Weather: Adverse Wind

Adverse wind is a category of hazardous weather that is responsible for many weather-related accidents. Adverse winds include: crosswinds, gusts, tailwind, variable wind, and a sudden wind shift. Takeoff and landing are the most critical periods of any flight and are most susceptible to the effects of adverse wind. The most at-risk group is general aviation (GA) pilots flying aircraft with lower crosswind and tailwind threshold values. Today, we’ll take a look at the subject with an excerpt from the new edition of Aviation Weather (AC 00-6B).

Crosswind. A crosswind is a wind that has a component directed perpendicularly to the heading of an aircraft. The potential of drift produced by crosswind is critical to air navigation, and can have its biggest impact during takeoff and landing. Airplanes take off and land more efficiently when oriented into the wind. The aircraft’s groundspeed is minimized, a shorter runway is required to achieve lift-off, and the pilot has more time to make adjustments necessary for a smooth landing. As the wind turns more perpendicular to the runway to become a crosswind, the airplane directional control is affected. If a pilot does not correctly compensate for the crosswind, the aircraft may drift off the side of the runway or sideload on landing gear might occur. In extreme cases, the landing gear may collapse (see the figure below).
Gust. A gust is a fluctuation of wind speed with variations of 10 knots or more between peaks and lulls.

Even if the airplane is oriented into the wind, gusts during takeoff and landing cause airspeed fluctuations which can cause problems for pilots. A gust increases airspeed, which increases lift, and may cause an aircraft to briefly balloon up. Once the gust ends, a sudden decrease of airspeed occurs, which decreases lift and causes the aircraft to sink. Gusty winds at the point of touchdown provide significant challenges to a safe landing.

Tailwind. A tailwind is a wind with a component of motion from behind the aircraft.

A tailwind can be hazardous during both takeoff and landing. A longer takeoff roll is necessary because a higher groundspeed is required to generate sufficient lift, and the aircraft may roll off the end of the runway before lift-off. Also, a smaller initial climb gradient occurs during takeoff, which may be insufficient to clear obstacles at the end of the runway. During a landing, a longer landing roll is required because the aircraft will touch down at a higher groundspeed. Wind should always be considered in takeoff performance planning.

Variable Wind/Sudden Wind Shift. A variable wind is a wind that changes direction frequently, while a sudden wind shift is a line or narrow zone along which there is an abrupt change of wind direction. Both, even at low wind speeds, can make takeoffs and landings difficult. A headwind can quickly become a crosswind or tailwind.

Wind Shear. Wind shear is the change in wind speed and/or direction, usually in the vertical. The characteristics of the wind shear profile are of critical importance in determining the impact for an aircraft on takeoff or landing. Please refer to the current edition of AC 00-54, Pilot Windshear Guide, for additional information.

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