation — Page 193, Lesson 310

I want to walk you through two important visual illusions that can affect your judgment during an approach at night, and then the correct way to maintain the glideslope using visual cues. Let's start with the Black Hole Effect, also called the Kraft Illusion. Imagine you're approaching an airfield at night over water, jungle, or desert. The only lights you can see are the distant runway or airfield lights, with a black hole — a complete absence of visual cues — in between. That absence of visual cue leads to an illusion that the aircraft is too high. Because you think you're high, you tend to fly the approach path at too shallow an angle. The result is that the aircraft touches down short of the runway. This phenomenon is also important to consider when designing simulator visual presentations: you need enough intermediate texture on the screen to allow correct height judgment. Now let's talk about Maintenance of the Glideslope using the Aiming Point and Aircraft Attitude Pitch Angle. Once the aircraft is established on the glidepath and is in the landing configuration, it's relatively easy to visually maintain the glidepath by keeping the aiming point at a fixed position on the windscreen. But here's a dangerous situation: Inadvertent Speed Loss. A situation can arise on the approach where, with an inadvertent speed loss and a gradual loss of altitude, the runway could remain in the same position on the windshield. That gives you the impression of a safe, stabilized approach — until you touch down some distance before the threshold. You should remember this particularly when visual flying at night around airports where the surrounding terrain is without lights. Let me explain why this happens. This technique of using the visual angle to the aiming point will only work providing the speed and configuration are maintained. If the speed decreases, the nose-up attitude of the aircraft will increase, and the selected spot on the windscreen will move towards the overshoot end of the runway. The danger is to descend to regain its position on the windscreen whilst maintaining the lower speed — and vice versa. Finally, let's look at Texture and Texture Flow. The approach is progressing normally as long as visual texture flows away from the aiming point and the visual angle between this point and the horizon remains constant. The texture change is particularly important when landing on a grass field. So to summarise: the black hole effect can make you fly too shallow and land short; an inadvertent speed loss can keep the runway in the same place on the windscreen while you descend below the glideslope; and the correct technique is to monitor texture flow away from the aiming point with a constant visual angle to the horizon.