Earlier I mentioned an accident in the USA where a Navajo crashed following an engine failure when it was ISA+ 30 or so. The NTSB queried Piper about its charts and the statement from Piper was that the lines were considered the environmental envelope for their aircraft. Therefore the aircraft was considered outside of it's environmental conditions if operated beyond the lines on the chart which equates to ISA +20. What that means is no performance could be guaranteed by Piper beyond those lines and they can not be held responsible if the aircraft did not climb, maintain altitude etc.... It becomes the PICs sole responsibility if they chose to fly on such a day. And if you should have an accident the responsibility and liability would fall on the PIC alone, not the manufacturer.