Of course... the weather at SLC just seemed to be extra crappy every time I deiced there (and I was based there), hence my response.

There is (was) a certain amount of frost (1/8") allowed under the CRJ fuel tank on the bottom of the wing.

waste of money to do both on a nice day

It's actually a cheaper procedure and you will see it being utilized at more Delta stations in the future.

Probably because the type 1 was not diluted with 90 percent water. The current mixture they used has a freeze point of -3 C. Type 4 does not protect water from freezing on the wing (which is pretty much what they are putting on the wing now with 90 water). The purpose of type 4 is to prevent FALLING precip from freezing on the wing by creating a barrier to which the precip lands then rolls off as the plane accelerates. It provides no protection for what is currently on the wing. In talking to the deicers they told us that they have a very short window to apply type 4 (in the 4 minute range) The application reduces the amount of water on the wing, however if it were to freeze prior to the application this would be no different than spraying type 4 on wing with out deicing it first.

re type 1 and frost, years ago I actually saw type 1 fluid freeze up..co. applied it several hours too early anticipating frost that night..now our holdover charts allow only 45min of protection in conditions of active frost when using type 1 only..