TAT Vs SAT
I'm possibly about to expose myself as a total charlatan pilot now... However, here goes -
The aircraft I fly (737NG) has a TAT reading on the Upper DU. To access the SAT reading, one has to go to Progress page 2 in the CDU.
We use TAT for determining when engine anti-ice needs to go on (ie below 10Deg) in the descent, climb and cruise. However we use SAT to determine when the temperature is sufficiently low that we turn the engine anti-ice off (colder than -40Deg).
Given that the TAT reading does not approximate to OAT (SAT being a closer approximation) and that I have heard many pilots (mostly Captains) refer to TAT as if it were OAT, I am wondering why we don't have SAT on the Upper DU and TAT in Progress Page 2 instead. Our TAT probes are not aspirated by the way.
Can anyone explain the logic of such a setup? Or is there some other earth shattering significance of TAT that I am completely missing?