Note that the Dornier 228 is certificated to FAR 23 up to amendment 23. That means it has no requirement whatsoever to withstand a bird strike. It could legally be downed by a well aimed sparrow! Once up on a time such aircraft were limited to max 9 pax. The story of how 19 occupants came to be put at risk dispels forever the myth that "safety is paramount"
The cerification that you speak of is no more than pencil whipping.
The More informative data is service history on similar age part 23 aircraft.