Does it help to have multiple flak units in a city to help guard against bombers?
I have seen my city get attacked by 5 bombers and then the sixth get shot down. Does this mean that my flak gets a shot at each bomber?
How does this work?
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AA combat formula
The real strength of each AA unit is 1/10th of the value shown in the editor. So Flak has a strength of 0.2.
Each AA unit in the tile being bombarded takes a shot against the defense of the bombarding air unit. The chance of shooting down the air unit is A/(A+D), as with all other forms of combat in Civ3. Remember, A is 10 times less than what is in the editor for the AA unit.
The maximum number of AA units that are given a chance to shoot down the plane is 4. Any AA units above 4 are ignored.
So, for example, if you want the formula for the probability of N units with a strength of A shooting down a plane with defense of D, here it is:
1 - (D / (D+A))^min(N,4)
D is the defense value of the bombarding air unit (2 for a bomber). No modifiers apply. A is the AA strength of the AA unit (0.2 for flak). N is the number of AA units, not the number of air units.
min(N,4) = N, if N less or equal to 4
min(N,4) = 4, if N greater than 4
The SAM battery works like this:
For each bomber, there is a chance equal to the interception chance for the SAM to fire (50% against conventional, 5% against stealth)
If the SAM doesn't fire, the bomber continues as if the SAM were not present. This includes the chance of getting shot down by AA fire.
If the SAM fires, it has a A/(A+D) probability to shoot down the plane (A=8 for the SAM, D=2 for the Bomber). If the plane is not shot down, the Bomber survives, but it loses its chance to do any damage as well.