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.
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.
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.