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Freezing rain

I have often wondered about departing in rain, because the height at which the rain stops, relative to the 0C level, is obviously very relevant!

In some cases it is obvious e.g. a cloudbase of 5000ft, 0C level of 4000ft, and it's raining. There has to be freezing rain between 4000ft and some level above that which will obviously be inside the cloud. In that situation I would never depart on an IFR flight.

But most freezing rain is found in circumstances where it is not expected. I have seen it twice so far but in both cases I was descending anyway. In the 2nd one I descended really fast - 2000fpm - as soon as I realised it was raining below 0C. So any exposure was very brief and nothing stuck.

One often sees airliners depart in rain and they just climb straight up, and at some point the rain stops.

How can this point be estimated?

Obviously it will be somewhere between the cloudbase and the cloud tops

Then you get more obscure scenarios where freezing rain is made by snow falling through a warm layer, melting, and then falling into cold air below.

Administrator
Shoreham EGKA, United Kingdom

This is a very difficult subject.

It is no exact science, but my experience is that flying from a zone with rain to a colder region above with below freezing temperatures, this does not automatically result in rain becoming freezing rain. Actually, most of the time or better almost always it doesn't give problems. Same for snow / wet snow.

Freezing rain is mostly associated with warm frontal weather, but is hard to predict. The freezing rain could fall from clouds above you. Then, when you expect it, it doesn't happen.

I have had some encounters with freezing rain and they are not fun. I might be flying a FIKI certified aircraft, but am sure it will not be able to handle severe cases of icing and freezing rain. Also, it is not certified for flying in freezing rain, which is excluded from the FIKI certification.

While climbing through the rain to higher altitudes and thus most likely lower temps, I carefully watch what the water is doing on my front window and wings. Then, I have the extra horsepower and FIKI system running and am ready to turn around.

You need to think of an escape all the time. I also look at the SIGMETS. If severe icing or freezing drizzle/rain is observed, a SIGMET is published where it was observed and how it is moving. It gives at least an extra warning. I talked to a KLM pilot that told me that they even fly with severe icing warnings and that this in almost all cases does not result in any problems. When he was flying the Fokker 50, he would have some problems with icing as the performance to climb out of it was not always there. With the airbus, they just climb straight through it even though on paper they shouldn't.

I personally don't fly through freezing rain, but as I cannot predict it fully and fly through snow, rain and in winter time, I must think of what to do in case I encounter it. Then, in reality I hardly ever encounter it.

EDLE, Netherlands

I think the freezing rain that causes most issues is the opposite situation - that is where rain is falling from a warm air mass into a below freezing air mass such that it is super cooled liquid which turns to a beautiful crystal coating on contact. As a child I remember examples of this on the ground (the whole world coated with a half inch of ice - no power, all roads closed, trees fragmenting under the weight, flying off the front step, etc.). My understanding of freezing rain in the aviation context is exactly the same.

The situation you describe is rain which is running back on an above 0 wing freezing as the air and wing cool below zero in the climb. I believe there have been examples of frozen controls due to poor lubrication, but this is not what is typically thought of as freezing rain. My experience with the rain/snow transition is normally in the mountains (bad weather skiing). Typically, the precip is falling as wet snow as much as 1000 feet below the freezing level and melts on contact. There is some element of freezing up as you go back above the freezing level, but this seems to be due to the absorption of water in clothes more than surface water in skis, helmets,etc (which blow dry at vey low speed relative to an aircraft). The precip is falling as snow well before the hard surfaces have cooled enough for any liquid to freeze.

EGTF

Usually freezing rain is associated with the approach of a warm front when cold air, at or below freezing temperature, is trapped in the lower levels of the atmosphere as warmth streams in aloft.

Now, this is a scenario that doesn't happen too often here in the UK or the Netherlands. Indeed, when you climb and the air around you plus the water/rain drops in temperature to the freezing level, you won't get into the freezing rain scenario (most likely) and can continue to climb through it.

EDLE, Netherlands

We had some rain running back and freezing on the wing coming into Oxford last week and the wing/fuel was very cold after soaking in -39 degree temps at altitude. Not an issue as we were descending into warmer weather and it soon melted. We didn't even cycle the boots. I think the bigger problem as mm_flynn indicates is SLD - supercooled liquid droplets that freeze on contact in a big, messy and drag-inducing way.

EGTK Oxford

We had some rain running back and freezing on the wing coming into Oxford last week and the wing/fuel was very cold after soaking in -39 degree temps at altitude. Not an issue as we were descending into warmer weather and it soon melted. We didn't even cycle the boots. I think the bigger problem as mm_flynn indicates is SLD - supercooled liquid droplets that freeze on contact in a big, messy and drag-inducing way.

That is another, different, scenario. You could get that on the ground, after a descent from a high altitude. Once, in the summer, +25C on the ground, I landed at Trieste following a rapid descent after flying over the Alps, and the condensation on the wings (which still contained a fair bit of fuel) was freezing and bits of ice were dropping off. Quite funny really...

Freezing rain is when you are flying through below-0C air and it is raining. This is extremely rare.

I think another scenario, if not already mentioned, is rain or snow falling through a straight temperature inversion. Inversions are very common but it is very rare to get one where the temperature at your flying altitude is below 0C and there is precipitation.

Administrator
Shoreham EGKA, United Kingdom

Which super cooled water droplets problem you most likely might get with the passing of a warm front and not while climbing through the rain or wet snow to top-of-clouds or at a cruise level in clouds at an altitude with an outside temp of lower than -20. Even when flying in the danger zone of +2 to -20 degrees and in the clouds, most of the time I do not encounter ice and if it is ice, then rime or mixed ice, but not freezing rain.

In this image I was flying at an outside air temperature of approx. -40 degrees Celsius at around FL240. That is the minimum allowed flying temp according to the POH of the Cirrus.

There seems to be ice on the wing, but that is not the case. It is a residue from the TKS fluid. Parts of it flash evaporated. I initially thought it was ice.

EDLE, Netherlands

At the various exams where questions were asked about freezing rain it always is about a situation where hot air is above a cold layer. The correct answer is to climb out of it into the warmer air which has to be above .. as it can otherwise not be raining???

Commander: that seems correct to me, but you thus need an aircraft with enough power to out-climb the buildup of the ice.

EDLE, Netherlands

I think most light GA types will never be able to outclimb an area of freezing rain, once it actually starts to stick, due to the loss of performance.

An immediate 180 has to be the only way, IMHO.

Administrator
Shoreham EGKA, United Kingdom
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