Where is the definition of Icing written down?

[Brilliant Stuff]

Where is the definition of Icing written down?
I looked in the ANO ,our operations manual and the Flight manual. But to no avail all it says is stay out of icing conditions.
I remember from the exams it's plus 4 degree celsius and moisture. But I can't find it written down in official documentation.

cheers for any help.

[sarboy]

Must depend on airframe then, bacause for mine the Manual says:

Airframe Icing - 0 degrees or less
Engine Icing - 10 degrees or less

But in both cases the clincher is less than 1000 m visibility in moisture or precipitation.

Perhaps that last bit is true generally?

[ShyTorque]

I have never seen a definition written down per se.

However, for icing to occur, 3 things are needed. Firstly the moisture, secondly a local temperature of zero degrees C or below and thirdly something for the ice to form around (a nucleus). The latter can be something as small as a dust particle or as large as your airframe, or parts of it.

You are therefore highly (!) unlikely to see significant airframe ice if both the OAT and the airframe are above zero.

However! Turbine engines / intakes are likely to suffer from icing at a higher OAT than the airframe because there is usually a pressure drop and therefore a temperature drop in the intake area. The design of the intake determines how prone or not it is to icing. (Always follow the RFM advice as the manufacturer will have done at least a basic investigation into the icing phenomena).

A petrol engine is even more prone because of the necessary fuel evaporation taking place inside the venturi causing even more of a temperature drop. Hence the requirement for judicious use of carb heat on most light aircraft, even in summer.

Be aware that late use of anti-icing on a turbine engine can cause flameout because of a quantity of loosened ice / slush suddenly going down it. It should be used as a preventative measure, which is why the RFM advice is normally to switch it on well above zero C.


It is possible for water to remain unfrozen at or below zero celsius (supercooled droplets), and it often does. These supercooled droplets are just waiting for a nucleus to form around - possibly your aircraft. Airframe ice build-up rate is affected by droplet size, temperature, airspeed and shape or profile of the nucleus. Small drops freeze instantly, giving whitish (air inside), brittle ice which may form and break off unevenly, causing vibration on rotor systems.

Large supercooled drops are more dangerous as they flow back and stick like glue (rain or clear ice), because of the very high rate of accretion. Beware of flying in rain if the OAT / airframe temperature is zero or below...a classic example is flying below an advancing "warm" front in winter. Large, supercooled raindrops (having fallen through colder air still in the "cold" sector on the way down), may remain unfrozen until they hit your aircraft when they will freeze on impact. I have seen this type of ice bring large branches off trees because of the weight increase. Any aircraft flying in those conditions would have severe problems to say the least.

My current aircraft manufacturer's advice is "visible moisture" / below plus 2C rather than a specific visibility, although 1000m with visibility reduced by moisture is probably good general advice.

Hope this helps but if I'm teaching egg-sucking then my apologies.

Isn't it annoying how you only get that last typo after it's up there, posted!

[ 05 January 2002: Message edited by: ShyTorque ]</p>

[heedm]

Good answer, ShyTorque. The only thing I would add is to get a very good understanding of all meteorogical phenomena.

Close to fronts, trowals, etc. you should know where there are and aren't potential icing conditions. ie ice pellets near a trowal normally have freezing rain above them.

The only significant icing I've encountered was flying over a ridge from the leeward side of a range to the windward side. Of course, the windward side had rising air that was moist and cooling, the leeward side had cooler air, so the airframe was cold. We crossed the ridge, instantly couldn't see forward, descended to sea level and melted the ice.

Afterwards we thought about the met classes we slept through and it all made sense.

[Nick Lappos]

Having been in enough actual and artificial icing, I can say that flying in atmospheres below 0 degrees and in visible moisture (cloud) is enough to create a high probability of icing accumulation.

The many factors that cause ice clouds are outlined in FAA Part 25, Appendix C, which has data based on hundreds of ice trial flights in airplanes back in the late 1940's, where the size and height of ice clouds was carefully measured, along with temperature and altitude. This appendix is the "real" definition of what we must design to withstand in a de-icing system, and so it is probably the best definition of what the FAA (and JAA) consider icing to be. Here is a pointer to a presentation that tells about changes to appendix c, it is 4 megs, however:

<a href="http://av-info.faa.gov/dst/C1presentations/07jbice1.ppt" target="_blank">http://av-info.faa.gov/dst/C1presentations/07jbice1.ppt</a>

[Brilliant Stuff]

Thanks for that. Grateful for the explantion of the dangers of Icing. What a shame nobody can write these down in some sort of legal document, so one has a guideline. Funnily it only says in our Flight Manual "Keep out of icing conditions."

cheers

[MightyGem]

Off subject I know, but I saw a documentary on Discovery the other night about the Comanche. There was some chap on it called Nick Lappos. Seemed a decent chap.
:)

[coalface]

For the purpose of RFM approved flight conditions, the UK CAA define airframe icing conditions as flight in visible moisture which reduces visibility to less than 1500m (or maybe it’s 1000m – I will check tomorrow) with OAT at or below 0 degrees centigrade. Flight in falling or blowing snow is not considered to be “icing conditions” for the purpose of Flight Manual approvals. Certification for flight in snow is dealt with under different certification requirements.

The above definition (or words to the same effect)is in the Flight Manual of any UK helicopter with an icing clearance. Why it is not in non icing cleared aircraft Flight Manuals I do not know. Maybe it is nowadays.

Certain aircraft may be prone to localised icing in certain areas above 0 degrees OAT (engine intakes for example) and again this is dealt with by the manufacturers and CAA by other requirements.

[ 07 January 2002: Message edited by: coalface ]</p>