Hmm...I thought when an an Ace was shot down they are out of action for a little bit?

One issue I see if HA's are mortal is that you could get to a stage (and possibly quickly) that there are none left...I think if they do go mortal they should only be able to be killed if the player shoots them down..and maybe Ace's in whatever Sqd your in be mortal too..though not sur eon this.