HUD in the Windscreen
 

We are seeing more and more cars with Heads Up Displays. Some are very good and only a small component mounted on the dash top. Couldn't this technology be applied to light aircraft as well.

Light ac use plastic for windscreens, not glass.

Think about the differences between showing speed on a car (and other bits) compared to the display of aircraft attitude (and specifically the horizon/heading) which will be used in a safety critical way... Massively critical that the users eye view symbology in precisely the correct location in the aircraft compared to it doesn't matter two hoots in the car...

My wife's 7 year old BMW has a HUD with a level adjustment feature to place the symbol as high, or low, as you want.

Of course you won't see it if you're wearing sunglasses with Polaroid lenses, but that's another story!

An aircraft HUD does some clever stuff in the combiner so that the focal length of the display is infinite even though the combiner is very close to your eyes. This is so you don't have to refocus when looking through the HUD.
Your car HUD doesn't do this.

The purpose of the infinite focal length is not really for eye focus, it is to position the display on the outside world, irrespective of head position. Otherwise your gun symbol, bomb fall line or vv symbol would be innaccurate and unstable if your head position moved. This only applies of course to pilots who move their heads whilst flying.

I hear you, but I can tell you that the car HUD does not require refocusing - it is absolutely clear and crisp at infinity requiring no adjustment that I can notice. (I have experience on HUD developments dating back to the 80's). I believe it is fixed at infinity, perhaps?

Of course, I don't have a head mounted display, so its only valid for the direct frontal view.!

Like others said the point of the combiner is that all information on the HUD always appears over the same part of the background irrespective of where you move your head. That being said you can't really move your head very far before you lose sight of the information completely.

Having recently moved to an aircraft with a HUD my major gripe is that there is distortion where the combiner glass ends. Now many people have told me to sit closer to the HUD but the issue is is that that would bring me way to close to the glass. Moving to this new aircraft has already completely changed my seating position due to the HUD and I think the distance required between the pilots eyes and the HUD is not designed realistically by the manufacturers. The only solution in my opinion is to increase the size of the combiner glass by around 50℅ in order to cover more space and allow the pilot to sit in a more natural position.