Originally Posted by
BFSGrad
The interview transcripts indicated that the 12th AB Blackhawk pilots used barometric altitude as the reference for flying the DC routes.
Thank you, I'd missed that. And on re-reading the preliminary report, I see that the NTSB described the routes using amsl. So I stand corrected on the route definition. That brings altimetry errors into play for erosion of the "designed" separation margin, which makes the design even more unsafe. But the point remains that PAT25's 78 foot deviation above the route maximum altitude is within the FAA's tolerance for commercial and instrument flying accuracy by helicopter pilots.