Because it takes less runway.
There are many factors that go into calculating airplane takeoff performance: airport elevation, air temperature, humidity, airplane weight, flap settings, and wind. While each of these are important, perhaps the most tangible factor is the wind.
You want a tailwind during cruise flight so you can get there faster, so why wouldn’t you want a tailwind to takeoff, too?! The answer lies in the difference between airspeed and groundspeed.
Before an airplane can get airborne, it must reach a certain airspeed (not groundspeed). The speed of the air flowing over the wings produces lift. When the force of the lift is greater than the force of the weight pulling the plane back down to earth, the plane will become airborne. Since the amount of lift generated is a function of airspeed, the plane’s airspeed is the key factor in determining when it can takeoff.
Prior to takeoff, pilots calculate what their takeoff speed will be. On a typical day, that’s about 55 knots for a Cessna 172. A 737 will be more like 135 knots (the takeoff speed variation for a larger plane can vary quite a bit). In either case, if there is 10 knots of wind blowing down the runway, the planes can reduce the length of runway required by using the wind to their advantage. For the Cessna, they only need to reach 45 knots groundspeed, and the 737 125 knots.
Now imagine they choose the opposite direction. They’ve essentially required an increase in takeoff by 20 knots each – 65 for the Cessna and 145 for the Boeing!
Given these two scenarios it is easy to see that the headwind will be advantageous over the tailwind (or even no-wind) for takeoff. The exact same reasoning applies for landing.
Sample Calculation
The Pilot’s Operating Handbook (POH) for a Cessna 172 S has this notes listed beneath its takeoff performance calculation table:
Decrease distances 10% for each 9 knots head wind. For operation with tail winds up to 10 knots, increase distances by 10% for each 2 knots.
Let’s say our calculated distance is 2,000 feet to takeoff in a no-wind condition. By choosing the 10 knot headwind we are able to decrease that by 11%, or 220 feet. Going the opposite direction would require an increase of 50%, or 1,000 feet! So that’s 1,780 feet for a 10 knot headwind, 2,000 feet for no wind, and 3,000 feet for a 10 knot tailwind. Yikes!
Summary
Wind affects takeoff performance because the speed an airplane becomes airborne is determines by it’s airspeed, not it’s groundspeed. The headwind decreases the distance required to accelerate to that speed, thus requiring less runway.
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