[Federal Register Volume 68, Number 88 (Wednesday, May 7, 2003)]
[Notices]
[Pages 24542-24550]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 03-11237]



[[Page 24542]]

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

[Docket No. FAA-2000-8560]


Icing Terminology

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Disposition of comments on proposed icing terminology.

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SUMMARY: The FAA is publishing its final icing terminology and the 
disposition of the comments received regarding this icing terminology. 
These comments were solicited on December 22, 2000, when the FAA 
published its proposal for new and revised icing terms in the Federal 
Register.

ADDRESSES: The complete docket for the notice on intent may be examined 
at the Federal Aviation Administration, Office of the Chief Counsel, 
Attn: Rules Docket (AGC-200), Room 914-G, Docket No. FAA-2000--8560, 
800 Independence Ave., SW., Washington, DC 20591, weekdays (except 
Federal holidays) between 9 a.m. and 5 p.m.

FOR FURTHER INFORMATION CONTACT: Dan Meier, AFS-220 Air Transportation 
Division, Flight Standard Service, Federal Aviation Administration, 800 
Independence Avenue, SW., Washington, DC 20591, telephone (202) 267-
3749.

SUPPLEMENTARY INFORMATION: 

Background

    Following the 1996 FAA international Conference on Aircraft In-
flight Icing, the FAA developed and implemented the FAA In-flight 
Aircraft Icing Plan during 1997 for responding to the recommendations 
and concerns which arose from that conference. Task 1.B of the FAA In-
flight Aircraft Icing Plan responded to recommendations and concerns 
expressed during the conference relative to consistent use of 
operational icing terminology in FAA regulations, guidance material, 
and manuals. Task 1.B addressed clarifying and redefining icing 
terminology applied to in-flight operations. In implementing Task 1.B, 
the FAA was to: First, ensure that this icing terminology (e.g., known, 
forecast, observed, trace, light, moderate, severe, and ``appendix C'' 
icing) is used consistently and clearly by the Flight Standards 
Service, pilots, dispatchers, the National Weather Service (NWS), 
Aviation Weather Center, the Aircraft Certification Service, and Air 
Traffic; and second, to update guidance related to icing reporting and 
pilot, Air Traffic Control, and dispatcher actions.
    To accomplish these objectives, the FAA established the Task 1B 
Working Group (WG), which comprised representatives from FAA, National 
Oceanic and Atmospheric Administration (NOAA), and the National Center 
for Atmospheric Research (NCAR). The goal of the WG was to review the 
definitions of all icing-related terms that appear in government 
aviation regulations, weather-related handbooks, aircraft flight 
manuals, etc. Based on its findings the WG was to make recommended 
changes to the definitions where they needed to be updated or improved. 
These recommendations would eliminate misunderstanding in their use 
among and between the previously mentioned sources.
    This work was accomplished through a series of meetings by the WG, 
and the result was a set of proposed definitions for in-flight icing 
terminology. The WG did not consider or propose any changes to the 
aviation regulations or icing forecasting procedures, although it 
became clear to the WG that existing regulatory wording and existing 
policy within the U.S. National Weather Service (NWS) and the 
International Civil Aviation Organization (ICAO) limited the freedom of 
the WG to change the icing-related terms in use. A public meeting was 
held in July of 1999 to solicit comments and input from industry 
representatives and interested members of the public concerning the 
FAA's proposal to clarify or add selected icing terminology. The FAA 
also proposed to amend the pilot-reporting format for icing PIREPs and 
append a table of icing effects. The terminology definitions developed 
by the WG were published in the Federal Register on December 22, 2000 
for public comment. The icing terminology definitions were 
appropriately revised during the disposition of the public comments.

Discussion

Summary of Significant Changes to Icing Terminology

    The new terminology excludes trace ice, eliminates former 
ambiguities about the meaning of known or forecast ice, and defines 
several new terms.
    The term ``trace ice'' has been deleted from the FAA in-flight 
icing terminology. The current definition of trace ice implied that it 
was not hazardous to flight, however, experience and research have 
shown that trace ice can be hazardous to some airplanes in certain 
conditions and that icing conditions can vary quickly and significantly 
in intensity. Also, National Transportation Safety Board (NTSB) Safety 
Recommendation A-98-88 recommended the following to the FAA: ``Amended 
the definition of trace ice contained in Federal Aviation 
Administration (FAA) Order 7110.10L, ``Flight Services'' (and in other 
FAA documents as applicable) so that it does not indicate that trace 
icing is not hazardous.'' Deletion of the term ``trace icing'' responds 
to the NTSB's Safety Recommendation A-98-88. However, the Task 1.B WG 
did acknowledge that deletion of the term ``trace icing'' may affect 
operation of airplanes without approved ice protection provisions in 
the heretofore defined ``trace icing'' conditions. ``Trace icing,'' 
previously defined as an icing intensity less severe than ``light'' or 
``moderate'' icing, is not addressed by the FAA operating rules (14 CFR 
91.527(b), 121.341(c), 125.221(c), and 135.227(c)). Therefore, the 
operation of some airplanes in ``trace icing'' without ice protection 
provisions may be inferred as acceptable since the term ``trace icing'' 
is not addressed by the rules. Definitions of the icing intensity terms 
are not included in the regulations definitions provided by 14 CFR part 
1. The WG concluded that the term ``trace icing'' should be deleted 
since: (1) The airworthiness of airplanes without ice protection 
provisions in any icing conditions was not addressed during type 
certification of such airplanes; (2) the operating rules fail to define 
light and moderate icing and fails to address ``trace icing;'' (3) the 
earlier discussion indicates that ``trace icing'' can be hazardous, 
especially without ice protection provisions; and (4) the NTSB Safety 
Recommendation A-98-88 states that FAA documents should not indicate 
that ``trace icing'' is not hazardous. Deletion of ``trace icing'' and 
re-definition of ``light icing'' will clarify and provide a means for 
showing compliance with the intent of the previously mentioned FAA 
operating rules.
    Airplanes having certification with ice protection provisions are 
approved for flight in icing conditions but do not have the capability 
of unlimited operation in all icing conditions. Currently, airplanes 
having certification with ice protection provisions, in compliance with 
14 CFR 23.1419 and CFR 25.1419, must be able to operate safely in the 
icing conditions defined in appendix C of 14 CFR part 25. Icing 
conditions in clouds, defined in appendix C of 14 CFR part 25, were 
established as being satisfactory standards for the design and 
certification of airplane ice protection

[[Page 24543]]

provisions, however atmospheric icing conditions are highly variable 
and can exceed these standards. Freezing precipitation (freezing rain 
and freezing drizzle), within and below clouds are examples of 
conditions that are not address by and exceed Appendix C. When 
encountering icing conditions that exceed appendix C of 14 CFR part 25, 
ice protection provisions may no longer be effective to provide safe 
operations and flight crew action may be required to promptly and 
safely exit those atmospheric environments, as required by 14 CFR 
91.13.
    The following is the list of terms recommended by the Task 1B WG as 
an updated replacement for current terminology used in reference to in-
flight icing of aircraft. The FAA intends to update the current 
terminology with the following terms.

Icing Terminology and Definitions

Icing Intensities

Light
    The rate of ice accumulation requires occasional cycling of manual 
deicing systems** to minimize ice accretions on the airframe. A 
representative accretion rate for reference purposes is \1/4\ inch to 
one inch (0.6 to 2.5 cm) per hour * on the outer wing. The pilot should 
consider exiting the condition.***
Moderate
    The rate of ice accumulation requires frequent cycling of manual 
deicing systems ** to minimize ice accretions on the airframe. A 
representative accretion rate for reference purposes is 1 to 3 inches 
(2.5 to 7.5 cm) per hour * on the outer wing. The pilot should consider 
exiting the condition as soon as possible.***
Heavy
    The rate of ice accumulation requires maximum use of the ice 
protection systems to minimize ice accretions on the airframe. A 
representative accretion rate for reference purposes is more than 3 
inches (7.5 cm) per hour * on the outer wing. Immediate exit from the 
conditions should be considered.***
Severe
    The rate of ice accumulation is such that ice protection systems 
fail to remove the accumulation of ice and ice accumulates in locations 
not normally prone to icing, such as areas aft of protected surfaces 
and any other areas identified by the manufacturer. Immediate exit from 
the condition is necessary.****

    * These rates can be measured by a suitable icing rate meter.
    ** It is expected that deicing or anti-icing systems will be 
activated and operated continuously in the automatic mode, if 
available, at the first sign of ice accumulation, or as directed in 
the Airplane Flight Manual. Occasional and frequent cycling refers 
to manually activated systems.
    *** It is assumed that the aircraft is approved to fly in the 
cited icing conditions. Otherwise, immediate exit from any of these 
intensity categories is required by regulations (14 CFR 91.13(a), 
91.527, 121.341, 125.221, and 135.227).
    **** Severe icing is aircraft dependent, as are the other 
categories of icing intensity. Severe icing may occur at any ice 
accumulation rate when the icing rate or ice accumulations exceed 
the tolerance of the aircraft. Icing certification implies an 
increased tolerance to icing intensities up through heavy.

Icing Types

    Note: Ice types are difficult for the pilot to discern and have 
uncertain effects on an airplane in flight. Ice type definitions 
will be included in the AIM for use in the ``Remarks'' section of 
the pirep and for use in forecasting.

Rime Ice
    A rough, milky, opaque ice formed by the rapid freezing of 
supercooled drops/droplets after they strike the aircraft. The rapid 
freezing results in air being trapped, giving the ice its opaque 
appearance and making it porous and brittle. Rime ice typically 
accretes along the stagnation line of an airfoil and is more regular in 
shape and conformal to the airfoil than glaze ice. It is the ice shape, 
rather than the clarity or color of the ice, which is most likely to be 
accurately assessed from the cockpit.
Glaze Ice
    Ice, sometimes clear and smooth, but usually containing some air 
pockets, which results in a lumpy translucent appearance. Glaze ice 
results from supercooled drops/droplets striking a surface but not 
freezing rapidly on contact. Glaze ice is denser, harder, and sometimes 
more transparent than rime ice. Factors, which favor glaze formation, 
are those that favor slow dissipation of the heat of fusion (i.e., 
slight supercooling and rapid accretion). With larger accretions, the 
ice shape typically includes ``horns'' protruding from unprotected 
leading edge surfaces. It is the ice shape, rather than the clarity or 
color of the ice, which is most likely to be accurately assessed from 
the cockpit. The terms ``clear'' and ``glaze'' have been used for 
essentially the same type of ice accretion, although some reserve 
``clear'' for thinner accretions which lack horns and conform to the 
airfoil.
Clear Ice
    See Glaze Ice.
Mixed Ice
    Simultaneous appearance or a combination of rime and glaze ice 
characteristics. Since the clarity, color, and shape of the ice will be 
a mixture of rime and glaze characteristics, accurate identification of 
mixed ice from the cockpit may be difficult.
Known or Observed or Detected Ice Accretion
    Actual ice observed visually to be on the aircraft by the flight 
crew or identified by onboard sensors.
Runback Ice
    Ice which forms from the freezing or refreezing of water leaving 
protected surfaces and running back to unprotected surfaces.
Residual Ice
    Ice which remains on a protected surface immediately after the 
actuation of a deicing system.
Intercycle Ice
    Ice which accumulates on a protected surface between actuation 
cycles of a deicing system.

Icing Conditions

Forecast Icing Conditions
    Environmental conditions expected by a National Weather Service or 
an FAA-approved weather provider to be conducive to the formation of 
in-flight icing on aircraft.
Potential Icing Conditions
    Atmospheric icing conditions that are typically defined by airframe 
manufacturers relative to temperature and visible moisture that may 
result in aircraft ice accretion on the ground or in flight. The 
potential icing conditions are typically defined in the airplane flight 
manual or in the airplane operation manual.
Known Icing Conditions
    Atmospheric conditions in which the formation of ice is observed or 
detected in flight.

    Note: Because of the variability in space and time of 
atmospheric conditions, the existence of a report of observed icing 
does not assure the presence or intensity of icing conditions at a 
later time, nor can a report of no icing assure the absence of icing 
conditions at a later time.

Freezing Rain (FZRA)
    Rain is precipitation at ground level or aloft in the form of 
liquid water drops which have diameters greater than 0.5

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mm. Freezing rain is rain that exists at air temperatures less than 0 
[deg]C (supercooled), remains in liquid form, and freezes upon contact 
with objects on the ground or in the air.
Freezing Precipitation
    Freezing precipitation is freezing rain or freezing drizzle falling 
through or outside of visible cloud.
Freezing Drizzle (FZDZ)
    Drizzle is precipitation at ground level or aloft in the form of 
liquid water drops which have diameters less than 0.5 mm and greater 
than 0.05 mm. Freezing drizzle is drizzle that exists at air 
temperatures less than 0 [deg]C (supercooled), remains in liquid form, 
and freezes upon contact with objects on the surface or airborne.
Icing in Precipitation
    Icing occurring from an encounter with freezing precipitation, that 
is, supercooled drops with diameters exceeding 0.05 mm, within or 
outside of visible cloud.
Icing in Cloud
    Icing occurring within visible cloud. Cloud droplets (diameter < 
0.05 mm) will be present; freezing drizzle and/or freezing rain may or 
may not be present.
Supercooled Large Drops (SLD)
    Liquid droplets with diameters greater than 0.05 mm at temperatures 
less than 0 [deg]C, i.e., freezing rain or freezing drizzle.
Supercooled Drizzle Drops (SCDD)
    Synonymous with freezing drizzle aloft.
Supercooled Drops or /Droplets
    Water drops/droplets which remain unfrozen at temperatures below 0 
[deg]C. Supercooled drops are found in clouds, freezing drizzle, and 
freezing rain in the atmosphere. These drops may impinge and freeze 
after contact on aircraft surfaces.
Appendix C Icing Conditions
    Appendix C (14 CFR, part 25 and 29) is the certification icing 
condition standard for approving ice protection provisions on aircraft. 
The conditions are specified in terms of altitude, temperature, liquid 
water content (LWC), representative droplet size (mean effective drop 
diameter [MED]), and cloud horizontal extent.

Disposition of Comments

1. Request for Statement That Icing Certification Does Not Imply 
Unlimited Safe Flight in All Icing Conditions

    One commenter requested that the FAA include in the final notice 
emphasis that certification for flight in icing conditions does not 
imply that an aircraft has the capability for unlimited safe flight in 
all icing conditions.
    The FAA concurs. The discussion section of the notice has been 
revised accordingly.

2. Drop Proposed New Icing Intensity Definitions

    The arguments in favor of dropping the icing intensity definition 
are as follows:
    (a) Any changes in the definitions would be too confusing (to 
pilots).
    The FAA does not concur. The FAA 1996 international icing 
conference concluded that the existing icing terminology is confusing. 
Reasons for this conclusion include:
    [sbull] The present definition of severe is contradictory to 14 CFR 
91.209(c) and 135.227(c) which allow icing-certificated airplanes to 
fly into severe (uncontrollable, by definition) icing conditions.
    [sbull] The definitions give no objective standard or rules for 
pilots to decide which icing intensity the aircraft is experiencing at 
the moment, or for distinguishing one intensity level from the next.
    [sbull] With the present definitions, icing intensities are neither 
measurable nor forecastable, because the definitions contain no 
quantitative relationship to anything that is calculable or observable, 
nor any connection at all to the icing atmosphere.
    [sbull] There is presently no way to relate the icing intensity 
reported by one aircraft make and model to another.
    The proposed terminology responds to the conclusions reached by the 
1996 conference.
    Icing intensities are of interest to pilots and forecasters, of 
course, but also to icing engineers, each group having its own 
experiences, needs, and perspectives. But the present icing definitions 
are useless to forecasters and engineers because the definitions 
contain nothing that can be measured or calculated, and they are of 
questionable value to pilots for the reasons bulleted above. During 
development of the proposed definitions, it became clear that the three 
groups often have difficulty comprehending the viewpoints of the 
others, and this contributes to the confusion. The public comments on 
the proposals were almost exclusively from the viewpoint of pilots, and 
the comments reflect their perspective. The proposed changes were 
intended to accommodate all three groups and to help overcome at least 
some of these problems.
    To minimize confusion on the part of the pilots, it was decided to 
keep some of the familiar wording while adding a quantifiable aspect to 
make the definitions more useful for engineering and forecasting 
purposes.
    (b) The definitions are intended to be reporting definitions and 
nothing else.
    The FAA does not concur. The original intent of the definitions was 
that they be used by pilots and flight crews to report encountered 
icing conditions. However, the pilot reports are now being used also by 
meteorologists to diagnose and forecast icing conditions. If a 
quantitative relationship between the intensity levels and something 
measurable and calculable is established, the definitions can be used 
for reporting, forecasting, and engineering purposes, and their utility 
can therefore be markedly improved.
    (c) The proposal to relate icing intensities to both the wing and 
tailplane, each with their own icing rates, will give rise to two icing 
intensities for the airplane instead of just one.
    The FAA concurs. The most icing-critical components of the aircraft 
are, of course, the ones to be concerned about. In the absence of any 
more critical components, the outer wing is now suggested in the 
interest of establishing a meaningful and uniform reference location 
for ice accretions on all airplanes. Typically, the outer wingspan, 
being the thinnest part of the wing, has the greatest droplet 
collection efficiency for the wing.
    (d) Large airplanes will report lesser icing intensities than small 
airplanes in the same icing conditions.
    The FAA concurs. This is already true with the existing 
definitions, and will remain true no matter what the definitions may 
be. However, because of this issue, the proposed definitions identify 
the leading edge as the reference surface and the PIREP format for 
icing is being revised to ensure reporting of the airplane type. This 
information can be interpreted by other pilots relative to anticipated 
ice accretion and flying quality effects for their aircraft and can 
meteorologically define the encountered icing environment. The new, 
quantified definitions are designed to take advantage of the difference 
in response between large and small airplanes and enable icing 
intensities to actually be

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computed (using modern software) for a given icing condition. This new 
feature is a major improvement because it will allow icing forecasts to 
be tailored to individual airplane makes and models, to the degree that 
the relevant variables are known. AIAA-98-0094 contains information on 
how the new definitions will permit this to be achieved.
    (e) The only way for accretion-based intensities to be useful for 
forecasting is to have every airplane carry a small ice accretion probe 
and base icing (PIREP) reports on that.
    The FAA partially concurs. Having a common ice accretion probe on 
all aircraft for reporting icing conditions would be advantageous, 
however, the FAA does not currently plan to mandate such a probe. The 
choice of the reference surface coupled with the aircraft model and 
estimated accretion rate provides useful information for forecasting.
    There are two ways to categorize icing conditions--describe the 
atmospheric conditions themselves, or describe their effects on the 
aircraft. The former would include liquid water concentration, static 
air temperature, and perhaps a representative droplet size in the 
clouds. But these are meaningless to pilots unless they are translated 
into effects on the aircraft. The effects may be qualitative or 
quantitative.
    Qualitative definitions focus on perceivable effects on the 
aircraft and are not quantifiable. The present icing intensity 
definitions are ultra-qualitative, using terms like occasional or 
frequent need to deice, and vague warnings that the icing may create a 
problem or is potentially hazardous, for example. (In comparison, see 
the AIM for definitions of turbulence intensities which, although 
qualitative, are much easier to characterize and distinguish).
    Quantitative definitions would specify ice accretion rates on 
components of concern or would list graduated intervals of speed loss, 
compensatory power increase, or other measurable effects of ice 
accretion. (A graduated table of operational effects was developed and 
will be submitted as a recommended change to the PIREP format.)
    The situation is complicated by the presence of functioning ice 
protection equipment. When the equipment is operating, it may be 
difficult or impossible to observe, estimate, or measure any ice 
accretion on the protected surfaces. For heated wings there should be 
no ice accretion to report anyway. For booted wings operated on 
automatic cycle intervals, the crew would have to estimate the rate of 
buildup between cycles. Newer boot models with embedded icing rate 
sensors may be useful here.
    Otherwise, an icing rate meter (not located on a critical surface) 
is a useful surrogate for quantitative measurements of both the icing 
atmospheric variables and the ice accretion on the aircraft components 
of concern. To a certain extent, it can be related quantitatively to 
both. By taking into account the differences in the droplet collection 
efficiency of the probe compared to the aircraft component, rates 
measured by the probe can be converted to proportional rates on the 
component.
    In a similar way and to a certain extent, icing rates measured by a 
probe on one airplane can be converted to expected accretion rates on 
components of other airplanes too, as long as the airspeeds are known. 
The new, quantitative definitions also take advantage of this fact and 
allow these measured icing rates to be converted to equivalent icing 
rates on the wings or tailplane of the reporting airplane and on any 
other make and model that may fly through the same icing conditions. 
(For a good explanation, see the technical paper ``A Workable, 
Aircraft-Specific Icing Severity Scheme'', AIAA-98-0094 (Jan. 1998) by 
R. Jeck.)
    It must be understood that the indicated icing rates are those to 
be expected on an unheated component.
    Gradually more and more airplanes may install icing rates probes 
that are already commercially available. Through icing PIREPS, this 
would greatly help the icing forecasters and, by means of conversion 
tables, could help even those airplanes without the probes.
    (f) There is no need to re-define something that well-trained 
pilots have known about for 30 years or more.
    The FAA does not concur. Concerns have been expressed about the 
ambiguities of the existing definitions. The existing definitions are 
outmoded in view of technological advances. The existing definition 
were formulated at a time when no suitable icing rate meters were 
available, and when computing icing rates on an airfoil was 
prohibitively difficult due to lack of computing power and to lack of 
the necessary experimental data on most airfoils. The FAA wishes to 
modernize the definitions consistent with current and anticipated 
technology.
    (g) Except for severe icing conditions, airplanes certificated for 
flight in icing conditions are supposed to be protected enough to allow 
safe transition out of icing, or to lesser icing intensities. 
Therefore, the changes in definitions are neither helpful nor necessary 
nor increase safety.
    The FAA does not concur. Even for icing-certificated airplanes, 
reported icing intensities are helpful for planning, forecasting of 
icing conditions, situational awareness, and compliance with operating 
rules and associated limitations (14 CFR parts 91.527, 121.341, 
125.221, and 135.227). The definition of severe icing conditions is 
being changed to be airplane-specific. Thus, reported less-than-severe 
icing conditions for one aircraft may indicate severe icing for other 
types of aircraft.
    (h) The new definitions endanger safety and introduce new 
ambiguities.
    The FAA does not concur. In light of the explanations given above, 
current ambiguities will be reduced for all users because of better, 
more versatile definitions of the icing intensities.

3. Revise Definitions of Light and Moderate Icing To Make Them 
Consistent With the ADs on the Operation of Pneumatic Boots

    One commenter requested that the FAA revise definitions of light 
and moderate icing to make them consistent with the ADs issued by the 
FAA in December 1999 on the operation of pneumatic boots.
    The FAA concurs. Therefore, the word ``use'' has been replaced by 
``cycling'' so as not to imply delayed activation of ice protection 
systems.

4. Include Characterization of Hazard to Aircraft in Icing Intensity 
Definitions

    Include characterization of hazard to aircraft in icing intensity 
definitions. (One commenter suggests that these be related to loss in 
indicated airspeed of percentage increase in power.)
    The FAA partially concurs. The definitions were modified to include 
characterization of the hazard, however, the aerodynamic effects of 
icing are aircraft-specific. Therefore, the PIREP icing report format 
currently contained in the AIM in being revised to include the 
characterization of the hazard being experienced by the reporting 
pilot.

5. Remove Footnotes

    The FAA does not concur. Although the footnotes have been removed, 
the information contained in them has been corrected and retained 
within the definitions themselves.

6. Correct Errors in Footnotes

    Several commenters noted that there were typographical errors in 
the footnotes.
    The FAA concurs. Typographical errors in the footnotes have been 
corrected and the information has been inserted in the definitions.

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7. Revise Icing Rates in Footnotes, as They Are Not Realistic or Not 
Consistent With Certification Standards

    (a) An icing rate of \1/4\-inch in five minutes, which would be 
severe based on these definitions, would not be considered hazardous 
with regard to the effect on aircraft flight characteristics on certain 
types of regional aircraft.
    The FAA concurs. The FAA now agrees that it is incorrect to assign 
an icing rate to severe. The FAA proposes to re-instate the term heavy 
for the greatest icing rate category. The FAA recommends that the term 
severe (without any icing rate attached) be retained to cover the 
situation where the ice protection system is inadequate, no matter what 
the icing rate.
    (b) What is the basis for the numerical icing rates assigned to the 
different intensities?
    The icing rates that were given in the footnotes were taken from 
the technical paper. ``A Workable, Aircraft-Specific Icing Severity 
Scheme,'' AIAA-98-0094 (Jan. 1998) by R. Jeck. While the reference 
rates are admittedly arbitrary, and are primarily based on the 
traditional operation of the pneumatic deicing boots, the AIAA paper 
clearly explains the rationale behind these rates and gives several 
application examples. Interested readers are referred to this paper.
    In AIAA-98-0094, occasional is interested as once every 15 minutes 
to an hour. Similarly, for moderate icing, which the present 
definitions associate with (frequent) use of deicers, the word frequent 
is interpreted as once every 5 to 15 minutes. Severe (or preferably 
heavy) icing rates must require even more usage, which would have to be 
more often than once every 5 minutes. In tabular form, the proposed 
rates have the following relationships:

Light: \1/4\-inch accumulation in 15-60 minutes, which is equivalent to

    0.1-0.4 mm./min, or
    \1/4\ to 1 inch per hour.

Moderate: \1/4\-inch accumulation in 5-15 minutes, which is equivalent 
to

    0.4-1.3 mm/min, or
    1 to 3 inches to hour.

Heavy: \1/4\-inch accumulation in less than 5 minutes, which is 
equivalent to

    more than 1.3 mm/min, or
    more than 3 inches per hour.


    This scheme preserves the ``1-hour'' separation between light and 
moderate intensities, as mentioned in the present definitions. It also 
relates the onset of heavy icing conditions with a rate that would, if 
continued, produce a 3-inch accumulation in an hour.
    Three inches of ice on unprotected surfaces is considered to be a 
critical accumulation for design, test, and certification purposes.
    (c) Commenters suggests changing the footnotes to read 30-60 
minutes for light, 15-30 minutes for moderate, and 5-15 minutes for 
heavy.
    The FAA does not occur. The commenter may be uncomfortable with the 
proposed 45 minutes spread in time allowed for light icing intensities, 
and prefer a 30 minute spread instead. But the commenter has not 
provided any justification for his preference.
    (d) Severe icing is (presumably) a condition outside of the 
Continuous Maximum envelope because, by rule, the (icing-certificated) 
aircraft must have protection throughout this envelope. But ``severe 
icing'' can be found on the Continuous Maximum icing chart in 14 CFR 
part 25, Appendix C. This is inconsistent.
    The FAA concurs that inconsistency could arise with the previous 
definition of severe for some conditions within the continuous maximum 
icing envelope. However, the FAA now agrees that it is incorrect to 
assign an icing rate to severe, so any inconsistency has been 
eliminated. The rate previously assigned to ``severe icing'' is not 
assigned to ``heavy icing.'' Depending on the airspeed of the aircraft 
and the collection efficiency of the monitored surface, the heavy icing 
rate can occur for some points within the continuous maximum icing 
envelope, particularly for shorter encounters. For encounters exceeding 
about 20 minutes, a heavy icing rate would ordinarily not be 
experienced in continuous maximum conditions by most aircraft because 
of the envelope correction reducing liquid water content for sustained 
encounters.

8. Base Icing Rates in Footnotes in Unprotected Surfaces to Preferably 
on a Representative Ice Detector Surface or Probe

    Base icing rates in footnotes on unprotected surfaces or preferably 
on a representative ice detector surface or probe. (That is, if the 
wing and tailplane are ice protected, then the pilot cannot observe or 
judge icing rates there while the protection systems are preventing or 
removing the ice. Therefore, only a separate probe or an unprotected 
part of the wing will be useful or observing ice accretion rates.)
    The FAA concurs. The outer wing may be used unless otherwise 
specified. The outer wing and tailplane were suggested as standard 
reference locations so that everyone involved (pilots, forecasters) 
would all be focused on the same spot on the airplane. Naturally, these 
locations may not even be observable due to darkness or line-of-sight 
obstruction, for example. Heated wings would not be expected to 
accumulated any ice anyway. In that case, the pilot would not report 
any icing intensity. Icing conditions may exist, but for adequately 
heated wings there should be no accretion and therefore no intensity! 
If the protected parts of the aircraft do collect ice, then it would be 
reported as severe if the equipment is unable to control it. This would 
apply to the windshield too, if it iced over uncontrollably.
    In any case, there is no substitute for a good measurement, and the 
proposed definitions anticipate the eventual use of icing rate meters 
for obtaining the measurements. Icing rate measurements on a probe can 
be converted to corresponding rates on the wing or tail. In the absence 
of an icing rate meter, the pilot is encouraged to estimate an 
accretion rate with the outer wing or tailplane in mind. This is no 
different from the present situation where pilots are instructed in the 
AFM to estimate when \1/4\-inch of ice has accreted as a signal for 
inflating the boots. Admittedly, without an icing rate meter there is 
no easy way to estimate ice accretion rates or amounts. This is a 
problem even with the current definitions. But by focusing on the same 
ice-critical components of the airplane, there can be uniformity in 
reporting and eventually in forecasting.

9. Retain the Term Trace Icing

    (a) The NTSB (A-98-88) did not recommend eliminating trace icing, 
but only to eliminate the ``not hazardous'' wording.
    The FAA does not concur and has decided to delete the term Trace 
ice for the following reasons:
    [sbull] Trace icing is not forecast.
    [sbull] Trace icing is not governed by the regulations.
    [sbull] Identification of trace icing is dependent on the 
capability of the pilot to judge. The FAA considers that estimating an 
ice accretion of a quarter of an inch or less per hour is outside the 
judgment of a pilot and questions how the instrumentation would handle 
it.
    [sbull] The definition of trace icing implies continued flight in 
icing by unprotected aircraft is acceptable.
    [sbull] An interpretation of 135.227 suggests that the proposed 
change may negate the current practice of flight in IFR icing 
conditions by unprotected aircraft and aircraft certified for icing 
under older rules (CAR-3, prior to amendment 23-14).

[[Page 24547]]

    [sbull] Removal of the term trace icing is consistent with the FAA 
position that all icing is hazardous.
    (b) Commenter recommends defining trace icing as: ``Icing becomes 
perceptible and the rate of accumulation is slightly greater than the 
rate of sublimation. Icing resulting from flight in a supercooled cloud 
with liquid water content less than 0.1 grams per cubic meter. A 
representative accretion rate for forecasting or reference purposes is 
\1/4\-inch or less in an hour or more on an outer wing or tailplane, 
prior to activation of any ice protection equipment.''
    The commenter is from the helicopter community where icing severity 
based on a LWC scale has been in use out of necessity. This is because, 
in hover, there is no forward flight and an artificially aspirated 
icing sensor must be used in order to assess the icing environment. In 
that case, the icing rate indicated by the sensor has no relation to 
what may be happening on the airframe. Rather, the icing rate, under 
the known aspirated air velocity, can be converted to LWC to gauge the 
icing propensity of the cloud or fog in which the helicopter may be 
embedded at the moment. In this case, the helicopter manufacturer may 
have to supply some relationship between LWC amounts and the expected 
effects on the helicopter.
    In any case, the FAA has no recommendations for an icing intensity 
scale for helicopters. The FAA proposals were intended for fixed wing 
airplanes. * * *

10. Retain Trace Icing If Its Elimination Will Result in Greater Aerial 
Coverage of Forecast Icing

    Another commenter requested that the term ``trace icing'' be 
retained if its elimination would result in greater aerial coverage of 
forecast icing.
    The FAA does not believe that the aerial coverage of forecast icing 
will be affected by the elimination of the term ``trace icing,'' since 
trace icing is not forecast by the NWS. Light icing is forecast by the 
NWS, and it will continue to be forecast under the same conditions 
whether or not trace icing is eliminated.

11. Change the Definition of ``Light Icing'' to ``The Rate of Ice 
Accumulation May Require Occasional Use of Ice Protection Systems To 
Remove or Prevent Accumulation''

    The FAA partially concurs. The recommended wording is reflected in 
the new wording proposed by the FAA.

12. Change the Definition of ``Light Icing'' So That It Is Icing 
``Represented by the Capability of the Aircraft To Safely Fly and Land 
Without the Ice Protection Turned On''

    The FAA does not concur. Light ice can accrete to the point where 
ice protection may be required. The pilot may not be able to judge ice 
accretion that results in reduced safety margins.

13. Change the Definition of ``Moderate Icing'' to ``The Rate of Ice 
Accumulation May Require Occasional to Frequent Use of Ice Protection 
Systems To Remove or Prevent Accumulation''

    The FAA partially concurs. The commenter retains the conditional 
``may'' from the definition of light icing in the original notice. The 
FAA now believes that both light and moderate icing connotes a definite 
need to activate ice protection equipment.
    The commenter also suggests retaining the word ``occasional'' in 
the description of moderate icing. According to the proposed revised 
definitions, moderate icing corresponds to \1/4\-inch of ice 
accumulation every 5 to 15 minutes. This has been interpreted (in AIAA-
98-0094) as frequent usage if the deicing system is activated at least 
each time \1/4\-inch accumulates.

14. Change the Definition of ``Moderate Icing'' to Anything Between 
Light and Severe

    The FAA does not concur. For clarity, the FAA prefers to provide an 
independent definition of moderate icing.

15. The Term ``Severe Icing'' Should Be Reserved for Ice Protection 
System Failure-To-Remove-Ice, and the Term ``Heavy Icing'' Used To 
Describe Ice Accretion Rates

    The FAA concurs. Although the term heavy has long been used by 
pilots to describe ice accretions greater than moderate, it has not 
been used for official forecasts or reporting. The FAA will propose 
that the National Weather Service cease forecasting severe icing and 
instead forecast heavy icing. Heavy icing should be based on reasonable 
scientific principles. The FAA agrees that severe icing is aircraft-
specific while heaving icing need not be and that severe icing should 
be limited to a failure-to-remove-ice condition until meteorological 
technology makes it possible to forecast severe ice conditions with 
reasonable accuracy which can be applied to specific aircraft.

16. Define ``Severe Icing'' To Be Anything Beyond What the Aircraft Has 
Demonstrated in Certification

    The FAA does not concur. Some airplane designs may be able to 
operate safely in icing conditions exceeding the certification 
standards, depending on airplane size and ice protection system 
capability.

17. New Definition of Severe Icing Conditions Is Not Consistent With 
Definitions in FAA Advisory Material

    The FAA concurs. When the new definition of severe icing, as well 
as the other proposed definitions, are approved, the FAA will revise 
all advisory material to include the new terminology.

18. The Term ``Heavy'' Should Be Included in the List of Definitions To 
Characterize an Accretion Rate Previously Associated With ``Severe 
Icing''

    The FAA concurs. The FAA will propose that the term ``heavy'' be 
included in the list of new definitions as the ice accretion rate 
associated with the current definition of severe.

19. The Term ``Heavy'' Should Be Used To Provide Another Icing Level 
Between Moderate and Severe

    The FAA concurs. The term ``severe icing'' will be reserved to 
refer to that condition where the pilot determines that his/her 
aircraft cannot safely continue flight. Revise note for accuracy or 
delete note.
    The FAA concurs that as written the note was unclear, and it has 
been revised.

20. Request To Use Shape as the Primary Descriptor, and Clarity and 
Color (if at all) as Secondary Descriptors in the Definitions of Ice 
Types

    Several commenters requested that shape be included as a descriptor 
in the ice type definitions. It was further requested that shape be 
identified as the primary descriptor, and clarity and color as 
secondary descriptors, on the grounds that shape is more likely to be 
accurately identified from the cockpit than clarity or color.
    The FAA partially concurs and has added shape to the definitions of 
rime and glaze ice. Furthermore, the definitions now include 
acknowledgment that shape, rather than clarity or color, is more likely 
to be accurately assessed from the cockpit.

21. Request To Relate Aerodynamic Effects to Ice Type in Definitions

    One commenter requested that statements relating ice type to

[[Page 24548]]

aerodynamic effects be included in the ice type definitions.
    The FAA does not concur. The FAA acknowledges that glaze ice, 
particularly if horns are present or if the ice is relatively rough, is 
likely to be more detrimental to flying qualities than rime ice, 
particularly if the rime is conformal to the airfoil and relatively 
smooth. However, determination of ice type from the cockpit is 
challenging and may be extremely difficult. Thus, misidentification of 
ice type by pilots, particularly when visibility is limited by night or 
other circumstances, may be a common event. If such misidentification 
is associated with erroneous expectations as to the aerodynamic effect 
of the ice, potential hazards to the safety of flight may be increased.

22. Request To Reword Definition of ``Rime Ice''

    One commenter noted inconsistencies in the wording of the 
definitions of rime and glaze ice, and requested that these 
inconsistencies be corrected.
    The FAA concurs and the wording has been clarified.

23. Request To Not Include Clear Ice as a Separate Term in List of 
Definitions

    One commender requested that clear ice be referenced in the 
definition of glaze ice, and that it be deleted as separate entry in 
the list of definitions. The commenter noted that the proposed 
definitions indicate identical formation mechanisms for glaze ice and 
clear ice, and provided no reason to differentiate between the two.
    The FAA partially concurs. Clear ice is a commonly used term within 
the aviation community. It is retained, therefore, as a separate entry 
in the list but the reader is referred to the definition of glaze ice 
which has the same formation mechanism.

24. Request To Reword Definition of Mixed Ice

    One commenter requested that the word ``characteristics'' be added 
at the end of the first sentence in the definition of mixed ice.
    The FAA concurs. Accordingly, the word ``characteristics'' has been 
added at the end of the first sentence in the definition of mixed ice.

25. Request To Either Delete the Term ``Known or Observed/Detected 
Icing'' From the List of Definitions or To Combine It With the Term 
``Known Icing Conditions''

    One commenter requested that the FAA delete the term ``Known or 
Observed/Detected Icing'' or else combine it with the term ``Known 
Icing.'' The commenter believed that there was not a sufficiently clear 
distinction between the two terms and that retention of both would 
cause confusion.
    The FAA does not concur, but agrees that there is a possibility of 
confusion. Therefore, it has replaced the term ``Known or Observed/
Detected Icing'' with ``Known or Observed/Detected Ice Accretion'' in 
order to avoid such confusion. The FAA believes that there is a clear 
distinction between Known Icing Conditions and Known or Observed/
Detected Ice Accretion.

26. Request To Clarify the Meaning of ``Approved'' in Definition of 
``Forecast Icing Conditions''

    The FAA concurs that clarification is needed and has revised the 
definition to state that the weather provider must be FAA-approved.

27. Request To Revise Definition of ``Potential Icing Conditions''

    Two commenters request that the definition of ``potential icing 
conditions'' be revised for improved clarity and accuracy and so that 
it will not be confused with ``forecast icing conditions.''
    The FAA concurs. Potential icing conditions are typically defined 
by airframe manufacturers relative to temperature and visible moisture 
that may result in aircraft ice accretion on the ground or in flight. 
Because the airframe manufacturers are aware of areas on the aircraft, 
such as the engine induction system, that may accrete ice under certain 
atmospheric conditions, aircraft manufacturers are considered to be the 
best source for this information. The potential icing conditions are 
typically defined in the airplane flight manual or in the airplane 
operation manual. Forecast icing conditions are predicted by weather 
providers.

28. Either Delete the Definition of ``Known Icing Conditions'' in the 
List, or Else Align the Definition With That Used in the Relevant NTSB 
Cases

    The FAA does not concur with this request because it believes that 
there is no conflict between the revised definition and the NTSB cases. 
Essentially the proposed definitions of ``Known Icing Conditions,'' 
``Known Ice,'' and ``Forecast Icing'' are in agreement with the recent 
court cases. In the case of Irmisch v. McLucas Civil No. 76-4273 (CA 2, 
filed May 2, 1977) the court understood Known Icing to mean icing that 
is known to the pilot. The FAA is not in conflict with the NTSB in its 
interpretation of forecast icing since forecasted icing conditions 
existed at the time of the aircraft icing events in the three cases 
cited by the commenter.
    Forecast icing conditions represent the best estimate by the 
National Weather Service that icing conditions will be present at a 
certain time over a certain geographic area. A forecast of icing 
conditions does not mean that there is an absolute certainty that icing 
will occur. It does mean, however, that a pilot must take into account 
forecasted icing conditions during flight planning so that the pilot, 
whose aircraft may not meet the requirements of the regulations, avoids 
actual icing. The FAA sees no conflict between the proposed definitions 
and what is required by the regulations.

29. Reword Definition of ``Freezing Rain (FZRA)'' for Improved Clarity, 
Etc.

    Several commenters requested that the FAA reword the definition of 
``freezing rain (FZRA)'' for improved clarity, accuracy, utility, and 
consistency with other terms.
    The FAA partially concurs. One commenter stated that there is no 
mention of size distribution in the definition. There ought not to be--
the definition applies to individual drops. There is no ``freezing rain 
distribution'' and to attempt to define such would add unnecessary 
complexity to this definition. This applies to freezing drizzle as 
well.
    Another commenter notes the work that is being done to characterize 
freezing rain and freezing drizzle in terms of drop size, liquid water 
content, etc. and is concerned with possible conflicts between this 
definition and what may come of that work. The size definitions 
included in the proposal are those that appear in the Glossary of 
Meteorology; these are generally accepted by meteorologists and have 
been for some time. It is highly unlikely that new definitions of these 
terms will arise from the characterization work. Rather, the results of 
the environmental characterization will serve to provide envelopes of 
possible environmental conditions where freezing rain and freezing 
drizzle are found, in enough detail to enable engineering 
specifications for possible compliance to an expanded icing envelope, 
and avoid conflict with existing terminology.
    Another commenter suggests that the freezing rain and freezing 
drizzle definitions be expanded to include the atmospheric conditions 
often associated with them. The FAA believes this could be misleading 
since the conditions a) can overlap for freezing rain and freezing 
drizzle and b) can be present

[[Page 24549]]

with an absence of either freezing rain or freezing drizzle.
    It is probably not advisable to include a caveat in the definition 
specifying that freezing rain contain ``an appreciable amount of water 
in drops'' which have diameters greater than 0.5 mm, as one commenter 
suggested. The phrase ``appreciable amount'' adds ambiguity to the 
definition.
    There were also good suggestions for clarifying the language.
    The definition has been revised in the notice.

30. Reword Definition of ``Freezing Precipitation'' To Clarify 
Distinction Between ``Freezing Precipitation'' and ``Supercooled Large 
Drops''

    The FAA concurs that there was little distinction between the 
proposed definitions. The definition of ``Freezing Precipitation'' has 
been revised in the notice.

31. Reword the Definition of ``Freezing Drizzle (FZDZ)'' for Improved 
Clarity, Accuracy, Utility, and Consistency With Other Terms

    The comments are very similar to those for freezing rain; see above 
for some specifics. The FAA concurs with many of these and the 
definition has been revised in the notice.

32. Request To Revise Definition of ``Icing in Precipitation''

    Several commenters requested that the FAA reword the definition of 
``icing in precipitation'' for improved clarity, accuracy, utility, or 
consistency with other terms.
    The FAA concurs with most of the comments and has revised the 
definition accordingly.

33. Reword the definition of ``Icing in Cloud'' for Improved Clarity, 
Accuracy, Utility, and Consistency With Other Terms

    One commenter noted that even outside of visible cloud, the 
atmosphere will contain a distribution of droplet sizes and diameters 
of less than 50 microns will be present. Actually these smaller ``cloud 
droplets'' may be present, that is, they are not always there. The FAA 
prefers to include the ``visible cloud'' requirement, which implies 
substantial numbers of cloud droplets and is what differentiates this 
condition from ``Icing in Precipitation.''
    The FAA concurs with most of the remaining comments and the 
definition has been revised in the notice.

34. Reword the Definition of ``Supercooled Large Droplets'' for 
Improved Clarity, Accuracy, Utility, and Consistency With Other Terms

    The FAA concurs and proposes the definition has been revised in the 
notice.

    Note: The new definition provides a definition of an atmospheric 
phenomenon and is considered sufficient without reference to icing 
standards or possible effects on aircraft safety. The terms ``FZRA'' 
and ``FZDZ'' are used by weather providers to indicate SLD icing 
conditions.

35. Request To Delete ``Supercooled Drizzle Drops'' From List of 
Defined Terms

    One commenters requested that the FAA delete the term ``Supercooled 
Drizzle Drops'' from the list because the term has had only limited 
use.
    The FAA does not concur. Although it is true that the term has not 
appeared extensively, it has appeared with sufficient frequency to 
justify inclusion in the notice.

36. Request To Expand Definition of ``Appendix C Icing Conditions''

    One commenter requested that the FAA include in the final notice a 
definition of ``Appendix C Icing Conditions'' expanded to include and 
explain variables used in defining the icing envelopes.
    The FAA concurs that identification of these variables is 
appropriate in the notice, and the definition has been expanded 
accordingly. However, technical explanation and use of these variables 
are addressed in FAA advisory circulars on certification.

37. Request To Include Additional Meteorological Terms in List of 
Defined Terms

    One commenter requested that the FAA include ice crystals, hail, 
snow, sleet, graupel, and related meteorological terms in the list of 
definitions.
    The FAA does not concur. During the 1996 FAA icing conference the 
FAA was given the task of redefining those icing terms that, in the 
judgement of the FAA, were either confusing or were otherwise in need 
of clarification. The terms proposed for redefinition and clarification 
are those icing terms which fit the criteria expressed in the FAA Icing 
Plan developed using the recommendations from the conference.
    The FAA does not agree that the terms proposed by the commenter are 
confusing or are unclear so as to require redefinition.

38. Request for Removal of Contradictions in CFR Material Pertaining to 
Severe Icing

    One commenter requested that the FAA remove contradictions that 
exist in the CFR material (in particular, with respect to usage of term 
``severe icing''), so that the material presented in the docket does 
not continue to sanction these contradictions.
    Atmospheric icing conditions are highly variable and can exceed in-
flight icing standards defined by the airplane airworthiness 
requirements. Therefore, the FAA concurs, and plans to revise the FARs 
which are in conflict with the proposed definition of severe icing. The 
National Weather Service, however, is required to forecast and report 
severe atmospheric conditions, including thunderstorms and severe 
icing. Pilot reports, experience, and other parameters are used by 
meteorologists to define severe icing conditions, regardless of 
airplane ice protection provisions, size, or performance. Severe icing 
conditions for small airplanes may not be severe for large air 
transports. The FAA will provide the requirement that the National 
Weather Service replace the term ``severe icing'' with ``heavy icing.'' 
Resolution of the terminology conflict requires that the FAA 
regulations be revised and successful collaboration with the National 
Weather Service be achieved.

39. Request To Include ``Sandpaper Ice'' in List of Defined Terms

    Several commenters requested that the term ``sandpaper ice,'' as 
defined in Advisory Circular AC 25-7A, Para. 20(a)(3), be added to the 
list of definitions.
    The FAA does not concur. The notice is intended as a compendium of 
operational definitions. Inclusion of a technical term pertaining to 
the certification of aircraft is deemed inappropriate in this 
compendium.

40. Request To Include ``Runback Ice'' and ``Residual Ice'' in List of 
Defined Terms

    The FAA concurs. Definitions of runback ice, residual ice, and 
inter-cycle ice have been added to the notice.

41. Include ``Supercooled Liquid Water'' in List. Use Term Exclusively

    One commenter requested that the term ``supercooled liquid water'' 
be included in the list of definitions and that this term be used 
exclusively where there are currently references to ``supercooled 
liquid water,'' ``supercooled water drop,'' or ``supercooled water 
droplets.''

[[Page 24550]]

    The FAA partially concurs. The term ``supercooled drops/droplets'' 
will be adopted as equivalent to ``supercooled liquid'' and 
``supercooled liquid water drops.'' The term ``supercooled drops/
droplets'' has been added to the list of definitions and references to 
``supercooled liquid water'' and ``supercooled liquid water drops'' 
have been deleted.

Conclusion

    After consideration of the comments submitted in response to the 
notice of intent, the FAA has determined that the icing terminology, as 
amended following review of the comments, does not conflict with the 
current regulations and the criteria set forth in the FAA Icing Plan.

    Issued in Washington, DC, on April 30, 2003.
Louis C. Cusimano,
Deputy Director, Flight Standards Service.
[FR Doc. 03-11237 Filed 5-6-03; 8:45 am]
BILLING CODE 4910-13-M