[Federal Register Volume 66, Number 148 (Wednesday, August 1, 2001)]
[Notices]
[Pages 39815-39818]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-19154]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

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


Proposed Issuance of Policy Memorandum, Compliance With Induction 
System Icing Protection (14 CFR part 23, Sec. 23.1093(b)) for Part 23 
Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of policy statement; request for comments.

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SUMMARY: This document proposes to adopt new policy for compliance with 
induction system icing protection for certification of normal, utility, 
acrobatic, and commuter category turbine powered airplanes with 
propeller beta mode pitch settings.

DATES: Comments sent must be received by August 31, 2001.

ADDRESSES: Send all comments on this proposed policy statement to the 
individual identified under FOR FURTHER INFORMATION CONTACT.

FOR FURTHER INFORMATION CONTACT: Randy Griffith, Federal Aviation 
Administration, Small Airplane Directorate, Regulations and Policy 
Branch, ACE-111, 901 Locust, Room 301, Kansas City, Missouri 64106; 
telephone (816) 329-4126; fax (816) 329-4090; email: 
[email protected]>.

SUPPLEMENTARY INFORMATION:

Comments Invited

How Do I Comment on the Proposed Policy?

    We invite your comments on this proposed policy statement, ACE-01-
23.1093(b). You may send whatever written data, views, or arguments you 
choose. Mark your comments, ``Comments to policy statement ACE-01-
23.1093(b)'' and send two copies to the above address. We will consider 
all comments received on or before the closing date. We may change the 
proposals contained in this notice because of the comments received.
    You may also send comments using the following Internet address:

[[Page 39816]]

[email protected]>. Comments sent by fax or the Internet must 
contain ``Comments to policy statement ACE-01-23.1093(b)'' in the 
subject line. You do not need to send two copies. Format in Microsoft 
Word 97 for Windows or ASCII text any comments you send via the 
Internet as attached electronic files.
    Send comments using the following format:
    Organize comments issue-by-issue. For example, discuss a comment 
concerning design evaluation and a comment about maintenance as two 
separate issues.
    For each issue, state what specific change you are seeking to the 
proposed policy memorandum.
    Include justification (for example, reasons or data) for each 
request.

The Proposed Policy

Purpose

What is the Purpose of this Policy Statement?

    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) 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 23.1419-2A, relevant to 
Sec. 23.1093(b) compliance, is still applicable.
    Applicants and FAA Aircraft Certification Offices (ACO) 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).
Advisory Circular 23.1419-2A, Certification of Part 23 Airplanes for 
Flight in Icing Conditions.
Advisory Circular 20-73, Aircraft Ice Protection.
Advisory Circular 29-2C, Certification of Transport Category 
Rotorcraft.

Flight into Icing Approval

    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 (Sec. 23.1419 compliance). Propulsion system items that were 
intended to be certificated to the level of flight into icing approval 
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. 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 flight into icing approval is 
not obtained. Therefore, compliance with Sec. 23.1093(b) is required 
even if flight into icing certification is not pursued.

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 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

    The airplane applicant should coordinate the installation of an 
engine with the engine manufacturer. Engine certification will 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 part 33 
engine certification would

[[Page 39817]]

fully address 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 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 
snow shed from wing surfaces. 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 
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

[[Page 39818]]

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. 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.
    As discussed earlier, 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). Therefore, in addition to 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 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 to get critical conditions 
can be very difficult and subjective. If artificial means is proposed 
as a means 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 July 18, 2001.
James E. Jackson,
Acting Manager, Small Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 01-19154 Filed 7-31-01; 8:45 am]
BILLING CODE 4910-13-U