If you have the following data:
W: window width,
h: the distance from the viewer to the window,
β: the angle between the flight direction and the window ( positive angle when it is getting away from and negative when its coming toward the window)
t: the measured time
L: the distance from the viewer to the plane when it first appears in the window frame,
then for the speed v, you would come up with the following expression:
It can be seen that for some large angles, v becomes infinite. This is because the plane doesn't disappear from the one sides of the window.