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Feb 7, 2005
For 737/757 drivers:

Why is it that at -40C or below, in climb or cruise condition, engine anti-ice must be turned off, but during descent at those temps it can be left on?

One pilot's elementary explanation: It's turned off below -40C because ice won't stick in temperatures below that. It's turned back on during descent because if you are in -40 air and you are descending, you probably won't be in -40 air for long. Better to get it on a little early rather than a little late.
It's not just the 757. The jets that I've flown have the same limitations: Wing and Engine anti-ice on in visible moisture between +10 and -40C or with an Ice Detected CAS. Plus there's some other times on slushy runways, etc. And for the reason mentioned above.

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