B737 outside temperature indication plus use of anti-ice

[pilotpilot]

According to Boeing limitations, engine anti-ice has to be used below 10 degrees Celius TAT and above -40 degrees SAT (with moisture present etc...). I get the TAT from the secondary EIS panel and the SAT from the FMC PROG page 3. How reliable are these indications? Are they to be counted on if you are at, for example, -35 degrees SAT for a long time?

oops, sorry meant to say, "-45 degrees for a long time"

[lomapaseo]

I tried several readings to decipher your post relative to temp ranges but gave up.

The key to anti ice is moisture content. At the warmer temperatures above freezing account is taken of some temprerature depression in the engine inlet (at low aircraft speeds) as a critical point while at the very low temperatures at altitude the key is presence of moisture. Typically at low enough temperatures below minus 45 deg there is no concern due to little or no moisture present.

I'll grant you that time is a factor since ice acretion is the worry here, but at very low moisture contents the ice adhesion ablates away as fast as it forms.

Thats a start but I admit I may have missed your question basis.

[LEM]

How reliable are these indications?
Well, if you don't trust your instruments, open the window and check for yourself! ;)

[Golden Rivet]

But which finger would you use ? Can the calibration of one finger suffice or would you need to stick all ten out and take an average. Does one need to take into account ram rise ?

:confused:

[arba]

ha...ha...ha..! LEM that's a good one !!

[wellthis]

The TAT temps are very reliable. Using the chart you'll find that normally around -16 TAT, you won't need anti ice in cruise and climb (meaning SAT is under -40). Keep in mind however that you'll always need it in descent, since engines are more vulnerable at low thrust and plus as you're descending you won't have to worry about robbing the engines from bleed air. Hope this helps.