User blog comment:Zel-melon/Kancolle Safety Guide: Preventing the "Oh shit!"/@comment-26337865-20160714110418/@comment-27783338-20160715063651

I think it was random...yesterday when I upgraded my recon plane from +7 to +8, I have 5 max proficiency recon planes and 3 no proficiency recon planes before (all unlocked), 2 max proficiency planes disappeared after I upgrade the plane, and that was my oldest planes too...but when I upgrade the plane from +1 until +7, the planes that used for sacrifice was the newest/no proficiency planes...so about the oldest one might be wrong and it was completely random...