Regarding Q226, when it says OAT = ISA + 10ºC does it mean that using ISA we must calculate the OAT, in this case: 15ºC at SL - (41*2) = -67ºC at FL410 but in ISA is -56.7ºC and then add 10º to this OR do we do this: 15ºC at SL + 10º C = 25ºC at SL - (41*2) = -57ºC but in ISA -56.7ºC? Using the CR-3 circular computer, when do we subtract the temperature rise from the OAT reading?
I think that you are overlooking the fact that the temp lapse rate (1.98 deg/1000 ft or approximately 2 deg/1000ft ) applies only up to the Tropopause at 36000 ft.
So at FL410 ISA Temp = 15 -(36*1.98) = -56.28)
using 15 deg at ISA msl and -1.98/1000 ft
So ISA +10 at FL410 = -56.28 +10 = -46.28
The examiners have picked a flight level above the Tropopause to try to get you to make this error.