This paper relates the different types of ice formations that can occur on airplanes and the dangers that they pose

Kopp, W

English

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MATIOI?ALADVISORY COWITTEE FOR A)ZROHAUT1C8
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DAHGER OF.ICE I’ORMATIONOl?AIRPLANES
By W. Ko~
Lindenberg AeronautloaZ Observato+, Berlin
,
Washington
February, 1929
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https://ntrs.nasa.gov/search.jsp?R=19930090860 2020-06-17T03:52:04+00:00Z
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NATIONAL ADVISORY’COMMITTEE FOR AERONAUTICS.
TECHNICAL tiORANDUM No. 499.
.-
DANGER OF ICE FORMATION ON AIRPLANES.*
By W. Kopp-
Aeronautioel experts exphaslze the fact that ice formation
on airplanes occurs comparatively seldom. It may, however, In
individual cases, become such a menace as to render a complete
account of”the ccmditions for its occurrence very desirable.
Such an account will help the meteorologists to issue timely
wexnlngs against this denger and may”also open the way toward
its control or prevention. In a previous paper, Dr. Hebner**
has already brought together a few viewpoints on this problem
without, however, aspiring to make his report ~ complete ac-
count. A few additional and correcting remexks may therefore
be permitted.
In commercial air transportation, cases of ice formation
are much less frequent than in trensooeanic or other long-
distance flights for special purposes, during which It may be
unavoidable to fly through extensive cloud exeas. It is neces-
sary, however, to consider the ice danger even in commercial
flying, since cases have occurred already where regular trans-
-. port airplanes have had to climb to higher levels, in lrder
-.
*From ‘fErf&hrungsberichtedes Deutschen Flugwetterdienstes”
(Reports of the German Aeronautical Weather Service). This
translation was made and submitted by The Daniel Guggenheim -d
for the Pro~otion of Aeronautics.
**Volume 1, No. 25, of this series of reports.
l
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N.A.C.A. Technical Memorandum No. 499 2
to avoid dangers of this type.
.Mr. ,Hebner:sreport dlsousees the danger df ioe formation
..
not only when flying in fog and Llouds, ‘K-atf:.’.:n :-l---.::‘--
the rain Is falling from warmer through colder strata with tem-
peratures below freezing. Unfortunately, no actual irv3tances
of ice formation ~e quoted, from which the nature of the ice
and the immediate oause of the eventual foroed landing may be
deduced. It would be a great help if the meteorologist had the
opportunity to gather information in this respect, so that he
.
would know just what particular point to keep in mind in reeking
his forecast. Due to this lack of data, it will be
to discuss the problem from a purely meteorological
As emphasized by Hebner, In fog (or clouds) at
necessary
angle.
low temper-
atures, the Ice coating is formed by subcooled water droplets
congealing as they strike the airplane, the temperature of which
is likewise below freezing. However, it seems to be of the
greatest importance to discuss the nature of the deposit in a
more general way. Three main types may be distinguished.
1. Hoarfrost (Rauhrelf) is formed by sublimation and
consists of fine crystals, arranged in the direction of flight
of the airplane, on the windwsxd edges of the wings, etc.
This type of deposit will h~dly affect the maneuverability of
the airplane, or cause any trouble by changing the pr”ofileof
the wings or propellers. The deposit is very seldom so thick
that the increased load would necessitate a lending. This type
—
. ..-—
.- .
N.A.C.A. Technical liemorand~ No. 499
%
3
of deposit ZEy form when the fog is very thin or even unnotice-
—.-
.“-”able. It is usually assumed that a temperature below -lO°C
(14°F) Is required for the formation of hoarfrost.
39 Rime (Ratifrost) occurs when the number of water .
droplets In the air is l~gez~. In this case the visibility
is, of couxse, poorer and a flight is therefore rarely started. -
However, since humidity conditions may show strong local vsxi-
tions, one cannot always avoid flying through dense fog banks.
Then a considerable deposit of rime may occur which, however,
affects the maneuverability of the airplane less than it adds
weight ~nd cl.angesthe profiles. The latter changes probably
affect the I?ingsless than the propellers. Of course the lat-
ter will not, as stated by Hebner,work irregularly and lose
R.P.:d.(which is the affair of the engine), but they will lose
in efficiency.
3. Ice (Itauheis)forms when the temperatures exe not
.
very low and the subcooled drops are so large that they flow
together before congealing. In deposits of this kind the bad
.
effects on the maneuverability must be considered as well as
the Increase in weight, while the effect on the profiles is of
less importance. It should also be added that, in case of ice
formation, the visibility may be relatively better, due to the
larger drops, than in the previous case, whioh may lead.to -
underestimation of the danger. However, as has been stated be-
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N.A.C.A. Technical Hemcrandum No. 499 4
fore, the experience of the pilots themselves must finally de- “, .... .. . .--.
-. .,..
-termine which type of deposit should be regarded as most denger-
.Ous9
It is hardly possible to state, even approximately, how
fax an airplane can fly through subcooled fog. In one case it
Is possible to travel long distances (even several hundred kil-
ometers); in ancther case the catastrophe may come within 5 or
10 kilometers (3 to 6 miles). At my rate, it is pecullex that,
under apparently similar conditions, ice will sometimes form,
but at other times will not. Often, after a flight which Is
trouble-free at the start, ice will form with great rapidity.
This seems to indicate that, besides the subcooling, another
factor must slsc be considered, namely, the strong supersatur>
tion of the fog with ice. Before enterin& intc a discussion of
this factor, we must, however, determine when ice formation in
fog is most likely to occur.
Few experimental data are available on this question. We
may, however, with a certain amount of justificatlen, make use
of the inaterlaloollected by W. Peppier* concerning
tion on kites and kite strings in clouds, since the
ice form~
principle
is the same, whether we speak of olouds or of fog. Peppler~s
-.
‘table is”reproduced below. .- .,,, .
*Beitr&e ~r physik der freien Atmosphere, Vol. 10.
N.A.C.A. Technical Memorandum No. 499 5
Frequency of ‘““---- ‘---‘– ‘ ‘- ‘ ‘- ““ - ‘-
. . T&p~r: “4;1
ature n
.In” O
1
I
o 10
I?E@
-0.1 -2.1 -4.1 -6.1
— —
-2.0 -4.0 m -8.0
\52 47 34 211.Frostformation 2Iceformation 1
From these figures i+ may be seen that fro~t or rime rhost
18 8 2 -4.0
1 1 0 -1.0
frequently occurs with temperatures around -4°, and ice with tem-
peratures around -1°) at the base of the cloud layer: The tezn=
perature in the interior of the cloud may therefore be assumed
to be approximately -6° in the former, and -4° in the latter case.
Pepplar slso gives the temperature
at times of ice or frost formation
clouds):
Fog Nimbus Stratus
-2.7° -1 l 0° -~.~o
for the vexious cloud form!
and finds
O&lulus
-3.2°
Rime “orfrost axe thereforq corresponding to
tures, more frequent with stratiform clouds,
frequently formed”in nimbus clouds. This is
the following table given by Peppier.
,.,
. . Stza.tus
Rime or Frost
formation 110
Ice
f~rmatlon 38
(at the base of the
Stratocumulus
+. lo
the lower temper-
while ice is more
clearly shown by
I Il!mlbus cumulus ::*:
62 6 40
56 4 18
Alto-Stratus &
Alto-Cumulus
1
3
Immmmmn II I II n— ——, . . . . . ..——. . .,. . . . — . .
F. —.. .—. .— . .....
*
N.A.C.A. Technical Memorandum No. 499 . 6
.
,The data below are ch~acteristio of a typloel rime-prdduc-
.-,.. .
--.... .
. . . .-
ing cloud:
Cloud Cloud Mean Temperature
base
Temperature lapse
top temper- lapse rate rate in cloud
ature ground-cloud
base
530 m 950 m -5.0° 0.6E1°C/100m 0.65
Judging from the lapse rates, this is a caee of an almost
saturated air mass from the ground up to the cloud. Frequently
fog actually occurs in the surfaoe layers. The conclusion is
therefore obvious that fogs and clouds at the
peratures are especially dangerous to flying.
above, under almost identical conditions, ice
cur
ice
the
and sometimes not.
The statisti~al material.,concerning the
above quoted tem-
However, as stated
may sometimes oc-
relatlon between
OF frost formation and the weather situation, shcws that
danger is greatest in the transitional zone between a 10%
pressure and a high-pressure area.
Cases of Ice or Frost
Cyclone I Zone oftransition I Anti cyclone
. West North East South West ‘North East South
* 41 , 14 16 76 95 17 16 15 33
The frost and ice formations here discussed may, of course,
also occur on the ground but, even if allowed
long-time intervals, do not become so intense
—- .
to continue for
as to Indicate
-— —
any
N.A.C.A. Technical Memorandum No. 499 .7
rq+, danger. We must therefore consider another element resgmn-
. ..-
.,, . ~
.+
.. . . .
. ..
sible for the suddenness of the ice formatlcn, This ‘elementis”
.
the occasional strong supersaturation of the atmosphere with ice.
At the negative”temperatures with which we are concerned,
the supersaturation is easily oomputed as the difference between
the observed relative humidity-and the relative humidity to be
expected in case of saturation over Ice.
If we represent graphically the mount of supersaturation
over ice at varicus negative temperatures, assuming the recorded
relative humidity to be 100%, we obtain the curve in Figure 1.
If an airplane should meet with such supersaturation, ice
or frost will form end we are probably justified In regarding
the curve as a measure of the flying possibilities in temperw
tures below 0° and with 1OO$ relative humidity.
$ubcooling and supersaturation are the usual phenomena at
temperatures belGw freezing. The intensity may vary, how~ver.
In typical winter stratus with very cold air below and warn air
above, the supersaturation often begins at the lower surface
and increases upwexd. Not Infrequently the winter stratus lo-
celly reaches clear down to the ground, in spite of which the
horizontal visibility in the surface layer may be sufficient
. ,.
from the viewpoint of flying. Exea@es of this nature wlllbe ~
given below. The following observation shows how strrng the
supersaturation may be, even at the ground. During the winter
.l
N.A.C.A. Technlfisl
of 1927, I had the, . .a,”& ,-.
1 om at the ground.
tops of the 80-meter
Asoendlng one of the
a l-centimeter thick
Memorandum Ho. 499 8
opportunity to observe a frcst deposit of
,,--
The horizontal visibility was 1.5 km. ,The
radto towers were occasionally obscured..
towers, I found that at 20 m elevation
rope had a m~imum diameter of 3 cm, while
the same rope at 80 m had a m~imum diameter of 9 om frnm the
deposit of rime. An airplane from the east might easily have
been forced to fly tow~d its goal.at & elevatim. Of 80 to
100 meters. Very intensive ioe formatlm would then have re-
sulted. Still thicker frost formations sxe often observed in
the 8g0 to 1000-meter mountains in central.Germany (Mittelgebirge)
where a deposit of 3 cm per h~ur may ofterlbe ~bserved. still
higher mountains are ofter,free from depcsits since they reach
through the frost clouds into the dry layers abovo.
The danger of flying in supersaturated air layers is in-
creased by the more or less rapid sublimation of water vapcr
(high pressure over water) on the already ice-coated airplane
(lower pressure over im). . This pr~cess is Intensified if the
ice-ooated airplane already has a lower temperature than the
surroundhg air. To illustrate this, me may point to an exam-
ple from the clcud theory. If an ice crystel from a oolder
. .
layer falls through a warmer but supersaturated water oloud
amorphous ice is then deposited very rapidly on the crystal.
A. Wegener believes that the greatest known hailstones have to
be explshed in this way. If} instead of an ice crystal, we
.N.A.C.A. Technicsl
assume an tirplane
*..- ..... ....,
Memorandum No. 499 9
to be coming from a colder into-a warmer
. ...
supersaturated layer, then an Intensive’-oreven”catastrophicd
ice formation is possible. One may ask if the airplane does not
. .%
very rapidly assume the temperature of the surrounding ~ir, es-
peolelly if it is en all-metal.one. However, in the first place~
the changes In elevation ususlly take place so rapidly as to
make a certain lag in the temperature unavoidable. In the seo-
ond place, the ioe ooating on the aluminum is strongly insulat-
ing, as is best seen from a comparison of the heat conduction l
coefficients of eluminum and ioe (aluminum, 3&35; ice 0.568,
0.5, 0.23, according to the nature of the ice).
The just-described conditions for sudden end violent ice
formation may occur:
.
1. During flights in surface fog.
2. When the airplane descends from colder through warmer
(supersaturated) layers.
There is special danger in these phenomena during trans-
oceanic flights.
It is therefore difficult to agree with Hebner, who hold~
the opinion that the subcooled and supersaturated layers in the
atmosphere zue of small extent. On the oontrary, these layers .t . .
may be very extensive, both verticeJ.lyand horizontally, especi-
ally at the beginning of a new period of cyolonic activity. As ‘
already shown, this is slso the time when the ice danger raaches
its maximum. It is just when the frost on the ground begins to
,
N.A.C.A. Technloal Memorandum No. 499 10
- melt and,rain,_or sleet begins to fall that warning against flight
-.
“
should be given, even If the temperat~e at the groun”dmay al-
ready be around or even above freezing.
Lastly, It.is @so desirable to take a stand with reg=d
to sleet (lisregen). Sleet (frozen raindrops) will not in it-
self lead to ioe formation. The rain that hits the airplane “
and stioks to It wI1l hardly produce any real change of profile,
but may affect the maneuverability of”the airplane to a consid-
erable degree. Eventually, it should be possible to guard
against this.result. It may be mentioned that fslling subcooled
water droplets have often been observed in spite of the friction
currents which might prevent the subcoollng. In the case of
subcooled rain, the ice coating on the airplane must necessarily
be thinner than the corresponding ice formation on the ground,
since its thickness depends only on the time during whloh the
.
still or moving object is e~osed to the rain. The thickest ice
, layers formed.on the ground by rain below the freezing point
.
“have had a thiokness of 2 to 3 centimeters. This would corre-
spond to a load of about 20 kilograms per square meter. This,
however, is an extreme case.
.%. en .airplsnes,rime plays much
In the problem of ioe fermation
the more hportsnt role.
.
H.A.C.A. Technical Memorrmdum 3G. 499 U
Now let us glance at a few examples of the various types
. ...1.
-.7. . . . . . .
of ice formation.
,, . . . .
Ascent of December 30, 1927, at Lindenberg
. Elevation Temper- Supersat- Vislbility
m ature Humldity uration
%.deg.
at ground
In $
~gg (ground} -6.0 95.0 0.6 71un
I -9.0 91.7 8.3
Stratus clouds, frost and Ice on kite stringti.
Tcke-off possible, but supersaturation increases rapidly “
with elevation, therefore danger of ice formation in spite of
rather good visibility.
December 3, 1927
Elevation 1 Temper- Supersat- Visibility
m ature Humidity uration at ground
deg. $$ in $ light fog
~_~ (ground) ~ -6.0 92
I
34 km
-8.0 98 5:4 ,
400 I 0.0 40
Stratus at less than 100 meters above the ground; frost on kite
6tring.
Take-off possible, but supersaturation increases upward,
therefore danger of ice formation in spite of rather good visi-
bility.
December 14, 1927
Elevation Temper- Supersat-
m“ ature uratlon visibility
.,, deg. in $
1~~ (ground) -0.2 “98 “ .- -At.ground
-2.0 100 1.9 800 m;
920 -6.0 100 5.6 Vertical
visibility
110 m.
Frost on string, 7 mm of Ice, kites will not stay in air.
,
ii”
N.A.C.A. Tecb@iosl Memorandum No. 499 12
...~~ho~h- both vertical and horizontal risibilities may be re-
.. ..-
..
garded as sufficient, a flight would be dangerous. I?otelespe- .
ci~ly that the temperature at the ground Is around 0°0, from
which fact one should not conclude that the dsnger is small.
The gree.testdanger of ioe formation is in flying downwind
through this layer of hir.
December 22, 1927
.
Elevation Temper- -“ Supersat- Visibility
m ature Eumldity uratiom
deg. $
at ground
In $ hazy
~Zl~(ground) r 7.0 92.5 4km
-10.0 95.0 4:1
.
600 - 2.0 100.0 1.9
Intensively suboooled rain (Eisregen), instrument and kites
covered with ice. Cloud base between 200.and 300 meters.
The visibility would permit a flight. Supersatwation rap-
idly increases with elevation- An atteMpt to fly throughthe
cloud layer in order to find layers with temperatures above
freezing would be a mistake. The drplane would reach supersat-
urated air at -2.0°C with a temperature difference of 8°U’and
with a coating of ice.
In summsrizlng, the followlng may be said:
1- Rime seems to play the more important roJ.eIn the for-
mation’-ofiv on drplanes, and forms rapidly in supersaturated
..-
layersti “
2, Rain fslling from warmer through colder layers, seems
M:A.C.A. Technical Memorand~ No. 499 13
.
.
to be of secondary importance.
. ..-~,.
3. ‘-The-te-fier-a\-ures’in case of ice formation are slight-
ly below O°C, in rime formatlcn between -20 ~d -IOOC. At very
lew temperatures ice formation is rather unusubl.
never occurs,4. In very high clouds ice formation almost
a~ e~erience In the airplane ascents at the observatory at
Lindenberg has shown.
5., Rime is deposited In
other mcteritis.
the same may on metal, wood and
disregard the possibility of. 6. It would be wrong to
Ice formation when the temperatures slong the whole flight path
are above freezing and rising. At a very low elevation above
the ground, the possibility of ice fcrmation may again occur~.
Very often the strcng winter temperature inversion has already
disappeared when the temperature, after a period of”frost, again
reaches freezing.. In other words, no layers with pos~tive tem-
peratures (above freezing) will then be found at higher elev~
tions.
7; If the instrumental arrangements
L... .
so allow, and in case of ioe danger under-a”
from.which there is no precipitation, it is
.
on board the airplane
tinter stratus layer
usually advisable to
fly through the clcud layer. Ususlly one will find very w8,rm
14N.A.C.A. Technloal Memorandum No. 499
and extremely dry air even below 1000 meters.
,-.. ,..
8; Foreca&%-#”02“warnt~s OY ice dsnger, which should
not be Issued more frequently than absolutely neoessary, are
possible only with the aid of temperature
of the ‘upperair.
.
and humidity soundings
90’ Lastly, it may agalmbe emphasized that as d?tailed
Infor?qationas possible regarding the charaoter of the ioe
coating is desirable.
,
Translation by
The Daniel Guggenheim -d
for the Promotion of Aermsutias.
.
.-.
. ..—
l—
N.A.O.A. Tednioal Memorandum No.499
.
*“, . . . . . ,.. -
.,.
,., -..
.-.. . . . . --
O(y
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
l’ig.l
.
I I 1
-,.
,.l
0.1
g/m3
Flg.1 Supersaturation
water per m3 of
0.2 0.3
Ingof
air at
. different temperatures
by 100$ reoordkd
relative humidity. .
JI
I
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[,, ,N~Tfiaiu~~l,l , L
3 l1760i440 2243 ‘
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Danger of ice formation on airplanes

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