first of all, as a perk, BIA only works if its on 100%. but unlike other perks, the whole crew must have it.
if a crew member dies, BIA will keep working.
from the wiki:
let me start with this question:
does it work like vents ? - except that vents doesn't improve repair/firefighting/camouflage.
-say, if all my crew is at 100% major qualification and I have vents, then my commander will be performing at 110% (100+5 vents+5 BIA) while the rest performs 121% (100+5 vents+5 BIA+11 commander) ?
-and if they also have repair/firefighting/camo at 100%, it becomes 105% (100+5 BIA)?
how many crew members does a tank/TD/SPG need to have for BIA to be a better choice over the other skills/perks ?