Document ID: FAA-2011-0398-0002
Agency: faa
Document Type: Rule
Title: Airworthiness Standards: Rotor Overspeed Requirements
Posted Date: 2011-07-18T04:00Z

[Federal Register Volume 76, Number 137 (Monday, July 18, 2011)]
[Rules and Regulations]
[Pages 42020-42024]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2011-18002]

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 33

[Docket No. FAA-2010-0398; Amendment No. 33-31]
RIN 2120-AJ62

Airworthiness Standards; Rotor Overspeed Requirements

    AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: This rule will amend the aircraft turbine engine rotor 
overspeed type certification standards. This action establishes uniform 
rotor overspeed design and test requirements for aircraft engines and 
turbochargers certificated by the FAA and the European Aviation Safety 
Agency (EASA). The rule also establishes uniform standards for the 
design and testing of engine rotor parts in the United States and in 
Europe, eliminating the need to comply with two differing sets of 
requirements.

DATES: This amendment becomes effective September 16, 2011.

FOR FURTHER INFORMATION CONTACT: For technical questions concerning 
this final rule, contact Tim Mouzakis, Engine and Propeller Directorate 
Standards Staff, ANE-111, Engine and Propeller Directorate, Federal 
Aviation Administration, 12 New England Executive Park, Burlington, 
Massachusetts 01803-5299; telephone (781) 238-7114; fax (781) 238-7199; 
e-mail timoleon.mouzakis@.faa.gov. For legal questions concerning this 
final rule contact Vincent Bennett, ANE-7, Office of Regional Counsel, 
Federal Aviation Administration, 12 New England Executive Park, 
Burlington, Massachusetts 01803-5299; telephone (781) 238-7044; fax 
(781) 238-7055; e-mail vincent.bennett@faa.gov.

SUPPLEMENTARY INFORMATION: 

Authority for This Rulemaking

    The FAA's authority to issue rules regarding aviation safety is 
found in Title 49 of the United States Code. Subtitle I, Section 106 
describes the authority of the FAA Administrator. Subtitle VII, 
Aviation Programs, describes in more detail the scope of the agency's 
authority.
    We are issuing this rulemaking under the authority described in 
Subtitle VII, Part A, Subpart III, Section 44701, ``General 
requirements.'' Under that section, the FAA is charged with promoting 
safe flight of civil aircraft in air commerce by prescribing 
regulations for practices, methods, and procedures the Administrator 
finds necessary for safety in air commerce, including minimum safety 
standards for aircraft engines. This final rule is within the scope of 
that authority because it updates existing regulations for rotor 
overspeed for aircraft turbine engines.

Background

    Part 33 of Title 14, Code of Federal Regulations, prescribes 
airworthiness standards for original and amended type certificates for 
aircraft engines. The European Aviation Safety Agency (EASA) 
Certification Specification--Engines (CS-E) prescribes corresponding 
airworthiness standards to certify aircraft engines in Europe. While 
part 33 and the CS-E are similar, they differ in several respects. 
These differences may result in added costs, delays, and time required 
for certification. This rule will harmonize applicable U.S. and EASA 
standards and clarify existing overspeed requirements for aircraft 
turbine engine rotor parts.

Summary of the NPRM

    The FAA published a notice of proposed rulemaking (NPRM) on April 
26, 2010 (75 FR 21523). The proposed changes establish a uniform 
certification basis for aircraft turbine engine rotor parts between the 
FAA and EASA. The proposal discussed requiring that rotor parts be 
designed with a safety margin large enough that the parts have an 
overspeed capability that exceeds the engine's certified operating 
conditions, including overspeed conditions which can occur in the event 
of a failure of another engine component and/or system malfunction. For 
failures that may result in an overspeed, the proposal limited rotor 
growth to that which would not lead to a hazardous condition as defined 
in Sec.  33.75. The comment period for the NPRM closed on July 26, 
2010.

Summary of the Final Rule

    There are minor differences between the proposal and this final 
rule. Sections 33.27(c) and (g) were changed in response to comments 
and our review of the proposal. This rule harmonizes rotor overspeed 
requirements found in part 33 with EASA CS-E 840, Rotor Integrity.

Summary of Comments

    The FAA received comments from Rolls-Royce, General Electric 
Aviation, Turbomeca, Pratt and Whitney, and General Aviation 
Manufacturers Association (GAMA). The commenters

[[Page 42021]]

suggested minor improvements in the following areas:
     Differences in the definition of ``extremely remote'' in 
Sec.  33.27(c);
     Exclusions of shaft sections from overspeed tests;
     Material properties of test rotors; and
     Validation of analytical tools.

Discussion of the Final Rule

    The final rule requires that rotor parts be designed with a safety 
margin large enough that the parts have an overspeed capability 
exceeding the engine's certified operating conditions, including 
overspeed conditions, which can occur in the event of a failure of 
another engine component and/or system malfunction. For failures that 
may result in an overspeed, the final rule limits rotor growth to that 
which would not lead to a hazardous condition as defined by Sec.  
33.75.
    To harmonize FAA and EASA standards, the FAA will:
     Change the current FAA overspeed design margin from 115 to 
120 percent of maximum permissible speed for all engine ratings except 
one engine inoperative (OEI) ratings of less than 2\1/2\ minutes;
     Change the current FAA overspeed design margin from 100 to 
105 percent for operating conditions associated with multiple failures;
     Introduce similar OEI overspeed design requirements;
     Require new similar rotor pass/fail design criteria;
     Require similar overspeed margin requirements;
     Allow the use of validated structural analysis tools to 
demonstrate compliance;
     Require that validated structural analysis tools be 
calibrated to actual overspeed tests of similar rotors; and
     Allow engine test durations of less than 5 minutes for 
failure conditions for which a 5-minute duration is not realistic.
    Like EASA's CS-E, the final rule specifies that rotors may not 
burst for overspeed conditions that do not involve component or system 
failure. For component or engine failures that result in an overspeed, 
the final rule specifies that rotors may not burst and limits the 
amount of rotor growth.

Differences in Definition of Probability of Occurrence in Sec.  
33.27(c)

    Section 33.27(c) proposed that overspeeds resulting from 
combinations of failures must also be considered unless the applicant 
can show that the probability of occurrence is not greater than 
10-9 per flight. Rolls-Royce, General Electric, Turbomeca, 
Pratt and Whitney, and GAMA commented that the proposed criteria in 
Sec.  33.27(c) is inconsistent with Sec.  33.75, CS-E 510, and CS-E 
840. The commenters also took issue with the FAA's criteria of 
probability of occurrence as not greater than 10-9 and FAA's 
use of the term ``per flight.'' They suggested that the probability of 
occurrence should follow the more flexible criteria of not greater that 
``extremely remote,'' which has been defined in the previous 
rulemakings as between 10-7 to 10-9. Finally, the 
commenters indicated that the term ``per engine flight hour'' should be 
substituted for ``per flight'' to be consistent with Sec.  33.75 and 
CS-E 840.
    We agree with the revised criteria proposed by the commenters. The 
final rule will reflect that overspeeds resulting from combinations of 
failures must also be considered, unless the applicant can show that 
the probability of occurrence is not greater than extremely remote 
(probability range of 10-7 to 10-9 per engine 
flight hour).

Exclusion of Shaft Sections From Overspeed Tests

    Proposed Sec.  33.27(f) allows exclusion of certain shaft sections, 
but not the whole shaft system, from the requirement when determining 
the terminal rotor speed due to shaft failure. Rolls-Royce commented 
that Sec.  33.27(c) allows exclusion on a probability basis only of 
overspeeds ``resulting from combinations of failures,'' whereas CS-E 
840(c) allows the probability exclusion for any cause if ``it can be 
shown to be Extremely Remote under the provisions of CS-E 850.''
    Rolls-Royce requested that the lead sentence of Sec.  33.27(c) be 
changed to, ``The highest overspeed which will result from a complete 
loss of load on a turbine rotor, unless it can be shown to be Extremely 
Remote or except as provided by paragraph (f) of this section.* * *''. 
The change proposed by Rolls-Royce would allow exclusion of the whole 
shaft system from consideration of failure, which is not the intent of 
the rule. Our changes to overspeed requirements due to shaft failures 
are consistent with those in CS-E-840 and CS-E-850(b). We did not 
change the rule due to this comment.

Material Properties of Test Rotors

    Section 33.27(a)(1) proposed that test rotors used to demonstrate 
compliance with this section that do not have the most adverse 
combination of material properties and dimensional tolerances must be 
tested at conditions which have been adjusted to ensure the minimum 
specification rotor possesses the required overspeed capability.
    Rolls-Royce claimed that determining the precise ``most adverse 
combination'' is not practical. Rolls-Royce noted that Advisory 
Circular (AC) 33.27-1, paragraph 7.g indicates that the applicant 
should consider ``the most adverse combination of dimensional 
tolerances and material properties,'' which allows the use of 
engineering judgment and best practices in lieu of an exhaustive 
assessment of all possible combinations and permutations. As a result, 
Rolls-Royce requested that the phrase ``that do not have the most 
adverse combination of material properties and dimensional tolerances'' 
be omitted from Sec.  33.27(a)(1).
    We disagree. We find that our proposed wording of Sec.  33.27(a)(1) 
is consistent with EASA's regulation CS-E 840(a) and that the suggested 
change would not meet the intent of the proposed paragraph. Our intent 
in Sec.  33.27(a)(1) is to ensure that the minimum specifications rotor 
is capable of meeting the test requirements of the proposed rule. 
Industry has been complying with this requirement, as stated in EASA 
regulations, for several years. The change proposed by Rolls-Royce 
would, therefore, diverge from EASA's rule and could increase cost to 
manufacturers. We did not change the final rule due to this comment.

Validation of Analytical Tools

    We proposed in Sec.  33.27(g) that if analysis is used to meet the 
overspeed requirements, then the analytical tool must be calibrated to 
prior overspeed test results of a similar rotor. The tool must be 
calibrated for the same material, rotor geometry, stress level, and 
temperature range as the rotor being certified. Calibration includes 
the ability to accurately predict rotor dimensional growth and burst 
speed. The predictions must also show that the rotor being certified 
does not have lower burst and growth margins than rotors used to 
calibrate the tool.
    Rolls-Royce commented that the requirements for validation of 
analytical tools eligible for use in showing compliance in lieu of 
testing are overly restrictive. Rolls-Royce said the language of Sec.  
33.27(g) appears to invalidate any potential for the applicant to 
propose analysis methods to the Administrator for acceptance per AC 
33.27-1, paragraphs 7.b and 7.c. Rolls-Royce noted that it seems 
unlikely that an applicant will have a tool calibrated for the same 
conditions and the same rotor as that being certified; such a 
certification appears redundant. Rolls-Royce requested that Sec.  
33.27(g) be

[[Page 42022]]

modified to read: ``If analysis is used to meet the overspeed 
requirements, then the analytical tool must be calibrated to prior 
overspeed test results of a similar rotor.''
    We agree that the language of proposed 33.27(g) appears overly 
restrictive. We changed the language to read the analytical tool must 
be ``validated'' instead of ``calibrated'' for each material. The 
analytical model must be validated using rotors which ``surround'' the 
rotor being certified in terms of ``shape, stresses and temperature.'' 
The final rule now reads: ``If analysis is used to meet the overspeed 
requirements, then the analytical tool must be validated to prior 
overspeed test results of a similar rotor. The tool must be validated 
for each material. The rotor being certified must not exceed the 
boundaries of the rotors being used to validate the analytical tool in 
terms of geometric shape, operating stress, and temperature.'' This 
changed wording is also consistent with EASA advisory material AMC E 
840.

Definition of Terms Used in the Final Rule

    The following definitions of terms used in the final rule are 
provided for clarity:
    Maximum permissible rotor speed. The maximum approved rotor speed, 
including transients, for the maximum approved rating, including One-
Engine-Inoperative (OEI) ratings.
    Overspeed Capability. The r.p.m. (revolutions per minute) at which 
the part fails or bursts.
    Rotor Growth. The total increase in a rotor part's radial 
dimensions caused by an overspeed condition. Total growth includes both 
the recoverable (elastic) and the permanent (plastic) change in rotor 
dimensions.

Rulemaking Analyses and Notices

Paperwork Reduction Act

    The Paperwork Reduction Act of 1995 (44 U.S.C. 3507(d)) requires 
that the FAA consider the impact of paperwork and other information 
collection burdens imposed on the public. We have determined there is 
no new requirement for information collection associated with this 
final rule.

International Compatibility

    In keeping with U.S. obligations under the Convention on 
International Civil Aviation, it is FAA policy to comply with 
International Civil Aviation Organization (ICAO) Standards and 
Recommended Practices, to the maximum extent practicable. We determined 
that no ICAO Standards or Recommended Practices corresponding to these 
proposed regulations exist.

Regulatory Evaluation, Regulatory Flexibility Determination, 
International Trade Impact Assessment, and Unfunded Mandates Assessment

    Proposed changes to Federal regulations must undergo several 
economic analyses. First, Executive Order 12866 and Executive Order 
13563 direct that each Federal agency propose or adopt a regulation 
only upon a reasoned determination that the benefits of the intended 
regulation justify its costs. Second, the Regulatory Flexibility Act of 
1980 (Pub. L. 96-354) requires agencies to analyze the economic impact 
of regulatory changes on small entities. Third, the Trade Agreements 
Act (Pub. L. 96-39) prohibits agencies from setting standards that 
create unnecessary obstacles to the foreign commerce of the United 
States. In developing U.S. standards, this Trade Act requires agencies 
to consider international standards and, where appropriate, that they 
be the basis of U.S. standards. Fourth, the Unfunded Mandates Reform 
Act of 1995 (Pub. L. 104-4) requires agencies to prepare a written 
assessment of the costs, benefits, and other effects of proposed or 
final rules that include a Federal mandate likely to result in the 
expenditure by State, local, or tribal governments, in the aggregate, 
or by private sector, of $100 million or more annually (adjusted for 
inflation with base year of 1995). This portion of the preamble 
contains the FAA's analysis of the economic impacts of this final rule.
    In conducting these analyses, the FAA has determined that this 
rule: (1) Has benefits that justify its costs; (2) is not an 
economically ``significant regulatory action'' as defined in section 
3(f) of Executive Order 12866; (3) is not ``significant'' as defined in 
DOT's Regulatory Policies and Procedures; (4) will not have a 
significant economic impact on a substantial number of small entities; 
(5) will not create unnecessary obstacles to the foreign commerce of 
the United States; and (6) will not impose an unfunded mandate on 
state, local, or tribal governments, or on the private sector by 
exceeding the threshold identified above.

Total Estimated Benefits and Costs of This Proposed Rule

    Presently, turbine aircraft engine manufacturers must satisfy both 
FAA part 33 and EASA CS-E regulations to certify their products in the 
United States and Europe. Certification to one standard will improve 
certification efficiency by eliminating duplicate testing and 
documentation. We have not attempted to quantify the cost savings that 
may accrue due to this improved certification efficiency beyond noting 
that these are expected to be minor. We have drawn that conclusion 
based on the consensus among potentially affected aircraft engine 
manufacturers.
    Industry must currently certificate to the two standards that are 
substantively similar, but have a few slightly different testing and 
documentation procedures and requirements. The rule harmonizes these 
procedures and requirements to the higher standard and, thereby, may 
increase safety. In addition, by reducing the amount of duplicative 
testing that would need to be either witnessed or analyzed by the FAA, 
the FAA is better able to prioritize its resources to other, more 
safety critical areas. Consequently, we determined that unquantifiable 
future minimal benefits from the rule may also accrue. We disagreed 
with a comment determining the precise ``most adverse combination'' of 
material properties and dimensional tolerances to establish the 
required overspeed capability. However, as noted in our response, the 
commenter's suggestion would result in a rule that is not consistent 
with the EASA regulations and the suggestion might increase costs to 
manufacturers. As a result, the FAA concludes that the combination of 
cost savings and potential increased safety benefits will make this 
rule cost beneficial. Further, we therefore determined that this rule 
is not a ``significant regulatory action'' as defined in section 3(f) 
of Executive Order 12866, and is not ``significant'' as defined in 
DOT's Regulatory Policies and Procedures.

Regulatory Flexibility Determination

    The Regulatory Flexibility Act of 1980 (Pub. L. 96-354) (RFA) 
establishes ``as a principle of regulatory issuance that agencies shall 
endeavor, consistent with the objectives of the rule and of applicable 
statutes, to fit regulatory and informational requirements to the scale 
of the businesses, organizations, and governmental jurisdictions 
subject to regulation. To achieve this principle, agencies are required 
to solicit and consider flexible regulatory proposals and to explain 
the rationale for their actions to assure that such proposals are given 
serious consideration.'' The RFA covers a wide-range of small entities, 
including small businesses, not-for-profit organizations, and small 
governmental jurisdictions.
    Agencies must perform a review to determine whether a rule will 
have a significant economic impact on a substantial number of small 
entities. If

[[Page 42023]]

the agency determines that it will, the agency must prepare a 
regulatory flexibility analysis as described in the RFA.
    However, if an agency determines that a rule is not expected to 
have a significant economic impact on a substantial number of small 
entities, section 605(b) of the RFA provides that the head of the 
agency may so certify and a regulatory flexibility analysis is not 
required. The certification must include a statement providing the 
factual basis for this determination, and the reasoning should be 
clear.
    The net effect of this rule is to provide regulatory cost relief. 
Further, all but one U.S. aircraft turbine engine manufacturer exceeds 
the Small Business Administration small-entity criteria for aircraft 
engine manufacturers of 1,500 employees. U.S. transport category 
aircraft engine manufacturers include: General Electric (GE); CFM 
International (a joint company of GE and Snecma); Pratt & Whitney 
(P&W); Honeywell; Rolls-Royce Corporation (formerly Allison Engines); 
International Aero Engines (a privately-held consortium that includes 
P&W, Rolls-Royce, Japanese Aero Engines Corporation, and MTU Aero 
Engines); and Williams International. Williams International is the 
only one of these manufacturers that is categorized as a U.S. small 
business by the SBA criteria. As this final rule reduces costs and 
there is only one small entity manufacturing part 33 aircraft engines, 
therefore, as FAA Administrator, I certify this rule will not have a 
significant economic impact on a substantial number of small entities.

International Trade Impact Assessment

    The Trade Agreements Act of 1979 (Pub. L. 96-39), as amended by the 
Uruguay Round Agreements Act (Pub. L. 103-465), prohibits Federal 
agencies from establishing standards or engaging in related activities 
that create unnecessary obstacles to the foreign commerce of the United 
States. Pursuant to these Acts, the establishment of standards is not 
considered an unnecessary obstacle to the foreign commerce of the 
United States, so long as the standard has a legitimate domestic 
objective, such as protection of safety, and does not operate in a 
manner that excludes imports that meet this objective. The statute also 
requires consideration of international standards and, where 
appropriate, that they be the basis for U.S. standards. We assessed the 
potential effect of this rule and determined that it uses European 
standards as the basis for regulation, and thus is consistent with the 
Trade Assessments Act.

Unfunded Mandates Assessment

    Title II of the Unfunded Mandates Reform Act of 1995 (Pub. L. 104-
4) requires each Federal agency to prepare a written statement 
assessing the effects of any Federal mandate in a proposed or final 
agency rule that may result in an expenditure of $100 million or more 
(in 1995 dollars) in any one year by State, local, and tribal 
governments, in the aggregate, or by the private sector; such a mandate 
is deemed to be a ``significant regulatory action.'' The FAA currently 
uses an inflation-adjusted value of $140.8 million in lieu of $100 
million. This final rule does not contain such a mandate, therefore, 
the requirements of Title II of the Act do not apply.

Executive Order 13132, Federalism

    The FAA has analyzed this final rule under the principles and 
criteria of Executive Order 13132, Federalism. We determined that this 
action will not have a substantial direct effect on the States, on the 
relationship between the national Government and the States, or on the 
distribution of power and responsibilities among the various levels of 
government, and therefore does not have federalism implications.

Environmental Analysis

    FAA Order 1050.1E defines FAA actions that are categorically 
excluded from preparation of an environmental assessment or 
environmental impact statement under the National Environmental Policy 
Act (NEPA) in the absence of extraordinary circumstances. We determined 
this rulemaking action qualifies for the categorical exclusion 
identified in Chapter 3, paragraph 312d, and involves no extraordinary 
circumstances.

Regulations That Significantly Affect Energy Supply, Distribution, or 
Use

    The FAA has analyzed this final rule under Executive Order 13211, 
Actions Concerning Regulations that Significantly Affect Energy Supply, 
Distribution, or Use (May 18, 2001). We determined that it is not a 
``significant energy action'' under the executive order and is not 
likely to have a significant adverse effect on the supply, 
distribution, or use of energy.

Availability of Rulemaking Documents

    You can get an electronic copy using the Internet by--
    1. Searching the Federal eRulemaking Portal (http://www.regulations.gov);
    2. Visiting the FAA's Regulations and Policies web page at http://www.faa.gov/regulations_policies/; or
    3. Accessing the Government Printing Office's web page at http://www.gpoaccess.gov/fr/index.html.
    You can also get a copy by sending a request to the Federal 
Aviation Administration, Office of Rulemaking, ARM-1, 800 Independence 
Avenue, SW, Washington, DC 20591, or by calling (202) 267-9680. Make 
sure to identify the notice, amendment, or docket number of this 
rulemaking.
    Anyone may search the electronic form of all comments received into 
any of our dockets by the name of the individual submitting the comment 
(or signing the comment, if submitted on behalf of an association, 
business, labor union, etc.). You may review DOT's complete Privacy Act 
statement in the Federal Register published on April 11, 2000 (Volume 
65, Number 70; Pages 19477-78) or you may visit http://DocketsInfo.dot.gov.

Small Business Regulatory Enforcement Fairness Act

    The Small Business Regulatory Enforcement Fairness Act (SBREFA) of 
1996 requires the FAA to comply with small entity requests for 
information or advice about compliance with statutes and regulations 
within its jurisdiction. If you are a small entity and you have a 
question regarding this document, you may contact your local FAA 
official, or the person listed under the FOR FURTHER INFORMATION 
CONTACT heading at the beginning of the preamble. You can find out more 
about SBREFA on the Internet at http://www.faa.gov/regulations_policies/rulemaking/sbre_act/.

List of Subjects in 14 CFR Part 33

    Air transportation, Aircraft, Aviation safety, Safety.

The Amendment

    In consideration of the foregoing, the Federal Aviation 
Administration amends Chapter I of Title 14, Code of Federal 
Regulations as follows:

PART 33--AIRWORTHINESS STANDARDS: AIRCRAFT ENGINES

0
1. The authority citation for part 33 continues to read as follows:

    Authority:  49 U.S.C. 106(g), 40113, 44701-44702, 44704.

0
2. Revise Sec.  33.27 to read as follows:

Sec.  33.27  Turbine, compressor, fan, and turbosupercharger rotor 
overspeed.

    (a) For each fan, compressor, turbine, and turbosupercharger rotor, 
the

[[Page 42024]]

applicant must establish by test, analysis, or a combination of both, 
that each rotor will not burst when operated in the engine for 5 
minutes at whichever of the conditions defined in paragraph (b) of this 
section is the most critical with respect to the integrity of such a 
rotor.
    (1) Test rotors used to demonstrate compliance with this section 
that do not have the most adverse combination of material properties 
and dimensional tolerances must be tested at conditions which have been 
adjusted to ensure the minimum specification rotor possesses the 
required overspeed capability. This can be accomplished by increasing 
test speed, temperature, and/or loads.
    (2) When an engine test is being used to demonstrate compliance 
with the overspeed conditions listed in paragraph (b)(3) or (b)(4) of 
this section and the failure of a component or system is sudden and 
transient, it may not be possible to operate the engine for 5 minutes 
after the failure. Under these circumstances, the actual overspeed 
duration is acceptable if the required maximum overspeed is achieved.
    (b) When determining the maximum overspeed condition applicable to 
each rotor in order to comply with paragraphs (a) and (c) of this 
section, the applicant must evaluate the following rotor speeds taking 
into consideration the part's operating temperatures and temperature 
gradients throughout the engine's operating envelope:
    (1) 120 percent of the maximum permissible rotor speed associated 
with any of the engine ratings except one-engine-inoperative (OEI) 
ratings of less than 2\1/2\ minutes.
    (2) 115 percent of the maximum permissible rotor speed associated 
with any OEI ratings of less than 2\1/2\ minutes.
    (3) 105 percent of the highest rotor speed that would result from 
either:
    (i) The failure of the component or system which, in a 
representative installation of the engine, is the most critical with 
respect to overspeed when operating at any rating condition except OEI 
ratings of less than 2\1/2\ minutes, or
    (ii) The failure of any component or system in a representative 
installation of the engine, in combination with any other failure of a 
component or system that would not normally be detected during a 
routine pre-flight check or during normal flight operation, that is the 
most critical with respect to overspeed, except as provided by 
paragraph (c) of this section, when operating at any rating condition 
except OEI ratings of less than 2\1/2\ minutes.
    (4) 100 percent of the highest rotor speed that would result from 
the failure of the component or system which, in a representative 
installation of the engine, is the most critical with respect to 
overspeed when operating at any OEI rating of less than 2\1/2\ minutes.
    (c) The highest overspeed that results from a complete loss of load 
on a turbine rotor, except as provided by paragraph (f) of this 
section, must be included in the overspeed conditions considered by 
paragraphs (b)(3)(i), (b)(3)(ii), and (b)(4) of this section, 
regardless of whether that overspeed results from a failure within the 
engine or external to the engine. The overspeed resulting from any 
other single failure must be considered when selecting the most 
limiting overspeed conditions applicable to each rotor. Overspeeds 
resulting from combinations of failures must also be considered unless 
the applicant can show that the probability of occurrence is not 
greater than extremely remote (probability range of 10-7 to 
10-9 per engine flight hour).
    (d) In addition, the applicant must demonstrate that each fan, 
compressor, turbine, and turbosupercharger rotor complies with 
paragraphs (d)(1) and (d)(2) of this section for the maximum overspeed 
achieved when subjected to the conditions specified in paragraphs 
(b)(3) and (b)(4) of this section. The applicant must use the approach 
in paragraph (a) of this section which specifies the required test 
conditions.
    (1) Rotor Growth must not cause the engine to:
    (i) Catch fire,
    (ii) Release high-energy debris through the engine casing or result 
in a hazardous failure of the engine casing,
    (iii) Generate loads greater than those ultimate loads specified in 
Sec.  33.23(a), or
    (iv) Lose the capability of being shut down.
    (2) Following an overspeed event and after continued operation, the 
rotor may not exhibit conditions such as cracking or distortion which 
preclude continued safe operation.
    (e) The design and functioning of engine control systems, 
instruments, and other methods not covered under Sec.  33.28 must 
ensure that the engine operating limitations that affect turbine, 
compressor, fan, and turbosupercharger rotor structural integrity will 
not be exceeded in service.
    (f) Failure of a shaft section may be excluded from consideration 
in determining the highest overspeed that would result from a complete 
loss of load on a turbine rotor if the applicant:
    (1) Identifies the shaft as an engine life-limited-part and 
complies with Sec.  33.70.
    (2) Uses material and design features that are well understood and 
that can be analyzed by well-established and validated stress analysis 
techniques.
    (3) Determines, based on an assessment of the environment 
surrounding the shaft section, that environmental influences are 
unlikely to cause a shaft failure. This assessment must include 
complexity of design, corrosion, wear, vibration, fire, contact with 
adjacent components or structure, overheating, and secondary effects 
from other failures or combination of failures.
    (4) Identifies and declares, in accordance with Sec.  33.5, any 
assumptions regarding the engine installation in making the assessment 
described above in paragraph (f)(3) of this section.
    (5) Assesses, and considers as appropriate, experience with shaft 
sections of similar design.
    (6) Does not exclude the entire shaft.
    (g) If analysis is used to meet the overspeed requirements, then 
the analytical tool must be validated to prior overspeed test results 
of a similar rotor. The tool must be validated for each material. The 
rotor being certified must not exceed the boundaries of the rotors 
being used to validate the analytical tool in terms of geometric shape, 
operating stress, and temperature. Validation includes the ability to 
accurately predict rotor dimensional growth and the burst speed. The 
predictions must also show that the rotor being certified does not have 
lower burst and growth margins than rotors used to validate the tool.

    Issued in Washington, DC, on June 30, 2011.
J. Randolph Babbitt,
Administrator.
[FR Doc. 2011-18002 Filed 7-15-11; 8:45 am]
BILLING CODE 4910-13-P