Frost is a bit of an issue , my understanding is that it is not allowed as there is no real clarification in the regulations between light frost or ice on the upper wing surface of any type as per FAA regs. Find this a bit of nuisance as see a regular problem with the A330 on a short turnround after a 7 hour sector , we end up with a couple of square feet of light frost over the main u/c mounting area which doesnt melt with fuel loaded if the OAT is a little cool . Unfortunately its in full view of the pax ! Understand the need for the regs due to various accidents , and certain types have had a problem , but it is a bit of a nuisance . As an LAE in these cases i'm more concerned for my colleagues at the pointy end if someone takes a photo !!!