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Aircraft Icing
“FAR 25, Appendix C charts”
Prof. Dr. Serkan ÖZGEN
Dept. Aerospace Engineering, METU
Fall 2015
Outline
• FAR 25 and FAR 29– Appendix C charts
• Using FAR 25 Appendix C charts
• Liquid water content as a function of horizontal extent
and ambient temperature
• Liquid water content as a function of horizontal extent
and droplet size
• Alternative ways to document test data and compare
with Appendix C
• Water catch rate (WCR) and total water catch (TWC)
• Icing severity definitions
• Variation of icing severity as a function of horizontal
extent and ambient temperature
• Comparing test data with natural probabilities
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FAR 25 and FAR 29 Appendix C charts
• FAR 25 App. C consists of 6 figures.
• Has been in use since 1964 for selecting values of
icing-related cloud variables for the design of inflight ice protection systems for aircraft.
• First 3 figures are known as “continuous
maximum” conditions representing stratiform icing
conditions or layer-type clouds.
• The last 3 figures are known as “intermittent
maximum” conditions representing convective or
cumuliform clouds and icing conditions.
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FAR 25 and FAR 29 Appendix C charts
• Traditionally, continuous maximum conditions
have been applied to airframe icing protection,
• Intermittent maximum conditions have been
applied to engine ice protection.
• Figures 1 and 4 indicate the probable maximum
(99%) value of cloud water concentration (liquid
water content – LWC) expected over a specified
reference distance for a given temperature and
representative droplet size in the cloud.
• Reference distance: 17.4 nm (20 statute miles) for
continuous maximum clouds,
• Reference distance: 2.6 nm (3 statute miles) for
intermittent maximum clouds.
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FAR 25 and FAR 29 Appendix C charts
• The actual drop size distribution (typically 1-30
microns) in clouds is represented by a single
variable – droplet median volume diameter (MVD).
• Overall MVD≈15 microns in stratiform clouds,
• Overall MVD≈19 microns in convective clouds.
• The MVD has proven useful as a simple substitute
for the actual droplet size distributions in ice
accretion computations.
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Continuous maximum (stratiform)
atmospheric icing conditions, Figure 1
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Continuous maximum (stratiform)
atmospheric icing conditions, Figure 2
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Continuous maximum (stratiform)
atmospheric icing conditions, Figure 3
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Intermittent maximum (cumuliform)
atmospheric icing conditions, Figure 4
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Intermittent maximum (cumuliform)
atmospheric icing conditions, Figure 5
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Intermittent maximum (cumuliform)
atmospheric icing conditions, Figure 6
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Using FAR 25 Appendix C charts
• There is no comprehensive guide for using,
interpretation and application of Appendix C.
• Design engineers typically select a conventionally
recommended MVD and a temperature
appropriate to the flight level of concern and use
them to obtain the probable LWC from Figure 1 or
4 of Appendix C.
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Using FAR 25 Appendix C charts
Selecting exposure distances (HE)
• LWC values obtained from Figure 1 or 4 are valid
only for the reference distances of 17.4 nm or 2.6
nm, respectively.
• If there is a reason to design for a longer or shorter
exposure distance, the LWC originally selected may
be reduced or increased by a factor obtained from
Figure 3 or 6 in Appendix C.
• Longer averaging distances will result in lower
maximum LWC.
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Using FAR 25 Appendix C charts
Selecting exposure distances (HE)
• Common applications:
– To estimate ice buildup amounts on unprotected
surfaces during a long exposure of 100-200 miles. LWC
obtained from Figure 1 is reduced by the factor obtained
from Figure 3.
– To estimate ice buildups on unprotected surfaces during
a 45 minute hold. LWC obtained from Figure 1 is used at
full value, without reduction. This assumes the worst
case in which the holding pattern happens to be entirely
within a 17.4 nm region of cloudiness containing the
maximum probable LWC.
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Using FAR 25 Appendix C charts
Selecting MVD values
• Common applications:
– For computing the impingement limits of droplets
(chordwise extent of ice accretion) on an airfoil an
absolute droplet diameter of 40 microns is used.
– In general, MVD=20 microns is used for the computation
of ice accretion amounts for standard exposure distance
(17.4 nm) or longer.
– Another reference recommends the use of the entire
range of MVDs. The designer is advised to consider
exposures to droplets with an MVD up to 40 microns
over distances up to 17.4 nm at least.
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Using FAR 25 Appendix C charts
Difficulties comparing with test data
• Users often wish to plot the points representing
combinations of LWC, MVD and temperature used in
–
–
–
–
Wet wind tunnel tests,
Computer simulations,
Test flights behind airborne spray tankers,
and test flights in natural icing conditions.
on Figures 1 and 4.
• The problem is that these figures are valid only for the
fixed averaging distances.
• A better way is to convert Figures 1 and 4 to
equivalent, distance based envelopes where the LWC
curves have already been adjusted for the distance
effect.
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Continuous maximum LWCs converted to
distance adjusted values
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Intermittent maximum LWCs converted to
distance adjusted values
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Appendix C curves converted to
distance based format (MVD=15m)
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LWC as a function of HE and Ta
(Continuous maximum, MVD=15μm)
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LWC as a function of HE and Ta
(Continuous maximum, MVD=20μm)
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LWC as a function of HE and Ta
(Continuous maximum, MVD=30μm)
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LWC as a function of HE and Ta
(Intermittent maximum, MVD=20μm)
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LWC as a function of HE and MVD
(Continuous maximum, Ta=0oC)
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LWC as a function of HE and MVD
(Intermittent maximum, Ta=0oC)
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The entire supercooled cloud database
(660 icing events, 28 000 nm in icing conditions)
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Graphing flight data
(texp=10 min, V=150knot)
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Graphing flight data
(texp=10 min, V=150knot)
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Sample flight data compared with Appendix C
Continuous maximum, Appendix C, MVD=15μm
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Sample flight data compared with Appendix C
Continuous maximum, Appendix C, Ta=0oC
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Icing tunnel test points on Appendix C envelopes
Continuous maximum
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Icing tunnel test points on Appendix C envelopes
Continuous maximum, MVD=20μm, V=174 kt
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Water catch rate
• In some applications, such as in testing thermal antiicing systems, the rate of water catch is important.
• For a given amount of LWC, the speed at which the
aircraft flies through it and the droplet collection
efficiency of the wing is important in determining how
much heat is required to keep the leading edges at a
required elevated temperature. Water catch rate is
calculated from:
WCR  V  tot LWC
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Water catch rate
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Total water catch
• Another item of interest for an icing encounter may be the
total amount of ice accreted on certain components, such
as unprotected surfaces.
• Here, the rate of water (ice) accumulation may not be
important, but rather the total water catch during the
encounter(s).
• The TWC may be useful for estimating the weight of ice
accreted on aircraft components, except for any losses due
to shedding or melting. Total water catch is calculated from:
TWC   tot HE LWC average
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Total water catch
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Acceptable Exposures
• What is an adequate exposure, or how much exposure is
enough?
• This can be set in terms of TWC.
• Maximum TWC from the envelopes for a 17.4 nm exposure
at the same temperature as the available icing conditions
during the test flight can provide a reference.
• This can be used as the target TWC to be achieved during
the test flight.
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Icing severity definitions
• Test exposures can be reported based on whether the
encounters correspond to trace, light, moderate or severe
icing conditions.
• Icing severity can be calculated from:
dB a V

dt
r
Icing severity
Trace
Light
Time expired for
0.25” ice formation
t > 1 hour
15 min < t < 60 min
Moderate
Severe
5 min < t < 15 min
t < 5 min
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Continuous maximum, Appendix C converted to
icing severity envelopes
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Sample icing intensity compared with cont. Max.,
Appendix C, converted to icing severity envelopes
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Comparing test data with natural probabilities
The differences between FAR 25 App. C and nature
• The envelopes in Appendix C do not show all the values
that can exist in nature.
• They also do not give information about the probability of
encountering various LWCs, MVDs, temperature durations
in icing conditions.
• Only the probable maximum (99% percentile) values of
LWC are shown. Designers of ice protection systems for
military aircraft would like to consider lesser percentile
values of LWC to accept more risk as a tradeoff against
extra weight, space and electrical power reqirements.
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Comparing test data with natural probabilities
Flight tests and icing wind tunnel tests
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Natural 99% limits vs altitude for highest
temperatures available at the altitude
(MVD=15-20m)
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Natural probabilities for LWC averages
at altitudes < 2500 ft AGL
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Natural probabilities for LWC averages
at altitudes 5000 ft ± 2500 ft AGL
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Natural probabilities for LWC averages
at altitudes 10000 ft ± 2500 ft AGL
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Natural probabilities for LWC averages
at altitudes 15000 ft ± 2500 ft AGL
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Natural probabilities for LWC averages
at altitudes 20000 ft ± 2500 ft AGL
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Sample flight data compared with natural
probabilities for LWC averages at altitudes< 2500 ft
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Natural HE limits and 99% LWC limits for different
MVDs in stratiform clouds at 0oC to -10oC
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Sample flight data compared with natural 99%
LWC limits for different MVDs
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