Shape Constancy
To appreciate how false shape constancy cueing can create orientational illusions in flight, consider the example provided by a runway that is constructed on other than level terrain. Figure 14a shows the pilot's view of the runway during an approach to landing and demonstrates the linear perspective and foreshortening of the runway that the pilot associates with a 3° approach slope. If the runway slopes upward 1° (a rise of only 35 m in a 2-km runway), the foreshortening of the runway for a pilot on a 3° approach slope is substantially less (the height of the retinal image of the runway is greater) than it would be if the runway were level. This can give the pilot the illusion of being too high on the approach. The pilot's natural response to such an illusion is to reshape the image of the runway by seeking a shallower approach slope (Fig. 14b). This response, of course, could be hazardous. The opposite situation results when the runway slopes downward. To perceive the accustomed runway shape under this condition, the pilot flies a steeper approach slope than usual (Fig. 14c).
Figure 14. Effect of runway slope on the pilot's image of runway during final approach (left) and potential effect on the approach slope angle flown (right). a. Flat runway -- normal approach. b. An up-sloping runway creates the illusion of being high on approach -- pilot flies the approach too low. c. A down-sloping runway has the opposite effect.