[Federal Register Volume 66, Number 4 (Friday, January 5, 2001)]
[Notices]
[Pages 1182-1184]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-347]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

[Policy Statement Number ACE-00-23.901(d)(2)]


Issuance of Policy Memorandum, Notice of Compliance with the 
Engine Ingestion Requirements Applicable to Turbine Powered, 14 CFR 
Part 23, Normal, Utility, Acrobatic, and Commuter Category 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 more information related to 
compliance with the engine ingestion 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: 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:

Background

    This notice announces the following policy statement, ACE-00-
23.901(d)(2). The purpose of this statement is to address compliance 
with the engine ingestion 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 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, and acrobatic 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 ingestion 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 September 1, 2000 
(65 FR 53338) and the public comment period closed October 2, 2000.

Was The Public Invited To Comment?

    The FAA encouraged interested people to join in making this 
proposed policy. We received one comment. The commenter, while fully 
agreeing with the content, noted that the policy would be better if in 
an FAA Advisory Circular. We have noted the commenter's concerns. We 
will eventually provide the pertinent information in this policy in a 
revision to Advisory Circular 23-16, Powerplant Guide for Certification 
of Part 23 Airplanes. In the interim, the issuance of a policy 
statement is more timely and effective. Additionally, experience with a 
recent certification project resulted in further clarification of the 
draft policy. As a result, we have explained compliance considerations 
related to critical conditions for turbopropeller engine installations 
as compared to turbojet/fan engine installations. If these added 
compliance considerations cause concern, please send your comments to 
[email protected]>.

The Policy

Background

    The current Sec. 23.901(d)(2) requirement was incorporated by 
Amendment 23-53. However, the basic requirement, which has evolved into 
the current Sec. 23.901(d)(2), was incorporated by Amendment 23-18.
    Amendment 23-18 required that the engine installation provide 
continued engine operation without a sustained loss of power when 
operated at flight idle in rain for at least three minutes. The rate of 
rain ingestion was to be not less than 4 percent, by weight, of the 
engine induction airflow rate. The rule was incorporated due to reports 
of turbine engine power loss while operating in heavy rain. The intent 
of the rule was twofold: (1) to ensure that installation effects do not 
result in deterioration of the engine's rain ingestion tolerance 
determined by engine certification; and (2) to evaluate the engine's 
capability for rain ingestion for engines that were certificated before 
Amendment 33-6 since rain ingestion requirements were not added to 14 
CFR part 33 until Amendment 33-6. Therefore, the rate of rain ingestion 
to be considered was based upon the part 33 engine certification 
requirement at the time.

Revisions of Standards

    Amendment 23-29 revised the requirement to consider rated takeoff 
power/thrust. Also, the preamble to Amendment 23-29 further defined the 
intent of Sec. 23.901(d)(2) by specifically stating that the rule is to 
ensure that installation effects do not result in any deterioration of 
the powerplant rain ingestion tolerance. Therefore, compliance with 
Sec. 23.901(d)(2) required a separate determination for engine 
installation other than the requirements addressed by part 33 (for 
example, engine certification without further installation 
certification is inadequate to demonstrate compliance with the part 23 
requirement).
    Amendment 23-43 added a requirement that the installation be 
evaluated at the maximum installed power/thrust for takeoff. This new 
requirement was due to engine installations where rated takeoff power 
could be less than installed takeoff power; for example, de-rate 
thrust. The amendment also added a requirement that the engine be 
accelerated and decelerated safely under the rain conditions; however, 
Amendment 23-51 removed this consideration.
    Amendment 23-53 added the current rule. The current amendment 
requires the installed engine to withstand ingestion of rain, hail, 
ice, and birds at a level not less than that established under engine 
certification. The significant changes with the new rule include 
operating concerns other than loss of power (for example, engine 
surges), the addition of hail, ice, and bird ingestion requirements, 
and replacement of specific rain quantification with the conditions 
used during engine certification. Under Amendment 23-53, the airplane 
applicant needs to evaluate the conditions used to address rain, hail, 
ice, and bird ingestion during engine certification and how the 
installation relates to these conditions.

Means of Compliance

    When showing compliance with the rain ingestion requirements for 
all amendment levels of Sec. 23.901(d)(2), compliance is typically 
accomplished with design analysis that identifies areas of concern and 
test when there are areas of concern. Part 33 engine certification 
testing may be used for compliance if the engine certification testing 
(1) addressed the areas of concern identified by the installation 
design analysis (for example, use of an installation representative 
test inlet system) and (2) specific conditions addressed in the rule 
were addressed

[[Page 1184]]

during engine certification testing. For airplanes with a certification 
basis prior to Amendment 23-53, test is typically required if the 
specific operating considerations contained in the part 23 rule were 
not addressed during engine certification.
    When evaluating areas of concern with the installation, consider 
areas where water pooling with subsequent ingestion or shed of 
localized ``slugs'' of water normally not addressed during engine 
certification might occur. Some examples are inlet system channels, 
indentations, and so forth. These are typical of turbopropeller or S-
duct type inlets that have complex geometry to allow water pooling. 
This consideration is usually not a concern with simple pitot style 
inlets typical of most part 23 turbofan/jet engine installations. 
However, due to the large diversity of turbine engine installations in 
part 23 airplanes, all installations should be evaluated to determine 
if areas of concern exist. For example, there are turbofan 
installations that use S-style inlet ducts that may have areas of 
concern.
    Therefore, part 23 turbine engine installations typically require 
testing since the vast majority of these are turbopropeller 
installations. However, if design analysis shows that the installation 
will not affect the water ingestion characteristics (for example, a 
simple and typical pitot style inlet installation) and engine 
certification addressed the specific conditions addressed in the part 
23 rule, this analysis combined with engine certification testing may 
be adequate to demonstrate rain ingestion compliance.
    Also, since the rain ingestion requirements in part 33 were not 
added until Amendment 33-6, the airplane applicant needs to evaluate 
the engine's certification basis to determine if the engine has been 
subjected to part 33 rain ingestion testing. If the engine does not 
have Amendment 33-6 or a subsequent amendment as part of the 
certification basis, in accordance with Sec. 23.903(a)(2)(iii), the 
engine must have a safe service history of rain ingestion in similar 
installations.
    If it is determined that testing for rain ingestion should be 
performed, flight test is not required. The intent of the part 23 rule 
is to ensure that the engine installation has not deteriorated the rain 
ingestion tolerance of the certificated engine. Since a ground static 
engine test normally demonstrates engine certification compliance, use 
of installation ground tests at the required power/thrust settings has 
been commonly accepted as a means of compliance.
    The applicant can use design analysis to determine critical 
configurations and conditions of the installation. This might reduce 
required installation tests to the critical configurations and 
conditions instead of repeating the entire part 33 test conditions. 
Engine certification should address the results of the critical point 
analysis for the engine; therefore, it is important for the engine 
installer to research the conditions and requirements used for engine 
certification.

Other Considerations for Compliance

    Amendment 23-53 also added requirements for ice, hail, and birds. 
Examples of installation issues normally not addressed by engine 
certification, but that should be addressed for installation 
compliance, include the following: ice build-up on areas where ice shed 
may be ingested by the engines (for example, ice shed from wings and 
airframe sources into aft mounted engines) and consideration of items 
such as inlet splitters, acoustic liners, and so forth, that may be 
damaged by impact with ice, hail, and birds.

    Issued in Kansas City, Missouri on December 14, 2000.
Michael Gallagher,
Manager, Small Airplane Directorate, Aircraft Certification Service.
[FR Doc. 01-347 Filed 1-4-01; 8:45 am]
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