Do realise that the air rules are somewhat "clunky" and this is one of the aspects that causes some confusion. Here are the relevant sections of the rules:-
Quote:
4.2.4 Flak Attacks
AA weapons are designed to fire defensively against an attacking enemy aircraft, and may therefore shoot immediately after an enemy aircraft formation makes an approach move but before it makes its attack.
and
Quote:
Ground units that are armed with AA weapons can shoot at enemy aircraft as they move past them. To represent this, they may shoot at an aircraft formation that moved within their weapon range during their approach or disengagement move, even if the aircraft is no longer within weapon range when the attack is made.
My emphasis
So firing (and suppression) is resolved at the end of the move, but units eligible for firing and suppression are determined during the move.
To be eligible for suppression, a unit only needs to be in range (and LoS) - it does *not* need to have the correct weapon type (AP/AT/AA) so in this case even units armed only with "small arms" that have a notional range of 15cm may qualify for suppression if the a/c passed within 15 cm of them. This is why the a/c must trace its flight across the table to allow the players to determine whether it passed through potential AA and suppression arcs of fire.
As an example, a formation of two units with one BM, one of which is a ground AA unit
- If the a/c passes directly over both units from front to back and ends nearer to the AA unit, the other unit may be used for suppression.
- If the A/c flew outside the notional arc of fire of the other unit, or if the a/c ends its move nearer to the other unit, then the AA unit is supressed and may not fire.