Complex sentence: A notable action occured on 28 May 1942 when he shot down and killed a "Feldwebel" Willi Langer, his total tally stood at 15 Axis aircraft destroyed.
Split into simpler sentences: A notable action occured on 28 May 1942, when he shot down and killed "Feldwebel" Willi Langer. At this stage, Frost's total tally stood at 15 Axis aircraft destroyed.