Every pilot knows the moment: you need ISA deviation for a performance calculation, the freezing level for icing risk, or a cold-temperature altitude correction — and you're doing it by hand at 0300 local time. Stop doing ISA deviation math on the back of a nav log. Set the ISA deviation in your FMC the right way. In under 10 seconds. On any day of the year. WHY IT MATTERS The FMC assumes standard atmosphere unless told otherwise. On an ISA+20 day, that assumption puts your TOD kilometres off