Just to say upfront that I think BKC an excellent ruleset, the balance of abstraction just right and that I play it and have played it a LOT, mostly 1940 France and 1943 east front, historic orbats and scenarios.
But I think AA fire is squiffy and it is far, far too easy to chase away aircraft that are already a challenge to call in via a FAC role, and might deviate anyway.
It is especially odd for early war battles.
My issue is that
a. All HQS get to roll 3 dice in an AA capacity , I believe to represent 'general AA fire from the battalion and built in AA guns'
b. All AA hits on 5+.
This is too much because...
With the possible - possible- exception of late war US Army tank or motor infantry with all the cupola mounted .50s MG, no unit has in built AA fire worth snot eg a UK inf btn through the war has a bren gun or two mounted on a tall pintle/back of a truck, a German motor btn might have a similar MG34 mount. How an earth does a French H35 btn with a battalion HQ of one or two tanks, tanks with a one man turret and a coax MG, no cupola mount put up any AA fire? Even late war germans have thin chance; yes I know Tigers, Panthers and Pz1Vs have cupola MG34 in the nice diagrams and pictures of training, but always missing in combat pictures - strangely never enough to go round...
I am not excluding the say, truck mounted 20mm, many german units had , but that would appear as seperate platoon in BKC (and be 'paid for' in points).
Looking from the other side, why are Stukas or Typhoons such a pain in real life? Because they are hard to stop with out fighter cover (dealt already with by the air superiorty modifiers in army lists). The RAF attacks on the pontoon bridges at Sedan are massacred by a LOT of well placed AA guns, not a few rifles and MGs.
I could go on - even more..
So;
1. Make hits and suppress on aircraft a 6 not 5+
OR make a supression add 1 deviation dice to attack not cancel an attack as AA fire could distract aim if not stop.
2. Give HQs an AA factor of 1. And/or apply a 'Stuka fright' or similiar characteristic to all army lists pre- 1943 that reduced all HQ AA to zero
If you want an AA capability 'buy' and AA unit off the lists or find it a historic orbat.