Clear Air Ice

Below freezing temperature + moisture = ice. I re-learned this basic lesson on a “VFR” flight last week to visit family for Thanksgiving to Oakland/Troy (KVLL).

Departed 3CK east to the lakeshore, level at 1700 feet, then turned south along VFR corridor. Head winds of about 45 knots put my ground speed at a painful 95 knots. The Hancock and Willis towers took a long time coming ... then we contacted KGYY tower requesting transition.

Clearing Gary, as I followed the shore about a mile off the coast and turned southeast, I noticed it became harder to see outside. Getting into scud? Then I noticed that it was actually ice frosting over on the windshield. In a matter of minutes the windshield became fully obscured. And I saw the wings were picking up ice too.

Because we don't have the option to scrape it away ...

I know you are trained to make a 180 and get out of IFR conditions. But it wasn't really IFR! The XM Weather display on the Garmin 496 did not indicate any precipitation over our location. At this point I was clear of KGYY airspace and listening to South Bend ATIS. I hit their approach frequency on the Garmin 430, and after identifying us, stated we were picking up light rime ice in clear air. KSBN approach provided vectors to runway 11 for a possible precautionary landing.

To my left was the soup of Lake Michigan, the front was fully obscured and I only had to navigate by seeing to the right. Thankfully there was land. Can you imagine if we had been flying straight ahead, we would have had an IFR experience in VFR conditions?

About this time I turned on defrost to full blast. It quickly punched a hole about 4-5 diameter width, and now I could see a little out the windshield. As we headed over to the land, the de-icing accelerated. Soon it was clear off both wings and the windshield. We cancelled our approach to South Bend, and resumed our flight to KVLL.

Looking back, I wondered what could have caused this. Then my non-pilot wife pointed out "you were flying too close above those smoke stacks". I was? All those chimneys had been putting out a lot of heated moisture collectively in that area. At our altitude, the OAT read 28 degrees F. Well, that's below freezing. That explains it. As we left that area, there was no more heated air rising and turning to ice.

Lessons: Pay attention to OAT at all times, particularly in the winter. Keep an eye out for conditions that could conspire to cause low visibility, then distinguish what could be causing that -- scud or frosting over? As always, don't be afraid to state your situation and ask for help with ATC.

Following the book and making a 180 could have made it worse, as we would continue to fly in those conditions for a lot longer. Exercise caution and prepare to land as quickly as possible. After all, a frosted windshield is better to look at from the outside.

1 comment:

Parth said...

And don't forget pitot heat. You don't have to wait to see ASI drop off, turn pitot heat on as a precaution.

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