[Federal Register Volume 67, Number 79 (Wednesday, April 24, 2002)]
[Notices]
[Pages 20422-20424]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 02-9846]



  Federal Register / Vol. 67, No. 79 / Wednesday, April 24, 2002 / 
Notices  

[[Page 20422]]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration


Advisory Circular 25.735-1, Brakes and Braking Systems 
Certification Tests and Analysis

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of issuance for advisory circular and disposition of 
comments.

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SUMMARY: This action announces the issuance of Advisory Circular (AC) 
25.735-1, Brakes and Braking Systems Certification Tests and Analysis, 
and documents the disposition of comments received in response to the 
notice of availability for the proposed AC, published in the Federal 
Register on August 10, 1999. This AC sets forth an acceptable means, 
but not the only means, of demonstrating compliance with the braking 
system requirements of Title 14, Code of Federal Regulations (14 CFR) 
for transport category airplanes. Like all AC's, it is not regulatory 
but is to provide guidance for applicants in demonstrating compliance 
with the objective safety standards set forth in the rule. The FAA will 
publish the Revision of Braking Systems Airworthiness Standards final 
rule and a Notice of Availability of Technical Standard Order (TSO) 
C135 in the Federal Register when they are issued.

DATES: The subject advisory circular was issued in the FAA Transport 
Airplane Directorate in Renton, Washington, on April 10, 2002.

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

SUPPLEMENTARY INFORMATION:   

How To Obtain a Copy of the AC

    Copies of this AC can be found and downloaded from the Internet at 
http://www.faa.gov/certification/aircraft/air--index.htm by taking the 
following steps: Under ``Aircraft Certification Related Information'' 
click on Advisory Circulars. Under ``Search Help'' click on Related 
Links. Then click on Advisory Circulars. You may also go to the 
Regulatory and Guidance Library web site at http:\\www.airweb.faa.gov/rgl, at the link titled ``Advisory Circulars.'' Paper copies of the 
AC's will be available in approximately 6-8 weeks from the U.S. 
Department of Transportation, Subsequent Distribution Office, SVC-
121.23, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 
20785.

Discussion of Comments

    Twenty-one commenters responded to the Notice of Proposed 
Rulemaking (NPRM), Proposed Advisory Circular (AC), and Notice of 
Availability of Proposed TSO and request for comments, published in the 
Federal Register on August 10, 1999 (64 FR 43579).
    Comments were received from eight foreign and domestic airplane and 
brake manufacturers, nine foreign airworthiness authorities, one 
operator, and three foreign and domestic industry organizations. Six of 
the twenty-one commenters have comments on the proposed advisory 
circular. The majority of the six 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 Aviation Rulemaking Advisory Committee 
(ARAC). Therefore, the FAA tasked the ARAC Braking Systems 
Harmonization Working Group (HWG) by letter dated February 8, 2000, 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 below is based on the 
agreement reached by the HWG and submitted to the FAA by letter, dated 
June 19, 2000. Several of the commenters address multiple issues, while 
many commenters address the same issue. As a result, the FAA responses 
to the comments are organized by paragraph number and subject.
    1. Purpose. One commenter suggests using ``14 CFR'' or ``Federal 
Aviation Regulations'' instead of ``FAR.'' Although most people in the 
aerospace industry understand the informal use of the acronym FAR as 
pertaining to the requirements of Title 14 of the Code of Federal 
Regulations (14 CFR), it is not correct. The acronym FAR is an 
abbreviation for Federal Acquisition Regulations.
    FAA's Response: The FAA concurs. The FAA is to use CFR to when 
referring to the Code of Federal Regulations. For Title 14, it appears 
as 14 CFR.
    2. Related Documents. The same commenter suggests including a 
statement that all section numbers, such as Sec. 25.735, refer to 
regulations in 14 CFR, unless otherwise noted.
    FAA's Response: As stated in the ``Purpose'' paragraph, this AC 
provides guidance material for demonstrating compliance with the 
braking system requirements of 14 CFR.
    3. Background. No comments were received for this paragraph.
    4. Discussion.
    Paragraph 4a(1)(e): One commenter explains the need to clarify the 
statement ``Combinations of any additional wheel and brake assemblies 
should meet applicable airworthiness requirements.'' In this context it 
is unclear what ``additional'' wheels and brakes mean. Also it is not 
recognized how meeting JAR 21.101(a) and (b) will eliminate situations 
that may have adverse consequence on airplane braking control and 
performance.
    FAA's Response: The FAA concurs that a clarification is necessary. 
The second sentence is revised to read: ``Following initial airplane 
certification, any additional wheel and brake assemblies should meet 
the applicable airworthiness requirements.'' The FAA has decided that 
reference to Sec. 21.101 and JAR 21.101 is appropriate.
    Paragraph 4a(2) and 4a(3)(b): One commenter suggests deleting 
``Refurbished and Overhauled Equipment'' from paragraph 4a(2) as this 
advisory material is not applicable for showing compliance to 
Sec. 25.735(a). The same commenter suggests deleting ``monitoring 
plan'' from paragraph 4a(3)(b), again because this advisory material is 
not applicable for showing compliance to Sec. 25.735(a).
    FAA's Response: The FAA does not agree. The FAA considers the 
advisory material to be relevant to the continued airworthiness of 
qualified equipment, reference paragraph 2.1 of the TSO-C135.
    Paragraph 4b(1): One commenter states that there is a significant 
difference between the Advisory Material Joint (AMJ) associated with 
the Joint Aviation Authority (JAA) rule and the AC associated with the 
FAA rule. The FAA material includes the word ``tire'' when referring to 
multiple failures originating from a single cause. It is pointed out 
that earlier drafts of the AMJ material also referred to tire failures 
potentially causing multiple hydraulic failures. The commenter 
recommends that the proposal should be re-worded to clarify that the 
rule refers to multiple failures from a single source occurring within 
the system itself. Failures from outside the system are adequately 
dealt with elsewhere in the regulations. Suggested text: ``Multiple 
failures resulting from a single cause shall be considered a single 
failure, for example, failure of a single component within the system 
leading to

[[Page 20423]]

the loss of two or more hydraulic supplies.''
    FAA's Response: The FAA concurs and the JAA agrees to modify the 
AMJ text to agree with the AC text. The FAA disagrees with the 
suggested rewording. The wording is correct as stated. The tire is 
considered a part of the braking system. The tire failure example is 
appropriate and relevant for a single failure leading to multiple 
failures of hydraulic power.
    Paragraph 4b(2)(c): One commenter states that if more than one 
fluid is allowed for the airplane hydraulic system, then the one 
resulting in the worst case scenario should be used for showing 
compliance. For example, LD-4 has a lower auto-ignition point than 
Skydrol 500B-4 and, if both are allowed for use on a particular 
airplane, the former should be used for showing compliance. A statement 
should be added accordingly. Note that the same comment has been made 
with respect to the proposed TSO-C135.
    FAA's Response: The FAA concurs that if the applicant can identify 
one fluid that exhibits all the worst case properties, then that one 
fluid may be used to show compliance. However, fluids are tested and 
qualified for a multitude of properties and the same fluid is unlikely 
to possess all worst case scenario properties. Therefore, the statement 
``If more than one fluid is allowed for the hydraulic system, 
compliance should be addressed for all fluids'' is added in the AC.
    Paragraph 4f(1) and (2): Two commenters state that if the most 
severe landing stop is not added to 14 CFR 25.735(f) or included in 
TSO-C135, then it should not be included in the advisory material.
    FAA's Response: The FAA concurs that the contents of the AC should 
reflect the regulation. The most severe landing stop test requirement 
is retained; hence no change in the AC is necessary.
    Paragraph 4f(2)(a): One commenter suggests replacing the text 
``conservative assessments of typical ambient conditions'' with 
``assessment of ambient conditions within the operational limits 
established by the applicant and the typical time the airplane will be 
on the ground.'' The commenter states that the use of a typical ambient 
condition is inconsistent with the general requirements for landing 
performance that requires horizontal distances to be determined for 
standard temperatures within the operational limits established by the 
applicant.
    FAA's Response: The FAA concurs and the text is revised 
accordingly.
    Paragraph 4f(2)(b): One commenter states that the phrase ``with the 
airplane in a configuration that would enable such a return to be 
made'' might seem to indicate that the analysis is not to consider an 
immediate return to land in cases where the airplane configuration is 
less than ideal. This is obviously not the intent as illustrated in the 
NPRM discussion for Sec. 25.735(f). Furthermore, there is no discussion 
about the acceptable probability of failure conditions in such cases 
(i.e. not extremely improbable), which is an important element of the 
rule. Finally, it should be specified how single failure cases are to 
be considered since their acceptability is linked to the effect, not 
the probability. For example, would it be acceptable that an applicant 
foregoes a most severe landing stop case test on the basis that it 
involves an extremely improbable single failure case resulting in a 
hazardous failure condition (such designs have been encountered in the 
past)? It is suggested that the discussion in the guidance material be 
expanded accordingly.
    FAA's Response: The FAA concurs. The following statement is added 
to the AC: The applicant should address effects and consequences of 
typical single and multiple failure conditions which are foreseeable 
events and can necessitate landings at abnormal speeds and weights. The 
critical landing weight for this condition is the Maximum Takeoff 
Weight, less fuel burned and jettisoned during a return to the 
departure airfield. A 30-minute flight should be assumed, with 15 
minutes of active fuel jettisoning if equipped with a fuel jettisoning 
system.
    Paragraph 4f(3)(b): One commenter states that the concern about not 
allowing a brake application speed higher than the ones used in the 
determination of the kinetic energy requirements to ensure that proper 
energy absorption rates are achieved is understood. However, it is felt 
that ``as close as practicable'' is too subjective and should be 
quantified. This would alleviate the certification office to have to 
argue with the applicant as to what a lesser but appropriate brake 
application speed can be for a particular project and help ensure a 
level playing field nation wide. Note that a similar comment has been 
made on the proposed TSO-C135.
    FAA's Response: The FAA disagrees. Quantifying the speed tolerance 
may lead to more restrictive inertia plate increments that may be 
incompatible with dynamometer setups ``As close as practicable'' 
provides the desired flexibility.
    Paragraph 4f(3)(b): The following comment is made relative to TSO-
C135 paragraph 3.3.1.3, but its disposition affects the AC paragraph 
4f.(3)(b) as follows: The reason stated for not exceeding the speed 
specified is that ``for a target test deceleration,'' the rate of 
energy input will decrease as speed increases. However, for the same 
stop distance or torque, the deceleration must increase for a higher 
initial brake-on speed, which causes the rate of energy input to 
increase as speed increases. In general, experience has shown that for 
the same energy and torque requirement, higher initial brake-on speed 
is a more severe condition. The commenter suggests adding the following 
statement to the TSO paragraph 3.3.1.3, which is also included in 
proposed AC 25.735-lX: ``However, a brake having a higher initial 
brakes-on speed is acceptable If the dynamometer test showed that both 
the energy absorbed and the energy absorption rate required by 
Sec. 25.735(f) had been achieved.''
    Alternatively, a tolerance on initial brake application speed of 
 2 knots could be included, which is currently allowed by 
at least one airplane manufacturer. This will allow continuation of the 
current practice of matching Inertia Equivalent (IE) as close as 
possible and varying speed slightly to achieve the required energy, 
which has worked well for many years. If the specified speed cannot be 
exceeded, combined margins to allow for tire energy absorption (when 
brake energy is specified) and the inability to initiate a stop at a 
precise brake-on speed may drive the IE and brake-on speed farther from 
the specified conditions, than for the current practice.
    FAA's Response: The FAA disagrees. The TSO wording for paragraph 
3.3.1.3 is correct as stated. The FAA agrees that there was confusion 
in the text of AC 25.735-1X, and that rearrangement of some text in 
paragraph 4f(3)(b) is necessary. As a result, the sentence: ``However, 
a brake having a higher initial brakes-on speed is acceptable if the 
dynamometer test showed that both the energy absorbed and the energy 
absorption rate required by Sec. 25.735(f) had been achieved.'' is 
removed from 4f(3)(b). This sentence is relocated under a separate 
paragraph, 4f(3)(c), to read as follows: ``(c). For certification 
purposes, a brake having a higher initial brakes-on speed is acceptable 
if the dynamometer test showed that both the energy absorbed and the 
energy absorption rates required by Sec. 25.735(f) had been achieved.'' 
Existing paragraph ``4f(3)(c)'' is redesignated as ``4f(3)(d).''

[[Page 20424]]

    Paragraph 4f(3)(c): One commenter suggests rewording this statement 
to say that brake qualification tests are not intended to verify 
expected performance on the airplane. Brake qualification tests are 
intended to predict expected performance on the airplane. This is the 
main reason for conducting these tests.
    FAA's Response: The FAA disagrees. Airplane performance is 
determined by airplane tests, therefore, no change in the AC text is 
necessary.
    Paragraph 4g(3): One commenter states that keeping the brake 
pressure applied throughout the 5-minute post stop period would help 
determine whether it might contribute to a fire hazard. It would, 
however, be acceptable for the parking brake pressure to fail to be 
maintained after 3 minutes, since the tires would most likely be 
deflated by that time anyway, thereby holding the aircraft stationary. 
It is important to determine whether the parking brake design aspects 
of the brake assembly could be potentially deficient at the time of 
qualification. Consequently, a statement to the effect that parking 
brake should remain applied throughout a 5-minute period should be 
added. Note that similar comments have been made about the proposed 
TSO-C135.
    FAA's Response: The FAA does not concur that the parking brake 
should remain applied for 5 minutes and reaffirms the 3-minute parking 
brake applied period for the dynamometer test. The FAA agrees that 
clarification of the parking brake set period is needed. There is no 
intent by the FAA to dictate that the parking brake must be released at 
3 minutes, but that it must be applied at least that long. Changes are 
made to the TSO Figures 3-1 and 3-2 and paragraphs 3.3.3.5 and 3.3.4.5 
to minimize ambiguity in this respect.
    The certification test on the airplane (worn brake RTO) need not 
follow the procedure prescribed in the TSO. But it is important that 
the brake manufacturer know early in the development period what 
procedure will be used on the airplane (i.e. the certification basis) 
since it can impact the design.
    The Transport Canada (representative) agrees that this is an 
acceptable harmonized solution allowing authorities that are not part 
of the harmonization process the needed flexibility. Also see the 
disposition of comments under NPRM proposal 13, and TSO paragraphs 
3.3.3.5 and 3.3.4.5.
    Paragraph 4h(4): One commenter suggests changing ``A full brake 
application * * *'' to ``A full brake application cycle * * *'' The 
term ``brake application'' or ``brake actuation'' is commonly used to 
mean increasing pressure from fully released to fully applied, while 
``brake retraction'' is commonly used to define returning brake 
pressure from fully applied to fully released. Using the term ``brake 
application cycle'' will help avoid the possibility of confusion.
    FAA's Response: The FAA concurs and the AC text is changed 
accordingly.

Conclusion

    With the exceptions of the changes noted in paragraph 4(a)(1)(e), 
4b(2)(c), 4f(2)(a), 4f(2)(b), 4f(3)(b), 4f(3)(c), and 4h(4), this AC is 
adopted as proposed.

    Issued in Renton, Washington, on April 10, 2002.
Ali Bahrami,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 02-9846 Filed 4-23-02; 8:45 am]
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