Flying in snow

[Pace]

ShyTorque

This is one of my earlier postings

Snow building up in/on the induction intakes/filters can have you reaching for hot/alternate air. The Partenavia and Apache are 2 types I've had this issue with.

Granite City Flyer

I almost posted this exception but decided not too. Yes if the snow has nowhere to go ie it hits a flat surface like a grille and doesnt pass through there is a danger that it will block the inlet.

Most aircraft dont have that sort of setup but ones which have a grill over something like an engine intake could block with snow if its unheated. The Partenavia did come to mind but that aircraft doesnt have a good reputation in any icing not just snow.

Pace

The fact is that this is not going anywhere fast and is becoming pointless and personal so its better we beg to differ and leave it and no hard feelings

Pace

[[email protected]]

Having just found this thread from another one it has been an interesting read but mostly ignored the original posters question.

He was not talking about Pace's pure snow, he was talking about the stuff we had in UK which, according to all the met reports and forecasts I read was mostly RASN - mixed rain and snow which can (even according to Pace's theories) adhere to the airframe because there is visible moisture.

So for the private pilots out there in UK - do not believe you can go flying in snow (the stuff we get here) and not expect to encounter icing conditions.

One assertion of Pace's which I do think is flawed is that only liquid will freeze - water vapour can sublimate straight to ice as hoar frost or, in fact, in the formation of snow.

I think the point was made to emphasise that ice must melt first before it can re-freeze onto an airframe so yes, a snowflake must melt before it becomes an icing risk but that is what RASN is all about.

Anyway it's nearly Summer

[bookworm]

He was not talking about Pace's pure snow, he was talking about the stuff we had in UK which, according to all the met reports and forecasts I read was mostly RASN - mixed rain and snow which can (even according to Pace's theories) adhere to the airframe because there is visible moisture.

I think you're missing the point. For ice to adhere to the airframe, you need an airframe that is at a temperature below freezing, otherwise the ice just melts off very shortly after it adheres by heat transfer from the airframe. Generally, the airframe will only be below freezing if the air temperature is below freezing. And if the air temperature is below freezing, you'll get snow, not sleet (RASN, in the UK sense of "sleet").

So why might there be liquid precip but the airframe still be below freezing? One possibility is an inversion, where the precip has passed through an above-freezing layer before falling into colder air again. But that will be freezing rain or ice pellets, not sleet. Another possibility is that the airframe is still cold, having come quickly from a colder environment with e.g. wings full of cold fuel that take a while to warm up to ambient. But again that's not an issue about snow, as the icing that you get in such conditions is likely to be as bad if not worse if the precip type is rain rather than snow.

[Droopystop]

Generally, the airframe will only be below freezing if the air temperature is below freezing. And if the air temperature is below freezing, you'll get snow, not sleet (RASN, in the UK sense of "sleet").

I wish I had kept the METARS but across northern Scotland last week, there was the scenario of SN and +ve air being reported simultaneously.

It is interesting that there are two very experienced IFR helicopter pilots who question the wisdom of discarding snow as a potential form of icing. I agree that the formal theory about snow means it won't stick. But in this country whilst we get SN forecast there is a risk it is not really SN, simply because it doesn't really get properly cold in the UK (unless you live in Braemar).

As a helicopter pilot myself with a similar number of hours to PACE, all of which are below 5000' and 150kts I have seen what looks like snow stick. In the context of this forum and the original post, I would say that SN on a UK TAF/METAR doesn't necessarily mean icing, but nor does it mean icing won't happen.

Whilst it may seem that helicopters are snow magnets, we probably have more in common with the Private fleet than it would first seem - similar speeds and operating altitudes.

[[email protected]]

Bookworm - but you don't have an airframe temperature gauge and the metal skin of your aircraft in a 100 kt wind (cruise) is likely to be lower than the OAT if you have melted snow evaporating from it. Add to that the variations in temp that will naturally occur in and around the base of convective cloud and you cannot in any way be sure that just because you see snow there is no risk of icing.

[bookworm]

Bookworm - but you don't have an airframe temperature gauge and the metal skin of your aircraft in a 100 kt wind (cruise) is likely to be lower than the OAT if you have melted snow evaporating from it.

Well there's a mechanism, at least. I'm not convinced that the latent heat of evaporation is an issue -- otherwise we'd see the same cooling effect in rain, and I don't think that's observed. There's also the latent heat of fusion of the snow as it melts that can take some heat away from the airframe.

Add to that the variations in temp that will naturally occur in and around the base of convective cloud and you cannot in any way be sure that just because you see snow there is no risk of icing.

And I would agree with that. But the point is that the circumstances have to be quite special for SN or RASN to pose a significant icing risk. The mere presence of RASN doesn't have to set off alarm bells like FZRA should.

[bookworm]

I wish I had kept the METARS but across northern Scotland last week, there was the scenario of SN and +ve air being reported simultaneously.

Isn't the reverse what you want to demonstrate your point -- that RASN can occur in negative temperatures? I don't think there's much doubt that SN can occur when the OAT is above zero -- it just hasn't had time to melt yet.

Let me help with the stats. Here are the northern Europe (codes beginning with E) METARs containing sleet for the last 30 days.

Code:

      wx       | temperature | count
----------------+-------------+-------
 -RASN          |          -2 |    3
 -RASN          |          -1 |    8
 -RASN          |          0 |  238
 -RASN          |          1 |  645
 -RASN          |          2 |  346
 -RASN          |          3 |  107
 -RASN          |          4 |   21
 -RASN          |          5 |    1
 -RASN          |          6 |    3

Code:

       wx       | temperature | count
 ----------------+-------------+-------
 RASN           |          -1 |    6
 RASN           |          0 |   35
 RASN           |          1 |  116
 RASN           |          2 |   87
 RASN           |          3 |   24
 RASN           |          4 |    6

The vast majority of RASN occur in positive temperatures. Compare that with FZRA

Code:

      wx       | temperature | count
----------------+-------------+-------
 -FZRA           |         -10 |    2
 -FZRA           |          -9 |    3
 -FZRA           |          -8 |   12
 -FZRA           |          -7 |   11
 -FZRA           |          -6 |   16
 -FZRA           |          -5 |    8
 -FZRA           |          -4 |   15
 -FZRA           |          -3 |   11
 -FZRA           |          -2 |   14
 -FZRA           |          -1 |   38
 -FZRA           |          -0 |   38
 -FZRA           |          1 |    1
 -FZRA           |          2 |    1

Is it worth paying attention to icing when there's RASN around? You bet it is. But most of the time, snow melting into sleet is not a problem.

[[email protected]]

Bookworm - you run the risk of arguing the same sort of semantics as Pace did about 'pure snow'.

If you want to go flying in the snow, crack on but it comes with several health warnings:

Poor visibility and the possibility of whiteout (can't distinguish land from cloud). This makes trying to continue VFR much more risky and if you have no TCAS or radar service makes deconfliction with other traffic far more difficult.

Possible icing in the RASN.

A high probability of icing in the cloud that is producing the snow/sleet, greatly reducing your IFR options. Once you are in it your airframe will be below zero amongst a mixture of supercooled droplets and snow.

All the ground based issues like slippery taxiways, runways, banks of slush and snow etc.

If you want to risk all that just to fly in falling snow...................

[bookworm]

This makes trying to continue VFR much more risky

Quite why anyone would attempt to fly VFR in snow is beyond me! That, and the ground risks you cite, are not at issue here.

A high probability of icing in the cloud that is producing the snow/sleet, greatly reducing your IFR options. Once you are in it your airframe will be below zero amongst a mixture of supercooled droplets and snow.

No, that's the whole point. A lower probability of icing in the cloud that is producing the snow/sleet, because the presence of snow is an indicator of glaciation and the probable absence of supercooled water. If you have a choice between a cloud in which it's snowing, and a cloud at the same temperature in which it's yet not snowing, pick the one that's snowing! And while your airframe may be below zero, it may not be even in dry snow, and it unlikely to be below zero in snow that has already started to melt to make RASN.

[ShyTorque]

If you have a choice between a cloud in which it's snowing, and a cloud at the same temperature in which it's yet not snowing, pick the one that's snowing! And while your airframe may be below zero, it may not be even in dry snow, and it unlikely to be below zero in snow that has already started to melt to make RASN.

Bookworm, in view of the fact that you won't know the temperature of a cloud until you're in it, how do you know which is the "best" cloud to fly in?

Before you answer the question, perhaps you would kindly outline your practical experience of winter operations and snow flying and what formal /paper qualifications you have to make such a statement?

[bookworm]

Before you answer the question, perhaps you would kindly outline your practical experience of winter operations and snow flying and what formal /paper qualifications you have to make such a statement?

Woof. Perhaps my only paper qualifications and practical experience of winter operations are that I fetch the paper for my master in the snow? Would that make you less likely to believe me than if I claimed to have a PhD in physics and fly a booted light-twin around throughout the year? Should it? If you don't believe what I write about icing, why don't you do your own research?

in view of the fact that you won't know the temperature of a cloud until you're in it, how do you know which is the "best" cloud to fly in?

You're twisting the way I chose to express that. The simple point is that the higher icing potential comes from clouds that have high supercooled water content before glaciation, not the ones that are already glaciated and shedding snow at high rate.

There are no certainties when it comes to icing, and any flight in temperatures around freezing is going to have to pay significant attention to conditions. I've not, at any point in this discussion, doubted your practical observations that snow can pose a hazard, particularly to your helicopter. But, perhaps based on the limitations that your flight manual imposes, you seem to be extrapolating to overemphasise the effect of snow on other aircraft. There are other weather phenomena to be more concerned about.

[[email protected]]

Bookworm

No, that's the whole point. A lower probability of icing in the cloud that is producing the snow/sleet, because the presence of snow is an indicator of glaciation and the probable absence of supercooled water.

The weakness in your argument is the use of the word probable here - the fact is that to produce snow in the first place you must have supercooled droplets so that the water can evaporate from the droplets and form ice crystals on the freezing nuclei (the Bergeron-Findeisen process).
So, since both will exist in varying amounts depending on the conditions you cannot say with any certainty that the snow cloud will be a lesser icing hazard than the no-snow cloud.

The simple point is that the higher icing potential comes from clouds that have high supercooled water content before glaciation, not the ones that are already glaciated and shedding snow at high rate.

again - you talk about differences in potential which is probaby true but is not the hard and fast 'snowing therefore no icing' rule that you advocate.

And then your capitulation

There are no certainties when it comes to icing, and any flight in temperatures around freezing is going to have to pay significant attention to conditions.

which is what Shy and myself have been trying to get through and why we dislike ridiculous simplifications llike 'it's snowing so there is no risk of icing' which may influence the less experienced on this forum.

[bookworm]

but is not the hard and fast 'snowing therefore no icing' rule that you advocate

And if you read what I've actually written, rather than what you'd like me to have written, you'll see that I've not suggested anything of the sort.

I entered this debate because I objected to an assertion in a cited article that "wet mushy snow" necessarily contains supercooled water. Most of the time RASN does not: it's simply snow that has melted. We seem to be agreed that encountering liquid water precip, whether pure RA or RASN, with an airframe temperature below freezing, has a significant potential for icing.

Reading what you've actually written, I'm probably guilty of similarly misinterpreting you too in what I made of the following:

I think the point was made to emphasise that ice must melt first before it can re-freeze onto an airframe so yes, a snowflake must melt before it becomes an icing risk but that is what RASN is all about.

That RASN is "all about melting" is fair enough. That it's "all about re-freezing" (because it contains supercooled water) is not. I think you simply meant the former, didn't you? In the words that I previously quoted from you, you wrote "can ... adhere to the airframe" not "will". That's perfectly fair.

Finally...

The weakness in your argument is the use of the word probable here

The weakness in my argument is really in my use of "absence". Can we agree that glaciated cloud has a significantly lower supercooled water content than before glaciation?

[Pace]

Crab

Just to clear up a few points i think there is confusion as to what snow is what sleet is and what rain is? Snow is NOT sleet but a specific structure of dry ice particles which contain NO moisture. the particles are so fine that any warming converts those tiny structures into water and ice but that is NO longer snow.

You may see what appears to resemble snow to the pilot but it is likely to be sleet.

Yes if there is a mixture of rain ie visible moisture and snow flakes the rain can form ice but that is the case regardless of the snow.

Dont really want to start this one up again but if anyone can explain the science of snow forming ice please feel free

Pace

[Captain Smithy]

This has been an informative discussion which I have followed with interest; perhaps one of the better uses of PPRuNE.

Personally though, after my own experience mentioned earlier, I don't fancy flying in SN again, unless I was IFR in a de-iced machine; even though it was accidental it was a little foolish attempting VFR through SHSN.

P.S. Thanks for the Wiki link earlier, bookworm. I got a bit of a laugh at that.

Smithy

[ShyTorque]

Bookworm,

Please don't jump so strongly to the defence; I noted that your profile doesn't even say if you fly or not and I think it is important to know from which corner folk are arguing from.

I have already done my own research, some years ago and every winter. I've been required to operate in European winters since 1977. The aircraft I've flown most have had no icing clearance or de-icing equipment fitted. They have also, with one exception, been unpressurised and therefore unable to climb above snow or icing cloud. My job has been therefore "in it" or close to it.

A presentation I was asked to give on aircraft icing problems in the mid 1980s was purloined and used by the RAF for some years afterwards, most particularly by 33 (AMF) helicopter squadron, who operated in Norway in the winter. The brief's probably been replaced since Powerpoint etc came along but it might still be lurking in a box somewhere at RAF Odiham.

Although I no longer consider myself FW current I do have FW experience, light SEP and single jet, btw (hence my profile saying "QFI" as well as "QHI". I also hold a CPLA.

Before I bow out of this discussion, I repeat my sole purpose on this thread, which is to put out my own advice that generalisations, with regard to aircraft operating in falling snow and cold cloud conditions are best avoided.

Each manufacturer who wishes to obtain an icing clearance for a particular aircraft is required to test fly the aircraft and prove that it is safe to do so. There is NO "one size fits all" solution to safe operations in winter conditions.

[[email protected]]

Pace - just for you I have kept it simple

Bergeron Process - Geography For Kids - By KidsGeo.com

But if you google 'bergeron process' you will find plenty of answers - wikipedia has enough science for you I suspect.

You will see that, as I have indicated in previous posts, supercooled droplets (of varying sizes and formed on the condensation nuclei) co-exist with ice crystals (formed on the freezing nuclei) and the water evaporates making the ice crystals grow (into snow).

In the UK, sleet (referred to as RASN) is snow that has melted on its descent to Earth and accompanied by snow that has yet to melt. In the US, sleet is snow that has melted and re-frozen into solid, non crystalline particles.

Hope this helps.

[Pace]

Crab

have reposted this for you especially note the comments re sleet.

Pace

Quote:
A Snowflake Primer
... The basic facts about snowflakes and snow crystals ...

Snowflakes and snow crystals
Snowflakes and snow crystals are made of ice, and pretty much nothing more. A snow crystal, as the name implies, is a single crystal of ice. A snowflake is a more general term; it can mean an individual snow crystal, or a few snow crystals stuck together, or large agglomerations of snow crystals that form "puff-balls" that float down from the clouds.

The structure of crystalline ice
The water molecules in an ice crystal form a hexagonal lattice, as shown at right (the two structures show different views of the same crystal). Each red ball represents an oxygen atom, while the grey sticks represent hydrogen atoms. There are two hydrogens for each oxygen, so the chemical formula is H2O. The six-fold symmetry of snow crystals ultimately derives from the six-fold symmetry of the ice crystal lattice.


Quote:
Snowflakes grow from water vapor
Snowflakes are not frozen raindrops. Sometimes raindrops do freeze as they fall, but this is called sleet. Sleet particles don't have any of the elaborate and symmetrical patterning found in snow crystals. Snow crystals form when water vapor condenses directly into ice, which happens in the clouds. The patterns emerge as the crystals grow.
The simplest snowflakes
The most basic form of a snow crystal is a hexagonal prism, shown in several examples at right. This structure occurs because certain surfaces of the crystal, the facet surfaces, accumulate material very slowly (see Crystal Faceting for more details).
A hexagonal prism includes two hexagonal "basal" faces and six rectangular "prism" faces, as shown in the figure. Note that a hexagonal prism can be plate-like or columnar, depending on which facet surfaces grow most quickly.
When snow crystals are very small, they are mostly in the form of simple hexagonal prisms. But as they grow, branches sprout from the corners to make more complex shapes. Snowflake Branching describes how this happens.

The Morphology Diagram
By growing snow crystals in the laboratory under controlled conditions, one finds that their shapes depend on the temperature and humidity. This behavior is summarized in the "morphology diagram," shown at left, which gives the crystal shape under different conditions. Click on the picture for a closer view.

The morphology diagram tells us a great deal about what kinds of snow crystals form under what conditions. For example, we see that thin plates and stars grow around -2 C (28 F), while columns and slender needles appear near -5 C (23 F). Plates and stars again form near -15 C (5 F), and a combination of plates and columns are made around -30 C (-22 F).
Furthermore, we see from the diagram that snow crystals tend to form simpler shapes when the humidity (supersaturation) is low, while more complex shapes at higher humidities. The most extreme shapes -- long needles around -5C and large, thin plates around -15C -- form when the humidity is especially high.
Why snow crystal shapes change so much with temperature remains something of a scientific mystery. The growth depends on exactly how water vapor molecules are incorporated into the growing ice crystal, and the physics behind this is complex and not well understood. It is the subject of current research in my lab and elsewhere.

The life of a snowflake
The story of a snowflake begins with water vapor in the air. Evaporation from oceans, lakes, and rivers puts water vapor into the air, as does transpiration from plants. Even you, every time you exhale, put water vapor into the air.
When you take a parcel of air and cool it down, at some point the water vapor it holds will begin to condense out. When this happens near the ground, the water may condense as dew on the grass. High above the ground, water vapor condenses onto dust particles in the air. It condenses into countless minute droplets, where each droplet contains at least one dust particle. A cloud is nothing more than a huge collection of these water droplets suspended in the air.
In the winter, snow-forming clouds are still mostly made of liquid water droplets, even when the temperature is below freezing. The water is said to be supercooled, meaning simply that it is cooled below the freezing point. As the clouds gets colder, however, the droplets do start to freeze. This begins happening around -10 C (14 F), but it's a gradual process and the droplets don't all freeze at once.
If a particular droplet freezes, it becomes a small particle of ice surrounded by the remaining liquid water droplets in the cloud. The ice grows as water vapor condenses onto its surface, forming a snowflake in the process. As the ice grows larger, the remaining water droplets slowly evaporate and put more water vapor into the air.
Note what happens to the water -- it evaporates from the water droplets and goes into the air, and it comes out of the air as it condenses on the growing snow crystals. As the snow falls there is a net flow of water from the liquid state (cloud droplets) to the solid state (snowflakes). This rather complicated chain of events is how a cloud freezes.

[Pilot DAR]

Wow, This thread has an amazing life! It has outlasted snow in Canada in February! It is supposed to rain and freezing rain today (so I'm not going flying!).

I think that when I'm feeling hot in the mid summer, I'm going to ask a question about flying in snow. The opportunity to read the many posts which will follow, should give me cool thoughts for weeks!

In the mean time, the weather is kinda yuccy here today.... Does anyone have any thoughts on the flying challenges on a clear blue day in the summer, when you can see forever, and the temperatures soar to, say, 25C? Do you take any special precautions with respect to the weather for your flight? Are their any health hazards associated with the coconut fumes of sunscreen in the cockpit, when flying with the door off?

Pilot DAR

[[email protected]]

Pace - much of the post is irrelevant and to do with the different shapes snowflakes can come in with variations of humidity and temperature. I didn't post all the Meteorolgy 101 diatribe because I assumed as a pilot you knew it anyway.

Your definition of sleet, as I said before, is the US one not the UK Met Office one.

There is no such thing as a snow crystal - it is an ice crystal and when they increase in size and complexity (as water evaporates from the supercooled water droplets) they are called snowflakes.

I didn't say snowflakes are frozen raindrops - both water droplets and ice crystals in the atmosphere form the same way in that water vapour changes state and either condenses or deposits onto microscopic nuclei in the atmosphere. The only thing the jury is out on is exactly which types of nuclei trigger ice formation and which trigger condensation into water droplets.

There is some evidence to suggest that airborne bacteria may be a major contributor to ice crystal formation and subsequently snow formation.

I'm sure we'll get back to a 'pure snow' argument again soon.