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What are 'Icing Conditions'?

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What are 'Icing Conditions'?

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Old 19th Feb 2009, 19:37
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What are 'Icing Conditions'?

Ok, so before anyone replies "Conditions where icing exists", I want to know what is meant by the term in the flight manual 'operation in known icing condition is prohibited'. My company, as a general rule use 5 degrees or less in visible moisture but i have heard others say you are ok to 0 degrees in moisture. Can anyone, using their encyclopedic knowledge and experience illuminate me on what temperature you can LEGALLY fly in visible moisture to remain within the parameters detailed in the AFM????
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Old 19th Feb 2009, 19:45
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Obviously, water can't freeze until it meets a temperature of 0 degrees C.

Can this occur in the engine intakes, or plenums, for example? If so, you might encounter an engine icing situation although the airframe remains ice free.

Water can exist in a liquid state below that temperature, if there is nothing for droplets to freeze onto or around (supercooling). If the airframe, or part thereof, is below zero and it meets supercooled water droplets - you then almost certainly have icing conditions.

One other consideration - how accurate is the OAT guage on a particular aircraft?

In my outfit, if any build up is noticed (the wiper blades are an excellent indicator) THAT is icing conditions. However, as always, the Flight Manual is the paramount authority.
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Old 19th Feb 2009, 19:57
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According to some, snow is OK

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Old 19th Feb 2009, 19:59
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I would imagine it is when the met office report that they exist. And bear in mind that the FAA regard "forecast icing conditions" as "known icing conditions".

You could also infer it from the flight manual from when the anti-icing needs to be on - e.g. +4.4C for the 206, but Robinson have also issued warnings for +30 in humid conditions, so my own inclination to take the one the jury would take notice of!

Phil
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Old 19th Feb 2009, 20:46
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Can anyone, using their encyclopedic knowledge and experience illuminate me on what temperature you can LEGALLY fly in visible moisture to remain within the parameters detailed in the AFM????
I'm not sure that it is against the law to fly in visible moisture (icing) conditions or outside the parameters detailed in the AFM.

I am sure it would be unwise though.
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Old 19th Feb 2009, 20:59
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Like Phil says, the met reports are a great place to start looking for icing conditions, be it forecast icing or pireps showing icing encountered by others inflight.

Icing is an odd thing though and can be found in a range of conditions. I've flown IMC at -2C and never picked up any ice at all. On another day I flew thru a small localized rain shower at +5C and to my surprise watched the water freeze on contact with the window.
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Old 19th Feb 2009, 21:04
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Be careful. Icing is defined in Appendix C of Part 27 and 29, but it only covers water droplet sizes up to 50 microns - that's 0.5mm (or the thickness of a mechanical pencil lead). In that condition, the droplets are more or less suspended in mid-air.
Anything larger than 0.5mm is considered to be freezing precipitation - and no aircraft (fixed or rotary wing) is cleared for flight in those conditions. So if it's not snow, and you encounter something falling from the sky and freezing, stay on the ground.

Last edited by Shawn Coyle; 19th Feb 2009 at 21:04. Reason: minor correction
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Old 19th Feb 2009, 21:06
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Depends on the jurisdiction. Like Paco says, different authorities have different definitions. Some look only at an OAT gauge, some look for ice on sponsons or wipers, some look at the torque gauge, some look at a weather forecast. In some places regardless of what is forecast, if some brave ice-dummy soul ventures up and reports back that he isn't picking up ice then to the rest of the pilots it is no longer "Known Icing", in fact it is now "Known Not To Be Icing".

Pilots in places like Norway or the Canadian North that wouldn't turn a blade from September to April following the rules of more temperate locales, have built up a vast body of knowledge based on experience. Such as Whirl's comment on snow. This is passed down from senior pilot to junior pilot. As it should be.
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Old 19th Feb 2009, 21:44
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The FAA has recently produced a legal interpertation defining known ICE for flights in the USA. As most guidance from the FAA it is not very usfull, if you get in trouble on a flight in iceing conditions you get hung out to dry. If the flight is completed ok you are ok, go figure.

The FAA also defines known ice in the Aeronautical Information Manual as ice observed accumulating on the aircraft.

The link for the interpertaion is below.






Regulations
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Old 19th Feb 2009, 21:54
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up to 50 microns - that's 0.5mm

Isn't that 500 microns? Or 0.05 mm?
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Old 19th Feb 2009, 22:04
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Shytorque, I guess you do know already that ice can form at *ambient* temperatures above zero where there is a cooling effect from expansion - the obvious example being carb icing. But would such an effect occur on the airframe? Any low pressure area could be a candidate, such as behind slots in flaps, behind objects projecting into the airstream etc.
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Old 19th Feb 2009, 22:24
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Yes, I am very much aware of the individualities of airframes. Follow Whirls' link and look at my argument on that discussion, which was in a nutshell that generalities are dangerous things to believe. In that instance one contributor was quite adamant that snow could not stick to an airframe - I disagreed. Icing trials, particularly on helicopters, need to be very carefully carried out for a meaningful icing clearance to be issued. Subsequent experience in a variety of conditions may result in a clearance being further restricted or eased.
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Old 19th Feb 2009, 22:36
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Don't quote me, but I think there is a UK definition of Icing Conditions as "flight in visible moisture with an OAT of 0C or less, with a visibility of 1km or less".
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Old 20th Feb 2009, 07:26
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JHR - you are right, that letter from the FAA isn't overly helpful. On one hand it says that known icing conditions only exist when you see ice accumulating on the airframe but on the other they say a pilot who flys into an area where icing would probably be encountered (based on met forecasts and reports) is OK if they don't encounter ice but might be prosecuted if they do!

If potential icing conditions exist ie less than 1000m vis with OAT below zero - then as long as you have what the FAA call 'an icing exit strategy' you are pretty much acting within the letter and the spirit of the RFM.

The problem with helicopters is that once you start to accrete ice you often don't have the power/performance to climb and only a limited ceiling anyway so you are forced into descent into clear/non icing air or a turn back to a known clear/non icing environment.

So if your enroute met looks icy all the way to the deck it might be time to say no.
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Old 20th Feb 2009, 08:58
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once you start to accrete ice
err... accrue, shurely?
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Old 20th Feb 2009, 09:33
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fkelly

ice 'accretes' to an aircraft ie attaches itself to the airframe. your bank account 'accrues' interest or more likely if like most of us on here...your scarily increasing overdraft accrues charges!!!

biz
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Old 20th Feb 2009, 12:16
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Icing Conditions

Shytorque mentioned that "you might encounter an engine icing situation although the airframe remains ice free" ... the following definitions appear in one of my company's limitations documents:

"Rotor and airframe icing conditions are defined as cloud or fog in the temperature range 0C to 30C when the visibility is less than 1,000 metres" (where 30C is the bottom end of the aircraft environmental envelope). However ... "Engine icing conditions are defined as cloud or fog with visibility less than 1,000 metres, or rain, all in the temperature range of +10C to 30C". (The increased upper limit (+10C) recognises the cooling effect of the engine intakes ...)

The same document also includes the following comment: "Due account must be taken of the limit of accuracy of the OAT indication, which is 1C".

CY
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Old 20th Feb 2009, 13:16
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"Flight in icing conditions prohibited", so says the Flight Manual.
I read this, and luckily so does my company as "conditions conducive to icing" i.e. 0 degrees, visible moisture, viz below 1000m etc. If such conditions are forcast on your route you should avoid it. By penitrating the area you are in breach of the limitation section in the flight manual.

The word "conditions".

As a verb: To make dependent on a condition or conditions.
As a noun: a circumstance indispensable to some result; prerequisite; that on which something else is contingent.

Both come up with the same answer. a number of prerequisites occuring together to give in our case Icing.

There is a subtle difference between flight in icing conditions and flight in icing.
The first, your going to get it, the second, you've got it.
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Old 21st Feb 2009, 07:23
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Check - yes but Eurocopter beans' wording at the start of this thread was 'known icing conditions' which is more specific then 'icing conditions'.

You interpret your RFM to read 'possible icing conditions' which I have no problem with but, just because you are in cloud below zero degrees, it doesn't mean that you will pick up ice because more often than not you don't.
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Old 21st Feb 2009, 07:53
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Crab, your right, I never re-read the original post. However I still stand by my post.

We have three conditions available, no icing, icing conditions, and icing. The only way of being informed of known icing conditions are a) entering into the condition of flight in icing conditions where you may or may not pick up ice, or from reports from other aircraft that have done just that and picked up ice. If the met report reports icing then what's the question?

The wise will carefully read met reports and if icing conditions exist will ensure that if he/she decides to procede then they have to ensure enough space below to leave the icing and continue or return as the case may be.

This icing thing comes round every winter and most of us seem to handle it well, survival has something to do with it!
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