Well Nick I've had to rethink my original reply to your post and subsequently deleted it (I was in a dazed and confused state!
). I did what you suggested using the line of constant 12,600 ft DA on the graph and here are the results...
12,000ft PA, OAT -4 degrees = DA 12,600ft, MAUW = 1100 lbs
10,000ft PA, OAT +17 degrees = DA 12,600ft, MAUW = 1150 lbs
8,000ft Pa, OAT +40 Degrees = DA 12,600ft, MAUW = 1200 lbs
You said that you believed the lower temperature conditions should have higher HIGE performance. But it seems judging by these figures, that DA being equal, performance is increased at the lower altitudes/higher temperatures. This is reflected by the increase in maximum allowed all-up weight. Does that make sense?? Is the piston engine performance therefore more adversely affected by higher altitudes than by higher temperatures for a constant DA? That's the only way it seems to make sense to me. Am I still in a dazed and confused state?
That apart, thanks for throwing light on the error of my assumptions. As usual, it all seems so obvious in hindsight. I always took for granted that an aircraft would perform identically irrespective of the pressure altitude/temperature combination as long as the density altitude didn't change. If only life were so simple! I guess this assumption is only correct for non-powered flight, gliders etc. This for me is a real eye opener and something I thought I understood well needs a bit of reevaluation.
Thanks!
Irlandés
