[Federal Register Volume 66, Number 216 (Wednesday, November 7, 2001)]
[Notices]
[Pages 56374-56378]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-28000]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

[Policy Statement Number ACE-01-23.1093(b)]


Issuance of Policy Statement, Compliance with Induction System 
Icing Protection for Part 23 Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of policy statement.

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SUMMARY: This document announces an FAA general statement of policy 
applicable to turbine powered, normal, utility, acrobatic, and commuter 
category airplanes. This document advises the public, in particular 
small airplane owners and modifiers, of information related to 
compliance with the engine induction system icing protection 
requirements applicable to turbine powered, part 23, normal, utility, 
acrobatic, and commuter category airplanes. This notice is necessary to 
tell the public of FAA policy.

FOR FURTHER INFORMATION CONTACT: Paul Pellicano, Federal Aviation 
Administration, Small Airplane Directorate, Regulations and Policy 
Branch, ACE-111, 901 Locust, Room 301, Kansas City, Missouri 64106; 
telephone (770) 703-6064; fax (770) 703-6097.

SUPPLEMENTARY INFORMATION:

Background

    This notice announces the following policy statement, ACE-01-
23.1093(b). The purpose of this statement is to address compliance with 
the engine induction system icing protection requirements applicable to 
turbine powered, part 23, normal, utility, acrobatic, and commuter 
category airplanes.

What is the general effect of this policy?

    The FAA is presenting this information as a set of guidelines 
suitable for use. However, we do not intend that this policy set up a 
binding norm; it does not form a new regulation,

[[Page 56375]]

and the FAA would not apply or rely on it as a regulation.
    The FAA Aircraft Certification Offices (ACO's) and Flight Standards 
District Offices (FSDO's) that certify changes in type design and 
approve alterations in normal, utility, acrobatic, and commuter 
category airplanes should try to follow this policy when appropriate. 
Applicants should expect the certificating officials would consider 
this information when making findings of compliance relevant to 
compliance with the engine induction system icing protection 
requirements applicable to turbine powered, part 23, normal, utility, 
acrobatic, and commuter category airplanes.
    As with all advisory material, this statement of policy identifies 
one way, but not the only way, of compliance.

General Discussion of Comments

    Has FAA taken any action to this point?
    We issued a notice of policy statement, request for comments. This 
proposed policy appeared in the Federal Register on August 1, 2001 (66 
FR 39815) and the public comment period closed August 31, 2001.

Was the public invited to comment?

    The FAA encouraged interested people to join in making this 
proposed policy. We received comments from two airplane manufacturers.
    The comments were related to similarity of part 25 guidance, the 
severity and subjectivity of the falling and blowing snow criteria, 
applicability of auxiliary power units (APU), policy differences 
between inlet styles, an ice shedding example in the policy which 
contradicts operational regulations, and making the policy into an 
Advisory Circular. The comments on the subjectivity of the falling and 
blowing snow criteria and the ice shedding example resulted in 
revisions to the policy.
    The proposed policy was coordinated with the appropriate technical 
specialists at the Transport Airplane Directorate and the Engine and 
Propeller Directorate and it does not contradict any part 25 policy.
    The \1/4\ mile visibility criteria for falling and blowing snow 
comes from the definition of heavy snow in the FAA Aeronautical 
Information Manual (AIM) and agrees with transport and rotorcraft 
directorate policy. The policy states that other criteria may be 
applicable, such as that provided in Advisory Circular 29-2C. Another 
resource is FAA Report DOT/FAA/AR-97/66, ``Snow and Ice Particle Sizes 
and Mass Concentrations at Altitudes Up to 9 km (30,000 ft.)'' and this 
is added to the policy.
    The proposed policy states that all turbine installations, 
regardless of inlet type, should have a design analysis performed and 
if no accumulation sites of concern exist, then the analysis may be 
sufficient. A typical part 23 turbopropeller installation has 
accumulation sites of concern that may not exist for a typical turbojet 
or turbofan installation with a pitot style inlet. Also, the policy 
does not distinguish between inlet styles in the applicability of 
service history.
    The policy will be incorporated into the next revision to Advisory 
Circular 23-16 and in the interim will be posted on the Internet to 
provide a wide circulation.

The Policy

    The purpose of this policy statement is to provide compliance 
guidance for the engine induction system ice protection requirements 
contained in 14 CFR, part 23, Sec. 23.1093(b), which is applicable to 
part 23 turbine powered airplanes. Except for the information contained 
in Advisory Circulars (AC's) 20-73 and 23.1419-2A, this guidance 
cancels and supersedes previous guidance on Sec. 23.1093(b) compliance 
for part 23 normal, utility, acrobatic, and commuter category 
airplanes.
    The guidance contained in AC 20-73 and AC 23.1419-2A, relevant to 
Sec. 23.1093(b) compliance, is still applicable.
    Applicants and FAA Aircraft Certification Offices (ACO's) involved 
with certification of small airplanes should generally follow this 
policy. Applicants should expect that the ACO would consider this 
information when making findings of compliance. However, in determining 
compliance with certification standards, each ACO has the discretion to 
deviate from these guidelines when the applicant demonstrates a 
suitable need. To ensure standardization, the ACO should coordinate 
deviation from this policy with the Small Airplane Directorate.

References

    FAA Aeronautical Information Manual (AIM).
    FAA Report DOT/FAA/AR-97/66, Snow and Ice Particle Sizes and 
Mass Concentrations at Altitudes Up to 9 km (30,000 ft.).
    AC 23.1419-2A, Certification of Part 23 Airplanes for Flight in 
Icing Conditions.
    AC 20-73, Aircraft Ice Protection.
    AC 29-2C, Certification of Transport Category Rotorcraft.

Considerations Regarding Approval for Flight in Known Icing

    It is important to know that compliance with Sec. 23.1093(b) for 
induction system icing protection, the initial requirement being 
incorporated by Amendment 23-7, is independent of approval for flight 
into icing conditions (Sec. 23.1419 compliance). Propulsion system 
items that were intended to be certificated for approval for flight 
into icing conditions are addressed under Sec. 23.929, initially 
adopted by Amendment 23-14. Service experience has shown that airplanes 
encounter icing conditions even if the airplane is not approved for 
flight into icing conditions. This is particularly true with turbine 
powered airplanes, which typically have an expanded operating flight 
envelope as compared to reciprocating engine powered airplanes. To 
provide a minimum level of ice protection for all for part 23 normal, 
utility, acrobatic, and commuter category airplanes, compliance with 
all the requirements contained in Sec. 23.1093 must be demonstrated 
even if the aircraft is not approved for flight into icing conditions. 
Therefore, for turbine powered airplanes, compliance with 
Sec. 23.1093(b) is required even if approval for flight into icing 
conditions is not sought.

Use of Similarity and Service Experience

    The use of similarity and service experience is appropriate to 
lessen the design risk associated with an installation. Once an 
applicant has developed data on an installation, then the applicant may 
use this data, when suitable, for substantiation on later projects with 
similar installations. It is common and proper for an applicant to base 
analytical methods and test point definitions on experience and testing 
of previous, similar certification programs performed by the applicant. 
However, since certification data helps define the type design of an 
airplane, for one applicant to use data from another applicant's 
certification program as substantiation, access to the specific design 
and test considerations used by the second applicant would be required. 
Therefore, the proper use of similarity data by an applicant to support 
analytical methods and testing requirements would be difficult if the 
data was not based on the applicant's past projects or if the project 
is not being performed in cooperation with another applicant.
    Even if previous experience and data are used, each inlet/engine 
installation and the associated operating

[[Page 56376]]

characteristics can be different and should be considered individually. 
Therefore, it is not appropriate to use similarity or service 
experience by itself for the purpose of demonstrating compliance to the 
Sec. 23.1093(b) requirement. Rather, such means as similarity or 
service experience should be supplemented with either analysis, even if 
only basic design analysis to substantiate similarity, or testing, or a 
combination of both.

Use of Tunnel Test Data

    An area where there has been much discussion has been the use of 
tunnel test data instead of full-scale, airplane flight test data for 
showing compliance with Sec. 23.1093(b). The use of tunnel test data is 
a common, appropriate, and often efficient means to reduce the amount 
of testing required by the applicant for showing compliance. However, 
the extent that this data can be used for compliance is dependent upon 
how representative the test article and test conditions are to the 
installation and airplane operating conditions.
    It is not uncommon for tunnel testing to be performed on a 
prototype or test inlet that often has design differences from the 
production inlet used by an installer. When using tunnel test data, or 
any test data for that matter, as a basis for testing or certification, 
the applicant must address the differences and the impact of the 
differences. Three areas of difference usually addressed are:
    (1) Heated versus non-heated inlets;
    (2) inlets with movable or variable internal devices (for example, 
movable vanes used to select bypass modes on a number of turbopropeller 
inlets) versus fixed inlets; and
    (3) differences in geometry even if the inlet type (fixed versus 
variable) is the same.
    As an example, if tunnel testing is performed with a heated inlet 
and an applicant incorporates a non-heated inlet, ice runback/refreeze 
may be reduced, but items such as ice accretion characteristics will be 
different.
    Also, it must be ensured that the tunnel tests were performed at 
the critical points. Advisory Circular 20-73, Aircraft Ice Protection, 
provides guidance on critical points determination.

14 CFR Part 33 Engine Certification

    An applicant seeking airplane certification should coordinate the 
installation of an engine with the engine manufacturer. The engine 
manufacturer should be able to identify critical points, conditions, 
and operational requirements that may need to be addressed when showing 
compliance with the installation requirements. However, it is 
inappropriate to assume that any part 33 engine certification program 
would fully address all the part 23 engine installation requirements.
    It should be emphasized that it is the responsibility of the 
airplane applicant and not the engine manufacturer to show compliance 
with the part 23 induction system ice protection requirements. Items 
such as use of an inlet system recommended by the engine manufacturer 
would still require installation substantiation to show compliance with 
part 23 requirements.
    It is appropriate to use engine type certification data as the 
basis for reducing design risk, analysis, testing, and so forth; 
however, when showing compliance with Sec. 23.1093(b) it is still the 
responsibility of the installer to evaluate this data and demonstrate 
how the data is applicable to the particular application. Therefore, 
close coordination of the engine and airplane applicant can ease 
certification burdens and enhance the safety of a particular engine 
installation.

Falling and Blowing Snow Requirement

    The requirement contained in Sec. 23.1093(b)(1)(ii), incorporated 
initially by Amendment 23-15, is to evaluate the installed powerplant 
system to ensure no hazardous effects are encountered when operating in 
falling and blowing snow. A hazardous effect could be in the form of 
unacceptable engine operating characteristics (for example, adverse 
power loss, surges, and so forth) due to inlet blockage or engine 
damage resulting from conditions such as snow, which may accumulate, 
melt, refreeze, shed, and then be ingested by the engine. The 
requirement was incorporated separately from icing and water ingestion 
requirements due to the unique characteristics of snow. Therefore, it 
is inappropriate to assume that compliance with engine induction system 
icing requirements means that compliance with snow requirements have 
been met.
    Service experience has demonstrated that engine damage can occur as 
a result of prolonged ground operations in falling and blowing snow. 
Also, in-flight service experience has shown that snow, which has 
melted and refrozen, can shed from engine, inlet, or airplane 
accumulation sites, resulting in adverse engine operability or engine 
damage. Therefore, the effect of ingesting snow during ground 
operations and critical in-flight operations should be evaluated. The 
snow environment that has been seen to be critical is a ``wet, sticky 
snow,'' which accumulates on unheated exterior and interior surfaces 
subject to impingement.
    When showing compliance with Sec. 23.1093(b)(1)(ii), review of the 
installation should be performed to identify potential inlet, engine, 
and airframe sites where snow accumulation and shedding is possible. 
Also, review of the airplane operation should be performed to determine 
critical conditions that should be addressed.
    Although all turbine engine installations should be evaluated, 
turbopropeller installations generally have different areas of concern 
than turbofan/jet installations. Typical turbopropeller installations 
have inlets that incorporate complex geometry with features such as 
particle separators, plenum chambers, screens, oil coolers, and so 
forth, where hazardous snow accumulations may occur. Typical turbofan/
jet installations, using simple pitot (straight duct) inlets, have 
minimal, if any, areas for snow accumulation. For these inlets, in-
flight icing tests have been generally been found to be more critical 
than snow tests. Therefore, a turbofan/jet installation may be found 
acceptable by inlet design and airplane operation analysis, while 
turbopropeller installations will normally require testing in 
operationally representative conditions.
    However, it needs to be reemphasized that the installation should 
be evaluated to decide on the required level of substantiation. For 
example, aft mounted turbofan/jet installations may have concerns with 
accumulation and in-flight snow shed from wing surfaces after take-off. 
Also, there are turbofan installations with S-type inlet ducts that 
would have many of the same concerns as turbopropeller installations. 
Additionally, part 33 engine certification does not address snow 
ingestion and some turbofan/jet engines, in addition to turbopropeller 
engines, may have internal accumulation sites that may allow snow to 
melt, refreeze, and shed causing internal engine damage. Therefore, all 
turbine engines should be evaluated with close coordination with the 
engine manufacturers.
    When evaluating the conditions for showing compliance, the 
following airplane operations should be considered:
    1. Static operation with the engine at idle for 30 minutes, with 
the ability to attain take-off power. This condition is considered 
critical due to the operational consideration of idling an engine on 
the ground with minimal ability for de-ice/anti-ice. The primary

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concern is the loss of power at take-off roll.
    If found acceptable, the engine may be able to be run up at higher 
power settings during the 30 minute period for the purposes of ice/snow 
shed. If run-ups are performed during compliance demonstration, these 
procedures should be incorporated as limitations in the Flight Manual.
    Before run-ups are accepted, the practicality of the procedures 
should be evaluated. For example, if an engine must be run at a high 
power setting that may allow the airplane to slide or create hazards to 
other airplanes, then the procedures may not be acceptable.
    2. Higher power settings, which could result in increased snow 
ingestion, associated with taxi/hold ground operations.
    3. For airplanes with identified sites of possible hazardous snow 
accumulation and all inlets with bypass ducts (for example, typical 
turbopropeller inlets), a take-off run to take-off speed. This 
condition is considered critical since
    (a) accumulated snow may liberate at this dynamic condition; and
    (b) the static, idle point will not provide the ram effects that 
create bypass flow for bypass ducts.
    4. For airplanes with identified sites of possible hazardous snow 
accumulation, take-off climb. This condition is considered critical 
since accumulated snow may liberate at this dynamic condition.
    5. Extended in-flight operations such as hold patterns.
    6. Operation when engine rotor speeds are low, such as during 
descent from high altitudes. An engine is highly susceptible to snow/
ice accretion during this condition.
    It should be noted that the preceding conditions are operational 
considerations and not meant to require flight test at all the 
conditions. As mentioned earlier, each installation may have different 
critical operational considerations and only the critical conditions 
may need further substantiation than just analysis.
    Also, when appropriately substantiated by the applicant, some of 
the conditions can be, and have been, simulated and accepted by the 
FAA. For example, for a turbopropeller engine that incorporates an 
inlet screen that precludes the ingestion of hazardous quantities of 
materials, the critical concern to be addressed may only be the effect 
of snow accumulation and release from the inlet and screen. In this 
case, the inlet, bypass duct, inlet screen, and so forth, could be 
blocked to simulate snow accumulation on an identified area of concern. 
Since accumulation during dynamic operation would be simulated, the 
effects of snow ingestion could be determined through ground tests (for 
example, effects of operability on items such as reverse flow). Such 
methodologies need to be substantiated by means such as design 
analysis, operational review, tunnel tests, icing tests, and so forth, 
and coordinated early with the FAA.
    When testing in ``falling and blowing snow'' the actual snow amount 
is often difficult to quantify. The FAA Aeronautical Information Manual 
(AIM), an official FAA guide to basic flight information and air 
traffic control procedures, may be used as guidance for what 
constitutes falling and blowing snow. Per the AIM, paragraph 7-1-18, 
heavy snow, which is representative of what may be expected in 
operation, is defined as visibility of \1/4\ mile or less as limited by 
snow (not snow and fog). These conditions are usually indicative of the 
wet snow environment desired for test. When using the \1/4\ mile or 
less visibility for test, including flight tests, this value can be 
determined using ground conditions. Useful information on snow 
characterization can also be found in FAA Report DOT/FAA/AR-97/66, Snow 
and Ice Particle Sizes and Mass Concentrations at Altitudes Up to 9 km 
(30,000 ft.). Advisory Circular 29-2C (Certification of Transport 
Category Rotorcraft), section AC 29-1093, paragraph c(4)(iv) also 
provides information on snow quantification including desired snow 
concentration, which is acceptable for use on part 23 airplanes. 
However, whatever method is used to characterize the snow, as mentioned 
earlier, the design consideration that has been found to be critical is 
snow that accumulates on surfaces subject impingement. Therefore, the 
applicant should address this consideration when choosing the 
appropriate snow environment.
    The primary consideration is to demonstrate operability in a snow 
environment that is critical as far as snow accumulation on exterior 
and interior areas of impingement (for example, wet, sticky snow). 
However, for a snow environment indicative of a representative 
concentration expected for the airplane, temperature is also an 
important consideration. The applicant is responsible for defining the 
critical ambient temperatures for snow tests.
    Typically, in natural conditions, a temperature range between 25 
and 34 degrees Fahrenheit has been found conducive to the heavy snow 
environment. However, colder temperatures may be critical to some 
configurations. For example, in some installations, colder exterior 
surfaces may be bypassed, with snow crystals sticking to partially 
heated interior inlet surfaces, leading to melting and refreeze. In all 
cases, the applicant must identify and evaluate the critical 
temperature for the configuration proposed. Company developmental tests 
or experience with similar induction systems may be used to determine 
critical conditions.
    It should be emphasized that the purpose of the requirement is to 
evaluate the engine's induction system ice protection capability in 
snow environments that can be expected during the operational life of 
the airplane. Addressing the snow environment, detailed in resource 
materials such as the AIM, at critical operational conditions for a 
particular airplane, provides a good gauge to evaluate the system's 
capability. Most configurations will not require flight test in all 
operational conditions.
    Snow concentration corresponding to the visibility prescribed is 
often extremely difficult to locate naturally. Furthermore, it is often 
difficult to maintain the desired concentrations for the duration of 
testing. Because of these testing realities, it is very likely that 
exact target test conditions will not be achieved for all possible test 
conditions. Therefore, those involved in certification must exercise 
reasonable engineering judgement in accepting critical test conditions 
and alternate approaches, with early coordination between the applicant 
and the FAA addressing these realities.
    Artificially produced snow is an excellent developmental tool and 
has been used successfully to show potential problem areas and critical 
test points. When the desired natural snow concentration is not found, 
artificial means may be used to supplement the snow amount. However, 
when snow testing is required, the use of simulated snow is normally 
not used as the sole means of compliance. The desired heavy snow 
environment produces ``wet, sticky snow,'' which accumulates on 
unheated exterior and interior surfaces subject to impingement. Most 
artificial means (for example snow blowers) produce snow pellets that 
are dissimilar to the snowflakes associated with ``wet, sticky snow.'' 
Also, simulated snow produced indoors does not accumulate moisture from 
snow fall as seen in naturally created snow, with critical temperatures 
for simulated snow varying significantly from natural snow. Therefore, 
quantification of artificially produced snow for critical conditions 
can be very difficult and subjective. If artificial means is proposed 
as a means

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of compliance, the applicant should provide data and substantiation on 
how the artificial means will effectively simulate the critical, 
desired operational consideration.
    The concentration of snow entering the inlet in blowing snow will 
normally exceed the amount in falling snow; hence, the need to address 
``blowing snow.'' Therefore, the location of the inlets should be 
considered to determine critical directions of blowing snow in relation 
to snow accumulation on impingement surfaces. Snow blowing in excess of 
15 knots is the desired compliance condition. Means such as use of 
another airplane's propeller, taxiing the airplane in excess of 15 
knots, and so forth, may be used to simulate blowing.
    An additional area of emphasis for Sec. 23.1093(b)(1)(ii) 
compliance is the words in the regulation ``. . .within the limitations 
established for the airplane for such operation.'' As with all 
environmental considerations, such as rain, ice, hail, lightning, and 
so forth, operation in snow is considered an unavoidable, 
meteorological hazard that must be addressed. The only plausible Flight 
Manual limitation that may be acceptable would be prohibitions for 
ground operations such as taxi, take-off, engine runs, and so forth. 
However, the case of flying into snow after deployment must be 
considered.

Ice Fog

    The basic requirement contained in Sec. 23.1093(b)(2), also 
incorporated by Amendment 23-15, addresses the condition of idling the 
engine on the ground to ensure no adverse ice build-up (for example, no 
surges, adverse power loss, and so forth), commonly referred to as 
``ice fog.'' A way to view the Sec. 23.1093(b)(2) requirement is as an 
extension upon the 14 CFR part 25, Appendix C icing envelope addressed 
in Sec. Sec. 23.1093(b)(1)(i) and 23.1419. Therefore, the methodologies 
and analysis used for compliance with Sec. 23.1093(b)(1)(i) can be 
extended for Sec. 23.1093(b)(2) compliance.
    It is often difficult to encounter all the ambient conditions 
required by Sec. 23.1093(b)(2); therefore, when testing, one or more of 
the conditions is typically simulated. For example, a common and 
acceptable method of compliance is using water spray devices to 
simulate the water conditions required, while testing at the required 
ambient temperature conditions. Other manufacturers have used thermal 
analysis combined with dry air tests using ice shapes/simulated 
blockage to demonstrate compliance, which is also acceptable if 
properly substantiated.
    The rule allows an engine run-up periodically to higher power 
settings to shed ice. As with snow testing, if run-ups are performed 
during compliance demonstration, then these procedures should be 
incorporated as limitations in the Flight Manual. Also, before run-ups 
are accepted, the practicality of the procedures should be evaluated.

    Issued in Kansas City, Missouri, on October 23, 2001.
Michael Gallagher,
Manager, Small Airplane Directorate, Aircraft Certification Service.
[FR Doc. 01-28000 Filed 11-6-01; 8:45 am]
BILLING CODE 4910-13-P