Structural Icing without visible moisture

drizzt76

drizzt76

Pre-takeoff checklist
Joined
Jul 9, 2007
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Location
Cedar Falls, IA
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Display name:
Scott M.
I have been using the Supplementary Icing Information on ADDS and found something to question my weather knowlege. There are times with clear skies or at altitudes well below the clouds and no other visible moisture reported were are significant percentage of probable (over 50%) icing. Usually there is a front passing or high humidity but would that be enough to create structural icing? I didn't think so.....
 
I've picked up trace icing in nothing more than a light mist before, clearly VFR. I didn't stick around to see if it got any worse, though.
 
I've experienced air filters clogging during flight without any visible moisture. Never have known why, but the aux air doors opening on both engines was sufficient evidence that it happened.
kgruber said:
No. You have to be in cloud or freezing rain.
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kgruber said:
No. You have to be in cloud or freezing rain.
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I do agree. And this question goes out to all; then why show icing on the Supplementary Icing Information page when clearly there is not visible moisture? My guess is..
1. That maybe if a cloud was lingering in the area you would pick up ice.
2. Its Supplementary information and not to taken without other information...ie, reports of low ceiling, AIRMET, fog....
 
wabower said:
I've experienced air filters clogging during flight without any visible moisture. Never have known why, but the aux air doors opening on both engines was sufficient evidence that it happened.
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How did you know the aux air doors opened?
 
drizzt76 said:
I do agree. And this question goes out to all; then why show icing on the Supplementary Icing Information page when clearly there is not visible moisture? My guess is..
1. That maybe if a cloud was lingering in the area you would pick up ice.
2. Its Supplementary information and not to taken without other information...ie, reports of low ceiling, AIRMET, fog....
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Both your points sound reasonable. But...what is the valid time on the image? Is this CIP or FIP?

One of the things I learned from Scott D.'s excellent Ice Is Not Nice course is that the sources for CIP and FIP are different. CIP, being based on current data, uses satellite images to eliminate areas that are free of clouds when assessing where icing might be. FIP, being a forecast, is model based, so it can only rely on predictions of where clouds will be. Sometimes the model predicts clouds, and the possibility of icing, in areas that turn out to be completely clear.

But yes... always take into account all available information. None of these products should be used alone.

Hope I got most of that right. :)
 
Anything is possible, especially over Utah in February in a T-Aerostar. If you find a better answer, let me know. I've asked everybody who might know for 30-odd years.

kgruber said:
Must be that Texas air, or more likely, carburetor ice.
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Inverted said:
You have to have some form of visible moisture.
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I have picked up a trace, VFR, temp 23F, overcast, and I did not see it until it hits the windscreen. Yes, freezing rain, but light enough it wasn't visible to me...
 
wabower said:
Anything is possible, especially over Utah in February in a T-Aerostar. If you find a better answer, let me know. I've asked everybody who might know for 30-odd years.
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Well, I actually owned a 77' 601P. And agree, anything weird is possible, except carburetor ice.

OTOH, it could have been a hundred other things. It would be a tough deduction without any feedback other than fluctuating engine gages.
 
We asked everybody including the shop in Vegas after we landed. They said it has been reported by others so they didn't seem to surprised or upset. The fluctuations :(loss of power) were consistent with the way the internal doors would spring open and then ride almost open but with less airflow. It doesn't matter now anyway.

kgruber said:
Well, I actually owned a 77' 601P. And agree, anything weird is possible, except carburetor ice.

OTOH, it could have been a hundred other things. It would be a tough deduction without any feedback other than fluctuating engine gages.
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scottd said:
You betcha! Saved me a lot of typing!
In CIP (an analysis valid in the recent past), METARs are available to help determine the cloud base. Unless it is precipitating, it's virtually impossible to have ice anywhere from the surface up to the cloud base even if the RH is extremely high (98 percent, for example). CIP can use the surface observations to figure out that there isn't visible moisture despite the high RH. Observations are so wonderful. And if it's precipitating, that makes it more challenging, but I can't give away all the secrets. ;)
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Thank you! I was using the CIP and that was my question...the CIP should know where the bases(by the observations) are and still the CIP will show icing potential below the clouds. The CIP may need some more work...
 
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