Hi Gosd,
You can look inside the xdp file there are damage values for each system. (open with notepad) scroll down until you see <DamageBoxes> it usually comes just after the loadout section of the xdp.
example:
<Box ID="damagebox_nose" Parent="damagebox_fuselage">
<BoxMap SystemID="nose_structure" Probability="2" Points="20"></BoxMap>
<BoxMap SystemID="left_guns" Probability="24" Points="28"></BoxMap>
<BoxMap SystemID="right_guns" Probability="24" Points="28"></BoxMap>
<BoxMap SystemID="engine_one" Probability="31" Points="364"></BoxMap>
<BoxMap SystemID="oil_reservoir" Probability="6" Points="71"></BoxMap>
<BoxMap SystemID="coolant_reservoir" Probability="8" Points="98"></BoxMap>
<BoxMap SystemID="hydraulics_reservoir" Probability="5" Points="60"></BoxMap>
</Box>
under each section the probability values when combined need to add up to 100, so you can increase the probability of a hit to a given part. or under the next set of numbers in each line "points" you could reduce this number.
Usually these values are set by the airfile creation process to correspond with supplied aircraft data. But there is nothing to stop you making them as strong or as weak as you like. I personally found the 109's shed their tails way to easily so I beefed up that part of the plane in my install.
hope this helps, regards Rob.
PS as always back up the original file first in case you decide you want to go back.