Icing conditions

[double_barrel]

I would appreciate some help to understand the physics of icing risks.

I read that the icing zone is +2C to -20C and that low clouds represent a greater icing risk than high clouds.

I take that to mean that only liquid water striking an airframe colder than 0C will form ice. Is that correct? Otherwise why does the icing risk disappear at temps below -20C? And that the reason that high clouds are low risk is simply the assumption that they are in air temps below -20?

And does it follow from that, that even with very high tops which you cannot get above, an IFR flight, that didn't have a descend option, would be better to stay in cloud but climb into lower temps?

Thanks

[Jim59]

Browsing the Aircraft Icing Handbook (New Zealand CAA) will give you a good start. Typing 'Aircraft Icing' into a search engine will find plenty of other authoratative sources.
https://www.caa.govt.nz/safety_info/...g_Handbook.pdf

[double_barrel]

Originally Posted by Jim59

Browsing the Aircraft Icing Handbook (New Zealand CAA) will give you a good start. Typing 'Arcraft Icing' into a search engine will find plenty of other authoratative sources.
https://www.caa.govt.nz/safety_info/...g_Handbook.pdf

Thanks, that's very helpful.

[foxmoth]

Actually you can get icing right down to -40c, below that any water is likely to be frozen already and will not adhere to the aircraft. Water above zero CAN freeze if the airframe is below zero but it would be an unusual situation for this to be a problem as the airframe would normally then be warmed. Worst icing is just below zero because only a little of the water freezes on impact, the rest flows back and freezes forming a dense hard layer, when it is colder more freezes on impact which is lighter and not as hard.

[bookworm]

Originally Posted by double_barrel

I would appreciate some help to understand the physics of icing risks.

A couple of good articles here:
Supercooled liquid water and airframe icing
Cloud liquid water content, drop sizes, and number of droplets

I read that the icing zone is +2C to -20C and that low clouds represent a greater icing risk than high clouds.

I take that to mean that only liquid water striking an airframe colder than 0C will form ice. Is that correct? Otherwise why does the icing risk disappear at temps below -20C?

In essence, yes.

And that the reason that high clouds are low risk is simply the assumption that they are in air temps below -20?

It's a bit more than that. As you'll see from the second article, the liquid water content in a cloud depends on the cloud base temperature.

And does it follow from that, that even with very high tops which you cannot get above, an IFR flight, that didn't have a descend option, would be better to stay in cloud but climb into lower temps?

Usually yes, though note the strong peak in LWC around minus 6 or 7 degC.

[ASRAAMTOO]

I would also recomend reading Safety Sense leaflet 14 on Piston Engine Icing

Safety Sense Leaflet 14: Piston Engine Icing

[BigEndBob]

I took off into super cooled droplets at sea level, in a Warrior, temperature was probably about 3-4 degrees c.
Interesting next half hour to deice a patch in the windshield to see through to land.
And only time ever seen carb. icing in the Warrior was when on the power check, visible moisture forming at the tip of the prop blades on a misty damp day.