There have been lots of aviation HUD projects fail to gain any traction over the years. The reason is that a HUD is a very specialized military design, aimed at allowing the pilot to maintain a visual connection with something in the outside world while reading instrument data.

In civilian use, there's little practical reason to look outside the aircraft rather than look at an instrument on the panel. You can fly all the way from wheels up to wheels down without caring about what's visible through the windows. In fact, in some cases the outside world can be a nuisance / distraction. LED strobe and nav lights for example are a great improvement in reliability, but they're also brighter than the old incandescent lights at a short distance. The effect of that is when flying inside a cloud at night, the flashing LED strobe is like a constant lightning bolt going off in front of your face. The solution is to put your IFR training goggles (blindfold) on, so that you can't see outside. By extension, you wouldn't see a HUD either.

The most useful UI improvement in airplane panels in the past 20 years has been color-coded multi-function displays. [1]

Grey or Green: don't care.

Yellow: you're close.

Red: you're going to hit it, something is wrong.

You don't need to do any math, don't need to memorize anything on the approach procedure, you only need to glance at the moving map... if there are yellow things on it it's time to look up and see if you can find the runway, if not you need to think about missed approach. If you see red things on it, you have to have the runway visual, or call missed approach right now. Color codes on engine data panels are similarly useful. I don't need to know all of the numbers. I just need to know that it's making power, drinking a reasonable amount of fuel, and not on fire. If everything on the engine computer's panel is white or green that's enough, I don't care about the details. If it has a yellow I need to check and see what it is, if it has a red I need to give it my full attention right now.

Hitting some tall obstacle you forgot about immediately after takeoff or while descending to land is the most common aviation hazard most people will face. A close second is engine failure in flight that probably had warning signs the pilot failed to notice before the failure. The terrain display has saved me (maybe) once. I took off into the path of a mountain at night and had inadvertently kicked the emergency landing gear release with my foot prior to the takeoff roll. It's a hydraulic system in which the gear is normally extended when the pressure is lost. The pump pressure holds the gear up (most light aircraft designs are this way). It wasn't obvious right away due to the placement of things on the panel why we were not going very fast. There was a red light but no aural alarm on a gear fail condition in which the gear was fully extended, all of the alarm conditions were about improper gear extension that were not safe to land on. The panel has lots of different color lights and at night they're harder to discern. Guy in the right seat (an instructor friend of mine) started to grab his yoke and get ready to tell me he wanted control of the airplane when I pointed out something was wrong, but I could point to the color-coded map display showing all of the obstacles and tell him "stop it, the mountain is yellow, we're not turning 180 degrees here close to the ground. We're gonna clear it, we'll figure it out on the other side."

The post-mortem of all that was one's gut reaction in a crisis is to make the problem harder than it really is. We got into all sorts of fuel calculations, wind calculations, and where would we divert to with a 24 hour control tower rather than the destination we filed for (so that they could shine a spotlight on the gear and confirm it was down). After 5 minutes or so of hypothetical bullshit one of us had the bright idea to stop theorizing and troubleshoot the problem.

Took more like 30 seconds (rather than 5 minutes) to fix it and get the gear up once the problem was narrowed to solvable tasks.

1. https://ipadpilotnews.com/wp-content/uploads/2012/09/WingX-G...