go into the gunsights folder and delete all the files (including any NON image files so the complete folder is empty), then take and DL this sight:
http://www.hitechcreations.com/misc/sights/gadget/f4u1c.bmpright click the link and "save target as"
DO NOT modify this file in any way, except to rename it to "default"
then cut and paste it into the C:\Program Files\HTC\Aces High\Sights folder
then try to run the game.
DO NOT do this while the game is running and minimized in the background.
if that works, you had a corrupt file in the gunsights folder, or a gunsight that was the wrong file type.
all gunsights must be 64x64 (or 128x128 or 256x256) pixels, and 8 bit color (256 colors ONLY).
any gunsight that has an alpha must be named properly too. example:
default.bmp
default_alpha.bmp
otherwise the sight will not work correctly.
you can rename sight just like the sounds folders example:
p51d.bmp
then whenever you fly that plane that sight is used by default, without going into the game and "manually" setting a gunsight.
if you have a alpha for that sight it would need to be:
p51d_alpha.bmp
ALL alpha files must be the same size as the main sight bmp, AND be 256 color GRAYSCALE only.
hope that hleps.