The shadow falls when the plane flies in the sky , but the shadow is created for us to see if the position of the plane is subject to certain conditions. As the light source and the position of the object are fixed and the screen / surface is moved backwards (increasing the distance between the object and the screen) , the size of the shadow on the screen increases , but at the same time it becomes dimmer and lighter. So much for the general theory. Therefore, when the plane starts flying on a sunny-cloudless day, when its distance from the ground is less , then its clear dark shadow falls on the ground. Then, as the height of the plane increases, the shadow becomes distorted, larger and lighter. This is not the end !! The shadow of the plane is affected in many ways. Planes are not always at the lowest perpendicular distance from the sun. So most of the time the amount of light falling on the plane diagonally from the sun is high , so that the shadow of the plane spreads diagonally to the ground and becomes lighter. In addition, we need to keep in mind that our surface is not flat , but spherical. So where the shadow is supposed to fall on the plane , in reality it does not fall there and the shadow goes far and wide and becomes more obscure. Moreover, clouds , atmospheric weather, etc. often interfere with the formation of shadows. This is why the shadows of flying planes ( when at an altitude of 20000-40000 feet) are often not suitable for us to see. However, sometimes on a sunny day, if the amount of light falling on the plane perpendicular to the plane is high (simply put in the afternoon when the sun is overhead , if the plane flies over the head) a limited amount of clear shadow of the plane can be seen on the ground. For the same reason, sometimes when Rodela stands on the roof in the middle of the day, the shadow of a kite / crow or a big bird is visible to us.