[Federal Register Volume 66, Number 95 (Wednesday, May 16, 2001)]
[Rules and Regulations]
[Pages 27396-27403]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-12232]



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Part V





Department of Transportation





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Federal Aviation Administration



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14 CFR Part 25



Revision of Hydraulic Systems Airworthiness Standards To Harmonize With 
European Airworthiness Standards for Transport Category Airplanes; 
Final Rule

  Federal Register / Vol. 66, No. 95 / Wednesday, May 16, 2001 / Rules 
and Regulations  

[[Page 27396]]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. 28617; Amendment No. 25-104]
RIN 2120-AF79


Revision of Hydraulic Systems Airworthiness Standards To 
Harmonize With European Airworthiness Standards for Transport Category 
Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: This amendment revises the hydraulic systems design and test 
requirements of the airworthiness standards for transport category 
airplanes. The amendment adds appropriate existing Joint Aviation 
Requirements (JAR) standards to achieve harmonization; moves some of 
the existing regulatory text to a new advisory circular (AC) 25.1435-1; 
consolidates and/or separates certain subparagraphs for clarity; and 
revises airplane static proof pressure test requirements to allow a 
complete functional (dynamic) airplane test at the hydraulic system 
relief pressure. These revisions were developed in cooperation with the 
Joint Aviation Authorities (JAA) of Europe, Transport Canada, and the 
U.S. and European aviation industry through the Aviation Rulemaking 
Advisory Committee (ARAC). These changes benefit the public interest by 
standardizing certain requirements, concepts, and procedures contained 
in the airworthiness standards without reducing, but potentially 
enhancing, the current level of safety.

DATE: Effective June 15, 2001.

FOR FURTHER INFORMATION CONTACT: Mahinder K. Wahi, Propulsion/
Mechanical Systems Branch, ANM-112, Transport Airplane Directorate, 
Aircraft Certification Service, FAA, 1601 Lind Avenue SW., Renton, WA 
98055-4056; telephone (425) 227-2142; facsimile (425) 227-1320.

SUPPLEMENTARY INFORMATION:

Availability of Final Rules

    You can get an electronic copy using the Internet by taking the 
following steps:
    (1) Go to the search function of the Department of Transportation's 
electronic Docket Management System (DMS) Web page (http://dms.dot.gov/search).
    (2) On the search page type in the last four digits of the Docket 
number shown at the beginning of the notice. Click on ``search.''
    (3) On the next page, which contains the Docket summary information 
for the Docket you selected, click on the final rule.
    You can also get an electronic copy using the Internet through 
FAA's web page at http://www.faa.gov/avr/arm/nprm/nprm.htm or the 
Federal Register's web page at http://www.access.gpo/gov/su_docs/aces/aces140.html.
    You can also get a copy by submitting 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 amendment number or docket number of this final 
rule.

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. Therefore, any small entity that has a 
question regarding this document may contact their local FAA official, 
or the person listed under FOR FURTHER INFORMATION CONTACT. You can 
find out more about SBREFA on the Internet at our site, http://www.gov/avr/arm/sbrefa.htm. For more information on SBREFA, e-mail us at [email protected].

Background

    This amendment is based on notice of proposed rulemaking (NPRM) 
Notice No. 96-6, which was published in the Federal Register on July 3, 
1996 (61 FR 35056). The related background leading to Notice No. 96-6 
is as follows:
    In 1988, the FAA, in cooperation with the JAA and other 
organizations representing the American and European aerospace 
industries, began a process to harmonize the airworthiness requirements 
of the United States and the airworthiness requirements of Europe, 
especially in the areas of Flight Test and Structures.
    In 1992, the FAA harmonization effort was undertaken by the 
Aviation Rulemaking Advisory Committee (ARAC). A working group of 
industry and government hydraulic systems specialists of Europe, the 
United States, and Canada was chartered by notice in the Federal 
Register (57 FR 58843, December 12, 1992) to harmonize requirements and 
the associated test conditions for hydraulic systems, installed in 
transport category airplanes (Sec. 25.1435). The harmonization task was 
completed by the working group and recommendations were submitted to 
FAA by letter dated November 6, 1995. The FAA concurred with the 
recommendations and proposed them in Notice No. 96-6. A Notice of 
availability of proposed AC 25.1435-1 and request for comments was also 
published in the Federal Register on July 3, 1996 (61 FR 35062). In 
August 1996, the JAA issued its Notice of Proposed Amendment (NPA) 25F-
273: ``Hydraulic Systems'' which included the proposed advisory 
material AMJ 25.1435. The amendments proposed in NPA 25F-273 and the 
advisory material proposed in AMJ 25.1435 were substantively the same 
as the amendments proposed in Notice No. 96-6 and the advisory material 
in proposed AC 25.1435-1.
    As a result, although the FAA (and JAA) has received two sets of 
comments from the public, in response to the proposed rule and the 
proposed AC, the comments are interlinked and addressed jointly. 
Therefore, the FAA has considered both sets of comments in preparing 
the final rule contained herein and the new AC. The announcement of the 
FAA's issuance of the new AC will be published in the Federal Register 
once it is available to the public. Interested persons have been given 
an opportunity to participate in this rulemaking, and due consideration 
has been given to all matters presented. Comments received are 
discussed below.

Discussion of Comments

    Eight commenters responded to the request for comments contained in 
Notice No. 96-6, the notice of availability of proposed AC 25.1435-1 
and the corresponding JAA document NPA 25F-273 and AMJ 25.1435. 
Comments were received from foreign airplane manufacturers, foreign 
airworthiness authorities, and both foreign and domestic industry 
organizations. The majority of the commenters agree with the proposal 
and recommend its adoption. However, some commenters disagree with the 
proposal while providing alternative proposals that appear to merit 
further consideration by the ARAC. Therefore the FAA tasked the ARAC 
Hydraulic Systems Harmonization Working Group (HWG) by notice in the 
Federal Register (62 FR 38187, July 16, 1997) to consider the comments 
and provide recommendations for the disposition of the comments along 
with any recommendations for changes to the proposal. The disposition 
of the comments that follows is based on the agreement reached by the 
HWG. Several

[[Page 27397]]

of the comments address multiple issues and some of the issues were 
addressed by many commenters. As a result, the FAA responses to the 
comments are organized by individual comment under each proposal, i.e., 
proposals 1 through 12.
    Proposal 1, Sec. 25.1435(a)(1). One commenter states that the 
structure of the punctuation in the first sentence appears to allow 
leakage under proof pressure, providing that such leakage does not 
prevent the element from performing its intended function. The proper 
intent should be to prohibit any leakage under proof pressure. The 
commenter suggests to revise the regulatory text of the first sentence 
as follows: ``(1) Withstand the proof pressure without leakage and 
without permanent deformation that would prevent it from performing its 
intended function, and withstand the ultimate pressure without 
rupture.'' The FAA agrees that some clarification of the rule text is 
necessary. The FAA does not however agree with the commenter's 
suggested text because under proof pressure, some external seal leakage 
is allowed as long as the element's ability to perform its intended 
function remains unaffected once the design operating pressure (DOP) is 
restored. Accordingly, the final rule text of the first sentence and 
the associated advisory circular text have been revised to read:
    Rule text: ``(a) Element design. Each element of the hydraulic 
system must be designed to:
    (1) Withstand the proof pressure without permanent deformation that 
would prevent it from performing its intended functions, and the 
ultimate pressure without rupture.''
    Advisory circular text: The following text has been added to 
Paragraph 4a(1)(d), Ref. Sec. 25.1435(a)(1), of the AC: ``At proof 
pressure, seal leakage not exceeding the allowed maximum in-service 
leak rate is permitted. Each element should be able to perform its 
intended function when the DOP is restored.'' For consistency, in 
Paragraph 4a(2), Ref. Sec. 25.1435(a)(2), of the AC, the following text 
will be added: ``At limit load, seal leakage not exceeding the allowed 
maximum in-service rate is permitted.''
    Another commenter recommends that consideration be given to address 
system ``return pressures'' in addition to the ``design, proof, and 
ultimate pressures'' in the table presented as part of 
Sec. 25.1435(a)(1). The FAA did consider the return pressures but 
decided that since the factors specified in the table apply to both 
high/supply pressures and low/return pressures, it was unnecessary to 
specify two sets of factors.
    A third commenter recommends that the advisory material should 
include guidance for determination of the DOP for elements in the low 
pressure side of the system. The FAA concurs and has added the 
following definition in the AC, Paragraph 4a(1)(b), Ref. 
Sec. 25.1435(a)(1): ``The DOP for low pressure elements (e.g., return, 
case-drain, suction, reservoirs) is the maximum pressure expected to 
occur during normal user system operating modes. Included are transient 
pressures that may occur during separate or simultaneous operation of 
user systems such as slats, flaps, landing gears, thrust reversers, 
flight controls, and power transfer units. Short term transient 
pressures, commonly referred to as pressure spikes, that may occur 
during the selection and operation of user systems (e.g., those 
pressure transients due to the opening and closing of selector/control 
valves) may be excluded, provided the fatigue effect of such transients 
is addressed in accordance with Paragraph 4a(4) of this AC.''
    A fourth commenter proposes to replace the term DOP with the term 
``nominal pressure'' claiming that this terminology was consistent with 
MIL-standards and the commenter's own country's practices where 
operating pressure of 3000 psi corresponds to the nominal pressure. The 
FAA notes that consideration was given by the working group to use the 
term ``nominal pressure'' but no agreement could be reached on its 
definition because the term ``nominal'' could involve tolerances, 
fluctuations, and other interpretations; the term ``DOP'' is more 
specific.
    This commenter also proposes that the same safety factor be used 
for all elements, i.e., not less than 1.5 for proof pressure and not 
less than 3.0 for burst pressure. The FAA does not agree. Existing U.S. 
and European industry standards/practices were used to arrive at these 
factors and to harmonize with current JAR 25, Appendix ``J'' (Appendix 
``K'' effective May 27, 1994) requirements. The commenter's suggestion 
would simplify the requirements but does not reflect the acceptable 
industry standards.
    For the above reasons, the proposed Sec. 25.1435(a)(1) has been 
modified by deleting reference to ``without leakage'' and text added in 
the advisory circular regarding leakage and a definition of the DOP for 
elements in the low pressure side of the system.
    Proposals 2 and 3, Secs. 25.1435(a)(2), (3). One commenter states 
that in spite of the guidance material, there is still room for 
misunderstanding the meaning of structural loads in the context of 
hydraulic system elements. The commenter states that the intent is that 
the designer must consider those loads arising when the airplane 
responds to the relevant critical loading conditions of subpart C, in 
which case it would improve clarity to say so. Also, the strength 
analysis of hydraulic system elements must not stop at consideration of 
inertia, dynamic and aerodynamic loads, but must also include 
consideration of strains imposed by the deformation (bending, twist, 
etc.) of the structure to which the elements are attached. Furthermore, 
thermal stresses are likely to be important at the normal operating 
temperature of the hydraulic system. To address these factors, the 
commenter proposes the following amendment:
    ``Sec. 25.1435(a)(2)--Withstand, without deformation that would 
prevent it from performing its intended functions, the design operating 
pressure in combination with the loads and structural deflections 
arising from the critical limit loading conditions of subpart C. Where 
appropriate, thermal effects must also be taken into account.
    Sec. 25.1435(a)(3)--Withstand, without rupture, the design 
operating pressure multiplied by a factor of 1.5, in combination with 
the ultimate loads and ultimate structural deflections arising from the 
critical loading conditions of subpart C. Where appropriate, thermal 
effects must also be taken into account.''
    The commenter also suggests that the third sentence of Paragraph 
4a(2) of the proposed AC be modified to read: ``The loading conditions 
to be considered include, but are not limited to flight and ground 
maneuvers, and gust and turbulence conditions. The loads arising in 
these conditions should be combined with the maximum hydraulic 
pressures, including dynamic transients, that could occur 
simultaneously. Where appropriate, thermal effects should also be 
accounted for in the strength justification.''
    The FAA has considered these comments. The commenter's suggested 
amendments to Secs. 25.1435(a)(2) and (a)(3) are inherent in the 
regulations as stated in the original document and do not have to be 
itemized in the rule. Therefore, it was determined that it would be 
more appropriate that the texts for Secs. 25.1435(a)(2) and (a)(3) 
should remain as proposed in the NPRM but that the associated advisory 
material should be improved, as suggested by the commenter, to more 
adequately reflect the intent of the proposed requirements.

[[Page 27398]]

The AC, Paragraph 4a(2), is therefore amended to read as follows:
    ``(2) (Ref. Sec. 25.1435(a)(2)) Limit structural loads are defined 
in Sec. 25.301(a). The loading conditions of Part 25, Subpart C to be 
considered include, but are not limited to flight and ground maneuvers, 
and gust and turbulence conditions. The loads arising in these 
conditions should be combined with the maximum hydraulic pressures, 
including transients, that could occur simultaneously. Where 
appropriate, thermal effects should also be accounted for in the 
strength justification. For hydraulic actuators equipped with hydraulic 
or mechanical locking features, such as flight control actuators and 
power steering actuators, the actuators and other loaded elements 
should be designed for the most severe combination of internal and 
external loads that may occur in use. For hydraulic actuators that are 
free to move with external loads, i.e., do not have locking features, 
the structural loads are the same as those produced by the hydraulic 
actuators. At limit load, seal leakage not exceeding the allowed 
maximum in-service leak rate is permitted.'' For consistency, the 
statement ``Where appropriate, thermal effects should be accounted for 
in the strength justification'' will also be added at the end of 
Paragraph 4a(3) of the AC.
    The same commenter further adds ``The final sentence in Paragraph 
4a(3) of the proposed AC specifically allows operational/functional 
failure under (when subjected to) ultimate load conditions. However, 
the use of the word ``under'' in this context could give rise to 
confusion as to whether operational/functional failure is allowed 
``below'' ultimate load. If so, this would be inconsistent with the 
safety objectives set by the structural requirements that prohibit 
failure at any load level up to and including ultimate. If a hydraulic 
component is essential for continued safe flight then it must not be 
allowed to fail, or lose operational functionality, at or below, 
ultimate load conditions. For example, the hydraulic system powering an 
elevator would be critical for recovery from the design maneuvering 
condition, and must not be allowed to fail below the ultimate loads 
associated with this condition. To improve clarity and remove 
confusion, the wording should be changed to state positively that 
operational/functional failure is not allowed at any load level up to 
and including ultimate.'' The FAA agrees with the commenter that no 
structural failure (rupture) may occur up to ultimate load. However, 
the commenter seems to be suggesting that operational/functional 
failures should not be allowed up to ultimate loads. The FAA disagrees. 
Section 25.1435(a)(3) of the regulation requires that elements of the 
hydraulic system not rupture (structural failure) up to ultimate loads. 
Section 25.1435(a)(2) requires operational/functional integrity only up 
to limit load. Paragraphs 4a(2) and (3) of the AC properly capture this 
relationship and no change is necessary.
    Another commenter suggests that time limits for proof and burst 
pressure tests be included in the regulation, not just in the AC. The 
FAA does not agree. The recommended time limits in the proposed AC, 
paragraph 4(a)(1)(e), are an industry standard and one method, but not 
the only method, of demonstrating compliance and therefore not 
appropriate for inclusion in the regulation. The commenter also states 
that the definitions of pressures and/or pressures and times given in 
the proposed AC do not appear to match the current JAA criteria and 
wondered whether they had been fully harmonized. The FAA notes that the 
proposed pressures and times have been fully harmonized although they 
may differ from the current JAA criteria (Appendix J to JAR 25). The 
regulatory agencies have agreed to use the criteria proposed in the AC.
    A third commenter states that the proposed advisory material for 
Sec. 25.1435(a)(3) was simply rephrasing of the regulation and not a 
means of compliance as expected; a more detailed clarification in the 
AC of the methods of implementing this requirement was desirable. The 
FAA notes that the first statement in the AC references regulations 
relevant to the requirement, however the remainder is advisory and 
gives details of the methods of pass/fail of a test of the requirement 
but allows flexibility for the applicant to propose any method 
acceptable to the FAA.
    In light of the above discussion, Secs. 25.1435(a)(2) and (a)(3) 
are adopted as proposed with clarifying text added in the AC to account 
for thermal effects in strength justification and to allow some leakage 
at limit load.
    Proposal 4, Sec. 25.1435(a)(4). One commenter states that in order 
to provide sufficient safeguard against the possibility of a premature 
failure in the operational life of an airplane, it will be necessary to 
consider the effects of material fatigue variability on life. 
Conventionally, this would be done through application of an 
appropriate scatter factor to the result of the fatigue analysis or 
fatigue test (See JAR ACJ 25.571(a)(3)). To ensure that the effects of 
variability are properly taken into account in the interpretation of 
the fatigue analysis and test data required by this paragraph, the 
commenter proposed the following:
    ``Sec. 25.1435(a)(4)--Withstand, without failure, the fatigue 
effects of repeated loads of variable magnitude expected during its 
service life, including pressure cycles, pressure transients, 
externally induced loads, structural deformations and, where 
appropriate, thermal effects. Appropriate safe-life scatter factors 
must be applied.''
    The FAA understands the concerns expressed here, but does not agree 
with the linkage to Sec. 25.571, which applies to safe-life components, 
e.g., landing gear, and not to hydraulic components. However, the 
intent of this comment is already addressed in AC Paragraph 4c(1), Ref. 
Sec. 25.1435(c)(1).
    The same commenter adds, ``The term ``cyclical loads'' in Paragraph 
4a(4) of the proposed AC, is usually associated with a periodic force. 
It would be better to use the term ``load cycles.'' This paragraph 
would be an appropriate place to give guidance on the need to cover 
scatter in fatigue properties--JAR ACJ 25.571(a) has some relevant 
guidance material.'' The FAA agrees with the use of the term ``load 
cycles'' and the advisory text has been modified accordingly. The FAA 
does not agree that any advisory material is needed for scatter factors 
or the relevance to Sec. 25.571, as discussed earlier.
    Another commenter states that it is not understood how the current 
JAR 25.1435(a)(6) requirement--means of providing flexibility--comes 
into the new Sec. 25.1435(a)(4) requirement. The FAA notes that it is 
not the new Sec. 25.1435(a)(4) but rather the new Sec. 25.1435(a)(5) 
that addresses the current JAR 25.1435(a)(6) requirement and NPRM 
Proposal 5 clearly stated that. The new Sec. 25.1435(a)(5) addresses 
the environmental factors, including the vibrational & acceleration 
effects of the elemental installation as discussed in the associated 
advisory material. The commenter also suggested including in the 
advisory material, a recommendation for the scatter factor to be used 
when conducting the fatigue testing (for example 4.0 for non-critical 
parts, 6.0 for critical parts). The FAA notes that as stated in 
Paragraph 4c(1) of the AC, the manufacturer may select design factors 
identified in accepted manufacturing, national, military, or industry 
standards provided that it can be established that they are suitable 
for the intended application. It is not appropriate to give scatter 
factors since they are more suitable for safe life components, e.g., 
landing gear and not hydraulic system components.

[[Page 27399]]

    This same commenter also wonders whether there should be an 
allowance for the fact that a component might be fitted on more than 
one airplane in its lifetime, and hence the fatigue cycles could well 
be considerably more than predicted for a part which is assumed to be 
on the airplane for its entire life; it would be very useful to have a 
consistent policy for this issue. The FAA notes that the requirements 
of Secs. 25.671 and 25.1309 specify that the failure of no single 
element shall jeopardize the continued safe flight and landing of the 
airplane. Section 25.1435(a)(4) specifies the design requirements of 
the element and its failure consequences that should be understood and 
addressed by the designer. The existing requirements adequately cover 
the overall safety of an airplane at the time of certification and part 
25 regulations are not intended to deal with parts tracking. 
Furthermore, reliability of hydraulic systems is based on redundancy of 
the design architecture rather than safe-life of components.
    Yet another commenter suggests that Society of Automotive Engineers 
(SAE) document ARP 1383 ``Impulse Testing of Hydraulic Actuators, 
Valves, Pressure Containers, and Similar Fluid System Components'' be 
included as a reference in the AC. The FAA notes that ARP 4752 
``Aerospace-Design and Installation of Commercial Transport Aircraft 
Hydraulic Systems'' listed in the advisory circular in turn refers to 
ARP 1383. All of the relevant SAE documents are referenced in ARP 4752 
and are too numerous to be individually listed in the AC.
    For the reasons stated, Sec. 25.1435(a)(4) is adopted as proposed.
    Proposal 5, Sec. 25.1435(a)(5). One commenter recommends that the 
advisory material state that thermal effects be particularly considered 
for accumulators which are isolated from the hydraulic system by non-
return valves. The FAA notes that Sec. 25.1435(a)(5) addresses the 
environmental factors that are to be considered when designing the 
element and that in the AC, temperature effects are specifically stated 
as one of the variables to be addressed. For the stated reasons, 
Sec. 25.1435(a)(5) is adopted as proposed.
    Proposal 6, Sec. 25.1435(b)(1). One commenter expresses a concern 
that the requirements of (b)(1)(i) could be open for interpretation by 
different airworthiness authorities, particularly with respect to fluid 
level quantity indication. The commenter further states that there were 
occasions when the warning/indication philosophy that had been agreed 
to with one airworthiness authority had not been agreed to by other 
authorities and this therefore led to redesign and/or other additional 
costs. The FAA notes that the commenter's concern of eliminating 
differences in interpretations is the basic reason for harmonization 
effort. The intent of the harmonized rule is to specify what type of 
indication is required from the point of view of what the pilot can 
use, without specifying the design of the indication. Adoption of a 
harmonized rule and guidance material will enhance the likelihood of 
similar interpretations during the certification process. But neither 
the rule nor the guidance material can assure precision in 
interpretation and application absent detailed listings of acceptable 
methods for particular applications. Since the rule and guidance 
material are intended to establish performance based standards, useful 
for future applications and developments, as well as current 
certifications, it is impossible, and undesirable to provide the detail 
that would assure uniformity. The FAA has determined that the currently 
agreed upon language is as likely to produce uniform interpretations as 
any other language which can prudently be adopted.
    Another commenter states that ``moving from prescriptive to general 
indication requirements is considered to be sensible, but to be truly 
meaningful, the requirement should be stated more objectively. Section 
25.1435(b)(1)(ii) is close to being an objective requirement but 
Sec. 25.1435(b)(1)(i) is not. In fact it is not apparent what 
indication might be required by Sec. 25.1435(b)(1)(i) that would not be 
required by Sec. 25.1435(b)(1)(ii).'' The FAA does not agree. Changing 
the existing requirement of JAR 25.1435(a)(2), which refers to the 
provision of indications of system pressure and fluid quantity, to 
Sec. 25.1435(b)(1) which is less prescriptive in that it requires the 
provision of indications of only the appropriate parameters, 
establishes an objective statement of the requirement. The FAA has 
determined that, to ensure continued safe flight and landing, each 
hydraulic system that either (1) performs an essential function or (2) 
that requires corrective action by the flightcrew following a 
malfunction (irrespective of whether it performs an essential 
function), must be associated with the appropriate flight-crew 
indications. The associated AC clarifies that the ``appropriate 
indications'' are not limited other than that they should be 
appropriate. As discussed in the response to the preceding comment, the 
FAA believes the current language is as close to a performance based 
standard as is possible, while avoiding dictating design and allowing 
flexibility in future design development.
    The second commenter also points out that ``in Paragraph 4b of the 
proposed AC, the statement `These requirements are unique to hydraulic 
systems' is questioned.'' The FAA agrees with the commenter. 
Accordingly, the first sentence ``These requirements are unique to 
hydraulic systems, and may compliment Sec. 25.1309'' has been deleted 
and the second sentence has been modified to read ``Design features 
that should be considered for elimination of undesirable conditions and 
effects are:''
    The first commenter also points out that the NPRM cited this 
requirement as Sec. 25.1435(a)(1) when it should have been 
Sec. 25.1435(b)(1). The FAA concurs that the preamble of the proposed 
rule had a typographical error but not the proposed rule text.
    For the stated reasons, Sec. 25.1435(b)(1) is adopted as proposed 
with clarifying changes made in the AC text including deleting 
reference to Sec. 25.1309.
    Proposals 7, 8, and 9, Secs. 25.1435(b)(2), (3), and (4). No 
comments were received. Sections 25.1435(b)(2), (3), and (4) are 
therefore adopted as proposed.
    Proposal 10, Sec. 25.1435(b)(5). One commenter states that the 
means to identify the hydraulic fluid may not always be practical--
particularly for small components such as in line non-return valves. 
The FAA notes that the intent of the requirement is not that every 
component be so identified but rather that suitable placarding be 
provided as practical so that servicing of the hydraulic system(s) is 
done with the specified fluid. As pointed out by another commenter, 
typical/acceptable marking locations for the hydraulic fluid used are 
hydraulic actuators, refill points, reservoirs, and applicable 
servicing documents. The second commenter recommends specifying these 
typical locations in the AC. The FAA concurs and appropriate wording 
has been included in Paragraph 4b(5), Ref. Sec. 25.1435(b)(5), of the 
AC.
    A third commenter suggests that FAA consider clarifying the 
language in proposed Sec. 25.1435(b)(5) to address the situation of 
fluid mixtures. The FAA infers that the commenter is referring to 
Paragraph 4b(5) of the AC which states: ``If more than one approved 
fluid is specified, the term suitable hydraulic fluid is intended to 
include acceptable mixtures.'' The FAA notes that acceptable fluids 
and/or mixtures are those listed in the airplane

[[Page 27400]]

manufacturer's maintenance manuals as approved for that airplane model. 
These maintenance manual provisions, coupled with the proposed AC 
language, seem to provide adequate clarity. For the stated reasons, 
Sec. 25.1435(b)(5) is adopted as proposed.
    Proposal 11, Sec. 25.1435(c). One commenter states that as written, 
this section continues the practice of including some means of 
compliance within the main code rather than in the advisory material. 
The commenter believes Sec. 25.1309(d) currently contains the same 
anomaly, but understands that the decision has been taken in the 
Sec. 25.1309 Working Group to rectify this by moving Sec. 25.1309(d) 
into the advisory material. The commenter recommends that the same 
thing could be done here. The FAA partially accepts this comment, and 
is amending the opening paragraph by deleting the words ``To 
demonstrate compliance with Sec. 25.1435 and support compliance with 
Sec. 25.1309.'' The paragraph would commence ``Tests must be conducted 
. . . .'' and would otherwise remain unchanged from the original 
proposal. Except for qualifying statements that bring immediate 
clarification to the primary regulatory statements, the remainder of 
the paragraph is considered regulatory and not advisory. Section 
25.1435(c) has been revised accordingly.
    Proposal 11, Secs. 25.1435(c)(1) and (c)(2). One commenter states 
``Although it is considered that an endurance test of a complete 
airplane hydraulic system is a very useful test, there are 
circumstances where a full endurance test is an expensive exercise with 
no benefit to the integrity and safety of the airplane. Particular 
examples of this are: (1) the airplane hydraulic system is 
substantially based on an existing, well proven in-service airplane, 
and (2) the number and/or nature of services which are powered 
hydraulically are such that the loss of the system has no significant 
effect on the airworthiness of the airplane.''
    The FAA concurs with the commenter in that testing may not always 
be necessary and notes that the proposed requirement test criteria 
already include the provision ``except that analysis may be used in 
place of or to supplement testing, where the analysis is shown to be 
reliable and appropriate.'' The type and extent of testing guidance 
covered in AC adequately address commenter's concern. In addition, full 
system testing is not required; subsystem or element testing is 
allowed. The commenter further states:
    ``It should be noted that American engineers quite often think of 
endurance as fatigue testing. It is therefore recommended that 
`pressure impulse' be added after fatigue in this section.'' The FAA 
notes that Paragraph 4c(1) of the AC adequately defines these terms 
(for all engineers) and the associated testing. For the stated reasons, 
Secs. 25.1435(c)(1), and (c)(2) are adopted as proposed.
    Proposal 12, Sec. 25.1435(c)(3). One commenter states: ``It is 
proposed that this requirement be dispensed with. This is because (1) 
in the course of an airplane production run, the hydraulic system can 
undergo many modifications (including the introduction of a cargo door 
system) which affect the system installation. Yet, it is the norm that 
this is a once only test which is conducted on an early production 
airplane during the certification test program, and (2) each airplane 
should be inspected with respect to clearances with the hydraulic 
system unpressurized and then pressurized. It is doubtful whether there 
will be any significant movement of the piping, hoses, components, etc. 
as a result of increasing the pressure by 25%.''
    The FAA notes that this test is conducted only once per 
installation. However, the FAA requires that any significant 
modification(s) such as introduction of a cargo door system, a ram air 
turbine (RAT), or a tail-skid system be assessed along with any 
associated/affected system(s) to meet this requirement. There are 
several recent examples of such modifications that will require 
additional testing. Such testing may also be supplemented by analysis 
if appropriate.
    Regarding the commenter's statement (2) about insignificant 
movement of the piping etc., when pressurized at 25% above nominal, the 
FAA concurs and notes that both the proposed and the final rule states 
that instead a full range-of-motion, testing be conducted at just below 
the system relief pressure setting. The commenter goes on to state 
``There are some reservations with the new test proposal, as follows:
    (1) The requirement to check clearances may not be easily achieved 
for those parts of the system which are actuated, for example, landing 
gear, flight controls.
    (2) As the system is pressurized to a higher value, there may be 
concerns about safety, particularly as the services may operate 
quicker.
    (3) The validity of the test results could be queried as the flight 
control actuators are unloaded.''
    The FAA notes that: (1) Paragraph 4c(3) of the AC adequately 
addresses this concern by stating: ``it may be permissible that certain 
components of the system need not be tested if it can be shown that 
they do not constitute a significant part of the system with respect to 
the evaluation of adequate clearances or detrimental effects.''
    (2) The system(s) relief valve(s) protect against over-
pressurization. Standard safety precautions on the factory floor while 
the testing is being conducted must be practiced. There are no 
appreciable differences from full functional test(s) conducted by the 
manufacturer.
    (3) The intent is not to check/verify structural deflections or 
motion of surfaces for flight controls. Loading is not anticipated to 
cause surface deflections.
    Lastly, the commenter states: ``With respect to the low pressure 
side of the hydraulic system, it is proposed that the tests be 
conducted with a dummy return filter element installed, thereby forcing 
all the fluid through the return filter bypass. This meets the same 
criteria as for increasing the system pressure to 125%, that is 
increasing the pressure levels to that which could conceivably occur in 
service.''
    The FAA notes that the commenter's scenario may be applicable to 
some of the hydraulic system architectures (layouts), but not all. It 
is the FAA's policy to allow flexibility for the applicant to propose a 
method of compliance which is acceptable to the cognizant certification 
office. Specifying the proposed dummy filter installation may be 
misinterpreted by an applicant as too restrictive. The commenter's 
suggested method is one means but not the only means of demonstrating 
compliance.
    In light of the discussion above and the explanations already 
provided in the AC, Sec. 25.1435(c)(3) is adopted as proposed.
    General: One commenter states: ``It is considered that the 
harmonization of the Sec. /JAR 25.1435 has produced a good set of 
airworthiness requirements. However, there is still a concern that 
there are areas within the new requirements, which could be subject to 
interpretation by airworthiness surveyors. It is recommended that the 
FAA/JAA review the advisory material and ensure that there are no areas 
where misinterpretation can occur. The reason for this comment is not 
to direct concern at the professionalism of the JAA and FAA, but rather 
there is a concern that other national authorities could read in 
additional requirements where none were intended.'' The HWG together 
reviewed and developed these regulations and the associated advisory 
material. Both the regulations and the advisory material are fully 
harmonized. A considerable amount of time was

[[Page 27401]]

spent discussing the very issues and concerns raised by the commenter 
to arrive at the final rule and the AC. As discussed previously in 
addressing the comment on Proposal 6, the FAA believes that these rules 
are as likely to produce uniform interpretations as any other language 
which can be prudently adopted.
    Another commenter points out that the word ``must'' in Paragraphs 
4a(1), a(2), and a(4) of the AC, and the word ``shall'' in Paragraph 
4a(3) of the AC should be replaced by ``should.'' The FAA agrees and 
the text in the AC has been revised. The commenter adds that in 
Paragraph 4a(1), third paragraph, reference to ``structural loads'' 
should be replaced by ``external loads.'' The FAA does not agree since 
reference to structural loads is appropriate as used in Secs. 25.301 
and 25.1435(a)(1).
    With the exception of the changes noted in Secs. 25.1435(a)(1) and 
(c) this final rule is adopted as proposed in Notice 96-6.

Paperwork Reduction Act

    In accordance with the Paperwork Reduction Act of 1995 (44 U.S.C, 
3507(d)), there are no requirements for information collection 
associated with this amendment.

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. The FAA has 
reviewed the corresponding ICAO Standards and Recommended Practices and 
has identified no differences with these regulations.

Executive Order 12866 and DOT Regulatory Policies and Procedures

    Executive Order 12866, Regulatory Planning and Review, directs the 
FAA to assess both the costs and benefits of a regulatory change. We 
are not allowed to propose or adopt a regulation unless we make a 
reasoned determination that the benefits of the intended regulation 
justify its costs. Our assessment of this proposal indicates that its 
economic impact is minimal. Since its costs and benefits do not make it 
a ``significant regulatory action'' as defined in the Order, we have 
not prepared a ``regulatory impact analysis.'' Similarly, we have not 
prepared a ``regulatory evaluation,'' which is the written cost/benefit 
analysis ordinarily required for all rulemaking proposals under the DOT 
Regulatory and Policies and Procedures. We do not need to do the latter 
analysis where the economic impact of a proposal is minimal.

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

    Changes to Federal regulations must undergo several economic 
analyses. First, Executive Order 12866 directs that each Federal agency 
shall 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 requires agencies to analyze the 
economic impact of regulatory changes on small entities. Third, the 
Trade Agreements Act (19 U.S.C. section 2531-2533) 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 also requires the consideration of international standards and, 
where appropriate, that they be the basis of U.S. standards. And 
fourth, the Unfunded Mandates Reform Act of 1995 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 the private sector, of $100 million or more (adjusted 
annually for inflation) in any one year.
    In conducting these analyses, the FAA has determined this rule: (1) 
has benefits which do justify its costs, is not a ``significant 
regulatory action'' as defined in the Executive Order and is ``not 
significant'' as defined in DOT's Regulatory Policies and Procedures; 
(2) will not have a significant impact on a substantial number of small 
entities; (3) reduces barriers to international trade; and (4) does not 
impose an unfunded mandate on state, local, or tribal governments, or 
on the private sector of 100 million or more in any one year. These 
analyses, available in the docket, are summarized below.

Economic Evaluation

    Manufacturers contacted by the FAA (in its preparation of the 
economic evaluation for the NPRM), estimated that three of the 
revisions to Sec. 25.1435 (corresponding to proposals 1, 4, and 12) 
would impose additional costs--revision 1 (regarding design load 
factors for proof and ultimate pressure conditions), 4 (regarding 
induced loads, pressure transients, and fatigue), and 12 (regarding 
functional testing of the complete hydraulic system). However, based on 
new information from the same manufacturers related to their 
experiences with recent type certifications, the FAA has determined 
that none of the provisions will impose incremental costs. The rule 
changes codify current industry practice and or conform Sec. 25.1435 to 
corresponding sections of the JAR. Adoption of the rule changes 
increases harmonization and commonality between American and European 
airworthiness standards. Harmonizing airworthiness standards reduces 
manufacturers' certification costs for testing, report preparation, 
certification-related travel abroad, etc. One manufacturer of part 25 
large airplanes estimates that such cost savings could range between 
$65,000 and $650,000 per type certification (pertaining to hydraulic 
systems' requirements as discussed in this rulemaking). Since this 
estimate has such a wide range and represents only one manufacturer, 
the FAA used the midpoint of approximately $360,000 for a conservative 
estimate of harmonization cost savings for a part 25 large airplane 
type certification. A manufacturer of part 25 small airplanes estimates 
such savings at $65,000 per type certification. The FAA believes these 
industry-provided numbers are reasonable estimates of potential 
harmonization cost savings. Potential safety benefits resulting from 
specification of minimum accepted standards will supplement these cost 
savings. Thus, with the described benefits and no associated 
incremental costs, the FAA finds the rule cost-beneficial. (Note: All 
estimates in this analysis are expressed in 1999 dollars).

Regulatory Flexibility Determination

    The Regulatory Flexibility Act of 1980 (the Act) establishes ``as a 
principle of regulatory issuance that agencies shall endeavor, 
consistent with the objective of the rule and of applicable statutes, 
to fit regulatory and informational requirements to the scale of the 
business, organizations, and governmental jurisdictions subject to 
regulation.'' To achieve that principle, the Act requires agencies to 
solicit and consider flexible regulatory proposals and to explain the 
rationale for their actions. The Act 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 proposed or 
final rule will have a ``significant economic impact on a substantial 
number of small entities.'' If the determination is that it will, the 
agency must prepare a regulatory flexibility analysis as described in 
the Act.

[[Page 27402]]

    However, if an agency determines that a proposed or final rule is 
not expected to have a significant impact on a substantial number of 
small entities, section 605(b) of the Act 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 rule will affect manufacturers of transport category airplanes 
produced under future new airplane type certifications. For 
manufacturers, a small entity is one with 1,500 or fewer employees. 
Since no part 25 airplane manufacturer has 1,500 or fewer employees, 
and, in addition, the rule imposes no incremental costs, the FAA 
certifies that the rule will not have a significant economic impact on 
a substantial number of small manufacturers.

International Trade Impact Assessment

    The Trade Agreement Act of 1979 prohibits Federal agencies from 
engaging in any standards or related activities that create unnecessary 
obstacles to the foreign commerce of the United States. Legitimate 
domestic objectives, such as safety, are not considered unnecessary 
obstacles. The statute also requires consideration of international 
standards and where appropriate, that they be the basis for U.S. 
standards. In addition, consistent with the Administration's belief in 
the general superiority and desirability of free trade, it is the 
policy of the Administration to remove or diminish to the extent 
feasible, barriers to international trade, including both barriers 
affecting the export of American goods and services to foreign 
countries and barriers affecting the import of foreign goods and 
services into the United States.
    In accordance with the above statute and policy, the FAA has 
assessed the potential effect of this final rule and has determined 
that it will impose the same costs on domestic and international 
entities and thus has a neutral trade impact.

Unfunded Mandates Reform Act

    The Unfunded Mandates Reform Act of 1995 (the Act), enacted as Pub. 
L. 104-4 on March 22, 1995, is intended, among other things, to curb 
the practice of imposing unfunded Federal mandates on State, local, and 
tribal governments. Title II of the Act requires each Federal agency to 
the extent permitted by law, to prepare a written statement assessing 
the effects of any Federal mandate in a proposed or final agency rule 
that may result in a $100 million or more expenditure (adjusted 
annually for inflation) 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.
    This final rule does not contain such a mandate. Therefore, the 
requirements of Title II of the Unfunded Mandates Reform Act of 1995 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, or the 
relationship between the national Government and the States, or on the 
distribution of power and responsibilities among the various levels of 
government. Therefore, we determined that this final rule does not have 
federalism implications.

Regulations Affecting Intrastate Aviation in Alaska

    Section 1205 of the FAA Reauthorization Act of 1996 (110 Stat. 
3213) requires the Administrator, when modifying regulations in Title 
14 of the CFR in a manner affecting intrastate aviation in Alaska, to 
consider the extent to which Alaska is not served by transportation 
modes other than aviation, and to establish such regulatory 
distinctions as he or she considers appropriate. Because this final 
rule applies to the certification of future designs of transport 
category airplanes and their subsequent operation, it could affect 
intrastate aviation in Alaska. The Administrator has considered the 
extent to which Alaska is not served by transportation modes other than 
aviation, and how the final rule could have been applied differently to 
intrastate operations in Alaska. However, the Administrator has 
determined that airplanes operated solely in Alaska would present the 
same safety concerns as all other affected airplanes; therefore, it 
would be inappropriate to establish a regulatory distinction for the 
intrastate operation of affected airplanes in Alaska.

Environmental Analysis

    Federal Aviation Administration Order 1050.1D defines FAA actions 
that may be categorically excluded from preparation of a National 
Environmental Policy Act (NEPA) environmental impact statement. In 
accordance with FAA Order 1050.1D, appendix 4, paragraph 4(j), this 
amendment qualifies for a categorical exclusion.

Energy Impact

    The energy impact of the amendment has been assessed in accordance 
with the Energy Policy and Conservation Act (EPCA) Pub. L. 94-163, as 
amended (42 U.S.C. 6362) and FAA Order 1053.1. It has been determined 
that the final rule is not a major regulatory action under the 
provisions of the EPCA.

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.

The Amendment

    In consideration of the foregoing, the Federal Aviation 
Administration amends part 25 of Title 14, Code of Federal Regulations 
as follows:

PART 25--AIRWORTHINESS STANDARDS--TRANSPORT CATEGORY AIRPLANES

    1. The authority citation for part 25 continues to read as follows:

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

    2. Section 25.1435 is amended by revising paragraphs (a), (b), and 
(c) to read as follows:


Sec. 25.1435  Hydraulic systems.

    (a) Element design. Each element of the hydraulic system must be 
designed to:
    (1) Withstand the proof pressure without permanent deformation that 
would prevent it from performing its intended functions, and the 
ultimate pressure without rupture. The proof and ultimate pressures are 
defined in terms of the design operating pressure (DOP) as follows:

------------------------------------------------------------------------
                                                      Proof     Ultimate
                      Element                         (xDOP)     (xDOP)
------------------------------------------------------------------------
1. Tubes and fittings.............................        1.5        3.0
2. Pressure vessels containing gas:
  High pressure (e.g., accumulators)..............        3.0        4.0
  Low pressure (e.g., reservoirs).................        1.5        3.0
3. Hoses..........................................        2.0        4.0
4. All other elements.............................        1.5        2.0
------------------------------------------------------------------------

    (2) Withstand, without deformation that would prevent it from 
performing its intended function, the design operating pressure in 
combination with limit structural loads that may be imposed;
    (3) Withstand, without rupture, the design operating pressure 
multiplied by

[[Page 27403]]

a factor of 1.5 in combination with ultimate structural load that can 
reasonably occur simultaneously;
    (4) Withstand the fatigue effects of all cyclic pressures, 
including transients, and associated externally induced loads, taking 
into account the consequences of element failure; and
    (5) Perform as intended under all environmental conditions for 
which the airplane is certificated.
    (b) System design. Each hydraulic system must:
    (1) Have means located at a flightcrew station to indicate 
appropriate system parameters, if
    (i) It performs a function necessary for continued safe flight and 
landing; or
    (ii) In the event of hydraulic system malfunction, corrective 
action by the crew to ensure continued safe flight and landing is 
necessary;
    (2) Have means to ensure that system pressures, including transient 
pressures and pressures from fluid volumetric changes in elements that 
are likely to remain closed long enough for such changes to occur, are 
within the design capabilities of each element, such that they meet the 
requirements defined in Sec. 25.1435(a)(1) through (a)(5);
    (3) Have means to minimize the release of harmful or hazardous 
concentrations of hydraulic fluid or vapors into the crew and passenger 
compartments during flight;
    (4) Meet the applicable requirements of Secs. 25.863, 25.1183, 
25.1185, and 25.1189 if a flammable hydraulic fluid is used; and
    (5) Be designed to use any suitable hydraulic fluid specified by 
the airplane manufacturer, which must be identified by appropriate 
markings as required by Sec. 25.1541.
    (c) Tests. Tests must be conducted on the hydraulic system(s), and/
or subsystem(s) and elements, except that analysis may be used in place 
of or to supplement testing, where the analysis is shown to be reliable 
and appropriate. All internal and external influences must be taken 
into account to an extent necessary to evaluate their effects, and to 
assure reliable system and element functioning and integration. Failure 
or unacceptable deficiency of an element or system must be corrected 
and be sufficiently retested, where necessary.
    (1) The system(s), subsystem(s), or element(s) must be subjected to 
performance, fatigue, and endurance tests representative of airplane 
ground and flight operations.
    (2) The complete system must be tested to determine proper 
functional performance and relation to the other systems, including 
simulation of relevant failure conditions, and to support or validate 
element design.
    (3) The complete hydraulic system(s) must be functionally tested on 
the airplane in normal operation over the range of motion of all 
associated user systems. The test must be conducted at the system 
relief pressure or 1.25 times the DOP if a system pressure relief 
device is not part of the system design. Clearances between hydraulic 
system elements and other systems or structural elements must remain 
adequate and there must be no detrimental effects.

    Issued in Renton, Washington, on May 9, 2001.
Donald L. Riggin,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service, ANM-100.
[FR Doc. 01-12232 Filed 5-15-01; 8:45 am]
BILLING CODE 4910-13-P