Hi Widgeon,
The reference numbers you chose at 5,000 and ISA +10C on the HOGE chart do show the same weight capability since they are limited by Maximum gross weight and not aircraft performance at that altitude. I would like to mention though that your calculation of ISA +20 at 5,000 ft may be in error at 35C. Standard temperature at 5,000 ft is only 5C, therefore the figure should be 25C. Here again we have the gross weight limit line interfering, so never mind that either. I propose to go to 7,000 ft and ISA +20, which should be 21C. The 135 will do approximately 5,800 lbs with the P&W engines (which seem to give a few more lbs), the MD902 will do 6,250 lbs. That's 450 lbs. Now you have an excellent point. Maybe that extra performance is taken up in airframe or equipment weight. At this point we really don't know, but Eurocopter is listing the 135 empty weight at 3353 (1524Kg) and MD is listing theirs at 3375, a pretty nominal difference on the surface of things. You have to assume that both companies have done an equally misleading job of estimating empty weight, so they're just about even. I'd also go out on a limb and say that the installed equipment you would want on either machine would be equal too. That brings us back to the 450 lbs of difference in performance. Then again, I still may be reading the charts wrong, I usually do.