SAT vs TAT for anti-ice

[BMM389EC]

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?

[Fat Dog]

Think it's because in theory no ice can form below -40C SAT, so you could have AI off at all times in these circumstances, but Boeing recommends AI on during descent regardless as it's gonna get warmer. As to AI on below +10C TAT, thats because the TAT will vary with IAS at a constant SAT so there is no SAT at which AI on would be appropriate. My guess anyway.

Cheers
Fat Dog

[bookworm]

The lower temperature limit for airframe icing is based on the low concentration of supercooled liquid droplets in the atmosphere at low temperatures. Below a SAT of -40 degC, there's not enough water there in the liquid phase to cause (significant) icing. The temperature of the aircraft skin doesn't affect that concentration, and the limit is therefore an SAT.

(I'm surprised you saw ice at -40 degC. Is that a regular occurence?)

The upper limit is based on the highest temperature at which some part of the airframe is below zero. Because it's about skin temperature, the TAT is what's important. In aircraft with high wing-loading in low static pressure, this can happen at TATs up to +10 degC, because the part of the upper surface where most of the lift comes from is at lower pressure, and therefore lower temperature than the stagnation points.

[BOAC]

Just to clear up a possible confusion here, the 737 2/3/4/5 temperature requirements refer to ENGINE ANTI-ICE (not airframe) and I have always understood the +10 to allow for intake pressure drop.

For info the only time I have seen significant airframe ice at -40 and below is in strong mountain wave or in the top of an 'active' where extra moisture can be carried up to those levels (I personally recommend (and use) ENGINE anti-ice at -40 or below THERE for that reason!) The amount of engine ice normally accumulating at these temperatures is judged to be insignificant, but at idle there might not be enough centrifugal (am I allowed to say that?) to throw any ice off, hence the req for descent.