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What are 'Icing Conditions'?

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What are 'Icing Conditions'?

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Old 10th Feb 2010, 18:28
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B206 Icing: A Critical Question!

Hi guys,
I have been looking at the Bell 206BIII RFM and it says to turn on pitot heat and engine de-icing at OAT below 4.4 celsuis and visible moisture. Now, according to the manual flight is prohibited into 'known icing conditions'.

The questions that arise are:
1. What is visible moisture?
We have eyes for that, obviously and we know that mist, clouds, rain, snow and fog all are forms of visible moisture. There can also be an estimation of humid conditions by comparing the temp/dewpoint spread. I am looking for a specific definition, if any; as I have heard something related to visibility!
(I do not agree with having a definition when we have the word 'VISIBLE' in the manual)

2. What are known icing conditions?
Central Europe has icing conditions all winter. The zero-degree isotherm is always at SFC and icing conditions are always present. What would it be? PIREPs? AIREPs?
(To me its an area suspect-able to icing, like clouds, freezing rain or high altitude freezing areas)


Would really love to hear what other people have to say!
Thanks,
XR
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Old 10th Feb 2010, 19:12
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Visible moisture - what it says - mist, drizzle, hail, etc.

To the FAA, forecast icing conditions = known icing conditions. In court, in Europe, I suspect that known icing conditions would include where it could be reasonably expected that you would get icing, even if there wasn't a forecast to hand.

phil
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Old 10th Feb 2010, 19:44
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Known icing conditions are just that. If you fly and collect ice, that is known icing conditions. Some days the forecast can be incorrect, or just not accurate enough. A half a degree C can make the difference between collecting ice or not. You could change your track by a half a mile and be in (or out) of icing.

If the forecast indicates icing may take place, that should be used as a (strongly advisable) guide.

BTW, the engine has anti-icing protection, not de-icing. Anti-icing prevents ice formation. By definition, de-icing insinuates that you have already flown in icing and collected some ice.
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Old 10th Feb 2010, 20:08
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To add an importnt point. Anything larger than 0.5mm droplet size is freezing precipitation - and definitely must be avoided, even if you have an icing clearance.
A good rule of thumb for this is that if the moisture in the air is descending (as opposed to drifting around), then the droplet size is larger than 0.5mm.
Freezing mist is bad enough, freezing precipitation is much, much worse.
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Old 10th Feb 2010, 20:43
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Shawn, just to pick up on your point about freezing precipitation, I quite agree that flying in this stuff is a seriously bad idea, but I query whether its correct to say that no heli is cleared to fly in that stuff? There is nothing in the Eurocopter flight manuals about it (though flight in limited icing conditions is allowed with certain caveats). There used to be something in our Operations Manual prohibiting it, but its been lost along the way somewhere.

I have been asked about whether we can fly in freezing precipitation. Of course my answer is no, but I am at a loss to find written justification as to why!

So my question is, where does it specifically say that you can't fly in freezing precipitation? Or are you just taking the line that since it doesn't say you can, you can't.

I would also take the view that flight in snow or hail or ice crystal cloud is not icing conditions, provided the temperature is well below zero (if near zero, snow/hail can melt on impact and then re-freeze). In other words, if it doesn't stick it isn't icing conditions!

HC
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Old 11th Feb 2010, 07:58
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As the question is "LEGALLY ice conditions" I think that we shall consider the ufficials forecast informations. IMHO
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Old 11th Feb 2010, 10:16
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WX.............: SCT SN/SNRA, LATE NIL CENTRAL PARTS,
RISK LCA FG/FZFG
VIS............: +10KM, LCA 2-7KM IN WX,LCA 0100-1000 M IN FG/FZFG
CLD............: FEW-SCT 1000-1500FT, SCT-BKN 1500-5000FT,
LCA BKN/VV 0800-1500FT IN WX,
LCA VV002-009 IN FG/FZFG
0-ISOTHERM.....: SFC-1000FT, BECMG SFC-2000FT
ICE............: FBL/MOD, LOC MOD, LATE BECMG FBL/NIL

This is from today's IGA prognosis at the western part of Norway.
It is forecast icing conditions, but we are still flying VFR ops with 350's. The crew reported light icing in snowshowers on the last trip, but went out again.
One has to fly in visible moisture to get ice, unless we talk about carb.ice. That's another story.
But if you push it, and hover in light fog in minus-degrees, you may get icing very quick! There have been a few accidents because of that.

One R-22 was doing run-up in light freezing fog, and when they were ready to depart, they couldn't get the heli in the air. They shut down again, and revealed a lot of ice on the blades.... What they were supposed to do, flying in freezing fog, is another question....

A 350 flew in -20 c in Northern Norway, blue skies. They were to drop of some workers in the bush, and there was light fog at the landingsite. They landed, let of the passengers, and took off again. They didn't get enough lift, and crashed a couple of hundred meters away. They got icing during the short ground stop.

We are very aware of the OAT during the winter when there's visible moisture. As some has pointed out, ice on the wipers is a good indicator.
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Old 11th Feb 2010, 10:47
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I agree with those who say airframe icing conditions are generalised as below 0 degrees Celsius and in less than 1Km viz (i.e. fog).

I have heard that the term 'visible moisture' by definition is moisture causing a visibility of less than 1Km, although i would be hesitant to fly in mist (1-4km) at less than 0 degrees, which incidentally i would say is quite visible

with regards to engine/carb icing, the temp drop across the carb can (i've heard) be as much as 30 degrees so in humid conditions one must be very careful not to get caught out flying in +30C....

also there is obviously the possibility of freezing rain under the passage of a warm front, if you're in the cold sector, and rain from the warmer sector above cools to below zero (supercooled) and sticks upon impact, or your airframe is very cold and fly's into the rain... leading to particularly dangerous clear ice.

Aucky
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Old 11th Feb 2010, 11:16
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For what it's worth the CAA Supplement to the R22 POH says:

Icing conditions must be assumed to exist when in cloud, or when the visibility at takeoff is reduced to 1000 metres or less in visible moisture, with a true air temperature of 0°C or less. Icing conditions must also be assumed to exist whenever there are any visible signs of ice or slush build up on any part of the helicopter.

It goes on to state: Flight in freezing rain or freezing drizzle regardless of visibility is prohibited.

That'll do for me
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Old 11th Feb 2010, 11:17
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Helicomparator:
Like many things, it's not well defined. I know the Canadian FM for the Super Puma was amended to say No flight in freezing Precipitation, not sure about the others.
The logic is as follows:
The clearance for icing is based on testing to the requirements of one of the Appendices to Part 27 and Part 29 (can't remember which letter it is). This is the same wording and requirements as far as droplet size is concerned as the FW world uses. The maximum droplet size used for certification is 0.5mm, and anything larger is considered to be freezing precip. No one has a clearance to fly in freezing precipitation.
Hope that helps!
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Old 11th Feb 2010, 15:23
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I have been asked about whether we can fly in freezing precipitation. Of course my answer is no, but I am at a loss to find written justification as to why!
Because, if you haven't realised from some of the posts, of the rapid buildup of ice on the airframe/rotors in freezing rain/drizzle. This leads to an increase in weight and power(to cope with the extra weight) and loss of a nice aerodynamic profile(and hence lift) on the rotor blades.

It's bad stuff: don't go there.

AOPA Online: In-Flight Emergencies: Avoiding ice fright

Aircraft icing (xhtml w3c 11/09)
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Old 12th Feb 2010, 19:31
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More confusion.

I have it written down that the CAA in their publications believe Icing conditions are when the temperature is below +3°Celsius and visible moisture below 1500m sadly I can't put my finger onto the CAA document at the moment. Maybe after the weekend.

On the last page of the AS365 FLM it mentioned 4°Celsius if I remember correctly.
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Old 13th Feb 2010, 12:53
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Droplet Size and Implications

Shawn, believe you are correct re the 50 micron droplet size being the max for icing certification.

HC, the problem with freezing rain is what has been called "runback", wherein the precipitation runs back beyond the aft limit of the de-icing system chordwise coverage and freezes there*. Creates both lift and pitching moment problems.
* In fact preventing runback from occurring within the normal icing conditions envelope is a major controlling factor in the de-icing element timing cycle development.

Thanks,
John Dixson
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Old 13th Feb 2010, 13:46
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Some of you are probably aware of this but the standard weather service methodology and terminology used to characterize and classify the icing environment was developed from in-flight icing tests conducted on a Douglas DC4 and DC6 type aircraft. Thus labels such as trace icing, light icing, moderate icing and heavy icing related to the rate of ice accretion on a probe on a DC6, which does little to ascertain or predict the rate of ice accretion on a rotor system.
As an example, light icing is defined as an accumulation of one half inch of ice on a probe after 40 miles of flight. The rate of accretion is sufficient to create a hazard if flight is prolonged in these conditions, but insufficient to require a diversion.
The prior definition may well fit a large aircraft however there is no assurance that the rotating surfaces of a helicopter will accumulate only one half inch of ice over the same 40 miles, indeed there are many more factors in play with ice accretion to a rotor surface as opposed to a single non rotating airfoil that make it a much larger problem in much less time. Further while one half inch of ice on the wing of an airplane may correctly and appropriately be called “light icing” there is every reason to believe from testing that one half inch of ice on the leading edge of most helicopter rotor systems could result in tragic consequences if autorotation became necessary.
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Old 13th Feb 2010, 14:43
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Mightygem, JohnDixon et al

I guess I didn't make myself clear. In no way was I suggesting that flying in freezing precipitation was anything other than a very bad idea, and I am well aware of the seriously nasty effects that could rapidly arise if one foolishly did so.

My question only related to where in the legislation etc it is written down as being prohibited, so as to provide an answer to those that have asked me "but where does it say that?" when I have told them flying in FP is verboten.

HC
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Old 13th Feb 2010, 16:49
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HC - my answer would be that you can't because there is no specific clearance to say you can. Since every pilot from day one is taught the dangers of freezing rain and drizzle it seems reasonable to argue that therefore being able to fly in it would require some special equipment and/or clearance. You can anti-ice your engines and you can de-ice your MR and TR blades (with the right fit) but to my knowledge there is no method of de-icing either the airframe or the pitch change mechanisms of the rotors, both of which will be badly affected by freezing precipitation.

I have only encountered freezing rain once and then only briefly but it was more than enough to convince me that all the warnings are valid.

It is always interesting to see which pilots go for 'I can do it because it doesn't say I can't' versus 'I can't do it because it doesn't say I can' when quoting rules and regulations.
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Old 18th Feb 2010, 17:29
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HC, dug up some legislation wording used here in the great frozen north were nobody stops flying.

Transport Canada's latest position on freezing rain or drizzle is that it is equivalent to "severe icing". Quote from ASL 4/2009 "Takeoff into known freezing drizzle and/or light freezing rain is outside of the flight envelope for which any airplane currently operating today is certificated. ...".

Interestingly (or practically for operations in Canada), they differentiate between "forecast" icing, and "known" icing. In fact currently the regs 605.30 says
"no person shall conduct a takeoff or continue a flight where icing conditions are reported to exist or are forecast to be encountered along the route of flight unless
a) the pilot-in-command determines that the aircraft is adequately equipped to operate in icing conditions in accordance with the standards of airworthiness under which the type certificate for that aircraft was issued; or
b) current weather reports or pilot reports indicate that icing conditions no longer exist"

There is a proposal to change (b) further to permit more flexibility in operating in reported icing conditions:"b) current weather reports, pilot reports, or briefing information relied upon by the pilot-in-command indicate that the forecast icing conditions that otherwise prohibit flight will not be encountered during the flight because of changed weather conditions since the forecast." In other words, even if icing is forecast, if the commander has reason to believe his aircraft won't ice up along his route he can still fly.

So there you go for all you ice-dummy aircraft commanders. "heavy lies the crown..."
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Old 18th Feb 2010, 18:58
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My question only related to where in the legislation etc it is written down as being prohibited, so as to provide an answer to those that have asked me "but where does it say that?" when I have told them flying in FP is verboten.
Look in the RFM.
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Old 18th Feb 2010, 20:20
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Water can remain liquid in 'icing conditions' i.e. below 0 deg C or 32 deg F but impact with an object (like a main rotor blade or airframe) can cause freezing into a solid. However, the "freezing" does not occur all-at-once. In fact, there is a substantial lag.

I expect you all know this already, it forms part of the knowledge associated with ice detection systems like the Leigh Ice Detector.
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Old 18th Feb 2010, 21:20
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Icing Regs and Arguments

HC, of course the RFM is the document as called out in 29.1581, but I suspect that you were getting at some of the definition arguments.

See the following for examples of the associated legal tangles:

The Naked Truth About Known Icing Conditions.

What fun.

Thanks,
John Dixson
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