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I've had problems with 'blue on blue' airstrikes in the past.

What helped the most for me, was carefully picking the flight path (entry direction) of the plane; so it doesn't fly over friendlys until after the target area.

Secondly, the target should be a good distance from any friendly units (especially vehicles). The worse the visibility, the further to keep your air target from your own forces. This means I usually use air support for targets just behind the main line of resistance.

Use mortars and field artillery for close support.

Using the above approach, it's been a long time since I've had a 'friendly fire' incident.