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What am I seeing on the leading edge?

snopercod

snopercod

Well Known Member
Last year I was cruising along at 12,500' in clear weather over Colorado and started seeing this shiny gray stuff on the leading edge. It kind of looked wet. Needless to say I was concerned, so I pulled my iPad - the one I was using to navigate - out of its mount and snapped this photo. The camera didn't focus properly, but hopefully someone will be able to guess at what I was seeing. Was it just an optical phenomenon or ice? The OAT at the time was 5C and there was no visible moisture (except in the far distance), so how could it have been ice? Could it have been a thin layer of "fog" due to the sudden decrease in pressure? The wing itself is white. What was going on?

Mp2Jyz.jpg


I posed this same question on the Lancair forum last year but never got any meaningful responses. There is a lot more experience over here so I hope y'all won't mind...
 
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Lighting?

From the mountains, looks like it could be shadow.... maybe it is effectively, a little moisture followed by shadow.
 
not a shadow

Thanks for responding. Yes, it does like a shadow in the photo but trust me, it wasn't the shadow of the top of the wing. Maybe it could have been a shadow of the condensation - that's a good idea. I don't know what the dewpoint was, but from the clouds in the background, it must have been close to the OAT. It seems like the "gray stuff" was located right where the pressure on top of the airfoil was at a minimum.

veldist2.gif
 
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What i think

I have seen this before at a window seat om a airline. All i can think of is the water is starting to condense but instead of seeing a white cloud it is a reduction of light geting to the surface, like shadowing.
 
Did it look something like this?

Without the wingtip vortices, and viewed from the cockpit.
mr7jg7.jpg


If your wing had rivets in it, this might not happen ;):D

Cheers, David
 
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no rivets (wink)

koupster said:
If your wing had rivets in it, this might not happen ;):D
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LOL! Nah, it's all glued together, but I must have built it straight and true because the gray area was pretty constant all along the span. If I ever see this again, I'll pull some G's and see if the gray area expands. It should, right? I was stupid because I had my Garmin Virb video camera mounted right next to my head. I could have easily removed it and video recorded the phenomenon.
 
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It's a cloud;

just like a cap cloud on a mountain. Pressure drops as air passes over the wing and temperature drops below the dew point.

Cheers, David
 
That must have been what was happening. It looked for all the world like something wet was covering the leading edge. One correction, though. I took the photo near Grand View, ID, just past Mountain Home. I had spent the night in Brigham City and was on my way to Medford, OR. Man, that's some desolate country out there :eek:
 
Likely a very thin layer of ice is my guess. I've seen traces on my wing in clear air. On your picture there is some visible moisture (clouds) in the vicinity.
 
That's what I was worried about.

Vlad said:
Likely a very thin layer of ice is my guess.
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That's what I was concerned about. I've never flown in icing conditions and don't know what it looks like (other than photos). Neither did I know that one could experience icing in clear weather. My laminar flow airfoil doesn't tolerate icing well so I was real concerned. I didn't notice any degradation in my 165 Kt TAS at the time so I just shrugged and pressed on. I reviewed the ATC audio from my Garmin Virb and they were reporting a -1 deg C dewpoint on the ground. Thanks for your opinion.
 
OAT calibration unknown

BobTurner said:
If your OAT is accurate...
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I guess I need to do a single-point calibration check on my Davtron 5-in-1 to find out. Dipping the probe in some ice/water mix shouldn't be too difficult. That will give me something to do while I'm suffering through all this non-flying weather. Thanks for the idea.
 
Not convinced it's ice

I assume we are talking about the white area on the leading edge.

I have seen plenty of icing and that doesn't really look like icing. Notice how it doesn't come all the way to the root for one thing. It looks to stop about half way. Ice would be on all of the leading edge. The air in Colorado is pretty dry. Doubtful you would collect ice in that flight condition from what I'm seeing in the photo.

Again, if we are talking about the white area I would guess it's a reflection of the clouds in the distance. Light does funny things.

The plane I normally fly has heated (shiny) leading edges. When we go through clouds and the light is right you would swear there is a layer of ice when in reality it is simply a reflection.

Hard to say based on that picture.

Did it disappear all at once? Ice would break off in sections.
 
runt said:
Did it disappear all at once? Ice would break off in sections.
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Just as a point of fact: aircraft icing will indeed break off in sections particularly as it starts to melt. However, if it remains frozen while flown through clear dry air, it will tend to sublimate. Clear air sublimation is the conversion of matter from a solid (ice) directly to a gas (water vapor). Happens faster in jets, but more slowly in slower aircraft because slower speeds lesson the wind/ice friction.

I now return you to your originally scheduled topic. I can't tell what your photo is showing, sorry.
 
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OAT calibration OK

This morning I did a single point calibration of my OAT sensor/display. I dipped the probe in a bath of crushed ice and water, and the Davtron indicated 32F/0C. My Fluke read 32.1F. At ambient the Davtron read 43F and the Fluke read 43.1F. So the 5C reading was probably accurate. BTW, my OAT probe is located under the wing, well away from the cowling exit air. I wish I had known about Skew-T charts last year :confused:
 
snopercod said:
This morning I did a single point calibration of my OAT sensor/display. I dipped the probe in a bath of crushed ice and water, and the Davtron indicated 32F/0C. My Fluke read 32.1F. At ambient the Davtron read 43F and the Fluke read 43.1F. So the 5C reading was probably accurate. BTW, my OAT probe is located under the wing, well away from the cowling exit air. I wish I had known about Skew-T charts last year :confused:
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Please keep in mind that often times location of the probe matters and can change the temp reading during flight while a static reading can be accurate.
 
compressibility?

Bavafa said:
Please keep in mind that often times location of the probe matters and can change the temp reading during flight while a static reading can be accurate.
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Are you referring to "compressibility"? Most of my life I flew Cessnas and Pipers and at those speeds, compensating for mach number really wasn't necessary. :D Even in my Lancair, I never even considered adjusting indicated OAT for mach number... until you brought it up. Using the formulas found at Ed Williams Aviation Formulary - if I did the math correctly - I was flying along at mach .25 on that flight !!! (Yes, my chest is definitely puffed out now :D ) Because of my awesome speed ;), the 5C temperature reading on my Davtron needed to be adjusted down by 3.5C! So the OAT really was close to freezing. I learned something here. Thanks!

Mach numbers, true vs calibrated airspeeds etc.

Mach Number (M) = TAS/CS
CS = sound speed= 38.967854*sqrt(T+273.15) where T is the OAT in celsius.
TAS is true airspeed in knots.​

Because of compressibility, the measured IAT (indicated air temperature) is higher than the actual true OAT. Approximately:

IAT=OAT+K*TAS^2/7592

The recovery factor K, depends on installation, and is usually in the range 0.95 to 1.0, but can be as low as 0.7. Temperatures are Celsius, TAS in knots.​

Also:

OAT = (IAT + 273.15) / (1 + 0.2*K*M^2) - 273.15​
 
snopercod said:
Are you referring to "compressibility"? Most of my life I flew Cessnas and Pipers and at those speeds, compensating for mach number really wasn't necessary. :D Even in my Lancair, I never even considered adjusting indicated OAT for mach number... until you brought it up. Using the formulas found at Ed Williams Aviation Formulary - if I did the math correctly - I was flying along at mach .25 on that flight !!! (Yes, my chest is definitely puffed out now :D ) Because of my awesome speed ;), the 5C temperature reading on my Davtron needed to be adjusted down by 3.5C! So the OAT really was close to freezing. I learned something here. Thanks!

Mach numbers, true vs calibrated airspeeds etc.

Mach Number (M) = TAS/CS
CS = sound speed= 38.967854*sqrt(T+273.15) where T is the OAT in celsius.
TAS is true airspeed in knots.​

Because of compressibility, the measured IAT (indicated air temperature) is higher than the actual true OAT. Approximately:

IAT=OAT+K*TAS^2/7592

The recovery factor K, depends on installation, and is usually in the range 0.95 to 1.0, but can be as low as 0.7. Temperatures are Celsius, TAS in knots.​

Also:

OAT = (IAT + 273.15) / (1 + 0.2*K*M^2) - 273.15​
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What you said. Also the physical location of the probe in relation to the passing air. Let's exaggerate and pretend the probe is installed right at the egress air passage of the cowl, I would imagine the hot air existing the cowl will have an effect on the accuracy. In RVs, many people report false reading when the probe is installed in the NACA vent.

In regards to the shade in the pix, my initial reaction was that was going to be ice but the clear/dry air rejects that idea. Also, three times that I had minor ice on my plane, it started at the wing root area but don't know enough to say if that is common or not.
 
snopercod said:
the 5C temperature reading on my Davtron needed to be adjusted down by 3.5C! So the OAT really was close to freezing.

Still wondering where the moisture is coming from. Dry air doesn't usually cause icing.

I'm still going with the reflection theory. I know it isn't possible but it's too bad you don't have a better picture. If you were picking up ice in that air I would be interested in knowing why.
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Ice

Maybe what little moisture there is is turning to ice in the airstream casuing shadowing. But there is not enough moisture to cause the ice to stick. I bet you are making a small ice cloud with the particles so small they are following the airstream instead of impacting the wing. JMHO
 
must have been just a little moisture

runt said:
Still wondering where the moisture is coming from. Dry air doesn't usually cause icing.
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That's exactly what surprised me at the time. In retrospect, there must have been some moisture in the air as evidenced by the clouds in the background. I was flying at just about the altitude of the distant clouds, too, which tells me that the temp at my altitude must have been close to the dew point. Whether what I was seeing was ice or water, I guess I'll never know; All I can say is that the shiny gray stuff looked like it was actually coating the wing so I'm guessing it was an extremely thin layer of ice which - as someone posted earlier - immediately sublimated.

My Lancair isn't very stable; if I take my hand off the stick in cruise, I have only about 10 seconds before it wants to roll off in a spiral. I feel like I was lucky to get the photo at all, considering I was flying with my left hand and holding the iPad with my right while trying to aim the camera and tap the "button" with my thumb to snap the photo. I sure wish I had an autopilot for future situations like this ;) The iPad tagged the photo as taken near Mountain Home, ID, at a place called Grand View. I've been going through my cockpit videos trying to locate that segment, but haven't found it yet. If I find the video, it should show whether my airspeed dropped off at the time which would be one more piece of the puzzle.
 
Found the video

After going through hours and hours of shaky cockpit video, I finally found where I removed the iPad from its mount and took the photo of the mystery substance on the leading edge. For a while there, I thought I had lost my mind. First, the video is shaky because the mount that comes with the Garmin Virb is very flimsy. Since then, I've added a diagonal brace to steady the video. Second, the video was taken roughly 20 NM West of Twin Falls VOR, also known as "the middle of nowhere". I've synced the GPS data with the video as best I could, but there are no landmarks out there to allow me to do a good job of that. I didn't notice any airspeed dropoff as the "ice" built up, so maybe it wasn't ice. I had roughly a 15 KT headwind at the time, heading for Medford, OR. About an hour prior, there were thin puffy clouds at 12,500'.

Taking photo with iPad
 
Clear air

I never liked it during my Power add-on training, and ground school that I was always taught that ice could not form unless visible moisture was present.

I was flying my glider near Mono Lake (California) near 18k and picked up enough ice to easily be seen. I was flying with a friend, and he iced up too. We continued to fly, but dropped in altitude until we became ice free again.

Cloudbase that day was about 22k, and their were cumulus clouds within 5-10 miles, but none where we were.

We had been up high for awhile, and my guess is that our wings were colder than the air we were in, so that might explain what occurred, but that's just a guess.

I would love to know why they teach it isn't possible, when it does in fact occur, albeit fairly rarely.
 
Me too

cdeerinck said:
I was always taught that ice could not form unless visible moisture was present.
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Me too, and also that you only need carb heat if visible moisture was present and we all know that's not necessarily true. Many years ago when I got my PPL, my instructor advised me that "This is only a ticket to learn" and boy was he right. I learned a lot over the years but flying an Experimental aircraft these last two years has really upped my learning curve. My guess is we were taught about "visible moisture" because the truth would have been much too complicated to explain. Most instruction is designed to teach the student how to fly the airplane, not how to become a meteorologist; We all pretty much have to learn that stuff on our own as we take longer and longer trips. At this late date I've been trying to teach myself Skew-T Charts, but there is so much information in those that I don't think I'll ever understand them completely. Thankfully, there are some simplified graphics that depict temperature and relative humidity at various altitudes. Those, I can understand. Here's a sample "RAP Chart". (I think RAP stands for Rapid Refresh.) Well, that which doesn't kill us...
 
OAT sensors read TAT (total air temp). This needs to be converted to SAT (static air temp) but it is TAT that determines whether or not you are at risk of ice sticking to the airframe. I was involved in certification trials in natural ice on the CSeries and we had to fly as slow as possible in the icing cloud to get the TAT down low enough for the ice to stick to the plane. It woulf form, but then break off right away.

You have to be in some serious cloud to get ice on the airplane. I can't tell much from your photo but I suspect it is a reflection from condensation. You had the sun at just the right place to see it.
 
BobTurner said:
Lower, yes. 5 C (9 F) lower? Unlikely.
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I have seen ice form on the outside of a carb during a run-up on an 80 deg day. That's a 50 deg drop. A carb has a pressure drop just like a wing. We have all seen water droplets coming off fighters in turns on humid days. The temp drop can be massive!
 
sblack said:
I have seen ice form on the outside of a carb during a run-up on an 80 deg day. That's a 50 deg drop. A carb has a pressure drop just like a wing. We have all seen water droplets coming off fighters in turns on humid days. The temp drop can be massive!
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Most of the temperature drop in a carb (which, as you note, can be large) is due to the evaporation of fuel. Not directly comparable to airframe icing.
 
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