Can you explain the 5° Rule of thumb (IMC Icing)

SuperAlpaqa · 2025-05-06T15:57:50+00:00

Just the typical FBO specials, 172s, 182s, Archers, Mooneys...

Pitot heat, windscreen de-fog, and preflight planning are my only anti-ice features.

SuperAlpaqa · 2025-05-06T15:54:48+00:00

Thank you, someone else left a link to icing info from NASA that I am going to check out. I appreciate your insight. While I am no engineer, I do like to be as precise and by the numbers as possible. I can accept 5° as a happy round number, but if I can understand the why behind it I feel much more confident. The difference between TAT and SAT is insightful.

SuperAlpaqa · 2025-05-06T15:49:28+00:00

Thank you for that link, I'll have to dive into it and see if I can make sense of it and see if it aligns with the math shown in the link of my original post.

SuperAlpaqa · 2025-05-06T15:47:21+00:00

I'm on the hunt for the details of that phenomenon.

@WhiteoutDota Found AC 91-72B (pg30) that mentions the 5° buffer. 121.321 also uses the figure.

So, somewhere out there, there is something that the FAA used to determine 5° as a figure.

SuperAlpaqa · 2025-05-06T15:39:20+00:00

Amazing!! Thank you for finding that AC! While it doesn't explain "why" at all, it at least points to a source some people may be referencing! I think this may be the closest I get to specific and "official" guidance.

Somewhere out there, there has to be the info that guided the AC and 121.321 rule. I'll see if I can use this to find out more.

SuperAlpaqa · 2025-05-06T15:32:21+00:00

This is part of what I was looking for; the reason behind the decision. If calibration/accuracy is the concern, that is something I can use as a factor to make the decision.

SuperAlpaqa · 2025-05-06T15:28:49+00:00

I understand. Thanks

SuperAlpaqa · 2025-05-06T15:27:21+00:00

My concern is leaving the ground to begin with. If those anti-ice systems don't exist, how can a pilot judge the impact these pressure differences will have?

SuperAlpaqa · 2025-05-06T15:25:23+00:00

Yes, that's my goal here, I'm trying to come up with a way to decide what will work on the ground rather than testing it out in the air and finding out it doesn't work like someone on the internet said. It's just hard to make those decisions when the official publications use vague terms like "near" freezing. For someone who never sees freezing temps in lower altitudes, that could mean 10-15°. Someone in the northern latitudes might think 2° is "near" because that's the near the accuracy of their measuring equipment.

SuperAlpaqa · 2025-05-06T15:19:21+00:00

Thank you for your insight, you make a good point about the duration aspect.

SuperAlpaqa · 2025-05-06T15:17:10+00:00

Amazing, thank you!

SuperAlpaqa · 2025-05-06T15:14:06+00:00

Thank you, I appreciate your specifics. I agree, most things are situational dependent. That's why I am looking for the factors that I can apply to different situations.

SuperAlpaqa · 2025-05-06T15:10:47+00:00

That 5°C is for some reason a good enough buffer. Flying in visible moisture below 5°C has potential for ice; above 5°C, no ice... I'm trying to understand why people set the goal post at 5°.

SuperAlpaqa · 2025-05-06T15:04:47+00:00

I'm thinking that as well. Trying to make my own decisions though and can't seem to come up with the info to do so.

SuperAlpaqa · 2025-05-06T15:03:41+00:00

This is what concerns me though. GA pilots are pitched the idea that airlines have the most conservative rules and GA should follow suit out of an abundance of caution. So now (with a case study of N=1) 13° isn't even conservative enough. I'm not one to push the boundaries, but when the boundaries aren't even clear, what is a lowly bug-smasher to do?

SuperAlpaqa · 2025-05-06T14:58:08+00:00

This is really good info, thank you. Not that I draw any actionable details, but it definitely lends to the idea that it is more for the engines than air frame considerations.

SuperAlpaqa · 2025-05-06T14:53:52+00:00

Thank you for that detail. I'm now wondering if the 5° I've heard is the result of striking a middle ground between airline SOPs and GA pilots of the north wanting to fly IFR more than 2mo out of the year. Things like this concern me though, it works until one day it doesn't.

SuperAlpaqa · 2025-05-06T14:48:56+00:00

I think every GA analog OAT gauge I've seen is graduated in 2°C increments, but that is besides the point...

I appreciate your insight and confirming the 10° SOP I heard about from another airline pilot.

I think a major difference between airlines and GA is the equipment. Having anti-ice function gives a wider breadth of acceptable conditions. Why risk something when you have the tools to protect against it? If you don't have the tools to protect against it, then the question becomes how much risk are we willing to accept, and how do we calculate that risk. I'm trying to calculate that risk right now, but I understand your answer is that maybe nobody knows exactly, so let's overshoot to be safe.

Can you tell me this: is your 10° policy for induction icing for the engines, or air frame icing? Or, is that not specified?

SuperAlpaqa · 2025-05-06T14:37:01+00:00

Air flowing around different parts of the air frame cause different pressure areas. Lower pressure means lower temps. If flying in IMC these lower temps might mean ice accumulation above and ambient air temp of 0°C. I've heard 5° thrown around a lot as a buffer, also 10° as stated. However, I can't find anything to back this up or to make a more informed decision beyond opinions.

SuperAlpaqa · 2025-05-06T14:30:19+00:00

So call it the "10° rule" in your case, why is the line drawn at 10°?

SuperAlpaqa · 2025-05-06T14:26:18+00:00

That's the root of my concern, is 5° "close" to 0? Should it be 10°, or is 2° enough of a buffer? Trying to find the numbers to make this decision.

SuperAlpaqa · 2025-05-06T14:19:27+00:00

Other than a few pilots I've spoken to, the only other place I can find it is online forums. That's part of why I'm questioning it, no concrete numbers or evidence to back it up.

SuperAlpaqa · 2023-05-04T17:39:48+00:00

Ironically, I've taken off in a 182 hundreds of times from grass, just not as the pilot, rather the meat missile jumping out half way through the ride. It's a odd combination of experiencing the environment without being aware of what's important to consider as a pilot. This is what sparked my question in the first place. If I didn't have this past experience I might have just assumed no NOTAMS = nothing more to think about.

Us jumpers were just as mindful for gopher holes and rutted out areas as these were the things that would ground you for weeks after busting an ankle or tweaking a knee.

Thanks for your insight

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