Well, your site design looks uniform and works in Internet Explorer 7, Internet Explorer 8 Beta, Opera 9, Maxthon 2, Netscape 9, Firefox 2, and Firefox 3 Beta. There in one small issue with Safari 3 Beta For Windows - the blue area around the green search box (at the top of the page) is uneven - slightly higher than the rest of the blue bar. Also, in Internet Explorer 6, the bar falls apart entirely. I took a couple of screenshots so you can see what I'm talking about:

Now, when looked at in Lynx or a screen reader you run into some bumps in your navigation bar, as well as in a few other places, the worst spot being the "Aircraft Data" page, which falls completely apart. This is based completely on your use of lists all over the place. In the text/read versions, you end up with tons of bullets all lined up without any content attached, and other bullets with more than one item attached to them. You need to do some major reworking with your lists to clean it up. I would start with the navigation bar, as people will see it first, but definitely get the Aircraft Data page after that. (Obviously, this won't effect all of your visitors, but it rips up your site for anybody using a screen reader.)
Now, moving past the individual browsers - your code needs a little clean up. Your
CSS is valid, but you've got
49 errors in your (X)HTML. Mostly you just need to close a bunch of tags, and remove your extra closing "head" tag. General clean-up stuff.
Now for the design itself: I like the overall look quite a bit - it's very clean, easy to navigate, and overall pretty good looking. Just a few little things:
I'm not sure I like the boxes in the sidebar - they don't really seem necessary, and the upper one comes a bit too close for the search box's green area, so it looks a bit cramped. I'd try using a simple vertical dividing line between the sidebar and main content. This could just be me.
The centred text doesn't really fit - keep it left-aligned . Also, you seem to switch between blue text on the main page, and green text on the other pages, which looks a bit off (I believe SpooF mentioned all that).
The navigation bar - despite looking good - has two issues: 1) The word "Manufacturers" doesn't quite fit on it's button. (SpooF mentioned that too.) 2) No matter which page you are on, the link for the home page is highlighted (and the word home is in blue and underlined), rather than button for the page you are on.
Lastly, I'd do something about the tables. Maybe remove the borders altogether? Your design seems to work very well with white-space, (the navigation bar and logo are the only really "weighty" places in the design - the rest is very light) and using border-less tables would allow you to maximize your white-space - a big plus for your look, in my opinion (this is also part of why I'm not so sure about the sidebar boxes - without them, you would have more white space).
So, to sum it up: A little bit of code clean-up to get yourself valid and fix up your lists for text browsers/screen reader; a few tweaks to the navigation bar; make the text more uniform; and clean up your tables.
Overall, I really like your design - a little clean up, and it'll be fantastic. Keep it up!