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For a constant angle glidepath, the distance between the horizon and the aiming point remain constant. If a fi nal approach descent has been established but the distance between the perceived aiming point and the horizon appears to increase (aiming point moving down, away from the horizon), then the true aiming point and subsequent touchdown point is farther down the runway. If the distance between the perceived aiming point and the horizon decreases (aiming point moving up toward the horizon), the true aiming point is closer than perceived. 

When the aircraft is established on final approach, the shape of the runway image also presents clues regarding what must be done to maintain a stabilized approach to a safe landing. A runway is normally shaped in the form of an elongated rectangle. When viewed from the air during the approach, perspective causes the runway to assume the shape of a trapezoid with the far end appearing narrower than the approach end, and the edge lines converging in the distance. If the aircraft continues down the glidepath at a constant angle (stabilized), the image the pilot sees is still trapezoidal but of proportionately larger dimensions. 

During a stabilized approach, the runway shape does not change. [Figure 11-19] If the approach becomes shallower, the runway appears to shorten and become wider. Conversely, if the approach is steepened, the runway appears to become longer and narrower. [Figure 11-20] 

The objective of a stabilized approach is to select an appropriate touchdown point on the runway and adjust the glidepath so that the true aiming point and the desired touchdown point coincide. Immediately after rolling out of base leg and onto final approach, the pilot should adjust the speed so that the aircraft descends directly toward the aiming point. With the approach set up in this manner, the pilot is 

free to devote full attention to outside references. The pilot should not stare at any one place, but rather scan from one area to another, such as from the aiming point to the horizon, to the trees and bushes along the runway, to an area well short of the runway, and back to the aiming point. In this way, the pilot is more apt to perceive a deviation from the desired glidepath and whether or not the aircraft is proceeding directly toward the aiming point. 

If the pilot perceives any indication that the aiming point on the runway is not where desired, an adjustment must be made to the glidepath. This in turn moves the aiming point. For instance, if the pilot perceives that the aiming point is significantly short of the desired touchdown point and results in an undershoot, an increase in power is warranted. The minimum airspeed recommended by the manufacturer must be maintained. This results in a shallowing of the glidepath with the resultant aiming point moving toward the desired touchdown point. Conversely, if the pilot perceives that the aiming point is farther down the runway than the desired touchdown point and results in an overshoot, the glidepath should be steepened by an increase in speed with the throttle at idle. It is essential that deviations from the desired glidepath be detected early, so that only slight and infrequent adjustments to glidepath are required. 

If a situation arises in which the required corrections become larger (and possibly more frequent) as the aircraft draws closer to the runway, an unstabilized approach results. 

Common errors in the performance of normal approaches and landings include the following: 

• Not realizing there is a tailwind during downwind to complete an early base 

• Inadequate wind drift correction on the base leg 

