Icing Airmets

PaulaJay1 ) wrote:

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?

--
Peter












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Both sentences actually. Recent studies of airframe icing have not been
able to produce icing above 0C. (Sorry I don't have a reference). If the
static air temperature is above 0C then the water isn't supercooled, so even
if the airplane is at or slightly below freezing you won't get ice. The
wing's leading edge experiences a temperature increase, not a decrease.

I suppose that it is possible to locate the temp probe in an area with a ram
temperature rise greater than the wing. In that case, it is possible for
the temp guage to read above 0C but still have the static air temperature
and portions of the airplane below 0C. That would lead to icing but it is
an instrument error not icing above 0C.

Mike
MU-2


"Peter R." wrote in message
...
Mike Rapoport ) wrote:

You can't unless your thermometer is wrong.

Are you replying to Dave's first sentence, which is:

I'd like to know how you get in icing when the temperature is +5.

Or his second:

I have never seen ice until the the gauge reads 0 or below.

Or both?

If you are replying to his second sentence, this is contradictory to my
understanding that the temperature right at the leading edge of an airfoil
could be up to a few degrees colder than the surrounding air, which is why
I learned that airframe icing (excluding freezing rain) can occur in air
temperatures above freezing.

I should point out here that I am simply seeking clarification of your
comment, not challenging it (as I look to you as one of the more
experienced in these groups).

--
Peter












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Yes I am comfortable flying in temperatures above 0 and I do plan
accordingly. Although as soon as I see 0 on the OAT then I also plan
accordingly. (Like change altitude , turn around, land etc.)
"Peter R." wrote in message
...
Dave ) wrote:

How do you know your guage wasn't wrong?

I forgot to include the point of my adding to this thread: I use these
experiences (granted that they are somewhat limited to two winters of IFR
flying downwind of the Great Lakes, US) of encountering ice above 0c when
flight planning a cruise altitude, as well as when making a go/no decision
due to ice.

Are you implying that you are comfortable that ice will *not* occur above
0c and therefore plan accordingly?

--
Peter












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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. That's why you look at those surfaces
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

"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.




"ArtP" wrote in message
...
On 26 Jan 2004 10:08:44 -0800, (Andrew
Sarangan) wrote:

For example, right now the freezing level is at 5000 ft,
MEA is at 2500 ft, yet we have an airmet Z that goes all way from SFC
to 18k.

Since the published icing conditions occur from +5 to -20 C, a
freezing level at 5000 would mean icing potentially below 3000 feet.
The MEA may be 2500 but the normal minimum IFR altitudes would be
either 3000 or 4000 feet depending on direction and that is known
icing conditions. I am sorry this frustrates you but depending on
where you live a big chunk of winter is off limits to small GA
aircraft trying to fly IFR.

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. Don't know if the battery was weak or the electronics
just gave out. I didn't think that this was a problem since the inside of the
plane is at least 25deg F G or higher. The 45 deg was the refrigerator and
the 65 was outside temperature.

Chuck

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.


"Mike Rapoport" wrote in message hlink.net...
You can't unless your thermometer is wrong.

Mike
MU-2

"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.




"ArtP" wrote in message
...
On 26 Jan 2004 10:08:44 -0800, (Andrew
Sarangan) wrote:

For example, right now the freezing level is at 5000 ft,
MEA is at 2500 ft, yet we have an airmet Z that goes all way from SFC
to 18k.

Since the published icing conditions occur from +5 to -20 C, a
freezing level at 5000 would mean icing potentially below 3000 feet.
The MEA may be 2500 but the normal minimum IFR altitudes would be
either 3000 or 4000 feet depending on direction and that is known
icing conditions. I am sorry this frustrates you but depending on
where you live a big chunk of winter is off limits to small GA
aircraft trying to fly IFR.

This was gleaned form talking to a Phd (meteorology) instructor,
Citation, Super Cub pilot.

Water will not freeze until it is 0 C or below.
Just because it is 0 degrees or below doesn't mean water HAS to
freeze.
There is themometer error, but there are also lower pressure areas on
the wing/airframe, and due to the lower pressure, the temperature
lowers.
It is possible to get ice when your themometer reads above zero, but
it will be zero or below where the ice is.
If you get ice on your themometer and it reads above zero, your
themometer is out of calibration.


Peter R. wrote in message ...
Dave ) wrote:

How do you know your guage wasn't wrong?

The digital gauge was calibrated when new and the aircraft is about one and
one half years old. Is it possible that it slipped out of calibration a
degree or two? I suppose.

But since every icing article I have ever read states that icing can occur
at temperatures higher than 0c, I have no trouble believing that my icing
encounters above 0c actually did occur one to three degrees above 0c.



--
Peter












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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?




ArtP wrote in message . ..
On 26 Jan 2004 10:08:44 -0800, (Andrew
Sarangan) wrote:

For example, right now the freezing level is at 5000 ft,
MEA is at 2500 ft, yet we have an airmet Z that goes all way from SFC
to 18k.

Since the published icing conditions occur from +5 to -20 C, a
freezing level at 5000 would mean icing potentially below 3000 feet.
The MEA may be 2500 but the normal minimum IFR altitudes would be
either 3000 or 4000 feet depending on direction and that is known
icing conditions. I am sorry this frustrates you but depending on
where you live a big chunk of winter is off limits to small GA
aircraft trying to fly IFR.

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.

On Tue, 27 Jan 2004 20:33:12 -0500, Roy Smith wrote:


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.

That is all we are discussing, whether the FAA +5 to -20 is a
reasonable range for icing.