777 Anti Ice off below -40C?

[RRTrentSymphony]

Hi,

I've remembered that somewhere in the 777 FCOM, it was talking about the anti ice usage and that it had to be turned on below 10C when there is visible moisture........ and theres this part where they say the anti ice is not to be used below -40C (forgot it it was WAI or EAI and it this was the auto logic or it is a recommended switch to off).

Why would they not recommend to use anti ice below -40C? Is it something to do with there might be excessive ice and they might break off into the engine core causing damage or that opening the WAI/EAI valves would cause some sort of backflow of coolness/tube freezing leading to blockage/system malfunctions.

[GROUNDSTAR]

It's just because ice accumulation at temperatures of less than -40 degrees SAT is extremely unlikely.

[HighSpeedAluminum]

Boeing makes this recommendation because at temperatures below -40c there is no moisture (in the form of super cooled water droplets) that would adhere to the aircraft. The EAI/WAI system works fine if used in extreme cold but isn't necessary. There is a high temperature (>+10c) limit to prevent over heating the leading edge(s) and engine nacelle(s).

Hope this helps.

HSA

[GROUNDSTAR]

Use of engine anti ice at temperatures above +10 TAT is likely to result in 'compressor rub' which may cause damage to the engine and reduce long term life.

[compressor stall]

Boeing makes this recommendation because at temperatures below -40c there is no moisture (in the form of super cooled water droplets) that would adhere to the aircraft.

I would add a "usually" or "normally" to that statement.

With very clean air (read no condensation nucleii) in some remote parts of the world and unusual conditions, super cooled water droplets and associated severe clear icing have been observed below minus 40°C.

[de facto]

Ice crystal areas....close to storms in areas such as ASIA Pacific region.

[woodja51]

All correct about -40 as the lower limit to use it due to very low prob of icing occurring.....

HOWEVER..... Many recomendations now include using EAI when transiting areas of CBs ... Why ... Because it can be -40 one minute and - 35 for example the next with lots of super cool drops ready to,stick to moving bits.

Thus the cocncept of anti ice is to prevent ice build up, so unless you continually watch the SAT/TAT gauge, there is a chance you might encounter ice in these circumstances...

And then there is that other thing ( bulletin?) that applies to some 777s with RR engines if i remember correctly.... about seeing moisture on the windscreen sort of like rain ... That occurs in colder air usually around ITCZ s etc ... It is actually icecrystals melting on the windscreen I think ,...but it should lead you to throw on the anti ice. Think the temps could be below -40 in that case too...

Not a met scientist, but used to fly 146s too and they used to flame out near anything resembling ice so I have a healthy respect for using it earlier rather than later... Now if it was minus 60 I might think ice pretty well zero chance...

MW

[Check Airman]

Wasn't there something about A/I below -40 carried a risk of heating the wing back to the icing temperature range?