The lift source of the aircraft is the large wing, which moves relative to the air, and the pressure difference between the upper and lower surfaces provides lift for the aircraft. If the wind is downwind, the speed of the wing relative to the air will decrease, that is, the lift will decrease. Especially during take-off and landing, although you can fly with the wind, you need a longer running distance and a longer ground acceleration distance to provide a greater take-off speed. Otherwise, it is easy to lose altitude after takeoff, which is very dangerous. You can't land with the wind, otherwise once the lift is suddenly insufficient, the plane will easily land again, causing harm to the plane and the people on it.