Effect of Headwinds During Final Approach
A headwind plays a prominent role in the gliding distance over the ground. Strong headwinds decrease the glide as shown in the comparison in Figure 11-16A with no wind normal glide versus Figure 11-16B in headwind with steeper glide.
To account for a steeper glide in a headwind, the base leg must be positioned closer to the approach end of the runway than would be required with a light wind. Therefore, the base leg must be made closer to the runway to land in the intended area in a headwind. [Figure 11-16 C] However, if more headwind is experienced during final approach, increased power is required to make the intended landing area. [Figure 11-16 D]
Naturally, the pilot does not have control over the wind but may correct for its effect on the aircraft’s descent by adjusting the base leg of the pattern. The wind can vary significantly at different attitudes and locations in the pattern. If the pilot does not notice the headwind until the base leg, the base leg should be cut short and the pilot should head towards the runway sooner. This would provide the best possibility of making the runway if there is an engine failure in this situation. [Figure 11-17]
Additionally, during strong headwinds, more energy (power and airspeed) should be used since the wind gradient (slowing of the wind near the ground because of the friction of the ground) could reduce the airspeed and cause a stall on approach near the ground in higher winds.
Stabilized Approach Concept
A stabilized approach is one in which the pilot establishes and maintains a constant angle glidepath toward 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.
An aircraft descending on final approach at a constant rate and airspeed is traveling in a straight line toward a point on the ground ahead. This point is not the point on which the aircraft touches down because some float inevitably occurs during the roundout.
The point toward which the aircraft is progressing is termed the “aiming point.” [Figure 11-18]
It is the point on the ground at which, if the aircraft maintains a constant glidepath and was not rounded out for landing, it would strike the ground. To a pilot moving straight ahead toward an object, it appears to be stationary. 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 student pilot must acquire is the use of visual cues to accurately determine the true aiming point from any distance out on final approach. From this, the pilot is able not only to determine if the glidepath results in an undershoot or overshoot, but also to predict the touchdown point to within a few feet taking into account float during roundout.
For a constant angle glidepath, the distance between the horizon and the aiming point remain constant. If a final 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
- Overshooting or undershooting the turn onto fi nal approach
- Unstabilized approach
- Attempting to maintain altitude or reach the runway by slowing WSC aircraft below the minimum manufacturer’s recommended approach airspeed
- Gaining any altitude during the roundout
- Rounding out too fast during landing
- Focusing too close to the aircraft, resulting in an overly high roundout
- Focusing too far from the aircraft, resulting in an overly low roundout
- Touching down prior to attaining proper landing attitude
- Failure to lower the nose after the rear wheels touch down
- Failure to lower the nose after the front wheel touches down
- Excessive braking after touchdown