Cognition in Aviation — Page 193, Lesson 312

I want to walk you through ground proximity judgements during the approach phase. In the final stage of the approach, you as the pilot will need to assess your height above the ground so you can initiate the flare or reduce power. This is a critical judgment because getting it wrong can mean landing short of the runway or flaring too high. Let me define the cues you use. First, the apparent speed of objects on the ground increases as your height reduces — the lower you are, the faster things seem to rush past. Second, the size of objects, such as runway lights, increases with decreasing distance. That "distance" includes both your height above the ground and your range from the objects. Third, the apparent width of the runway increases as you get closer. Fourth, the texture of the ground changes. At height, grass appears only as a green surface — you cannot see individual blades. Only at low level does it actually look like grass. There are recorded cases where pilots have mistaken a green jungle canopy for a flat green field and attempted to land with an engine out. That is a real hazard. Now, let's talk about wheels and the touchdown point. Because most large aircraft have their main undercarriage a long way below and behind the pilot's eye level, it is self-evident that the wheels will not touch down at the visual aiming point but some way short. Figure 10.9 illustrates this. The shallower the approach path, the greater the distance between the aiming point and the actual touchdown point. If you have been misled by sloping terrain or a sloping runway to fly a lower-than-normal approach, and accurate height cues are missing, there is a danger of the aircraft touching down before the prepared surface — that is, landing short of the runway. This is one of the reasons the recommended approach for large aircraft is with an automatic ILS coupled to a radio altimeter input of wheel-height above touchdown whenever possible. The radio altimeter gives you precise height of the wheels above the ground, removing the guesswork. Finally, let's cover the missed approach somatogravic illusion. A missed approach in poor visual conditions involves linear acceleration, and this causes an illusion of pitch up or climb. This is due to inputs from the otoliths in your inner ear, which we discussed in Chapter 4. The key point here is that during a go-around in low visibility, your body senses forward acceleration and interprets it as a nose-up pitch, which can lead you to push the nose down inadvertently. You need to be aware of this illusion and rely on your instruments.