Another issue regarding performance is the environmental envelope on the PA31. The manufacturers P-Charts for take-off and single engine performance go up to 36c at sea level and fall away with altitude, just allowing ISA +20 operation. After an accident in the US where a Navajo did not maintain height after a failure at altitude with sea level temps of 39c the NTSB and FAA made a point that the aircraft was operating outside of its certified limits and performance can not be guaranteed. The lines ending on the charts signified the temperature limits of certified performance.