Ice warning - in VMC, below freezing level
Why do it if it's not fun?
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Ice warning - in VMC, below freezing level
In the thread about prop de-icing, it was commented that icing can occur anywhere, any time. On that subject, I thought my experiences from today would serve as a caution to others:
The weather was 6km vis in rain, few@2500', Bkn@4000' - distinctly VMC, although not the kind of weather that is fun to fly in.
I was planning an instrument training flight - the flight detail was to fly two instrument approaches. I expected to be cleared for the approach starting at 2000', but knew that if the airfield's overhead was not free I would need to climb to 3000'. There was a small chance of being put in the hold at 4000' if we weren't the only flight doing instrument training. So a relatively small chance of the flight entering IMC.
Bearing in mind the possibility of IMC, we began reviewing the icing conditions. There was a warm front sitting fairly stationary over the airfield. Metform 215 gave the freezing level before the front as 0', and after the front as 6000', so we figured that the actual freezing level was probably somewhere between the two.
Then we reviewed the actual weather. The temperature was +3, the dewpoint +2. I interpreted that to mean that the temperature at the cloudbase would be around about +2 degrees, still warm enough to not be a problem even if we entered IMC. But I knew I would have to be cautious about accepting a clearance to climb too high.
As we were taxying out, a Jet2 airliner was landing. I decided to take the extra precaution of asking the Jet2 pilot whether he encountered any icing on the approach, and he replied Negative. Good - I was now confident that we would be able to carry out the flight without risk of icing.
On departure, we were cleared for a standard missed approach, which consists of a climb to 2000' before making a turn back to the airfield's overhead - a procedure which would mean we maintained VMC the whole time. My student took off and put the foggles on, and then began flying the missed approach procedure.
As we levelled off at 2000' and began the turn, I noticed that the rain on part of the windscreen had stopped moving up the windscreen. I studied it a little closer, and realised that it had turned to ice....... in good(ish) VMC, well below where I expected the freezing level, and despite reports from another pilot who had just landed that he had not encountered any icing.
Needless to say, at this point I aborted the flight and we landed immediately. I am not one to mess around in ice, especially not rain-ice. I have seen ice once or twice before when I've been instructed to climb higher than the freezing level (and of course I have always descended immediately, leaving the holding area if that was necessary to avoid traffic in the descent) - but this is the first time I have ever experienced totally unexpected ice. It took me completely by surprise, and I thought others might be interested in learning from it.
Incidentally, the ice did not begin melting until we were down to 1000'. Fortunately, we were VMC at the time this occured. Had we been IMC, the lowest 10nm MSA for our airfield is 1600' which would still be in icing conditions. The first descent platform on the approach for the runway in use is also 1600', and that is to be maintained for 7nm outbound, through a base turn, and only descending below that after reaching 7nm inbound - so it would have been several minutes in rain-ice before we were able to descend out of it safely. That is quite scary!
As for the cause - it seems there was a sub-zero layer, with base of somewhere around 1000', which obviously caused the aircraft skin temperature to decrease to below freezing. No clouds formed in this layer, though, despite it being several degrees below the dewpoint.
Isn't the aim of this game to fill the bag of experience before you empty the bag of luck?????
Hope this post is useful to others, and maybe even generates some interesting debate...
FFF
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The weather was 6km vis in rain, few@2500', Bkn@4000' - distinctly VMC, although not the kind of weather that is fun to fly in.
I was planning an instrument training flight - the flight detail was to fly two instrument approaches. I expected to be cleared for the approach starting at 2000', but knew that if the airfield's overhead was not free I would need to climb to 3000'. There was a small chance of being put in the hold at 4000' if we weren't the only flight doing instrument training. So a relatively small chance of the flight entering IMC.
Bearing in mind the possibility of IMC, we began reviewing the icing conditions. There was a warm front sitting fairly stationary over the airfield. Metform 215 gave the freezing level before the front as 0', and after the front as 6000', so we figured that the actual freezing level was probably somewhere between the two.
Then we reviewed the actual weather. The temperature was +3, the dewpoint +2. I interpreted that to mean that the temperature at the cloudbase would be around about +2 degrees, still warm enough to not be a problem even if we entered IMC. But I knew I would have to be cautious about accepting a clearance to climb too high.
As we were taxying out, a Jet2 airliner was landing. I decided to take the extra precaution of asking the Jet2 pilot whether he encountered any icing on the approach, and he replied Negative. Good - I was now confident that we would be able to carry out the flight without risk of icing.
On departure, we were cleared for a standard missed approach, which consists of a climb to 2000' before making a turn back to the airfield's overhead - a procedure which would mean we maintained VMC the whole time. My student took off and put the foggles on, and then began flying the missed approach procedure.
As we levelled off at 2000' and began the turn, I noticed that the rain on part of the windscreen had stopped moving up the windscreen. I studied it a little closer, and realised that it had turned to ice....... in good(ish) VMC, well below where I expected the freezing level, and despite reports from another pilot who had just landed that he had not encountered any icing.
Needless to say, at this point I aborted the flight and we landed immediately. I am not one to mess around in ice, especially not rain-ice. I have seen ice once or twice before when I've been instructed to climb higher than the freezing level (and of course I have always descended immediately, leaving the holding area if that was necessary to avoid traffic in the descent) - but this is the first time I have ever experienced totally unexpected ice. It took me completely by surprise, and I thought others might be interested in learning from it.
Incidentally, the ice did not begin melting until we were down to 1000'. Fortunately, we were VMC at the time this occured. Had we been IMC, the lowest 10nm MSA for our airfield is 1600' which would still be in icing conditions. The first descent platform on the approach for the runway in use is also 1600', and that is to be maintained for 7nm outbound, through a base turn, and only descending below that after reaching 7nm inbound - so it would have been several minutes in rain-ice before we were able to descend out of it safely. That is quite scary!
As for the cause - it seems there was a sub-zero layer, with base of somewhere around 1000', which obviously caused the aircraft skin temperature to decrease to below freezing. No clouds formed in this layer, though, despite it being several degrees below the dewpoint.
Isn't the aim of this game to fill the bag of experience before you empty the bag of luck?????
Hope this post is useful to others, and maybe even generates some interesting debate...
FFF
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Metform 215 gave the freezing level before the front as 0', and after the front as 6000', so we figured that the actual freezing level was probably somewhere between the two.
Personally I never fly (without de-ice) in freezing conditions where ice is likely unless the temp at MSA will allow removal of said ice, or I know I can become VMC in order to descend further to a positive temp (ie base higher than MSA).
Sounds like you did your homework, got caught out a bit, and have learned from it. The airmet text often gives info on sub zero layers by the way. I think the balooning forecast does also.
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Good post FFF, reinforces what a crusty old sort in his 90's told me when I was doing my IR, he had been there in the begining as an air mail pilot in the 1920-30's and a production test pilot during WWII.
What he said about Icing.
You will find it where it is.
You may go flying in a clear sky not expecting it and find it and you may be in the clouds expecting it and not find it.
What he said about Icing.
You will find it where it is.
You may go flying in a clear sky not expecting it and find it and you may be in the clouds expecting it and not find it.
Originally Posted by FlyingForFun
There was a warm front sitting fairly stationary over the airfield. Metform 215 gave the freezing level before the front as 0', and after the front as 6000', so we figured that the actual freezing level was probably somewhere between the two.
...
- but this is the first time I have ever experienced totally unexpected ice. It took me completely by surprise, and I thought others might be interested in learning from it.
...
- but this is the first time I have ever experienced totally unexpected ice. It took me completely by surprise, and I thought others might be interested in learning from it.
BTW the Jet2 pilot has the benefit of higher speed, and therefore higher skin temperature. As a rule of thumb, never trust an icing pirep from anything faster than you.
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Freezing rain under a warm front, a classic ATPL exam scenario. Sounds fairly attention getting!
Not familiar with that correlation, can you expand on it further?
Then we reviewed the actual weather. The temperature was +3, the dewpoint +2. I interpreted that to mean that the temperature at the cloudbase would be around about +2 degrees
Grandpa Aerotart
it seems there was a sub-zero layer, with base of somewhere around 1000',
I would have been surprised if you didn't pick up ice
BTW in jets we put the engine anti ice on at 10 degree C or less in visible moisture.
Why do it if it's not fun?
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All good comments - thanks!
I will freely admit that icing is something which I don't know very much about. The reason for this, I think, is that I mainly fly aircraft with no de-icing, so I tend to be very conservative and stay on the ground if I feel there's a chance of icing. (Ironically, in all the time I've spent in IMC in de-iced aircraft - which isn't all that much - I've never once encountered any ice.) And since the best way of learning about something is to experience it, that means I've still got a lot to learn about ice.
A couple of posts on this thread have raised points which I'm interested in learning more about. Dr Eckener said:
Indeed, that fits in with my observations of what actually happened. But how does it fit in with a temperature of +3 degrees? This air temperature suggested to me a freezing level of between that quoted for before the front and that quoted for after the front, which seemed to fit in with the front being overhead..... which seemed quite logical at the time, but was obviously wrong.
And HWD picked up on a quote of mine, where I said:
Whatever the dewpoint is, if you take a parcel of air from the surface, and reduce the temperature of that air to the dewpoint, it will become fully saturated and start forming clouds. So what I have done (and it seems to have worked for me in the past) is reversed this, and said that, at the base of the clouds, the temperature and the dewpoint must be equal - or, to put it another way, the temperature at the base of the clouds is equal to the dewpoint as given on the ATIS. As I say, this has worked in the past (but maybe it's not actually correct?) It may well even have been correct yesterday - I don't know because we were never as high as the base of the clouds. But there was certainly a sub-zero layer below that - and what I don't quite understand is why no clouds formed in this sub-zero layer despite it being below the dewpoint.
There is always so much more to learn in this game......
(And just to lighten the tone a bit, while we're on the subject of how much there is to learn about the general subject of met, one of my colleagues, who is also a licensed met observer, has his own method of forecasting the weather. He reckons that whatever the weather is doing today, it will do the same tomorrow. He admits that this doesn't work all the time - but he reckons it's at least as reliable as the forecasts from the Met Office!)
FFF
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I will freely admit that icing is something which I don't know very much about. The reason for this, I think, is that I mainly fly aircraft with no de-icing, so I tend to be very conservative and stay on the ground if I feel there's a chance of icing. (Ironically, in all the time I've spent in IMC in de-iced aircraft - which isn't all that much - I've never once encountered any ice.) And since the best way of learning about something is to experience it, that means I've still got a lot to learn about ice.
A couple of posts on this thread have raised points which I'm interested in learning more about. Dr Eckener said:
It doesn't tend to work like that with fronts. The warm air behind is warm and the cold air in front is cold. If you are in the cold sector the freezing level will be as per the cold sector
And HWD picked up on a quote of mine, where I said:
Then we reviewed the actual weather. The temperature was +3, the dewpoint +2. I interpreted that to mean that the temperature at the cloudbase would be around about +2 degrees
There is always so much more to learn in this game......
(And just to lighten the tone a bit, while we're on the subject of how much there is to learn about the general subject of met, one of my colleagues, who is also a licensed met observer, has his own method of forecasting the weather. He reckons that whatever the weather is doing today, it will do the same tomorrow. He admits that this doesn't work all the time - but he reckons it's at least as reliable as the forecasts from the Met Office!)
FFF
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Originally Posted by FlyingForFun
Whatever the dewpoint is, if you take a parcel of air from the surface, and reduce the temperature of that air to the dewpoint, it will become fully saturated and start forming clouds. So what I have done (and it seems to have worked for me in the past) is reversed this, and said that, at the base of the clouds, the temperature and the dewpoint must be equal - or, to put it another way, the temperature at the base of the clouds is equal to the dewpoint as given on the ATIS. As I say, this has worked in the past (but maybe it's not actually correct?)
Grandpa Aerotart
Good grief...this is truly trawling the long lost depths of my brain
FFF the Saturated Adiabatic Lapse Rate and the Dry Adiabatic Lapse rates are different....thus the wet bulb temp and dry bulb temps on the ground are lapsed at two different rates...when you get to the altitude where those two temps are the same you get the theoretical cloud base. This is why the bigger the split the higher the cloudbase IF any cloud forms at all...which is not guaranteed.
In the case of your flight with only a 1 degree split you could be gauranteed IF there was any cloud the base would be quite low...as it was on the day...2500'
Now from memory dry adiabatic lapse rate is 2 degrees/1000' and saturated adiabatic lapse rate is 1.5 degree/1000'. Remember these are not exact but rounded numbers so dumb pilots can make calculations.
On the day in question;
0'............+3/+2
1000'.......+1/+.5
2000'.......-1/-1
So you can see the cloudbase was always going to be around 2000'
The ice problem is a seperate issue...you can see that the DALR hits 0 at 1500'...so in visible moisture you will experience icing...and by jove you did at 2000'...doesn't matter that you were in VMC...the rain started freezing on your airframe and unlike the jet you were not travelling at speeds sufficiently high for air friction to warm up the skin and alleviate that process.
I can't fecking believe I just pulled that ****e out of my head after 25 yrs
What happened to you was as close to a guaranteed outcome as aviation supplies....but a great bit of experience to tuck away
FFF the Saturated Adiabatic Lapse Rate and the Dry Adiabatic Lapse rates are different....thus the wet bulb temp and dry bulb temps on the ground are lapsed at two different rates...when you get to the altitude where those two temps are the same you get the theoretical cloud base. This is why the bigger the split the higher the cloudbase IF any cloud forms at all...which is not guaranteed.
In the case of your flight with only a 1 degree split you could be gauranteed IF there was any cloud the base would be quite low...as it was on the day...2500'
Now from memory dry adiabatic lapse rate is 2 degrees/1000' and saturated adiabatic lapse rate is 1.5 degree/1000'. Remember these are not exact but rounded numbers so dumb pilots can make calculations.
On the day in question;
0'............+3/+2
1000'.......+1/+.5
2000'.......-1/-1
So you can see the cloudbase was always going to be around 2000'
The ice problem is a seperate issue...you can see that the DALR hits 0 at 1500'...so in visible moisture you will experience icing...and by jove you did at 2000'...doesn't matter that you were in VMC...the rain started freezing on your airframe and unlike the jet you were not travelling at speeds sufficiently high for air friction to warm up the skin and alleviate that process.
I can't fecking believe I just pulled that ****e out of my head after 25 yrs
What happened to you was as close to a guaranteed outcome as aviation supplies....but a great bit of experience to tuck away
Last edited by Chimbu chuckles; 14th Mar 2006 at 15:20.
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Not a bad effort after 25 years, Chimbu, but from my hazy recollection of this esoteric subject, wrong nonetheless.
The Dry Adiabatic Lapse Rate (DALR) is actually 3 degC per 1,000 feet, and is the rate at which a parcel of unsaturated air will change temperature (solely due to change of pressure) when moved vertically through the atmosphere.
It is the Environmental Lapse Rate (ELR) that, in the International Standard Atmosphere, is 2 degC (well, actually 1.98 degC) per 1,000 feet. This is the variation in temperature with height within a non-moving column of air ... and in real life as opposed to the ISA may be greater or less than the DALR. Air forced to rise where the air mass ELR is greater than the DALR will continue to rise (unstable air mass).
The Saturated Adiabatic Lapse Rate (SALR) is the rate at which a parcel of air that has become saturated (i.e. visible moisture) will change temperature if it is forced to move vertically. Its value is half that of the DALR since latent heat is released as condensation occurs. The SALR has no bearing, as far as I'm aware, on the height of the cloud base.
But as with its temperature, the dew point of the air also normally drops with increased height, since its capacity to hold water vapour at a given temperature increases as its pressure is reduced. In a parcel of unsaturated air forced to rise, the dew point decreases by 0.5 degC per 1,000 feet.
So, on the day in question, cloud formed by adiabatic cooling would have a predicted base (from a tephigram) given by the convergence of temperature and dew point, where temperature starts at +3 degC and decreases by 3 degC per 1,000 feet, and dew point starts at +2 degC and decreases by 0.5 degC. This would suggest a low cloud base of circa 400 feet with an OAT of circa 1.8 degC. Markedly different from what was actually observed.
Well, that's as I remember the theory. Hopefully, a meteorologist will be along shortly to set us straight.
The Dry Adiabatic Lapse Rate (DALR) is actually 3 degC per 1,000 feet, and is the rate at which a parcel of unsaturated air will change temperature (solely due to change of pressure) when moved vertically through the atmosphere.
It is the Environmental Lapse Rate (ELR) that, in the International Standard Atmosphere, is 2 degC (well, actually 1.98 degC) per 1,000 feet. This is the variation in temperature with height within a non-moving column of air ... and in real life as opposed to the ISA may be greater or less than the DALR. Air forced to rise where the air mass ELR is greater than the DALR will continue to rise (unstable air mass).
The Saturated Adiabatic Lapse Rate (SALR) is the rate at which a parcel of air that has become saturated (i.e. visible moisture) will change temperature if it is forced to move vertically. Its value is half that of the DALR since latent heat is released as condensation occurs. The SALR has no bearing, as far as I'm aware, on the height of the cloud base.
But as with its temperature, the dew point of the air also normally drops with increased height, since its capacity to hold water vapour at a given temperature increases as its pressure is reduced. In a parcel of unsaturated air forced to rise, the dew point decreases by 0.5 degC per 1,000 feet.
So, on the day in question, cloud formed by adiabatic cooling would have a predicted base (from a tephigram) given by the convergence of temperature and dew point, where temperature starts at +3 degC and decreases by 3 degC per 1,000 feet, and dew point starts at +2 degC and decreases by 0.5 degC. This would suggest a low cloud base of circa 400 feet with an OAT of circa 1.8 degC. Markedly different from what was actually observed.
Well, that's as I remember the theory. Hopefully, a meteorologist will be along shortly to set us straight.
Last edited by Islander2; 15th Mar 2006 at 00:16.
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the dew point decreases by 0.5 degC per 1,000 feet.
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Originally Posted by High Wing Drifter
From what I recall, that is in the PPL Confuser, completely ignored in the Thom book and discredited by the ATPL course which teaches that the dewpoint lapse rate is quite unreliable and difficult to estimate
But it's all a mystery to me. Where's that meteorologist, there's never one on hand when they're most needed.
Grandpa Aerotart
I went hunting on google and found a formula for cloudbase;
Dry bulb - wet bulb x 125m
This would give a cloudbase on the day in question of 412' but the cloudbase was 2500'.
While it has been a VERY long time since I studied ATPL Met I have a strong recollection of using the method I used a few posts back. That it seems to give a closer to accurate answer, in this case, is interesting...but the formula espoused on the website I looked at exists too.
Just goes to show what a bag of worms predicting weather is
Dry bulb - wet bulb x 125m
This would give a cloudbase on the day in question of 412' but the cloudbase was 2500'.
While it has been a VERY long time since I studied ATPL Met I have a strong recollection of using the method I used a few posts back. That it seems to give a closer to accurate answer, in this case, is interesting...but the formula espoused on the website I looked at exists too.
Just goes to show what a bag of worms predicting weather is
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Meteorology is one of those problem sciences where there is no difficulty at all for a skilled expert to explain what happened or why something was the way it was. Predicting what is going to happen is orders of magnitude harder though!
It all starts going wrong for me when they talk about "parcels of air". Air doesn't come in parcels!
It all starts going wrong for me when they talk about "parcels of air". Air doesn't come in parcels!
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All good theoretical stuff but usually meaningless in the real world.
Just look at today's skew-t chart
http://weather.uwyo.edu/upperair/sounding.html
Select Europe, GIF-Skew-T and your favourite location.
Most days, there is little resemblance to "Trevor Thom weather forecast" conditions.
Just look at today's skew-t chart
http://weather.uwyo.edu/upperair/sounding.html
Select Europe, GIF-Skew-T and your favourite location.
Most days, there is little resemblance to "Trevor Thom weather forecast" conditions.
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Originally Posted by IO540
All good theoretical stuff but usually meaningless in the real world.
Just look at today's skew-t chart
Just look at today's skew-t chart
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Most days, there is little resemblance to "Trevor Thom weather forecast" conditions.
2D