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Freezing Fog

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Freezing Fog

Old 28th Sep 2004, 07:44
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Freezing Fog

Is there an international definiton for freezing fog. In the UK it is 1000m but i think that the US may use a statute mile?

thanks

stu
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Old 28th Sep 2004, 08:52
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Stu - it is 'FOG' that in the UK is less than 1000m vis due to water droplets. The 'freezing' bit just means that the droplets are 'super-cooled' below zero C. Don't know about the USA.
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Old 30th Sep 2004, 02:38
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Here's one;

A visible aggregate of predominantly supercooled water droplets which do not fall to the ground. The visibility must reduce to 1000 metres or less and the air temperature must be below 0 degrees Celsius.

mcdude
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Old 30th Sep 2004, 12:10
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I believe it is correct to say that subject to satisfying all other minima, you may land in freezing fog but may not take off in freezing fog, in the UK, that is?.
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Old 1st Oct 2004, 16:55
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Freezing fog departure?

Omark44, can you point to a regulation or limitation that supports that view? I'm not aware of anything that prohibits it.

I'm specifically talking about regulatory limits, not aerodynamic ones. If there's fog freezing onto the wings, I'm not going flying. But if it isn't (perhaps because the airframe is above freezing, or is adequately de-iced, or the air isn't quite as cold as the met man thinks it is) then can I go flying, and if not, why not?
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Old 1st Oct 2004, 19:10
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CJ - I am too lazy (or too busy) to check but I have a feeilng that hold-over times for FF are so tight that it is difficult to go flying!
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Old 1st Oct 2004, 20:16
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Type II Holdover

BOAC - obviously I have too much time on my hands, because I went and looked it up. According to my Ops Manual, type I fluid can give as little as 6 minutes of protection, so there would need to be a de-ice bay near the departure runway. Type II fluid however, in a typical 50:50 fluid/water mix, is good for 20 to 45 minutes of protection against freezing fog. Ample to get from the parking stand to the runway, I would have thought.

But in any case, I don't dispute that it is difficult, but the original question (particularly interesting given that my Ops Manual apparently tells me HOW to depart in freezing fog) was - is there anything that prohibits trying it?
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Old 1st Oct 2004, 23:15
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Another serious problem is ice on jet engine fan blades.
Freezing fog can cause very serious and expensive damage and there is no protection on common engines used in modern airliners.
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Old 2nd Oct 2004, 07:21
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There may be specific, individual airline restrictions on it, but "20 to 45 minutes" could make life very difficult at major airports, bearing in mind when the 'clock' starts running and typical taxi to airborne times.
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Old 2nd Oct 2004, 08:28
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BOAC,
Do you use Type IV fluid in europe now? Seems to have even better holdover times than Type II.

80/20,
You are correct, but when operating in such conditions we have to accelerate the engines to a mid-power setting to centrifuge the ice off the fan every few minutes....the RR Trent has a more complex procedure but the outcome is the same. Do you have a more limiting operating procedure?

idg
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Old 2nd Oct 2004, 08:57
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Yes, idg - all types 1-IV

OK I've been forced to dig out the Ops Manual - probably a good time for a bit of 'revision' anyway!

Firstly, for 'Stu', here is one (UK) company's definition
"A suspension of numerous minute water droplets which freezes
upon impact with ground or other exposed objects, generally
reducing the horizontal visibility at the earth's surface to less
than 1 km."

Secondly, FF hold-over times vary from 'difficult to get airborne' to 'easy', depending on the temperature/mix/de-icing facility. I guess you have to play it as you see it (as usual)
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Old 2nd Oct 2004, 10:13
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Looking at the holdover tables, I would tend to concur that freezing FOG is not that problematical (usually). After all, it's just like flying through cloud above the freezing level and we do that all the time. There are set/timed procedures for shedding ice build-up on fan blades and the holdover times using the better class of fluids are not too bad compared with other forms of precipitation.

The real 'nasty' is freezing RAIN and you wouldn't catch me even attempting to taxi in that...
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Old 4th Oct 2004, 12:56
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80/20,
You are correct, but when operating in such conditions we have to accelerate the engines to a mid-power setting to centrifuge the ice off the fan every few minutes....the RR Trent has a more complex procedure but the outcome is the same. Do you have a more limiting operating procedure?
idg
Not really - idg, Boeing introduced a pre-takeoff 70%N1 static run-up procedure after a freezing fog situation where most airlines cancelled all flights but a few decided to depart. Many of the departing carriers experienced severe vibrations which resulted in 12 CFM engines with serious blade damages.

The static run-up might help, but there continues to be cases where the fan icing is too severe to be shedded. Some airports, experienced with this problem, offers a procedure where engines are shut down, front covered and heated by hot air and inspected just prior to takeoff.

I once observed thick clear ice on each fan blade and an “icicle-like" spear of clear ice the size of a small coke bottle sticking straight out of the spinner after only 5 minutes of taxiing in freezing fog. Freezing fog got my respect since it can cause rapid fan icing too severd to be sheeded without engine damage.
It can even be just fog which becomes freezing fog as pressure drops (expansion cooling) in the engine inlet.

BOAC,
Do you use Type IV fluid in Europe now? Seems to have even better holdover times than Type II.
Both are used in Europe, IV will give better protection around freezing, but both II & IV can give you as little as 15 min freezing fog HOT (hold-over-time) at colder temps. The freezing fog that I have encountered has always been below -5°C with very strong inversions.

The real 'nasty' is freezing RAIN and you wouldn't catch me even attempting to taxi in that...
FullWings... I agree 100%
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Old 5th Oct 2004, 08:36
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is this 1000m a definition/requirtement for just the UK? Does Amercia have a different definition, e.g 1 statute mile?
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Old 5th Oct 2004, 12:44
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Stu,

I appreciate that your original question hasn't been answered...

I had a trawl through the FAA sites and cannot find a proper quantifiable definition of 'fog' - just the usual 'water droplets, reduced surface visibility, cloud within 50 ft of the surface' stuff. It's almost as if they don't want to join in with the rest of us and admit to a definition based on metric measurements.

HOWEVER, do go to

http://www.faa.gov/avr/AFS/afs400/ac00-45e.pdf

and look at their METAR constructions for RVR reporting in 'statute' measurements.

If someone else can point to an FAA definition, I'd be pleased to see it!!!!

The Odd One
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Old 5th Oct 2004, 18:55
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Stu it’s an interesting question.. Could you please expand a bit - are you asking for general definition, when it should be included in surface observation SA/METAR, forecast FT/TAF, ATIS and so on..
Where is the UK definition from (source)?
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Old 6th Oct 2004, 10:03
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I was looking to see if there was an international definition that everyone knew/used. The ICAO state that it is horizontal vis less than 1000m but that the temp doesnt nessesarily have to be below 0 degrees. The UK Met Office agrrees with this definition and the 1000m or less. However i know that Boeing use 1 statute mile. I presumed that evrybody used the same definition but it does not appear to be the case. Are there any other variations?


cheers

stu
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Old 6th Oct 2004, 16:26
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In Canadian METARS if there is an obscuration to visibilty of the "fog" variety less than 5/8 sm it's listed as FG (fog). More than or equal to 5/8 sm it's listed as BR (mist). The addition of the descriptor FZ (freezing) requires a temp of less than 0'C.
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Old 7th Oct 2004, 13:27
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non-pilot asking the obvious question:

What makes freezing rain nastier than freezing fog?

--------------------
"I tell you, we are here on Earth to fart around, and don't let anybody tell you any different." Kurt Vonnegut, Jr.
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Old 7th Oct 2004, 15:21
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Paxboy - in simple terms, fr fog is composed of small suspended 'supercoolled' water droplets which freeze instantly on meeting any (hard) surface. This makes a significant icing accretion rate. Freezing rain is larger, falling drops of same which do the same and REALLY accrete! Aircraft can very rapidly acquire a heavy coating of ice in fr rain with sometimes dire consequences. The rate of accretion can often defeat any deicing system on the aircraft and obscure windows. Best avoided if possible.
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