Freezing rain and drizzle

[Miserlou]

A gentle debate going on at work the other as to how a pilot should determine the actual existence of FZRA/FZDZ.
My opinion is that one must observe the outside of the aircraft but, and this is my question, how long does it take for freezing rain to freeze on the aircraft?

I check at 10 minute intervals which seems reasonable to me.

[BEagle]

One of the most dangerous forms of precipitation there is.

Intense clear ice can form almost instantaneously.

[Clandestino]

How long does it take for FZRA to freeze on A/C?

From my experience, anywhere between 0.1 and 0.3 secs.

For practical purposes, if ATC says there's freezing rain and we don't see any precip, we don't go because of ATC report. If ATC says nothing but we suspect there's possibility of FZRA, we don't go because of our suspicions.

[CoolHandleLuke]

Freezing rain and drizzle consist of super cooled droplets, so by the very definition they will freeze the very instant they strike an object such as an aircraft or whatever.

[JW411]

If you see FZRA on the forecast stay in the crewroom or go down to the pub. I encountered it one night about 40 years ago at night between Malta and Cyprus and despite doing an immediate 180° turn, we almost ended up in the Mediterranean.

[Dan Winterland]

Not quite freezing rain, but drizzle at close to zero freezing to a supercooled airframe. The aircraft became almost unflyable after a few seconds. Horrible experience and we were close to jumping out!

Take the advice already given and take the pub option!!!!!

[ZK-NSJ]

it brought down a convair 580 freight plane here a couple of years back

[Miserlou]

Well, the debate is still alive and well then.

My point is that if you are closer to borderline temperatures then the freeze will not be instantaneous. That's the reason it is so dangerous. It hits the leading edge, DOESN'T freeze but runs back past the de-icing system (boot or heat, depending on a/c type) and then freezes on unprotected surfaces.

The argument was that some people think that it freezes instantaneously which I don't believe to be correct. I am looking for an approximate but accurate 'time to freeze' for both normal and super cooled rain drops. Due to the latent heat of sublimation it must take notably longer for water at +0C to freeze than for water at -0C.

Looks like I'm going to have to do some experiments.

[CoolHandleLuke]

Hello Miserlou
I think you're right and I'm wrong!
Freezing rain and drizzle consist of super cooled droplets, so by the very definition they will freeze the very instant they strike an object such as an aircraft or whatever. Not true!
I just took the time to look in a book about flight meteorology and learned the following (amongst other things!):
Ice accretion is not only dependent on meteorological conditions but also aerodynamic factors such as: speed of ac, profile of the wing. No big surprise there!
The following is a summary of what my book writes:

During the freezing process 80 calories per gram heat energy is released and this induces a small temperature rise in the water droplet or ice crystal. It will, however, never rise to above 0 degrees C. How large a part of the droplet is freezing depends on its temeperature. At a given temperature an upper limit exist for how much water can be frozen via heat loss to the surroundings. Hence at impact, only 1/80 of the droplet will freeze for each 1 degree C of temperature below freezing point.
Thus at minus 5 degreesC, 1/16 of the droplet will freeze immediately, at -20 degrees C 1/4 will freeze and at minus 40 degrees C, it will be half.
And very short about clear ice:
When big cloud droplets hit a wing profile they will disintegrate and flow out before freezing completely.:ooh:
Thank you for correcting yourself, Luke. You're welcome!

[Miserlou]

Thanks for the numbers, I seem to recognize the 80 calories figure. Drove my physics teacher mad when he insisted that nobody used calories anymore in favour of SI units, joules and kilojoules etc. I produced the CPL books and corrected him. Few years ago now, of course!

[Mad (Flt) Scientist]

My point is that if you are closer to borderline temperatures then the freeze will not be instantaneous. That's the reason it is so dangerous. It hits the leading edge, DOESN'T freeze but runs back past the de-icing system (boot or heat, depending on a/c type) and then freezes on unprotected surfaces.

In addition to that mechanism, be aware that the impingement zone for SLD is MUCH larger than that for 'normal' icing conditions. Which means that there are droplets striking the wing outside the normal protected zone anyway - so even if your WAIS can somehow manage to remain fully evaporative (which would be quite impressive anyway) you still risk ice accretion on the further aft sections of the wing.

One of the tell-tales on our types is SIDE window icing for SLD - normal icing affects the front, wipers, etc., but SLD (also) affects the side windows. Indicating the greater area impacted by SLD.

[DK_FCI]

AFAIK the FZDZ and FZRA therms only apply to supercooled droplets when the are observed falling at ground level.

Any ice you pick up during flight is the result of supercooled droplets on varying intensity and size, once the water is frozen you dont pick up ice anymore.

It is perfectly possible the have normal rain falling at ground level, and severe icing from SLD at 500' agl, but that is not freezing rain.

[White Hart]

For those of you who fly into/out of Heathrow - how much notice do you take of the weather reports given out on the ATIS? Do you ever question or doubt the info being transmitted, especially with something as safety-relevant as FZDZ/FZRA.

I wonder where the buck stops if we (the new met people) don't give out FZDZ/FZRA in the reports when it is actually present. My Met instructions say I can only put FZ anything if the automatic temperature reading is 0deg or a minus reading. Can you get ice forming on aircraft surfaces when the temp is hovering very close to 0degrees? We've been discussing the issue of on-board aviation fuel actually cooling the wing surfaces sufficiently to cause ice to form - hence the evident need for de-icing even when the ATIS auto temp is reading slightly above zero.

Are we being led up the garden path with this fuel/icing issue? Comments appreciated.

[Founder]

For those of you who fly into/out of Heathrow - how much notice do you take of the weather reports given out on the ATIS? Do you ever question or doubt the info being transmitted, especially with something as safety-relevant as FZDZ/FZRA.
I wonder where the buck stops if we (the new met people) don't give out FZDZ/FZRA in the reports when it is actually present. My Met instructions say I can only put FZ anything if the automatic temperature reading is 0deg or a minus reading. Can you get ice forming on aircraft surfaces when the temp is hovering very close to 0degrees? We've been discussing the issue of on-board aviation fuel actually cooling the wing surfaces sufficiently to cause ice to form - hence the evident need for de-icing even when the ATIS auto temp is reading slightly above zero.
Are we being led up the garden path with this fuel/icing issue? Comments appreciated.

AEA has at least taken this into consideration, they've addressed the problem in their report regarding de-/anti-iceing... It might not give any real clarity but it's good reading anyway...

check page 35 I think it was on the "20th Edition September 2005" document

http://www.aea.be/AEAWebsite/Presentation_Tier/Pr_GroupMenuItem.aspx?NodeID=rootMenu398

[White Hart]

Founder

many thanks for that. yes, page35 does have details on the wing icing/fuel issues.

Is it an operational requirement to listen to the local met/ATIS broadcasts before deciding if de-ice checks are necessary? The reason I ask is because I'm trying to clarify if the accuracy of local Met reporting has any influence on such procedures, particularly when local conditions are borderline FZ, but not reported as being FZ.

[Ice-bore]

........Can you get ice forming on aircraft surfaces when the temp is hovering very close to 0degrees? We've been discussing the issue of on-board aviation fuel actually cooling the wing surfaces sufficiently to cause ice to form - hence the evident need for de-icing even when the ATIS auto temp is reading slightly above zero.
Are we being led up the garden path with this fuel/icing issue? Comments appreciated.

Not a particularly common occurrence but you can get frost/ice forming on both upper and lower wing surfaces at OAT's of up to +15 deg C and above, where sub-zero fuel is in direct contact with the wing skin.

The subsequent fuel uplift will normally increase the temperature of the fuel, however, it is often insufficient to raise the temperature of the wing enough to prevent the subsequent formation of frost or ice. It can lead to serious problems, with significant deposits of clear ice forming on the surfaces of wings when it's raining.

Due to the current tankering policy at LHR aircraft are arriving with relatively large quantities of very cold fuel (fuel temp may be as low as -20 deg. C on arrival) and are having to be de-iced, even when the OAT is above 0 deg. C.
An explanation may be found in 3.3.5 on page 5 in the latest edition of the AEA De-icing/Anti-icing Recommendations.