Hey Crab,
thanks for your reply and for taking the time to wade through my post. On a practical level (the one that really counts!) you are of course totally right and I couldn't agree more.
On a purely theoretical level however (please indulge me here), I would suggest that I'm not really guessing where the ISA +10 line is. If you consider that the ISA line intersects the 1300 lb vertical at approx -2 degrees OAT (pretty clear on the graph) which exactly corresponds to the ISA temp at 8,500ft (15-(8.5x2)=-2) then it seems logical that the ISA +10 line must intersect the same 1300 lb vertical at + 8 degrees OAT (-2+10=+8) which on this graph corresponds to 8000 ft PA and not the 7,300 ft I believe it should be. Do you see what I mean? My worry is that the graph tells you that your ceiling is higher than it actually is for ISA + values, i.e. if there is an error, then it's not erring on the side of safety. Obviously the reverse holds true for a 'minus' ISA day where performance improves. Also the same holds true for the R22's OGE graphs. The way I see it, on a 'hot and heavy' day, the graphs are not penalising you as much as they should for non-standard temperatures. Maybe I need glasses...
Ah well, back to the real world...
Irlandés