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  #51  
Old 03-05-2013, 01:26 PM
Weasel's Avatar
Weasel Weasel is offline
 
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Due to privacy concerns this post is presented as to protect the pilots identity.

Just as a data point.

I know of a pilot who has had some ice build up on a -4, -7a, and 2 different -10 airplanes.

All of the fuel tanks did not collapse and were vented per van design.

The -4 had ice up to 1/4in on it and had no pitot heat. Pitot never froze.

The -7 had ice up to 1/4in on it and had no pitot heat. Pitot froze over and airspeed indcator dropped to -0-. After 5-10min in thawing conditions the airspeed started working.

One of the -10's did not have heated pitot and airspeed never froze over but only had trace ice on that one.

The other -10 had a Dynon heated pitot and has had 1/4in of ice on it. Airspeed has never failed on it.
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  #52  
Old 03-05-2013, 01:44 PM
Bavafa Bavafa is offline
 
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FWIW, I have picked up ice twice on my 7A, once it was for a short duration but the rate was quicker than I had ever expected it. I quickly climbed higher where I would be out of the cloud. The second time, I was in the soup for a long duration as I had not much choice in scaping it but it was not much build up 1/8 or so. My pitot started freezing which my dynon pitot heat took care of it right away but no effect on the air vent to the tank.
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  #53  
Old 03-05-2013, 01:57 PM
Wayne Gillispie Wayne Gillispie is offline
 
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The more air in the tank upon freezing over, the better. At least for a few more minutes.
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  #54  
Old 03-05-2013, 02:09 PM
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ppilotmike ppilotmike is offline
 
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Thanks for the responses. I guess 1/4" line is far more capable than I thought. Perhaps the ID is small enough to keep bugs out too.
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  #55  
Old 03-05-2013, 04:05 PM
Mile High Relic Mile High Relic is offline
 
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Quote:
Originally Posted by Weasel View Post
Pitot froze over and airspeed indcator dropped to -0-.
I just read about how this can happen if the pitot freezes over slowly.

When your pitot gets completely frozen over quickly, it actually traps the pressure inside the tip of the pitot tube. Your airspeed starts to read erroneously, but not zero. As you climb with a frozen pitot, your indicated airspeed will increase. The opposite happens as you descend. You can go on quite a ways and not realize you even have a frozen over pitot tube if you are in straight and level flight.

And when you finally realize what is going on and turn on the pitot heat, the ASI will do things that make your eyes get as big as saucers.

Last edited by Mile High Relic : 03-05-2013 at 04:36 PM.
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  #56  
Old 03-05-2013, 05:00 PM
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Weasel Weasel is offline
 
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Quote:
Originally Posted by Mile High Relic View Post
I just read about how this can happen if the pitot freezes over slowly.

When your pitot gets completely frozen over quickly, it actually traps the pressure inside the tip of the pitot tube. Your airspeed starts to read erroneously, but not zero. As you climb with a frozen pitot, your indicated airspeed will increase. The opposite happens as you descend. You can go on quite a ways and not realize you even have a frozen over pitot tube if you are in straight and level flight.

And when you finally realize what is going on and turn on the pitot heat, the ASI will do things that make your eyes get as big as saucers.
Correct. and I will add. If it does drop slowly to -0- it probably indicates you have a leak in the pitot tube somewhere
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RV-4 715hr Sold
RV-10 "School Bus" - +1600hr counting
Fisher Classic Cassler Power VW sold
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  #57  
Old 03-05-2013, 09:46 PM
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Dave Bernard Dave Bernard is offline
 
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Default Ice at really low temps

In a former life as a CFII I too used to believe that once you got below a certain temp, you were not susceptible to icing... I even explained that to the examiner giving me my CFII rating to which he replied "You seem like a smart, experienced aviator!"

Fast forward a few years, I was holding above Jackson Hole at 15000' for 30-40 minutes in IMC with a temp around -35C (In a twin turboprop equipped with all the icing bells and whistles)- no worries, too cold for ice, right? I noticed my airspeed beginning to drop off and some light rime on the wing- Both my airspeed indicators slowed to 90 knots or so (slightly above stall in this airplane) but the GS was still showing 180.

I diverted to Idaho Falls and shot the ILS down to mins with no airspeed indications (not bragging, but it made for a great job interview story!)- turns out the aircraft had an ingenious design where the heated pitot tube lines route through the unheated forward baggage compartment, which is where the crystals refroze and created the blockage.

1st moral of this story: Even approved, somewhat sophisticated aircraft can get into trouble in any icing

2nd moral of this story: ALWAYS assume that you WILL get ice in visible moisture with temps near the freezing point and below!
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  #58  
Old 03-06-2013, 05:18 AM
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pierre smith pierre smith is offline
 
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Quote:
Originally Posted by Dave Bernard View Post
...........1st moral of this story: Even approved, somewhat sophisticated aircraft can get into trouble in any icing

2nd moral of this story: ALWAYS assume that you WILL get ice in visible moisture with temps near the freezing point and below!
Great advice...and thanks. That's why I stayed out of the clouds on the way home.

One other thought...this post has generated quite a bit of participation and confessions of some of you guys on here

I, for one, have learned quite a bit and will implement the fuel check valve system and a heated pitot. Aaaah, confession is good for the soul

Best,
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  #59  
Old 03-06-2013, 08:37 AM
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ppilotmike ppilotmike is offline
 
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Default +1 Pierre

This kind of stuff also helps new builds. I will also be utilizing a check valve setup for alternate vent air. I don't plan on flying into icing conditions, but then again, I've never met a pilot who did...
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  #60  
Old 03-06-2013, 03:23 PM
douglassmt douglassmt is offline
 
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Location: Missoula, MT
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Default Why planes?

Can someone explain why we almost never ice up our vehicles although we drive in visible moisture around and below freezing all the time? (at least up north we do)

The one time I picked up some ice on my -10 it accumulated about 1/8" on the leading edges and a short distance back on every surface facing relative wind, including leg fairings, wheel pants, HS, wings, air inlets. The windshield got a little rime ice but I could see forward OK, no ice on any other windows. Also the first 10-15" out from the spinner on the prop iced up, leading edges only. I could see through the moisture/virga (it was around 31 F) to the airport and had clear air behind so I was confident that I would be OK. I was through it in 5-10 minutes and landed immediately, never noticed any speed or control system changes. Some of the ice stayed on in my unheated hangar for several days.

Just wondering if the science of ice accumulation has to do with pressure drop, pressure increases, or something around those curved surfaces facing into the wind. And/or, does it increase with RV speeds?
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