Airliner HUD
Thanks for the super link to Boeing, PJ.
@ other.....
You don't need to know the exact AoA you are at or the exact degrees that will get you into trouble. You just need relative values to be displayed. The Boeing link discusses the mach correction and configuration effects.
I once bumped the flap switch in my SLUF on approach and had only leading edge flaps ( they really smoothed out the buffet). So a poor cross check with speed had me maybe 10 or 15 knots faster than I should have been although I had the AoA bracket nailed in the HUD. No problem with stall, but a rain-soaked runway and long landing created a serious problem. Had to take the departure end barrier with my trusty hook and all was well ( except for meeting the boss and taking the heat for being stupid).
Although a decent AoA display could allow an optimum cruise, my personal thoughts are it should primarily help the crew prevent a stall and have an optimum approach AoA.
Seems the commercial airline folks do not want to implement a wide field-of-view HUD, but I can tell you that the sucker is invaluable in bad weather. Apparently the 737 fleet had the option a few years back, but I don't know if there are many being used. Some of the commercial pilots here could comment.
The HUD's I used were a lot easier to interpret than some of the panel displays I have seen here. Sure, they can be cluttered, but the ones I used had de-clutter options to help out.
Secondly, with the increasing deployment of FBW systems, HAL should use all available inputs to make the flight safer and easier than the old days. Why the AoA is ignored below 60 knots befuddles me. There are too many other sensor inputs to use than the airspeed or the AoA by themselves. So it comes down to the reversion laws and such, huh?
Lastly, and @ several here, the zero gee trajectory vastly reduces the chance of a stall. The 'bus keeps trying for the one gee Nz, and the trim system doesn't allow a trim otherwise. Compared with the Viper, we could trim the basic system for any gee from minus whatever to zero or about 3.5 positive gees. So a neat demo for the nugget was to run the trim all the way back and watch the jet do a hands-off perfect loop. As speed got low and we reached the AoA limit we stayed at the AoA limit until flying back down, when the gee limit came into play.