Icing??

[Krallu]

Hi!

I've read that in the weather information there is a figure that tells me at what height it is 0 degrees. Today 3000 feet for example.

And then it is icing conditions in the rain and clouds. What does that 3000 feet tell me. Is it safe to fly below 3000 feet where it is plus degrees even tough it is icing in the rain and clouds over me?

Thanks!

[TonyR]

Icing is funny and unpredictable, and the weather information may not always be totally correct as things can change quite quickly.

I have encountered it in cloud just at and sometimes just below the "freezing level" and yet flown in cloud well above the forcast freezing level height and never got any.

The above was always in a multi engine with de-ice equipment.

You must plan to avoid icing unless your a/c is equipped to deal with it, even then I would always try to get out of the conditions ASAP.

In Northern Europe, two things really scare me in IMC conditions that is, ICE and HIGH GROUND, keep away from both

Stay safe

Tony

[FNG]

One of the instrument flyers will break cloud in a minute to tell you about the impervious horrors of icing in cloud, but the freezing level does not endanger your flight when clear of cloud and precipitation, even if you fly (visually) above it. Your calculations of true air speed will, of course, be affected by temperature. On a cold day in clear conditions, the aircraft leaps into the air, as its engine feasts on the scrummy air molecules and its wings squish them down onto Sir Isaac's head.

Edit: Tony came out of the clouds whilst I was still going around in circles in the sunshine.

[englishal]

Can't beat cold air ! I remember taking off from Big Bear (~6700') in minus 15°C, and the SEP, three up leapt into the air ;) We did have to find a hole to come back down through to get into the LA basin though (as BB was above the IMC), else we would have iced up in no time....

Generally speaking, if the OAT is above freezing at your level, you won't encounter icing. If you do, you must change level straight away, now whether this means climbing or descending depends on the air mass. Some instances (ie. warm front passing through) you may even be better off climbing if you start to ice (to get into the warmer air). The things that ensure icing are visible moisture (cloud, precip) and below freezing temps (though if extremely cold, you may not get any icing).

EA

[2Donkeys]

Icing is unpredictable.

You can expect ice to form on your aircraft with an OAT in the range +4 to -20 degrees C.

Generally speaking it will only form if you are in cloud or visible precipitation. The type that forms, and the rate at which it forms are determined by the type of moisture you fly though.

Stratus tends to be associated with rime icing, and cumulus and the stuff that falls out of it, with clear icing.

Rime tends to build slowly and although it should not be regarded as "safe", can be carried further for longer than clear ice.

Clear ice often builds quickly, is very heavy, and won't go even when you fly into warm air.

As a rule of thumb the freezing level can be predicted (in '000s of feet), by dividing the surface temperature in C by 2. This is not failsafe though ;)

2D

[IO540]

2D

You can expect ice to form on your aircraft with an OAT in the range +4 to -20 degrees C

Could one really get ice accumulation on the airframe in the range +4C to 0C, at typical GA airspeeds (below say 150kt)?

Assuming the OAT gauge is actually accurate (easy enough to test), it should not happen because the temp drop due to the pressure drop above the wing is less than 1C. This was discussed in the American newsgroups extensively some time ago and I recall seeing some data showing that the temp drop due to expansion is below 1C at GA speeds and with aerofoils in common use.

Having said that, the average UK training aircraft is not legally checked / maintained for other than VFR flight so the OAT gauge may never have been checked.

Perhaps the other thing worth mentioning is that even in clear air, the pitot tube can freeze up below 0C, so pitot heat should be used anywhere near freezing. I've seen this myself, and put the pitot heat on below +5C as a habit.

I find that the met forecast icing level being below the MSA is what stops most of my winter flying... TKS is probably the best money spent :O

[Flyin'Dutch']

Could one really get ice accumulation on the airframe in the range +4C to 0C, at typical GA airspeeds (below say 150kt)?

Yup, the FAA says so, therefore it has to be true.

Actually you seem to answer the question yourself a bit later:

Perhaps the other thing worth mentioning is that even in clear air, the pitot tube can freeze up below 0C, so pitot heat should be used anywhere near freezing. I've seen this myself, and put the pitot heat on below +5C as a habit.

TKS is a get out of jail card.

I have had a few encounters with icing one of which is a 'known icing certified' medium twin. Scary stuff.

FD

PS Thought you would have left the training process long enough now to stop you harping on about the poor quality of the training fleet and instructors. Jeepers.

[2Donkeys]

Could one really get ice accumulation on the airframe in the range +4C to 0C, at typical GA airspeeds (below say 150kt)?

The answer is yes, but the probability is obviously reduced at either end of the temperature spectrum.

Worth remembering that ice can even form on an airframe in above-0 conditions if it flies into freezing rain dropping from above.

2D

[IO540]

FD

I think if you ask pilots who have full TKS (not many around, yet) and who have experience of traditional twins with rubber boots with bicycle patches stuck all over them :O they will tell you that TKS is a LOT better than a get out of jail card. It is an anti-ice system, preventing ice forming in the first place and is an order of magnitude more effective than rubber boots.

I have a mere TKS prop and the resulting spray completely prevents ice forming on the front window.

No system is going to make it OK to fly around in icing conditions indefinitely, because ice will build up somewhere on the airframe, without a limit.

Re the pitot tube comment, I said "airframe". I don't think there is any physical mechanism to get ice BUILDING UP on an airframe which is flying through air whose temperature is above 0C (i.e. the airframe temperature will be above 0C too). A pitot tube is a different scenario because bits of ice can get stuck up the hole and more or less stay there; in any case you don't want ANY blockage in there at all.

[bookworm]

Worth remembering that ice can even form on an airframe in above-0 conditions if it flies into freezing rain dropping from above.

(I have a feeling of deja-news about this... Has it been discussed recently elsewhere?)

I'm not sure that's likely. The freezing rain is rain because at some point it was in above-zero air at high level. It then fell into below-zero air to become supercooled. Surely the issue is meeting it in that below-zero air, where it will freeze on contact with your airframe? I'm not sure it poses such a threat to an airframe above zero once it has dropped further into above-zero air.

[High Wing Drifter]

The safety sense leaflet says that severe icing is most likely to occure between 0 and -10 deg. However, in my ATPL notes somewhere, it states that severe icing conditions should be assumed between +3 and -10. Below -20 there is virtually nil/very light icing.

The freezing rain that bookworm alludes to is the ATPL examiners favourite hazard of flying in the sub-zero cold air below a warm front. To escape such a predicament, the tested resolution is to climb.

The only icing not related to cloud or precipitation is hoar frost that forms when descending quickly through an inversion.

[2Donkeys]

I'm not sure it poses such a threat to an airframe above zero once it has dropped further into above-zero air.

You'd better believe it! Not a common feature of flight in the UK, much more common elsewhere.

[bookworm]

Not sure how we settle this one, as it's obviously a rare event.

Wouldn't this mean that any rain that has fallen through a sub-zero layer would be hazardous to aircraft below that layer? That's not such an unusual event. It would have to spend sufficient time in the sub-zero layer to become supercooled, which generally takes a while with rain.

I would have thought that the heat capacity of an airframe above zero would melt any such ice in short order.

If we put the airframe in sub-zero air, without enough skin friction to heat it above zero, then obviously we've got a popsicle in the making. But that's conventional freezing rain, isn't it?

[sharpshot]

Krallu

http://img51.photobucket.com/albums/v156/Emirates/Icing.jpg

I hope you can clearly see what's on the leading edge in this photo. Having expected it and noticed it, I asked my friend in the R/H seat what he could see - nothing. Take another look and he realised what was happening. Cruising above the cloud over Wales and started descent. Pitot on in advance and this started as soon as we entered cloud. Surface OAT was about +4, at 5.5 it was about -7 deg c. Both Altimeter and VSI failed for a short time. This was Feb.

Don't stay in it if you're not equipped!
Been in similar over North Wales in June, so don't think it can't happen in summertime.

[TonyR]

Here (http://www.auf.asn.au/meteorology/section10.html#aircraft_icing) is some more info on icing



And also here (http://www.virtuallystrange.net/ufo/updates/2000/feb/m13-006.shtml)

Tony

[IO540]

High Wing Drifter

In the ATPL they would be talking about issues relevant at airliner speeds, and yes the temp drop around parts of the wings/airframe could be a few degrees C. But not at GA speeds.

Sharpshot

Did you calibrate your OAT gauge? Also it is VERY common to see an OAT of say +4C when VMC on top, dropping to below 0C the instant one descends into the cloud.

As I've said, icing below the MSA stops me flying. But I also think that some people are excessively worried about the shortest exposure to a cloud below 0C.

[sharpshot]

IO540

It was about -7 deg c at Fl 5,5 in bright sunlight and the temp inside the plane dropped dramatically as we entered cloud. It was physically noticeable - but anticipated I might add.

[High Wing Drifter]

In the ATPL they would be talking about issues relevant at airliner speeds
That's not actually true. The ATPL covers all aspects of flight. As a result, apart from Air Law and Ops Procedures, is not a chore to work through. The exams are a bit of a bind though.

, and yes the temp drop around parts of the wings/airframe could be a few degrees C. But not at GA speeds.
I've gone back to my notes and have spotted my mistake! The figure quote below +10deg TAT (Total Air Temp) which is, due to ram rise, always warmer than OAT. Ice does not form above 0deg. Sorry for the curve ball. Temprature probes read TAT if the speed is suffiently high. TAT = OAT + (TAS/100)^2

Runback icing, icing formed between +10 and -3 TAT, on a Jet is formed, so it says, due the high impact speed of supercooled droplet on the leading edge warming and running back and then re-freezes.

So says my notes!