In contrast to other 4th generation aircraft, the Su-25 lacks a HUD, and the pilot flies using the
cockpit instruments. The Su-25 is, however, fitted with an ASP-17 gunsight for aiming weapons.
the su-25 lacks a HUD,lacks这个词,我的理解是相对于其他四代机,su25的缺乏HUD的功能。但不是没有HUD,也许我的英语很差,理解有误。