[Federal Register Volume 60, Number 25 (Tuesday, February 7, 1995)]
[Rules and Regulations]
[Pages 7112-7115]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-2928]



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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 33

[Docket No. 94-ANE-18; Special Conditions No. SC-33-ANE-08]


Special Conditions; General Electric (GE) Aircraft Engines 
Model(s) GE90-75B/-85B/-76B Turbofan Engines

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final special conditions.

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SUMMARY: These special conditions are issued for the General Electric 
(GE) Aircraft Engines Model(s) GE90-75B/-85B/-76B turbofan engines. 
These special conditions contain the additional safety standards which 
the Administrator considers necessary to establish a level of safety 
equivalent to that established by the airworthiness standards of part 
33 of the Federal Aviation Regulations (FAR).

EFFECTIVE DATE: March 6, 1995.

FOR FURTHER INFORMATION CONTACT:
Tim Mouzakis at (617) 238-7114 or Karen Grant at (617) 238-7133, Engine 
and Propeller Standards Staff, ANE-110, Engine and Propeller 
Directorate, Aircraft Certification Service, FAA, New England Region, 
12 New England Executive Park, Burlington, Massachusetts 01803-5229; 
fax (617) 238-7199.

SUPPLEMENTARY INFORMATION:

Background

    On December 16, 1991, General Electric Aircraft Engines applied for 
type certification of Model(s) GE90-75B/-85B/-76B turbofan engines. 
These engines incorporate a first stage fan blade manufactured using 
carbon graphite composite material. This unusual design feature results 
in the GE90 fan blade having significant differences in material 
property characteristics when compared to conventionally designed fan 
blades using non-composite materials. For example, the probability that 
a composite fan blade will fail below the inner annulus flowpath line 
may be highly improbable, questioning the appropriateness of the 
requirement contained in Sec. 33.94(a)(1) to show blade containment 
after a failure of the blade at the outermost retention feature.
    The current requirements of Sec. 33.94 are based on metallic blade 
characteristics and service history, and are not appropriate for the 
unusual design features of the composite fan blade found on the GE90 
series turbofan engines. The FAA has determined that a more realistic 
blade out test will be achieved with a fan blade failure at the inner 
annulus flowpath line (only the airfoil) instead of the outermost 
retention feature as is currently required by Sec. 33.94(a)(1).
    The FAA has also determined that the composite fan blades 
construction presents other factors that must be considered. Tests and 
analyses must account for the effects of in-service deterioration of, 
manufacturing and materials variations in, and environmental effects on 
the composite material. Further, tests and analyses must show that a 
lightning strike on the composite fan blade will not result in a 
hazardous condition to the aircraft, and that the engine will meet the 
requirements of Sec. 33.75. Therefore, these special conditions are 
additional requirements which the Administrator considers necessary to 
establish a level of safety equivalent to that established by the 
Airworthiness Standards of part 33.

Type Certification Basis

    Under the provisions of Sec. 21.101 of the Federal Aviation 
Regulations (FAR), General Electric Aircraft Engines must show that the 
Model(s) GE90-75B/-85B/-76B turbofan engines meet the requirements of 
the applicable regulations in effect on the date of the application. 
Those Federal Aviation Regulations are Sec. 21.21, as amended through 
Amendment 21-68, August 10, 1990, and part 33, as amended 33-14, August 
10, 1990.
    The Administrator finds that the applicable airworthiness 
regulations in part 33, as amended, do not contain adequate or 
appropriate safety standards for the General Electric Aircraft Engines 
Model(s) GE90-75B/-85B/-76B turbofan engines because of unique design 
criteria. Therefore, the Administrator prescribes special conditions 
under the provisions of Sec. 21.16 to establish a level of safety 
equivalent to that established in the regulations.
    Special conditions, as appropriate, are issued in accordance with 
Sec. 11.49 of the FAR after public notice and opportunity for comment, 
as required by Secs. 11.28 and 11.29(b), and become part of the type 
certification basis in accordance with Sec. 21.101(b)(2).

Discussion of Comments

    Interested persons have been afforded the opportunity to 
participate in the making of these special conditions. Due 
consideration has been given to comments received.
    Two commenters express no objection to the adoption of these 
special conditions as proposed.
    Two commenters cite the apparent departure by the FAA from its 
general practice of involving industry prior to effecting significant 
changes to certification requirements, and recommend that the FAA 
evaluate the proposed changes in harmony with industry through the 
Aviation Rulemaking Advisory Committee (ARAC).
    The FAA has not determined that these special conditions will form 
the basis to a rulemaking change to amend 14 CFR part 33. These special 
conditions prescribe for a specific design, the testing and analyses 
necessary to achieve an equivalent level of safety. The FAA may 
consider whether it is necessary to revise Sec. 33.94 to include the 
requirements of these special conditions. The ARAC may be used to 
gather industry and public participation in that rulemaking project. 
For this specific application for type certification, however, the FAA 
has followed the rulemaking procedures provided by 14 CFR part 11 that 
allow for industry and public comment.
    Two commenters state that applying the maximum load criteria used 
for propellers to a fan blade, with significantly different mechanical 
arrangement and dynamic behavior, is technically unjustified.
    The FAA disagrees. The two times maximum load criteria test is 
designed to show the capability of the fan blade retention system to 
withstand without separation centrifugal loads significantly greater 
than will be seen in service. A safety factor of two is a reasonable 
safety factor as demonstrated by its success in propeller applications. 
The blade and its retention system must be capable of retaining the 
blade under this load condition.
    Two commenters state that the additional requirements, in 
conjunction with any available analyses, cannot guarantee that the 
failure probability will be extremely improbable. Inherent 
characteristics of complex composite hardware design, latent defects 
and susceptibility to manufacturing variations, and nonconformance are 
identified as reasons for the statement.
    The FAA agrees in part. The FAA has reviewed its position and 
concurs with the commenters that a failure [[Page 7113]] probability of 
extremely improbable can not be guaranteed. However, the FAA believes 
that the applicant has constructed a test program that demonstrates the 
blade retention features have sufficiently improved reliability to 
provide an equivalent level of safety to that provided by Sec. 33.94. 
While extensive testing is required for material certification in 
accordance with Sec. 33.15 to determine material characteristics and 
the effects of defects on blade life, additional test requirements were 
established within the compliance plan to determine the effect of 
defects and manufacturing variations on material capability.
    One commenter suggests adding an additional paragraph to these 
special conditions as follows:
    ``(a)(3) By appropriate test and analysis it must be shown that the 
most adverse blade vibratory stresses, as determined per Sec. 33.83, 
will not result in failure of the fan blade retention system when 
consideration is given to the most limiting manufacturing defect which 
could go undetected.''
    The FAA disagrees with the commenter that the suggested paragraph 
be added, as these considerations are well within the interpretation of 
Sec. 33.83 and no additional safety standards are deemed necessary.
    One commenter suggests adding an additional paragraph to the 
special condition to minimize the risk of hazard which would result 
from potential failure of the fan blade retention system as follows:
    ``(a)(4) Although the above test requires release of the fan blade 
at the inner flowpath, additional testing and/or analysis shall be 
performed to define the engine behavior for the case of a fan blade 
release at the outermost retention groove. The data obtained shall be 
used when establishing:
    (i) Any installation limitations to be included on the Type 
Certificate Data Sheet; and,
    (ii) Load requirements of Sec. 33.23.''
    The FAA disagrees. As stated in Sec. 33.75, Safety Analysis, the 
applicant must consider all probable malfunctions which will cause the 
engine to catch fire, burst, generate loads greater than those ultimate 
loads specified in Sec. 33.23(a), or lose the capability of being shut 
down. These special conditions also require such analyses and tests to 
show that the failure of the fan blade retention system is not a 
probable malfunction. Establishment of the maximum stop-start stress 
cycles for the blade retention system is also required to assure the 
structural integrity of the blade attachment system.
    One commenter states that the requirements should show that the 
failure rate of the fan blade retention system, for any cause, during 
the service life of the engine, be extremely improbable and can not be 
established at the time of type design approval for a new technology 
composite.
    The FAA agrees in part. While the FAA agrees that a failure 
probability of extremely improbable can not be guaranteed, the FAA 
remains receptive to advances in technology, approaches, and new test 
methods which adequately simulate those effects typically verified by 
in-service experience. Further, the FAA believes that these same 
principles have been successfully used by engine manufacturers to 
ensure the airworthiness of rotor structural parts. It should be 
recognized that failure to demonstrate acceptable reliability of the 
blade retention features, results in non-compliance with these special 
conditions and that would require testing to occur at the outer most 
retention groove.
    Two commenters suggest the energy levels and trajectories of any 
particles that would penetrate the engine cases by conducting an engine 
test in accordance with the test conditions of current Secs. 33.94(a) 
and 33.94(b) be defined in the Engine Installation Manual or on the 
Engine Type Certificate Data Sheet. The definition of results should 
also include determination of the loads that would be transmitted 
through the engine to airframe interface. One commenter states that the 
energy levels, trajectories and loads must be included in each airplane 
type's design precautions taken to minimize the hazards in the event of 
an engine rotor failure, as required by current FAR 25.903 and JAR 
25.903.
    The FAA agrees that the requirements for defining energy levels, 
trajectories of particles, and a resultant loads already exist in 
Secs. 33.19(a) and 33.23. The FAA also agrees that if such energy 
levels, trajectories, and resultant loads are defined, the appropriate 
data should be included in the Engine Installation Manual. The FAA does 
not agree with the commenters suggestion relative to complying with 
Secs. 33.94(a) and 33.94(b) in addition to these special conditions. 
These special conditions provide safety standards which apply to the 
composite blade design as an alternative to the requirements of 
Sec. 33.94. The applicant must demonstrate reliability of the blade 
root and the blade retention system.
    One commenter criticizes the explanations and logic presented for 
justification of these proposed special conditions. The commenter cites 
that there was insufficient information in the notice by which to test 
the validity of the FAA's determination.
    The FAA disagrees. The notice of proposed special condition 
identifies two bases on which the FAA determined that the current 
requirements of part 33 do not provide adequate or appropriate safety 
standards because of the novel or unusual design of the GE90 engine. 
The FAA also determined that additional safety standards were needed to 
ensure that the GE composite fan blades met an equivalent level of 
safety established by Sec. 33.94. Given the number and the nature of 
the comments received, the FAA believes that the notice gave an 
adequate description of the proposed action to allow critical comment 
on the basis for that action.
    One commenter states that they do not believe that use of graphite 
composite material for a turbofan blade retention system warrants a 
departure from the current requirements of Sec. 33.94.
    The FAA disagrees. The FAA supports the use of composite technology 
and the necessary methods of testing and analyses to show that the 
product meets an equivalent safety standard as established by 
Sec. 33.94.
    One commenter states that the demonstration means for showing 
``extremely improbable'' should be specifically part of these proposed 
special conditions. The commenter suggests to establish and define a 
methodology by which to rigorously assess the probability of fan blade 
retention system failure as extremely improbable, and by which to 
assess the associated level of confidence in the assessment, 
particularly at the time of initial certification.
    The FAA agrees in part. The FAA agrees that the assessment of the 
fan blade retention system should be conducted rigorously, but 
disagrees with the need to establish and define a methodology in these 
special conditions. The FAA believes it should not define a specific 
means to meet a safety standard, or publish an applicant's proprietary 
methodology. To publish a specific demonstration means would presume 
the FAA has predetermined the composite blade material property 
characterization. The methodology for assessing the fan blade retention 
system will be proposed by the applicant, and will be evaluated by the 
FAA.
    One commenter states that lightning test conditions should be 
specifically identified in the special condition.
    The FAA disagrees. Existing regulatory guidance material and 
standard industry practices for lightning [[Page 7114]] tests may be 
used to develop appropriate test criteria.
    One commenter suggests that the term ``inner annulus flowpath 
line'' be substituted for ``inner flowpath diameter'' to eliminate 
ambiguity of definition.
    The FAA concurs. The inner annulus flowpath line provides a better 
description of the flowpath contour because flowpath diameter suggests 
a line of constant radius. These Final Special Conditions will be 
revised to include this term.
    One commenter states it is an issue of unnecessary additional risk 
that, in the absence of full compliance to Sec. 33.94, these proposed 
special conditions are insufficient in scope and detrimental to 
aviation safety.
    The FAA disagrees. The FAA has concluded that upon compliance with 
all of the requirements of these special conditions, together with 
additional testing beyond that typically employed for metallic blades 
within the scope of 14 CFR part 33, an equivalence to the safety 
standard provided in Sec. 33.94 has been achieved and no additional 
risk has been assumed.
    One commenter states that the most significant feature of the 
notice is the proposed probability of fan blade retention system 
failure of ``extremely improbable'' is a reduction in severity of the 
effects of a blade failure.
    The FAA agrees. The FAA recognizes that certain loads associated 
with a blade release at the inner annulus flowpath line may be less 
than the loads associated with release of a fan blade at the outermost 
retention. Those loads imparted to the engine mount system based on the 
inner annulus flowpath line will be identified in the Engine 
Installation Manual. Since there is potential for a reduction in 
certain loads, it is imperative that the blade retention system 
demonstrates sufficiently improved reliability to provide an equivalent 
level of safety to that provided by Sec. 33.94.
    One commenter requested on what basis has it been decided that a 
failure along the inner flowpath line is the most critical for failures 
which are not assessed as being extremely improbable.
    The FAA selected the inner annulus flowpath line as the critical 
location for blade release based on design, blade stresses, and 
demonstrated fatigue and impact testing.
    One commenter states that these proposed special conditions make no 
mention of the design and construction requirements of either 
Sec. 33.19 relating to containment design and uncontained blade 
fragments, or Sec. 33.23 relating to mounting attachments and 
structure.
    The FAA concluded that the requirements of Secs. 33.19 and 33.23 
were adequate and appropriate when applied to this design of the GE90 
engine, and no additional special conditions were necessary.
    One commenter suggests that these special conditions should also 
address the effects of possible detachment of those metallic portions 
of the blade.
    The FAA disagrees. These special conditions provide an alternative 
to the release failure location on the blade. The metal to composite 
blade bonding capability has been addressed through tests conducted 
under 14 CFR part 33. There were no additional special conditions that 
are required.
    One commenter suggests that the text of these proposed special 
conditions paragraph (a), has been mis-compiled.
    The FAA concurs. The intent of the paragraph (a) is to identify the 
location of the release point for the fan blade containment test and to 
prescribe the additional safety standards to be demonstrated. These 
special conditions will be modified by reorganizing paragraph (a) to 
more clearly express this intent.
    One commenter states that some re-wording is also necessary to make 
it clear that the fan blade test must be conducted as a full engine 
test.
    The FAA concurs. These special conditions will be modified to 
incorporate this change.
    One commenter states that these special conditions ought to make 
more visible how there can be meaningful confidence in ``extremely 
improbable'' as the assessed probability of fan blade retention system 
failure if the stress levels are not so conservative as to result in an 
infinite fatigue life.
    The FAA disagrees. The intent is to assure that within the service 
life of the blade, that the fan blade retention system is not likely to 
fail due to manufacturing and material variations, in-service 
deterioration, and environmental effects.
    One commenter asks how will it be established that any large bird 
ingestion is not a possible cause of fan blade retention system 
failure, a mode of failure that is likely to be much more severe than 
an airfoil only fan blade containment tests.
    The damage effects on the blade retention system will be 
substantiated by developmental and certification testing. It is 
incumbent upon the applicant to demonstrate that the blade attachment 
system is designed to withstand the affects of an eight pound bird 
impact on the blade airfoil, and is less severe than the effects from 
fan blade release.
    One commenter requests a definition of ``without failure,'' with 
regard to the two times centrifugal load test.
    The FAA definition for ``without failure'' in this context is to 
demonstrate the blade root is retained within the disk dovetail slot, 
and that there are no conditions present which would indicate impending 
release.
    One commenter suggests relative to paragraph (a)(2) of the proposed 
special conditions, that there is a need for explicit reference to 
consideration of both high cycle and low cycle fatigue during start 
stop stress cycles.
    The FAA concurs. The determination of the life cycle of the 
composite fan blade must include the effects of combined high cycle and 
low cycle fatigue with enhanced load factors. These special conditions 
will be modified to include the requirement for high cycle and low 
cycle fatigue tests.
    One commenter requests clarification of the term ``extremely 
improbable.''
    For the purpose of these special conditions, ``extremely 
improbable'' refers to the unlikelihood that a failure will occur 
during the engine's operational life.
    One commenter questions why paragraph (d) of these proposed special 
conditions is applicable only to the tests and analyses required by 
paragraphs (a)(1) and (a)(2) of the proposed special conditions.
    The effects of in-service deterioration, manufacturing and material 
variations, and environmental effects must be accounted for during the 
centrifugal load test and in lifting determinations. The intent is to 
determine the effects on material capability under centrifugal loads 
significantly greater than will be seen in service. Combined high cycle 
and low cycle tests will further determine the effects on material 
capability. The blade releases demonstration, however, may or may not 
be conducted accounting for these effects.
    After careful review of the available data, including the comments 
noted above, the FAA determined that air safety and the public interest 
require the adoption of these special conditions as proposed with the 
changes as noted above.

Conclusion

    This action affects only General Electric Aircraft Engines on 
Model(s) GE90-75B/-85B/-76B turbofan engines. It is not a rule of 
general applicability and affects only the manufacturer who applied to 
the FAA for approval of these engines containing this novel or unusual 
design feature. [[Page 7115]] 

List of Subjects in 14 CFR Part 33

    Air transportation, Aircraft, Aviation safety, Safety.

    The authority citation for these special conditions continues to 
read as follows:

    Authority: 49 U.S.C. App. 1354(a), 1421, 1423; 49 U.S.C. 106(g); 
and 14 CFR 11.49 and 21.16.

The Special Conditions

    Accordingly, pursuant to the authority delegated to me by the 
Administrator, the following special conditions are issued as part of 
the type certification basis for the General Electric Aircraft Engines 
Model(s) GE-90-75B/-85B/-76B turbofan engines:

    (a) In lieu of the fan blade containment test with the fan blade 
failing at the point specified in Sec. 33.94(a)(1), conduct the 
following:
    (1) An engine fan blade containment test with the fan blade 
failing at the inner annulus flowpath line.
    (2) The following must be shown by test and analyses, or other 
methods acceptable to the Administrator, that:
    (i) The disk and fan blade retention system can withstand 
without failure a centrifugal load equal to two times the maximum 
load which the engine could experience within approved operating 
limitations, and
    (ii) By a procedure approved by the Administrator, an operating 
limitation must be established which specifies the maximum allowable 
number of start-stop stress cycles for the fan blade retention 
system. The stress cycle shall include the combined effects of high 
cycle and low cycle fatigue. The fan blade retention system includes 
the portion of the fan blade from the inner annulus flowpath line 
inward to the blade dovetail, the blade retention components and the 
fan disk and fan blade attachment features.
    (b) It must be shown that the probability of fan blade retention 
system failure, for any cause, during the service life of the engine 
to be extremely improbable.
    (c) It must be shown by test or analysis that a lightning strike 
to the composite fan blade structure will not result in a hazardous 
condition, and that the engine will meet the requirements of 
Sec. 33.75.
    (d) The tests and analyses required by (a)(2)(i) and (a)(2)(ii) 
of these special conditions must account for the effects of in-
service deterioration, manufacturing and material variations, and 
environmental effects.

    Issued in Burlington, Massachusetts, on February 1, 1995.
James C. Jones,
Acting Manager, Engine and Propeller Directorate, Aircraft 
Certification Service.
[FR Doc. 95-2928 Filed 2-2-95; 9:32 am]
BILLING CODE 4910-13-M