The recipe for ice

[flyingcheesehead]

OK, I feel really stupid, but...

When does ice form? Will it always form with visible moisture at below-freezing OAT's? What's your personal "formula" for the icing portion of the go-no go decision?

The reason I ask: I watch the weather quite a bit while I'm on the road, and I'm always hoping for flyable IFR when I get home. I've seen several instances in the last couple of months where I check ceilings/vis and see good IFR weather, then I check airmets and there are no airmets for icing... But then I check temps at the fields and winds/temps aloft and see temps slightly below freezing, and lots of PIREPs with ice. Why, in that scenario, are the airmets for ice not present???

Also, I've read the "Dr. Bruce icing protocol" which talks about having surface temps above 35 (and I remember Ed G suggesting 40 for Mooneys and other laminar-flow wet-wing aircraft). What about when it's too cold for ice... When does it get cold enough to go anyway?

Trying to learn,

 

[gismo]

OK, I feel really stupid,

After effects of the long drive from the left coast?

but...

When does ice form?

IME, when you can least afford it.

Will it always form with visible moisture at below-freezing OAT's?

Absolutely not. "Visible moisture" can already be frozen in which case you won't get ice on the airplane. It also appears that there are plenty of clouds that don't generate airframe ice even though the OAT is below but near 0 C. I don't know if this always means that the water is already frozen, or if there's some other reason it doesn't form ice.

What's your personal "formula" for the icing portion of the go-no go decision?

Since my plane is "full deice" (not KI, but close) and twins have considerable excess climb capability compared to most singles, my requirements are a little different than if I was flying an unprotected Skyhawk.

No freezing rain or drizzle in the forecast. No reports of moderate icing (by credible sources), and two or more options for escape if the wx reports/forecasts prove optomistic. In general if temps suggest ice is possible, I avoid the NE quadrant of any significant low pressure area.

The reason I ask: I watch the weather quite a bit while I'm on the road, and I'm always hoping for flyable IFR when I get home. I've seen several instances in the last couple of months where I check ceilings/vis and see good IFR weather, then I check airmets and there are no airmets for icing... But then I check temps at the fields and winds/temps aloft and see temps slightly below freezing, and lots of PIREPs with ice. Why, in that scenario, are the airmets for ice not present???

Also, I've read the "Dr. Bruce icing protocol" which talks about having surface temps above 35 (and I remember Ed G suggesting 40 for Mooneys and other laminar-flow wet-wing aircraft). What about when it's too cold for ice... When does it get cold enough to go anyway?

Trying to learn,

IME too cold for ice means at least -20 C or lower, but a lot depends on the amount of moisture and where the airmass came from. I think that with a zonal flow from the Pacific NW the chances are that the moisture in the clouds is already frozen if the temps are below about -10C, but a SW flow from the Gulf seems to support unfrozen water at lower temps.

There's also a big difference between stratus and cumulous clouds with the latter having less predictability and considerably more vertical range on the ice. In stratus conditions it's widely accepted that a 4000 ft change of altitude will almost always get you out of (or into) ice.

To be honest, I'm usually more concerned about escape routes than predictions as it seems the predictions are rarely correct.

 

[poadeleted20]

When does ice form?

When there is liquid moisture present and the air/skin temp is at/below freezing.

Will it always form with visible moisture at below-freezing OAT's?

No, but it happens often enough that I don't go messing in subfreezing clouds unless it's so cold the moisture is already thoroughly frozen. The issue is that moisture generally won't actually freeze unless it has some sort of nucleus on which to freeze or the temperature is really, really cold. Thus, in -5C air, the water is still liquid until it touches your airplane and freezes. If the water is truly frozen already, it will just bounce off. Scott Denstaedt can explain it better.

What's your personal "formula" for the icing portion of the go-no go decision?

I just don't fly into clouds or precip when the OAT is freezing or below, unless it's so cold the moisture is already frozen (usually -10C to -20C, depending on conditions, but dry snow is OK, too).

The reason I ask: I watch the weather quite a bit while I'm on the road, and I'm always hoping for flyable IFR when I get home. I've seen several instances in the last couple of months where I check ceilings/vis and see good IFR weather, then I check airmets and there are no airmets for icing... But then I check temps at the fields and winds/temps aloft and see temps slightly below freezing, and lots of PIREPs with ice. Why, in that scenario, are the airmets for ice not present???

You'll have to let Scott explain why there may not be an AIRMET for icing even when all the necessary conditions seem to be present.

 

[Ed Guthrie]

Also, I've read the "Dr. Bruce icing protocol" which talks about having surface temps above 35 (and I remember Ed G suggesting 40 for Mooneys and other laminar-flow wet-wing aircraft).

The only reason you care about surface temperatures being above freezing is that you want some sort of escape route should you encounter icing. IOW, you are hoping the ice will shed before you hit the ground. FWIW, if you are hoping for such an "out", 36 degrees F. is nothing more than unsubstantiated, wishful thinking. Here is an NTSB accident report that proves the point quite nicely (emphasis added):

IAD05LA051​

​

On April 2, 2005, at 1630 eastern standard time, a Cessna 182N, N3292S, was substantially damaged when it impacted terrain during an instrument approach to the Marion Municipal Airport (MNN), Marion, Ohio, following an encounter with in-flight icing.

The director of aviation for MNN stated during a telephone interview, that at the time of the accident, very wet, large snowflakes were falling at the airport. When he arrived at the accident site, about 5 to 6 minutes after it had occurred, he estimated that the airplane's leading edges were covered with 1 to 1.5 inches of ice.

A weather observation taken at MNN, about 7 minutes after the accident, recorded the winds as 340 degrees at 21 knots, gusting to 29 knots, 4 statute miles visibility in unknown precipitation and mist, overcast clouds at 700 feet, temperature 36 degrees Fahrenheit, dew point 34 degrees Fahrenheit, and an altimeter setting of 29.61 inches of mercury.

 

[gismo]

The only reason you care about surface temperatures being above freezing is that you want some sort of escape route should you encounter icing. IOW, you are hoping the ice will shed before you hit the ground. FWIW, if you are hoping for such an "out", 36 degrees F. is nothing more than unsubstantiated, wishful thinking. Here is an NTSB accident report that proves the point quite nicely (emphasis added):

IAD05LA051​

On April 2, 2005, at 1630 eastern standard time, a Cessna 182N, N3292S, was substantially damaged when it impacted terrain during an instrument approach to the Marion Municipal Airport (MNN), Marion, Ohio, following an encounter with in-flight icing.

The director of aviation for MNN stated during a telephone interview, that at the time of the accident, very wet, large snowflakes were falling at the airport. When he arrived at the accident site, about 5 to 6 minutes after it had occurred, he estimated that the airplane's leading edges were covered with 1 to 1.5 inches of ice.

A weather observation taken at MNN, about 7 minutes after the accident, recorded the winds as 340 degrees at 21 knots, gusting to 29 knots, 4 statute miles visibility in unknown precipitation and mist, overcast clouds at 700 feet, temperature 36 degrees Fahrenheit, dew point 34 degrees Fahrenheit, and an altimeter setting of 29.61 inches of mercury.

I agree. If the temp at 1500 AGL is above freezing by a couple degrees and it's at least that warm all the way to the ground you might shed ice in time to avoid crashing, but even that presumes that you are able to maintain controlled flight during a normal descent through that last 1500 ft. With ground temps at +1C the chances are pretty good that perfect icing conditions exist at 1000 AGL if there's enough moisture and all that +1C at ground level is going to do is make it easier to remove the ice once the plane's on the ground.

When I've picked up ice and descended into air that's barely above freezing, it usually takes several minutes for the ice to disappear from the unprotected collectors. If you spend less than that in the "warmer" air, you won't accomplish much in terms of cleaning up the airplane.

Also, in the 182 accident it sounds like the area weather might have been convective and that pretty much always means that icing will be present if the temps are below freezing.