Towards a working Heads Up Display

The elusive goal. Readable characters with a full daylight scene visible behind.

A relatively small number of Boeing 737 aircraft are equipped with a Heads Up Display (HUD), but last year at Oshkosh I had the opportunity to sit in the captain’s seat in one of them: an American Airlines 737-800 carrying Honor Flight veterans from Wisconsin to Washington, DC.

Real HUD mounted in an American Airlines B737-800
When American opened up their 737-800 for cabin and cockpit tours, airshow attendees mobbed the aircraft, leading to this suboptimal image of the projector, combiner/splitter glass and bracket

The HUD definitely looked like it belonged in a modern NG cockpit, so when an old HUD from a 727-100 came up for auction on eBay, I figured this was the closest I was going to get to the real thing for a while, so I snapped it up.

What arrived several weeks later was a crate with about 25 pounds of gear, including the CRT projector, optics, the combiner/splitter glass (without the bracket), as well as the selector module.

A number of other builders including David Allen have acquired this same model HUD, and are in various stages of making it functional. David in particular has been working on the original manufacturer, trying to obtain a wiring diagram. After reading the patent, it occurred to me that the biggest technical challenge involved with a functional HUD is the optics, which must have a very precise alignment. The combiner/splitter in particular has very specific optical characteristics that allow it to display the projected information on the glass while the actual reality out the window appears unaltered behind it, all in full sunlight.

Given that I never plan to operate my sim outdoors (it will only ever fly in my basement), I thought I might be able to substitute a cheap LCD screen for the power-hungry CRT and its complicated, proprietary electronics. Given that Project Magenta now has a working, highly configurable HUD module of its own in software, all that I would have to do is find an appropriately sized LCD screen with a VGA connector, and I would be well on my way.

So I started by disassembling the CRT/projector unit in such a way that it could be easily reassembled later if the actual wiring diagram becomes available. This turned out to be no problem as the CRT is attached to the optics with several captive screws, and the logic boards come out by detaching several pin connectors. There were three shims in place between the CRT and the optics, so I used one of the shims as a template to make a mounting bracket for my inexpensive 4.3 inch LCD screen.

Real 727-100 HUD, disassembled
Metal shims found between the CRT and projector
Comparison of old and new screens. I think I'm about to gain some useful load!

What appears below is a proof of concept photo showing the HUD symbology through the optics. I have yet to work out the exact angles, but this shows what is possible.

Image generated with the original optics, new LCD screen and Project Magenta


6 thoughts on “Towards a working Heads Up Display”

  1. I jsut purchased my 737 HGS and want to start the replcement proces using an LCD also. I’ll send you pics after it arrives so you can see what I have and perhaps give me pointers on where you are with your project.

  2. Jim, thanks for your post. I’ve put my HUD aside for a while as I am trying to get my sim reassembled in my basement, but I don’t think I’m that far off from a solution to this problem. All I should need is a small LCD projector that can focus at the same plane of the original CRT screen shown above. The projector should not have to be that bright. Definitely keep me posted on your progress!

  3. Actually – the projector won’t work at all. You need to have a focus point that is set for infinity along with several other optical path alignment issues that I could go into detail with you on. I have a very experienced optics guy that works with me and he has assured me you won’t get the image you are seeking. Also, my “Flight Dynamics” HUD (Rockwell HGS) requires a specific color of green to get the mirror collimator to reflect properly. So that will also be a challenge.

    1. Interesting, but none of the problems sound insurmountable. After all, Rockwell Collins did it in the first place. I haven’t made any progress on this particular piece since I did the proof-of-concept, but I did show earlier that I could focus the image on the collimator with the LCD screen. Maybe the focus won’t be perfect, but it can definitely be done!

      1. Peter, I have made significant progress on my HGS. The combiner is mounted and operational and I will be mounting the modified upper unit (projector) this weekend. Test show excellent response on glass with an LCD instead of the CRT as you have shown on your site. I have a custom written HUD software gauge which runs under P3D as a undocked window.

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