Insights

Wind shear, a hot subject in recent years, is a knowledge element of the Private Pilot Practical Test Standards. Unfortunately, wind gradients are not included, even though this phenomenon is required knowledge if you want to eliminate long landings.

Wind shear is a major hazard to large Transport-category airplanes because of inertia of their large mass. The wind shear illustration shows the flight path of such an airplane on short final. Normal approach speed is 140 knots, and when 225 feet above the runway, groundspeed is 80 kt because of the 60-kt headwind component.

Moderate to severe turbulence is suddenly encountered, but the airplane's inertia causes the ground speed to remain at 80 kt for a few moments. The headwind component, however, quickly decreases to 20 kt at 175 feet above the runway, and indicated airspeed immediately decreases to an alarming 100 kt. The pilot firewalls the thrust levers and decreases pitch attitude in a hopeless attempt to regain flying speed. Recovery is impossible as many past accidents have proven.

Professional flight crews are highly trained in wind-shear recognition, avoidance, and recovery, if recovery is possible. Avoidance is quite simple - go around at the first indication that severe wind shear might exist. Wind shear is not as critical for light airplanes because of their low mass as compared to Transport-category airplanes.

Wind gradients on final approach exist whenever there is a difference between the surface wind and the low-level winds aloft. The wind gradient illustration shows the flight path of a light airplane on short final. Normal approach speed is 65 kt, and when 200 feet above the runway, groundspeed is 45 kt thanks to the 20-kt headwind component. At 100 feet groundspeed is 55 kt due to the 10-knot headwind component; at 10 feet it is 65 kt because of the calm wind. The groundspeed increase causes the glide path to flatten out, and a long landing occurs.

To eliminate the long landing, you must maintain the glide path that will take you to your intended touchdown spot. That requires a slight decrease in pitch attitude in order to maintain that glide path and a slight power reduction in order to maintain the proper airspeed. Allowing airspeed to increase will cause floating and an additional extension of the landing distance. Remember, all of this occurs on short final.