James, I saw this question earlier and was holding my answer for someone more qualified than myself to answer. However, I believe responses so far are incorrect. My understanding is that air wings defend federation members, in accordance with federation rules, regardless of the war treaty. This appears to be backed by docs that state:
from:
https://www.simcountry.com/cgi-bin/cgid?faq#federations_11
"How does it work when a federation member is attacked?
If a federation member is attacked and the federation rules include mutual defense of the members of the federation, then the air force of the federation members will assist the attacked member by sending their defense wings. These forces might be damaged in such attacks.
Federation members can also setup automatic war declarations if their friends in the federation are attacked. This feature reduces the time between a war declaration by an enemy and the intervention by allies."
As you see, mutual defense is talked about separately from the war treaty issue. Because of this, federation members of sub war level 3 or of a war level not directly able to attack an aggressor of a fed member can still contribute to the defense.