Question about anti-ice

[RestUnknown 0]

Hi all In the documentation it says that engine anti-ice does not need to be used during climb and cruise segments when the temperature is less than -40°c SAT.Isn't this wrong? I mean the colder it gets, the more chance of ice? Or is it just not necessary to anti-ice the engines during these segments because of the heat they produce then? And the wing anti-ice should always be on if SAT is less than 10°c? No matter what part of the flight you're in? Thanks!

[yos233 1]

Around that height (I'd say 26,000 and up) the air is too thin for ice to form on the wings at the speed you are going. So why waste power.

[sheepfu 3]

Engine Anti-Ice is taken off of a very early stage in the engine. Therefore, it reduces engine thrust and changes the flight profile. During climb phase, the engines are at high RPM and are less prone to icing. Once at cruise, the friction of the aircraft moving through the air creates enough heat to eliminate the need to use anti icing of any kind. In decent phase, the entire dynamics of the aircraft has changed. For the most part the aircraft is traveling slower and slower as it descends. Engines are at low to medium RPM and you are more likely to encounter weather. Regardless of aircraft, if the temperature is 10 degrees C or less AND there is visible or likely precipitation wing and engine anti-ice should be turned on. In final approach phase, engine anti-ice should be turned off in case of the need to go-around which requires maximum power. Most modern aircraft automatically disengage engine anti-ice if TO/GA mode is initiated by the flight crew.

[RestUnknown 0]

Had no idea ice couldn't form at that altitude and that was the reason, thanks! But Eric says to disable all anti-ice systems at 26000 and Branton very early.So if I do this, is it correct? (Bare in mind temperatures are below 10°C and there is precipitation)Taxi => onTakeoff => onClimb => off at 26000? or earlier?Cruise => offDescent => onApproach (lets say 30nm out) => on (?)Final => offTaxi => on

[scandinavian13 5,192]

The assumption that colder = more ice is a reasonable thought until you think about it: If the temperature is below 0, the chance is that the water is already ice, and would therefore not stick to the wing (icing occurs when supercooled water freezes to the airframe, or water freezes to a colder than freezing airframe). Want to test that? Toss an ice cube at something - it'll bounce. Now toss supercooled water on something else that's cold - some part of that water will freeze to the object. The number isn't exactly 0, however, because even above freezing, the water can still be supercooled (colder than freezing yet still liquid), and even below freezing, there can be parts of the water that haven't frozen. It really has nothing to do with air density. It's all about temperature. The air at altitude is less dense, sure, but the bigger reason why icing isn't as frequent at cruise is the temperature is too cold for any liquid to be present to freeze.

[sheepfu 3]

Many factors go into ice forming on an airframe. It is a very complicated thing to predict which is why there are general rules. Things come into play that you might not think about. For instance, imagine your a 747 coming off of a long flight. If the ambient air temperature at cruise is -40, eventually, your fuel will get pretty close to that temperature. Once you descend for landing, the fuel takes equally as long to warm back up. This means that fuel you had in reserve for your long flight is now causing the wings to stay colder than the ambient temperature. So even when it is above freezing outside, ice can form on the wing which is being cooled by the fuel. The only hull loss of a 777 was due to ice build up and cold fuel. That time it was a clog to the engines, but it just shows that air temperature doesn't tell the whole story as to when Anti-ice is needed.