Living Manual

[Wiedrock]

There are some rules regarding SU committments (likely Artillery) different than what is described in the manual.

, p.413f]23.6.1. Support Unit Commitment
The maximum number of attached support units that can be committed by
headquarters units to a single battle is 6, with the exception where the defending
combat units are in a light urban or heavy urban hex, where the maximum is 18.
Note this limit is in addition to the commitment of Support Units that
are directly attached to Combat Units involved in the combat.

[...]

23.6.2. Defender Artillery Special Commitment
Artillery support units attached to a defending HQ unit have priority to be committed
into a battle during a special commitment phase. During this round of commitments,
defending HQ units have a chance of committing 3 more than the normal limit of
Support Units (so 9 or 21 instead of 6 or 18). After this round, the normal commitment
round is conducted.

[Wiedrock]

19.3.3. Barrage Balloons
All ports are assumed to have barrage balloons that will impact any raid that is
bombing anything in the port's hex. Aircraft bombing under 3000 feet have a chance
of being destroyed by the barrage balloon equal to two times the size of the port (port
3 means a 6% chance that bombing aircraft will be destroyed). For night missions, the
percentage chance is tripled (so port level 3 means that each plane has an 18%
chance of being destroyed).

But don't worry, here the results at 3k with the 50kg which finds us another Manual error since Balloons seemingly also affect planes flying at 3k.

Barrage Balloons seem to affect planes flying up to 4000ft (at least) and not just planes under 3k.
At 5k I have not encountered any anymore.