Icing Airmets

On 27 Jan 2004 17:05:49 -0800, (Andrew
Sarangan) wrote:

This is not about frustration, but about trying to understanding how
FAA comes up with airmets. An airmet is supposed to be issued when
moderate icing is expected. If they are issuing an airmet every time
there are clouds and freezing temperatures (or even above freezing),
then what about light icing?

Unless your plane is certified for known icing then airmet or not any
icing potential means you can't legally fly. Arguing over moderate
versus light is academic since either is prohibitive..

"Dave" wrote in message
ink.net...
| I'd like to know how you get in icing when the temperature is +5. I have
| never seen ice until the the gauge reads 0 or below.

Icing occurs when liquid water freezes on an airplane surface that is below
freezing. The aircraft may have been cooled earlier when it flew through a
layer.

Per your next reply -- if you don't trust Peter R.'s gauge to say it is +3
when he is getting icing, what makes you think you can trust your gauge?

"PaulaJay1" wrote in message
...
| In article , Peter R.
| writes:
|
| I brought it
| home to calibrate it and found that it read 2 deg high at 25, 45, and
65
| deg.
|
| How did you calibrate your thermometer for 25 degrees?
|
|
| I put it and two reference thermometers in the freezer part of the
| refrigerator. I put them in the full freezer that is about 0 deg F and
the
| digital did not display.

Digital displays usually fail in cold temperatures. There is nothing odd
about that.

"Andrew Sarangan" wrote in message
om...
The temperature above the wings and below the elevators could be
slightly below ambient due to the lower pressure. I don't have a
number on what the temperature drop is on these surfaces, but
technically it is possible to have icing on the lifting surfaces when
the ambient temperature is above freezing. Sort of like carb icing in
above-freezing temperatures.


There has been a lot of hypothisizing about this in the past but NOBODY has
EVER been able to reproduce it.

You don't get icing just because a portion of the airplane is below 0C. I
doubt if droplets even touch any part of the airfoil where the
temperature/preasure is below ambient without running back. If they run
back, the first encountered the heated portion of the wing (leading edge).
You need supercooled water to get airframe icing.

It is another of aviations myths

Mike
MU-2
..

"Bob Gardner" wrote in message
news:XKARb.132385$nt4.579289@attbi_s51...
Any time air is accelerated, as it is when passing over a small-radius
surface, its temperature drops...so it is entirely possible to accrete ice
when the temp is above zero.
first...OAT guage, struts, lower edge of windscreen where there is a lip
rather than a flush surface, etc. That is also why tail feathers begin to
accrete ice before the wing's leading edge does.

Bob Gardner


While small radius objects do collect ice better than larger redius objects,
temperature drop has nothing to do with it. Small radius objects have a
higher "collection efficiency" meaning more of the droplets in their path
will impact the surface. They have a higher collection efficiency because
they don't project a "bow wave" as far in front of them as larger. You NEED
supercooled water for airframe icing.

Mike
MU-2

"C J Campbell" wrote in message
...

"Dave" wrote in message
ink.net...
| I'd like to know how you get in icing when the temperature is +5. I have
| never seen ice until the the gauge reads 0 or below.

Icing occurs when liquid water freezes on an airplane surface that is
below
freezing. The aircraft may have been cooled earlier when it flew through a
layer.

Almost. Icing occurs when a below freezing aircraft encounters supercooled
water. Supercooled water does not exist above 0C.

Mike
MU-2

This is not about frustration, but about trying to understanding how
FAA comes up with airmets. An airmet is supposed to be issued when
moderate icing is expected. If they are issuing an airmet every time
there are clouds and freezing temperatures (or even above freezing),
then what about light icing?
Unless your plane is certified for known icing then airmet or not any
icing potential means you can't legally fly. Arguing over moderate
versus light is academic since either is prohibitive..

The other crazy thing is people are arguing over 2 degrees F. I don't
care how accurate your thermometer is, but if you are that close
to potential icing, get out of there. +5 certainly gives you the
margin and sure the FAA will err on the larger side but still,
are you watching the OAT in your instrument scan....."one more
degree drop and I turn around....." Crazy.

Gerald

Gerald Sylvester ) wrote:

are you watching the OAT in your instrument scan....."one more
degree drop and I turn around....." Crazy.

LOL!

--
Peter












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Roy
Thanks for that explanation. I did not realize that it was evaporative
cooling that was the cause of cooling in the carb. A lot of texts out
there describe that the cooling is due to pressure drop. It makes
sense that evaporation gives out a lot more energy than adiabatic
expansion.


Roy Smith wrote in message ...
In article ,
(Andrew Sarangan) wrote:

The temperature above the wings and below the elevators could be
slightly below ambient due to the lower pressure. I don't have a
number on what the temperature drop is on these surfaces, but
technically it is possible to have icing on the lifting surfaces when
the ambient temperature is above freezing. Sort of like carb icing in
above-freezing temperatures.

You're talking about two very very different things.

When a gas undergoes adiabatic expansion, it gets cooler. There is no
doubt that this happens at the leading edges of airfoils, but at the
pressure drops we're talking about in any kind of airplane I'm likely to
fly is very small. How small is very small? I'm not sure, but I can't
imagine more than a degree or two.

Yes, you in the back? What's that? You think I'm trying to befuddle
the issue with big-sounding words like "adiabatic"? OK, all adiabatic
means is that there's no exchange of heat. We all know that gasses get
hotter when you compress them. You probably learned Boyle's Law and
Charles's Law in high school chemistry, or maybe the Ideal Gas Law.
These are all just different ways of saying that if you've got a certain
amount of gas which contains a certain amount of energy, if you know any
two of pressure, volume, and temperature, you can figure out the third.
As long as you don't add or subtract energy (i.e. heat), you can play
with those three variables to get all sorts of different combinations.

All those "no heat lost or gained" transitions are adiabatic. That's
what happens at the leading edge. The air moves from an area of high
pressure to an area of lower pressure on top of the wing. As it does,
it expands and cools, but the total amount of energy in a given parcel
of air stays the same.

This is not to say that adiabatic cooling can't cause very large
temperature drops. Anybody who has ever fired off a CO2 fire
extinguisher knows that the gas coming out is VERY cold, and that is an
adiabatic process. But it's also undergoing a pressure drop orders of
magnitude bigger than what goes on at the leading edge of a spam can.

Carb icing is a totally different fish. What's going on inside a
carburator is liquid gasoline is evaporating and turning into vapor.
There's a phase change. It takes a huge amount of energy to effect a
phase change. The air that enters the carburator is NOT undergoing an
adiabatic process; it's giving up energy to the gas to make it vaporize.
That's why you get huge temperature drops inside the carb, and can get
carb icing at ambient temperatures way above freezing.

ArtP wrote in message . ..
On 27 Jan 2004 17:05:49 -0800, (Andrew
Sarangan) wrote:

This is not about frustration, but about trying to understanding how
FAA comes up with airmets. An airmet is supposed to be issued when
moderate icing is expected. If they are issuing an airmet every time
there are clouds and freezing temperatures (or even above freezing),
then what about light icing?

Unless your plane is certified for known icing then airmet or not any
icing potential means you can't legally fly. Arguing over moderate
versus light is academic since either is prohibitive..

But what if the airplane were certified for known ice? Would it not matter then?