Normal Approach and Landing (Part Six) – Stabilized Approach Concept

Stabilized Approach Concept

A stabilized approach is one in which the pilot establishes and maintains a constant angle glide path towards a predetermined point on the landing runway. It is based on the pilot’s judgment of certain visual clues and depends on the maintenance of a constant final descent airspeed and configuration.

 

An airplane descending on final approach at a constant rate and airspeed is traveling in a straight line toward a spot on the ground ahead. This spot is not the spot on which the airplane touches down because some float occurs during the round out (flare). [Figure 8-9] Neither is it the spot toward which the airplane’s nose is pointed because the airplane is flying at a fairly high AOA, and the component of lift exerted parallel to the Earth’s surface by the wings tends to carry the airplane forward horizontally.

Figure 8-9. Stabilized approach.

Figure 8-9. Stabilized approach. [click image to enlarge]

The point toward which the airplane is progressing is termed the “aiming point.” [Figure 8-9] It is the point on the ground at which, if the airplane maintains a constant glide path and was not flared for landing, it would strike the ground. To a pilot moving straight ahead toward an object, it appears to be stationary. It does not appear to move under the nose of the aircraft and does not appear to move forward away from the aircraft. This is how the aiming point can be distinguished—it does not move. However, objects in front of and beyond the aiming point do appear to move as the distance is closed, and they appear to move in opposite directions. During instruction in landings, one of the most important skills a pilot must acquire is how to use visual cues to accurately determine the true aiming point from any distance out on final approach. From this, the pilot is not only able to determine if the glide path results in either an under or overshoot but, taking into account float during round out, the pilot is able to predict the touchdown point to within a few feet.

For a constant angle glide path, the distance between the horizon and the aiming point remains constant. If a final approach descent is established and 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, meaning that the aiming point is moving up toward the horizon, the true aiming point is closer than perceived.

When the airplane is established on final approach, the shape of the runway image also presents clues as to what must be done to maintain a stabilized approach to a safe landing.

Obviously, runway is normally shaped in the form of an elongated rectangle. When viewed from the air during the approach, the phenomenon known as perspective causes the runway to assume the shape of a trapezoid with the far end looking narrower than the approach end and the edge lines converging ahead.

 

As an airplane continues down the glide path at a constant angle (stabilized), the image the pilot sees is still trapezoidal but of proportionately larger dimensions. In other words, during a stabilized approach, the runway shape does not change. [Figure 8-10]

Figure 8-10. Runway shape during stabilized approach.

Figure 8-10. Runway shape during stabilized approach. [click image to enlarge]

If the approach becomes shallow, the runway appears to shorten and become wider. Conversely, if the approach is steepened, the runway appears to become longer and narrower. [Figure 8-11]

Figure 8-11. Change in runway shape if approach becomes narrow or steep.

Figure 8-11. Change in runway shape if approach becomes narrow or steep. [click image to enlarge]

The objective of a stabilized approach is to select an appropriate touchdown point on the runway, and adjust the glide path so that the true aiming point and the desired touchdown point basically coincide. Immediately after rolling out on final approach, adjust the pitch attitude and power so that the airplane is descending directly toward the aiming point at the appropriate airspeed, in the landing configuration, and trimmed for “hands off” flight. With the approach set up in this manner, the pilot is free to devote full attention toward outside references. Do not stare at any one place, but rather scan from one point 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. This makes it easier to perceive a deviation from the desired glide path and determine if the airplane is proceeding directly toward the aiming point.

 

If there is any indication that the aiming point on the runway is not where desired, an adjustment must be made to the glide path. This in turn moves the aiming point. For instance, if the aiming point is short of the desired touchdown point and results in an undershoot, an increase in pitch attitude and engine power is warranted. A constant airspeed must be maintained. The pitch and power change, therefore, must be made smoothly and simultaneously. This results in a shallowing of the glide path with the aiming point moving towards the desired touchdown point. Conversely, if the aiming point is farther down the runway than the desired touchdown point resulting in an overshoot, the glide path is steepened by a simultaneous decrease in pitch attitude and power. Once again, the airspeed must be held constant. It is essential that deviations from the desired glide path be detected early so that only slight and infrequent adjustments to glide path are required.

The closer the airplane gets to the runway, the larger and more frequent the required corrections become, resulting in an unstable approach. Common errors in the performance of normal approaches and landings are:

  • Inadequate wind drift correction on the base leg.
  • Overshooting or undershooting the turn onto final approach resulting in too steep or too shallow a turn onto final approach.
  • Flat or skidding turns from base leg to final approach as a result of overshooting/inadequate wind drift correction.
  • Poor coordination during turn from base to final approach.
  • Failure to complete the landing checklist in a timely manner.
  • Unstable approach.
  • Failure to adequately compensate for flap extension.
  • Poor trim technique on final approach.
  • Attempting to maintain altitude or reach the runway using elevator alone.
  • Focusing too close to the airplane resulting in a too high round out.
  • Focusing too far from the airplane resulting in a too low round out.
  • Touching down prior to attaining proper landing attitude.
  • Failure to hold sufficient back-elevator pressure after touchdown.
  • Excessive braking after touchdown.
  • Loss of aircraft control during touchdown and roll out