Talk of HUDs makes me thing of a question that has puzzled me for a long time: How do you keep the HUD aligned with the scene outside? I mean, if you are looking directly through the HUD, and the lines on the HUD match the contours of the runway ahead exactly, what happens if you move your head to one side? Don't the lines on the HUD shift out of alignment then?
Seems to me that either you'd need something to clamp your head in place relative to the combiner of the HUD, or the HUD would need some sort of optical system that somehow keeps the lines on the HUD in the right place no matter what the position or angle of your view through the combiner. So how is it done?
I'm not actually that keen on the idea of HUDs and I'm glad that they are not widely used.