B737NG Engine TAI
 

Boeing 737NG limitations says that Engine TAI must be on if icing conditions exists or are anticipited except during climb and cruise below -40C SAT.

My question is, why? what happens at temperatures below -40C SAT that doesn't require the use of Engine TAI?

hi wof,

this is because air at low temperatures (i.e. below -40C) can only absorb a certain mass of water, which is determined by maximal humidity (saturation concentration). The actual value of maximal humidity depends on SAT; the lower the temperature, the less water can be absorbed.
This is basically why there is no risk of icing at SAT below a certain value, in this case -40C (which is in fact defined by the manufacturer).
I hope this clears things up a bit.

There is NO no risk....but it is much lower during climb and cruise as the engine is operating at high compressor speeds.
During descent,when thrust is normally on the low side,you just need to encounter some ice crystal to sneak and build up around the compressor and there comes your bang bang :ooh:

You're totally right. There is actually never "no risk". Under given circumstances, though, you may say that there is no risk of icing, because there is only little water absorbed by the air and the fan as well as the compressor are rotating at higher speeds, compared to power settings close to or at idle during descent.

If the air is saturated doesnt that mean it has water already and hence EAI must be turned on also?