SAT vs TAT for anti-ice
On the B737-200/400 the FCOM says anti-ice on in visible moisture if TAT below +10C. Except during climb and cruise when SAT below -40C. Why the change from TAT to SAT? Why not use just one for the anti-ice requirement? Surely TAT would be the better one seeing as that is the temp affecting the actual surface of the aircraft. I have been in the cruise in cloud with a SAT of below -40C but then one still gets ice forming on the wipers, so therefore perhaps elsewhere? Any ideas?