THE FREEZING RAIN
The Freezing Rain is one of the critical environmental issues that still today are the cause of serious
incidents. I’ve personally experienced the effects of Freezing Rain in flight: I’d like to describe my
experience and offer an account of the current activities on the subject.
1. Foreword
We were flying at 4000 ft, IMC 1, without any external reference in a dense cloud and with no
visible ice accretion on the wings. Suddenly one strange thing happened: having gradually
augmented the power up to the MCP 2, the IAS 3 appeared to be only 110 kts. This would have been
perhaps normal for a helicopter, but not for a twin-engine turboprop.
What was happening? The pilot, an expert in anti-ice systems evaluation fortunately knew the
answer: we were flying into a layer of freezing rain, which is rain that becomes super cooled and
freezes upon impact with cold surfaces. The aircraft was now almost completely enveloped in a
layer of ice that increased its weight and, above all, drag.
What to do? This situation has caused many accidents. Thanks to an accurate flight plan, the pilot
was aware of the presence of an inversion of temperature; after an authorization by the traffic
control we climbed to 6000 ft were the warmer air restored the normal conditions.
We decided to land, rapidly passing trough the zone affected by the freezing rain and, after a
temporary lost of visibility, we performed a normal landing.
As a National Aviation Authority’s inspector, I was in the United States to certificate an anti-ice
system on a turboprop twin engine FAR 23 aeroplane after a series of flight tests in natural icing
conditions performed by the manufacturer. Flight tests with artificial shapes of ice had been already
performed to verify the aeroplane performance such as controllability, manoeuvrability, and
stability.
It is not so easy to find icing conditions when you need them for testing an aircraft. Sometimes you
have to spend weeks to have a favourable encounter. I was particularly lucky because one day after
my arrival the weather condition were “favourable”.
During a first flight in the morning we had “classic” example of consistent ice accretion on the
wings and empennages leading edge, without any problem of controllability and so on, confirming
the test already performed.
However, the freezing rain surprised us during a flight performed in the afternoon of the same day.
I like to point out that this condition is extremely risky especially for aircraft provided with standard
de-icing systems capable to de-ice the wing and empennages leading edge but not the rest of the
aeroplane.
As a further personal experience, I remember an accident investigation for an ATR 42 in the 80s - I
was a member of the commission. The chain of factors leading to the accident began with a stall in
icing conditions during the climb. The anti ice boots were working, but according to the voice
recorder registrations, some ice formation was noted behind the boots. Nevertheless, in order to
justify the massive degradation of the rate of climb of the aeroplane, an assumption of a very big
accretion of ice was advanced.
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Instrumental Meteorological Conditions
Maximum Continuous Power
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Indicated Air Speed
2
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After a few years and that flight experience, I was convinced that the ATR 42 had a nasty encounter
with freezing rain.
On October 31, 1994, an accident involving an Aerospatiale Model ATR72 series airplane occurred
in which icing conditions, believed to include freezing drizzle drops, were reported in the area. The
FAA, Aerospatiale, the French Direction Général de l’Aviation Civile, Bureau d’Enquête et
d’Analyses, National Aeronautics and Space Administration, National Transportation Safety Board,
and others conducted an extensive investigation of this accident. This investigation led to the
conclusion that freezing drizzle conditions created a ridge of ice on the wing’s upper surface aft of
the de-icing boots and forward of the ailerons
Freezing rain can rapidly affect the surfaces of an aircraft and cause big problems because most
helicopters and light aeroplanes do not have de-icing equipment to cope with such a situation. Even
sophisticated de-icing systems on large airplanes could not be able to handle the problem of heavy
freezing rain.
2. The current airworthiness regulations (aeroplanes)
Current airworthiness regulations deal with icing problems.
A type certificate could be obtained without special anti-ice certification, but such a T. C. does not
allow operation in icing condition, for which compliance with the appropriate requirements is
mandatory.
The paragraph 1525 of the FAR/CS 23 and 25 standards establishes (even if with different wording)
that the kind of operation authorized (e. g. VFR, IFR, day or night) and the meteorological
conditions (e. g. icing) to which the operation of the aircraft is limited or from which it is
prohibited, must be established appropriate to the installed equipment.
For the certification of the appropriate equipment for icing protection, the paragraph 1419 of the
above-mentioned FAR/CS standards gives the relevant key requirements.
To give an example, I quote the relevant FAR paragraph 4
§ 25.1419 Ice protection.
If the applicant seeks certification for flight in icing conditions, the airplane must be able to safely operate in the
continuous maximum and intermittent maximum icing conditions of appendix C. To establish this:
(a) An analysis must be performed to establish that the ice protection for the various components of the airplane is
adequate, taking into account the various airplane operational configurations; and
(b) To verify the ice protection analysis, to check for icing anomalies, and to demonstrate that the ice protection system
and its components are effective, the airplane or its components must be flight tested in the various operational
configurations, in measured natural atmospheric icing conditions and, as found necessary, by one or more of the
following means:
(1) Laboratory dry air or simulated icing tests, or a combination of both, of the components or models of the
components.
(2) Flight dry air tests of the ice protection system as a whole, or of its individual components.
(3) Flight tests of the airplane or its components in measured simulated icing conditions.
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The CS 25. 1419 has the same content.
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(c) Caution information, such as an amber caution light or equivalent, must be provided to alert the flight crew when the
anti-ice or de-ice system is not functioning normally.
(d) For turbine engine powered airplanes, the ice protection provisions of this section are considered to be applicable
primarily to the airframe. For the powerplant installation, certain additional provisions of subpart E of this part may be
found applicable.
(e) One of the following methods of icing detection and activation of the airframe ice protection system must be
provided:
(1) A primary ice detection system that automatically activates or alerts the flight crew to activate the airframe
ice protection system;
(2) A definition of visual cues for recognition of the first sign of ice accretion on a specified surface combined
with an advisory ice detection system that alerts the flight crew to activate the airframe ice protection system;
or
(3) Identification of conditions conducive to airframe icing as defined by an appropriate static or total air
temperature and visible moisture for use by the flight crew to activate the airframe ice protection system.
(f) Unless the applicant shows that the airframe ice protection system need not be operated during specific phases of
flight, the requirements of paragraph (e) of this section are applicable to all phases of flight.
(g) After the initial activation of the airframe ice protection system—
(1) The ice protection system must be designed to operate continuously;
(2) The airplane must be equipped with a system that automatically cycles the ice protection system; or
(3) An ice detection system must be provided to alert the flight crew each time the ice protection system must
be cycled.
(h) Procedures for operation of the ice protection system, including activation and deactivation, must be established and
documented in the Airplane Flight Manual.
This paragraph 1419 offers the requirements to demonstrate safe operation in the continuous
maximum and intermittent maximum icing conditions of Appendix C of FAR/CS 25. 5
2.1 The Appendix C
Appendix C addresses continuous maximum and intermittent maximum icing conditions within
stratiform and cumuliform clouds ranging from sea level up to 30,000 feet.
Appendix C defines icing cloud characteristics in terms of mean effective drop diameters 6, liquid
water content 7 temperature, horizontal and vertical extent, and altitude.
Icing conditions that contain drops with mean effective diameters that are larger than the cloud
mean effective drop diameters defined in appendix C are typically referred to as freezing drizzle or
freezing rain.
Icing conditions containing freezing drizzle and freezing rain are not currently considered when
certifying an airplane’s ice protection systems.
Because of the larger diameter, drops typically impinge farther aft on the airfoil than the drops
defined in appendix C; exposure to these conditions can result in ice accretions aft of the ice
protection area, and may negatively affect airplane performance, flutter characteristics, handling
qualities and engine and systems operation.
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Other paragraphs of FAR/CS 25 must be complied with.
Mean effective drop diameter:`The calculated drop diameter that divides the total liquid water content present in the
drop size distribution in half. Half the water volume will be in larger drops and half the volume in smaller drops. This
value is calculated, as opposed to being arrived at by measuring actual drop size. The MED is based on an assumed
Langmuir drop size distribution. The fact that it is a calculated measurement is how it differs from median volume
diameter, which is based on actual drop size.
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Liquid Water Content (LWC): The total mass of water contained in liquid drops within a unit volume or mass of air,
usually given in units of grams of water per cubic meter (g/m3).
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Likewise, mixed phase (supercooled liquid and ice crystals) and 100% ice crystal icing conditions
are not currently considered when certifying an airplane’s ice protection systems. Exposing engines
and externally mounted probes to these conditions could result in hazardous ice accumulations
within the engine that may result in engine damage, power loss, and loss of or misleading airspeed
indications.
The certification regulations for transport category airplanes and engines do not address the safe
operation of airplanes in SLD, mixed phase, or ice crystal icing conditions and the operating rules
do not specifically prohibit operations in these conditions.
The accident and incident history indicates that flight crews of certain types of aircraft have had
power losses and in some cases lost control of their aircraft in such conditions.
3. The future FAA airworthiness regulations (Transport cat. aeroplanes).
With reference to the ATR 72 accident in October 1994, the atmospheric conditions (freezing
drizzle) that may have contributed to the accident are outside of the icing envelope specified in
Appendix C of FAR 25. The FAA has not required that aeroplanes be shown to be capable of
operating safely in freezing drizzle or freezing rain icing conditions.
After the 1980s, a few amendments of Paragraph 1419 and other paragraph of FAR/CS have been
produced, but the problem of the freezing rain is still unsolved.
The above-mentioned accident brought to the forefront of public and governmental attention safety
concerns about the adequacy of the icing certification standards.
The FAA became aware that the types of icing conditions considered during the certification of
transport category airplanes and turbine aircraft engines needed to be expanded to increase the level
of safety during flight in icing. The FAA determined that the revised icing certification standards
should include supercooled large drops (SLD), mixed phase, and ice crystals.
This is the kind of amendments requiring an engagement of specialists in the long term. An
international working group led by the Aviation Rulemaking Advisory Committee (ARAC), tasked
by the FAA, worked between 1997 and 2009 to prepare recommendations for a regulation change.
So, it is therefore not surprising if only at the end of June 2010 the FAA was able to issue a Notice
of Proposed Rulemaking (NPRM) that is still in a discussion phase.
3.1 Summary of the proposals
The FAA proposes to review FAR 25 (Airworthiness Standards: Transport Category Airplanes) and
FAR 33 (Airworthiness Standards: Aircraft Engines) related to the certification of transport
category airplanes and turbine aircraft engines in icing conditions.
It also propose to create new regulations:
 § 25.1324—Angle of attack systems;
 § 25.1420 SLD icing conditions;
 part 25, Appendix O (SLD icing conditions);
 part 33, Appendix C
 and part 33, appendix D (Mixed phase and ice crystal icing conditions).
To improve the safety of transport category airplanes operating in SLD, mixed phase, and ice
crystal icing conditions, the proposed regulations would:
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 Expand the certification icing environment to include freezing rain and freezing drizzle.
 Require airplanes most affected by SLD icing conditions to meet certain safety standards in
the expanded certification icing environment, including additional airplane performance and
handling qualities requirements.
 Expand the engine and engine installation certification, and some airplane component
certification regulations (for example, angle of attack and airspeed indicating systems), to
include freezing rain, freezing drizzle, ice crystal, and mixed phase icing conditions.
NOTE. The new §1420 will be applicable to aeroplanes most affected by SLD icing conditions 8 or
with reversible flight controls 9
3.1.1 Advisory Material
A new proposed AC and revisions to existing ACs would provide guidance material for one
acceptable means, but not the only means, of demonstrating compliance with the proposed
regulations contained in this NPRM. The guidance provided in these documents is directed at
airplane manufacturers, modifiers, foreign regulatory authorities, and FAA transport airplane type
certification engineers, flight test pilots, and their designees.
4. The future EASA airworthiness regulations (Large aeroplanes)
These FAA recommendations have been reviewed and analysed by the EASA and an update of CS25 is proposed: NPA N° 2011-03 of 21 Mars 2011.
The Agency considered and analysed the IPHWG 10 recommendations, the above-mentioned FAA
NPRM and all the lessons from in-service large aeroplanes.
The EASA NPRM contains an interesting analysis and risk assessment in which is observed that
the most severe risk is the loss of control of the aeroplane in SLD icing conditions, which can lead
to a hull loss of the aeroplane. At least five accidents happened with this scenario, and four of them
involved fatalities.
Engine power losses or flameouts are also a safety threat, especially in ice crystal and mixed phase
icing conditions. More than one hundred documented cases exist.
Furthermore, service experience indicates that flight crews have experienced temporary loss of or
erroneous airspeed indications, malfunctioning and/or damage to temperature probes in severe icing
conditions (in areas of deep convection). The main suspected cause is ice crystals in high
concentration.
In-flight snow (and mixed phase) conditions have also caused power interruptions on some turbine
engines and APUs.
Finally, service experience of different engine types has identified the potential for a multiple
engine failure during take-off, after prolonged ground operation in freezing fog.
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Aeroplanes with a maximum takeoff weight less than 60,000 pounds
The size of an airplane determines the sensitivity of its flight characteristics to ice thickness and roughness. The relative
effect of a given ice height (or ice roughness height) decreases as airplane size increases.
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Reversible Flight Controls: Flight controls in the normal operating configuration that have force or motion
originating at the airplane’s control surface (for example, through aerodynamic loads, static imbalance, or trim tab
inputs) that is transmitted back to flight deck controls. This term refers to flight deck controls connected to the pitch,
roll, or yaw control surfaces bydirect mechanical linkages, cables, or push-pull rods in such a way that pilot effort
produces motion or force about the hinge line.
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(IPHWG) Ice Protection Harmonization Working Group led by the Aviation Rulemaking Advisory Committee
(ARAC)
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4.1 Differences with FAA’s proposal
The EASA proposal mainly differs from the FAA’s proposal on the following points:



The new proposed SLD environment would be applicable to all new large aeroplanes
(not limited to a category of large aeroplane),
The mixed phase and ice crystals environment for flight instrument external probes:
EASA propose to use the Part 33 Appendix D proposed by the IPHWG, which would be
applicable to all flight instrument external probes (not limited to Pitot tubes and Angle
of attack sensors),
EASA propose to clarify and extend the existing provisions requiring alerting flight
crews when an installed flight instrument external probes anti-ice or de- ice system is
not operating normally.
5. Conclusions
These proposals are still in a discussion phase and it is difficult to forecast the timing for the
amendments. Furthermore, FAA and EASA would certainly harmonise such important amendments
of FAR and CS 25 and that will require further time.
In the meantime, special conditions tailored to some of the new requirements might be added to the
certification basis of new projects, but the final rules would fully apply to the certification of future
designs of transport category airplanes and will not concern the thousands of existing aeroplanes.
Consequently, alternative means should be provided “now” to cope with the problem, such as better
meteorological information, but above all, crew training: sound knowledge about freezing rain and
how to avoid it if it all possible and how to behave if that happens. We have seen too many
accidents caused, for instance, by poor stall recovery.
NOTE: The FAR/CS 23 aeroplanes.
If we look at the FAR/CS 23, they present a common approach to the problem, but the last
amendment of FAR 23 § 1419 is of April 9, 1993, while the same paragraph of FAR 25 has been
amended several times (last amendment is dated Aug. 3, 2009) and it is more demanding. 11
The AC No: 23.1419-2D dated June 28, 2007 “Certification of part 23 airplanes for flight in
icing conditions” sets forth an acceptable means of showing compliance with FAR 23, for the
approval of airplane ice protection systems for operating in the icing environment defined by FAR
25, Appendix C.
This is a valuable document also containing a list of related regulations, documents and reading
material. But the problem of the SLD, mixed phase, and ice crystals, not covered by the appendix C
remains open.
In any case, the AC gives some important advice about the Pitot Probe Ice Protection and Stall
Warning Ice Protection for conditions within and outside FAR 25 Appendix C Icing Envelope.
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CS 23 was issued in Nov. 2003, but the content of paragraph 1419 is similar to that of FAR 23.
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ACTIVATION of ICE PROTECTION
The proposals of the above-mentioned FAA and EASA amendments are related to airworthiness
regulations as FAR/CS 25 and 33.
Now, on the basis of the latest research of aircraft icing, the FAA has issued a new rule to enhance
aviation safety by requiring to install ice detection equipment in their existing fleets or to update
their flight manuals to make sure crews know when they should activate their ice protection
systems.
This action revises the operating rules for flight in icing conditions and in particular amends the
paragraph 321 of FAR 121. This amendment becomes effective on the FAR 121 in date October
21, 2011, but the operating rule is mandatory after October 21, 2013.
According to the criteria for amending the FAR 25, this amendment of the FAR 121 is related to
aeroplanes with a certificated maximum takeoff weight less than 60 000 pounds.
The discussion about this amendment can be found here:
<http://www.ofr.gov/OFRUpload/OFRData/2011-21247_PI.pdf>
Filippo De Florio
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