Question
: I've noticed that some aircraft like the F-15 and F/A-18 have a HUD which consists of two pieces of glass (or whatever it is). Why is this used instead of one piece like the F-16? Curtis
Answer : Curtis, You are very observant and that is a great question. I looked at some of my old flight test documents, and contacted some of my engineering friends for the answer to your question.
The simple answer is technology. The original HUDs were designed in the 1970s and were not simply very bright for daytime use. By using two combining glass panels (that’s what the glass is called, and yes, it IS optically coated glass), they could intensify the image. This was especially true when trying to present video images in the HUD (such as forward looking infrared or FLIR). The F-16s I flew had both single combiners (in the Block 5/10/15/25/30/50) and dual combiners (in the Block 40).
Let’s look at some examples: compare the F-15C (early 1970s) with the F-15E (1980s), the F-18A/C (1970s) with the F-18E/F (1990s), and the F-16 (non Block 40) with the F-16 Block 40. The interesting exception here is the F-16 Block 40. It was designed later than the early block F-16s with a one piece combiner, but it was the only model to project FLIR video to the pilot, and only a dual combiner would give the required brightness. The F-16 HUD is also very think, as much as four times the thickness of the F-18 because if the F-16 lost its one-piece canopy, the strong HUD protected the pilot from windblast.
Finally, let’s look at modern fighters. The F-22 is similar to the advanced F-16 designs with a one piece combiner. The F-35, which is still in development, will have the most interesting HUD of all…mounted in the pilot’s helmet! The video and symbology will be displayed on the helmet visor and will not be limited to just in front of the pilot, but anywhere he looks! Thanks again for a great question; I hope you enjoyed the answer.
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