[Federal Register Volume 59, Number 179 (Friday, September 16, 1994)]
[Unknown Section]
[Page 0]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 94-22903]
[[Page Unknown]]
[Federal Register: September 16, 1994]
_______________________________________________________________________
Part III
Department of Transportation
_______________________________________________________________________
Federal Aviation Administration
_______________________________________________________________________
14 CFR Part 25
Revised Discrete Gust Load Design Requirements; Proposed Rule
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. 27902; Notice No. 94-29]
RIN 2120-AF27
Revised Discrete Gust Load Design Requirements
AGENCY: Federal Aviation Administration, DOT.
ACTION: Notice of proposed rulemaking.
-----------------------------------------------------------------------
SUMMARY: This notice proposes to revise the gust load design
requirements for transport category airplanes. The proposed changes
would: (1) replace the current discrete gust requirement with a new
requirement for a discrete tuned gust; (2) modify the method of
establishing the design airspeed for maximum gust intensity; and (3)
provide for an operational rough air speed. These changes are proposed
in order to provide a more rational basis to account for the
aerodynamic and structural dynamic characteristics of the airplane.
These proposed changes would also provide for harmonization of the
discrete gust requirements with the Joint Aviation Requirements (JAR)
of Europe as recently amended.
DATES: Comments must be received on or before December 15, 1994.
ADDRESSES: Comments on this notice may be mailed in triplicate to:
Federal Aviation Administration, Office of the Chief Counsel,
Attention: Rules Docket (AGC-200), Docket No. 27902, 800 Independence
Avenue SW., Washington, DC 20591; or delivered in triplicate to: Room
915G, 800 Independence Avenue SW., Washington, DC 20591. Comments
delivered must be marked Docket No. 27902. Comments may be examined in
Room 915G weekdays, except Federal holidays, between 8:30 a.m. and 5
p.m. In addition, the FAA is maintaining an information docket of
comments in the Transport Airplane Directorate (ANM-100), Federal
Aviation Administration, 1601 Lind Avenue SW., Renton, WA 98055-4056.
Comments in the information docket may be examined weekdays, except
Federal holidays, between 7:30 a.m. and 4 p.m.
FOR FURTHER INFORMATION CONTACT:
James Haynes, Airframe and Propulsion Branch, ANM-112, Transport
Airplane Directorate, Aircraft Certification Service, FAA, 1601 Lind
Avenue SW., Renton, WA 98055-4056; telephone (206) 227-2131.
SUPPLEMENTARY INFORMATION:
Comments Invited
Interested persons are invited to participate in this proposed
rulemaking by submitting such written data, views, or arguments as they
may desire. Comments relating to any environmental, energy, or economic
impact that might result from adopting the proposals contained in this
notice are invited. Substantive comments should be accompanied by cost
estimates. Commenters should identify the regulatory docket or notice
number and submit comments in triplicate to the Rules Docket address
above. All comments received on or before the closing date for comments
will be considered by the Administrator before taking action on this
proposed rulemaking. The proposals contained in this notice may be
changed in light of comments received. All comments received will be
available in the Rules Docket, both before and after the comment period
closing date, for examination by interested persons. A report
summarizing each substantive public contact with FAA personnel
concerning this rulemaking will be filed in the docket. Persons wishing
the FAA to acknowledge receipt of their comments must submit with those
comments a self-addressed, stamped postcard on which the following
statement is made: ``Comments to Docket No. 27902.'' The postcard will
be date/time stamped and returned to the commenter.
Availability of NPRM
Any person may obtain a copy of this notice by submitting a request
to the Federal Aviation Administration, Office of Public Affairs,
Attention: Public Inquiry Center, APA-230, 800 Independence Avenue SW.,
Washington, DC 20591; or by calling (202) 267-3484. Communications must
identify the notice number of this NPRM. Persons interested in being
placed on a mailing list for future rulemaking documents should also
request a copy of Advisory Circular No. 11-2A, Notice of Proposed
Rulemaking Distribution System, which describes the application
procedure.
Background
The National Advisory Committee for Aeronautics (NACA), the
predecessor of the National Aeronautics and Space Administration
(NASA), began an inflight gust measurement program in 1933 to assist in
the refinement of gust load design criteria. Using unsophisticated
analog equipment, that program resulted in the development of the
improved design requirements for gust loads that were issued in part 04
of the Civil Aeronautics Regulations (CAR) in the 1940's. The
corresponding Civil Aeronautics Manual (CAM) 04 provided a simplified
formula from which to derive the design gust loads from the specified
design gust velocities. These criteria were based on an analytical
encounter of the airplane with a discrete ramp-shaped gust with a
gradient distance (the distance necessary for the gust to build to a
peak) of 10 times the mean chord length of the airplane wing. An
alleviation factor, calculated from wing loading, was provided in order
to account for the relieving effects of rigid body motion of the
airplane as it penetrated the gust. With the development of the VGH
(velocity, load factor, height) recorder in 1946, NASA began collecting
a large quantity of gust load data on many types of aircraft in airline
service. Although that program was terminated for transport airline
operations in 1971, the data provided additional insight into the
nature of gusts in the atmosphere, and resulted in significant changes
to the gust load design requirements. The evolution of the discrete
gust design criteria from part 04 through part 4b of the CAR to current
part 25 of Title 14 of the Code of Federal Regulations (CFR) (which
contains the design requirements for transport category airplanes)
resulted in the establishment of a prescribed gust shape with a
specific gust gradient distance and increased peak gust design
velocities. The prescribed shape was a ``one-minus-cosine'' gust shape
with a specified gust gradient distance of 12.5 times the mean chord
length of the airplane wing. The gust gradient distance, for that
particular shape, was equal to one-half the total gust length. A
simplified analytical method similar to the methodology of CAM 04 was
provided along with an improved alleviation factor that accounted for
unsteady aerodynamic forces, gust shape, and the airplane rigid body
vertical response.
The increasing speed, size, and structural flexibility of transport
airplanes resulted in the need to consider not only the rigid body
response of the airplane, but also structural dynamic response and the
effects of structural deformation on the aerodynamic parameters. Early
attempts to account for structural flexibility led to a ``tuned'' gust
approach in which the analysis assumed a flexible airplane encountering
gusts with various gradient distances in order to find the most
critical gust gradient distance for use in design for each major
component. A tuned discrete gust approach became a requirement for
compliance with the British Civil Airworthiness Requirements.
Another method of accounting for the structural dynamic effects of
the airplane involved the power spectral density (PSD) analysis
technique which accounted for the statistical distribution of gusts in
continuous turbulence in conjunction with the aeroelastic and
structural dynamic characteristics of the airplane. In the 1960's, the
Federal Aviation Administration (FAA) awarded study contracts to Boeing
and Lockheed for the purpose of assisting the FAA in developing the PSD
gust methodology into continuous gust design criteria with analytical
procedures. The final PSD continuous turbulence criteria were based on
those studies and were codified in Appendix G to part 25 in 1980.
Recognizing that the nature of gusts was not completely defined,
and that individual discrete gusts might exist outside the normal
statistical distribution of gusts in continuous turbulence, the FAA
retained the existing criteria for discrete gusts in addition to the
new requirement for continuous turbulence. The current discrete gust
criteria in Subpart C of part 25 require the loads to be analytically
developed assuming the airplane encounters a gust with a fixed gradient
distance of 12.5 mean chord lengths. For application of the current
criteria, it is generally assumed that the airplane is rigid in
determining the dynamic response to the gust while the effects of wing
elastic deflection on wing static lift parameters are normally taken
into account. The minimum value of the airplane design speed for
maximum gust intensity, VB, is also established from the discrete
gust criteria.
Recent flight measurement efforts by FAA and NASA have been aimed
at utilizing measurements from the digital flight data recorders (DFDR)
to derive gust load design information for airline transport airplanes.
The Civil Aviation Authority (CAA) of the United Kingdom has also been
conducting a comprehensive DFDR gust measurement program for transport
airplanes in airline service. The program, called CAADRP (Civil
Aircraft Airworthiness Data Recording Program), uses data sampling
rates that allow the measurement of a wide range of gust gradient
distances. The CAADRP program is still continuing and has resulted in
an extensive collection of reliable gust data.
In 1988, the FAA, in cooperation with the JAA and 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 in regard to gust
requirements. The objective was to achieve common requirements for the
certification of transport airplanes without a substantive change in
the level of safety provided by the regulations. Other airworthiness
authorities such as Transport Canada have also participated in this
process.
In 1992, the harmonization effort was undertaken by the Aviation
Regulatory Advisory Committee (ARAC). A working group of industry and
governmental structural loads specialists of Europe, the United States,
and Canada was chartered by notice in the Federal Register (58 FR
13819, March 15, 1993). The harmonization effort has now progressed to
a point where some specific proposals have been developed by the
working group for the discrete gust requirements and these proposals
have been recommended to FAA by letter dated October 15, 1993. The FAA
is also considering other proposals for future rulemaking.
Discussion
The continued evolution of gust design requirements among the
various world aviation authorities has resulted in many separate gust
load design criteria with which the transport airplane manufacturer
must comply in order to export its product. Recent efforts between the
FAA and the Joint Aviation Authorities (JAA) of Europe in cooperation
with the transport manufacturers has resulted in a proposal to refine
the criteria and consolidate them into a common set of gust
requirements. A review was made of analytical methods to find a single
method that would simulate both discrete gusts and continuous
turbulence and produce design loads that could be used directly for
structural analysis. However, no single method was found to be
satisfactory for accounting for both the discrete gust and continuous
turbulence; therefore, separate criteria for these conditions will be
retained in the requirements. This notice addresses only the discrete
gust criteria. If revisions to the continuous turbulence criteria are
deemed necessary, they will be proposed in a future notice.
A tuned discrete gust methodology would replace the current
discrete gust requirement of Sec. 25.341 in order to provide a more
rational basis that accounts for the aerodynamic and structural dynamic
characteristics of the airplane. This methodology would take into
account the expected operation of the airplane by allowing multiplying
factors, based on fuel loading and maximum operating altitude, to be
used to adjust the required design gust velocities. This method is
considered to be more rational in that it more accurately reflects the
actual conditions experienced by the airplane and is therefore less
likely to lead to either overdesigning or undesigning of structure. An
effort has been undertaken by the industries and governments of the
United States and Europe to evaluate the new proposed criteria and
ensure that the provide reasonable design loads for current
conventional transport airplanes as well as for new technology
airplanes that may include systems that react in a nonlinear manner.
Furthermore, the proposed gust gradient distance and design gust
velocity distributions are believed to represent the best available
measurements of the gust environment in which the airplane is likely to
be operated. In this regard, the CAADRP gust measurement data (CAA,
Safety Regulation Group, Research Note Number 74, November 30, 1990,
``Investigation of Derived Gust Velocities from CAADRP Data'') have
been used to support the design gust velocity and gradient distance
distributions for the new proposed discrete gust design criteria.
The method for establishing the minimum value of the design speed
for maximum gust intensity, VB, which is currently predicated on
the discrete gust criteria of the current Sec. 25.341, would also be
revised. The proposed tuned gust criteria would replace the static
discrete gust criteria of Sec. 25.341 which are used in the calculation
of the minimum value of VB. Therefore, a revised criterion for the
minimum VB is also proposed.
The proposal does not include a discrete gust design condition at
VB, although the speed VB would continue to be used in
determining the criteria for continuous turbulence. The design gust
velocity and gradient distances established for the gust design
conditions at VC, ``structural design cruising speed,'' and
VD, ``structural design diving speed,'' were developed in
consideration of the full operational envelope so that a specific
discrete gust condition at VB is not considered necessary,
provided an adequate speed margin is retained between VB and
VC, and provided the current practices for operating in severe
turbulence are continued. In this regard, it is also proposed that the
recommended operational turbulence penetration speed of
Sec. 25.1585(a)(8) be based on a new operational rough air speed,
VRA, which would be no greater than the VB chosen for
structural design. In the interest of developing a common requirement
for part 25 and JAR-25, the current JAR requirement (JAR 25.1517) for a
rough air speed, VRA, for which there is a satisfactory service
history, would be the basis for the new proposed Sec. 25.1517. The FAA
considers the level of safety provided in this notice to be the same as
in the current rules.
Several changes are also proposed to other related rules to
implement the new criteria and to consolidate the general gust
requirements into a single section. Gust requirements are located in
several different sections of part 25 that pertain to continuous
turbulence, lateral gusts, etc. This proposal would consolidate many of
these gust requirements into a revised Sec. 25.341. In this regard,
several changes to other sections are proposed to transfer requirements
and to revise references to these requirements. These include the
relocation of Sec. 25.305(d) to Sec. 25.341(b) and the transfer of
Secs. 25.331(a)(1) and 25.331(a)(2) to Sec. 25.321 ``General'' and
changing the title of Sec. 25.331 to ``Symmetric maneuvering
conditions.'' Also the lateral gust requirements of Sec. 25.351 would
be removed since the proposed Sec. 25.341 addresses both vertical and
lateral gusts. The gust envelope would no longer be needed with the
proposed criteria so it would be eliminated from Sec. 25.333 and the
title of this section would be changed to ``Flight maneuvering
envelope.''
Changes are also proposed to adapt the tuned gust criteria to the
cases of unsymmetrical loads in Sec. 25.349 ``rolling conditions,''
Sec. 25.427 ``Unsymmetrical loads,'' and to Sec. 25.445 ``Outboard
fins.'' These rules would be revised in order to provide criteria for
calculating unsymmetrical external airloads for dynamic discrete gust
conditions and to provide for the effects of lateral gusts acting on
auxiliary aerodynamic surfaces such as winglets and outboard fins. To
be more general, it is proposed to change the title of Sec. 25.445 from
``Outboard fins'' to ``Auxiliary aerodynamic surfaces.''
Regulatory Evaluation Summary
Preliminary Regulatory Evaluation, Initial Regulatory Flexibility
Determination, and Trade Impact Assessment
Proposed 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 effect of regulatory changes on small entities.
Third, the Office of Management and Budget directs agencies to assess
the effects of regulatory changes on international trade. In conducting
these analyses, the FAA has determined that this rule: (1) Would
generate benefits that justify its costs and is not a ``significant
regulatory action'' as defined in the Executive Order; (2) is not
significant as defined in DOT's Policies and Procedures; (3) would not
have a significant impact on a substantial number of small entities;
(4) would not constitute a barrier to international trade. These
analyses, available in the docket, are summarized below.
Cost-Benefit Analysis
The proposed changes would have economic consequences. The costs
would be the incremental costs of meeting the tuned discrete gust
requirements rather than the current static discrete gust requirements.
The benefits would be the savings from not meeting two different sets
of discrete gust requirements, i.e., the requirements in the current
FAR and the requirements in the JAR. In order to sell their transport
category airplanes in a global marketplace, manufacturers usually
certify their products under both sets of regulations. Harmonizing
these discrete gust requirements would result in a net cost savings.
Industry sources provided information on the additional costs and
cost savings that would result from the proposed rule. Based on this
information a range of representative certification costs and savings
are shown below. The costs and savings per certification are those
related to meeting discrete gust load requirements, including related
provisions of the proposed rule.
Per Certification Costs and Savings Associated With Proposed Discrete
Gust Load Requirements
[In thousands of dollars]
Costs of current FAA certification........................... $29-$115
Costs of current JAA certification........................... 70-145
Costs of current joint certification......................... 100-150
Costs of proposed FAA certification.......................... 70-145
Costs of proposed joint certification........................ 70-145
Savings (current joint certification costs minus proposed
joint certification costs).................................. 5-29
The costs and cost savings of specific certifications may vary from
these estimates. In all cases where a manufacturer seeks both FAA and
JAA certification, however, the cost savings realized through
harmonizing the requirements would outweigh the expected incremental
costs of the proposal. The FAA solicits information from manufacturers
and other interested parties concerning the costs and savings
associated with this proposal.
In addition to the cost savings expected from harmonization, the
proposed rule would result in airplane designs that are based on more
rational evaluations of conditions expected in flight.
Regulatory Flexibility Determination
The Regulatory Flexibility Act of 1980 (RFA) was enacted by
Congress to ensure that small entities are not unnecessarily and
disproportionately burdened by Federal regulations. The RFA requires A
Regulatory Flexibility Analysis if a proposed rule would have ``a
significant economic impact on a substantial number of small
entities.'' FAA Order 2100.14A outlines FAA's procedures and criteria
for implementing the RFA.
An aircraft manufacturer must employ 75 or fewer employees to be
designated as a ``small'' entity. A substantial number of small
entities is defined as a number that is 11 or more and which is more
than one-third of the small entities subject to a proposed or final
rule. None of the manufacturers of transport category airplanes qualify
as small entities under this definition. Therefore, the proposed rule
would not have a significant economic impact on a substantial number of
small entities.
International Trade Impact Assessment
The proposed rule would not constitute a barrier to international
trade, including the export of American goods and services to foreign
countries and the import of foreign goods and services into the United
States. The discrete gust load requirements in this rule would
harmonize with those of the JAA and would, in fact, lessen the
restraints on trade.
Federalism Implications
The regulations proposed herein would not have substantial direct
effects on the states, on the relationship between the national
government and the states, or on the distribution of power and
responsibilities among the various levels of government. Thus, in
accordance with Executive Order 12612, it is determined that this
proposal does not have sufficient federalism implications to warrant
the preparation of a Federalism Assessment.
Conclusion: Because the proposed changes to the gust design
criteria are not expected to result in a substantial economic cost, the
FAA has determined that this proposed regulation would not be
significant under Executive Order 12866. Because this is an issue that
has not prompted a great deal of public concern, the FAA has determined
that this action is not significant under DOT Regulatory Policies and
Procedures (44 FR 11034; February 25, 1979). In addition, since there
are no small entities affected by this rulemaking, the FAA certifies
that the rule, if promulgated, would not have a significant economic
impact, positive or negative, on a substantial number of small entities
under the criteria of the Regulatory Flexibility Act, since none would
be affected. A copy of the regulatory evaluation prepared for this
project may be examined in the Rules Docket or obtained from the person
identified under the caption FOR FURTHER INFORMATION CONTACT.
List of Subjects in 14 CFR Part 25
Air transportation, Aircraft, Aviation safety, Safety, Gusts.
The Proposed Amendments
Accordingly, the Federal Aviation Administration (FAA) proposes to
amend 14 CFR part 25 of the Federal Aviation Regulations (FAR) 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. 1344, 1354(a), 1355, 1421, 1423, 1424,
1425, 1428, 1429, 1430; 49 U.S.C. 106(g), and 49 CFR 1.47(a).
Sec. 25.305 [Amended]
2. By amending Sec. 25.305 by removing and reserving paragraph (d).
3. By amending Sec. 25.321 by adding new paragraphs (c) and (d) to
read as follows:
Sec. 25.321 General.
* * * * *
(c) Enough points on and within the boundaries of the design
envelope must be investigated to ensure that the maximum load for each
part of the airplane structure is obtained.
(d) The significant forces acting on the airplane must be placed in
equilibrium in a rational or conservative manner. The linear inertia
forces must be considered in equilibrium with the thrust and all
aerodynamic loads, while the angular (pitching) inertia forces must be
considered in equilibrium with thrust and all aerodynamic moments,
including moments due to loads on components such as tail surfaces and
nacelles. Critical thrust values in the range from zero to maximum
continuous thrust must be considered.
4. By amending Sec. 25.331 by revising the title and paragraph (a)
to read as follows, and by removing and reserving paragraph (d).
Sec. 25.331 Symmetric maneuvering conditions.
(a) Procedure. For the analysis of the maneuvering flight
conditions specified in paragraphs (b) and (c) of this section, the
following provisions apply:
(1) Where sudden displacement of a control is specified, the
assumed rate of control surface displacement may not be less than the
rate that could be applied by the pilot through the control system.
(2) In determining elevator angles and chordwise load distribution
in the maneuvering conditions of paragraph (b) and (c) of this section,
the effect of corresponding pitching velocities must be taken into
account. The in-trim and out-of-trim flight conditions specified in
Sec. 25.255 must be considered.
* * * * *
5. By amending Sec. 25.333 by revising the title and paragraph (a)
to read as follows, and by removing and reserving paragraph (c).
Sec. 25.333 Flight maneuvering envelope.
(a) General. The strength requirements must be met at each
combination of airspeed and load factor on and within the boundaries of
the representative maneuvering envelop (V-n diagram) of paragraph (b)
of this section. This envelope must also be used in determining the
airplane structural operating limitations as specified in Sec. 25.1501.
* * * * *
6. By amending Sec. 25.335 by revising paragraph (d) to read as
follows:
Sec. 25.335 Design airspeeds.
* * * * *
(d) Design speed for maximum gust intensity, VB.
(1) VB may not be less than
TP16SE94.011
where--
VS1=the 1-g stalling speed based on CNAmax with the flaps
retracted at the particular weight under consideration;
Vc=design cruise speed (knots equivalent airspeed);
Uref=the reference gust velocity (feet per second equivalent
airspeed) from Sec. 25.341(a)(5)(i);
w=average wing loading (pounds per square foot) at the particular
weight under consideration.
TP16SE94.012
=density of air (slugs/ft3);
c=mean geometric chord of the wing (feet);
g=acceleration due to gravity (ft/sec2);
a=slope of the airplane normal force coefficient curve, CNA per
radian;
(2) At altitudes where VC is limited by Mach number--
(i) VB may be chosen to provide an optimum margin between low
and high speed buffet boundaries; and,
(ii) VB need not be greater than VC.
* * * * *
7. By revising Sec. 25.341 to read as follows:
Sec. 25.341 Gust and turbulence loads.
(a) Discrete Gust Design Criteria. The airplane is assumed to be
subjected to symmetrical vertical and lateral gusts in level flight.
Limit gust loads must be determined in accordance with the following
provisions:
(1) Loads on each part of the structure must be determined by
dynamic analysis. The analysis must take into account unsteady
aerodynamic characteristics and all significant structural degrees of
freedom including rigid body motions.
(2) The shape of the gust must be:
TP16SE94.013
for 0 s 2H
where--
s = distance penetrated into the gust (feet);
Uds = the design gust velocity in equivalent airspeed specified in
subparagraph (a)(4) of this paragraph; and
H = the gust gradient which is the distance (feet) parallel to the
airplane's flight path for the gust to reach its peak velocity.
(3) A sufficient number of gust gradient distances in the range 30
feet to 350 feet must be investigated to find the critical response for
each load quantity.
(4) The design gust velocity must be:
TP16SE94.014
where--
Uref = the reference gust velocity in equivalent airspeed defined
in subparagraph (a)(5) of this paragraph.
Fg = the flight profile alleviation factor defined in subparagraph
(a)(6) of this paragraph.
(5) The following reference gust velocities apply:
(i) At the airplane design speed VC: Positive and negative
gusts with reference gust velocities of 56.0 ft/sec EAS must be
considered at sea level. The reference gust velocity may be reduced
linearly from 56.0 ft/sec EAS at sea level to 44.0 ft/sec EAS at 15000
feet. The reference gust velocity may be further reduced linearly from
44.0 ft/sec EAS at 15000 feet to 26.0 ft/sec EAS at 50000 feet.
(ii) At the airplane design speed VD: The reference gust
velocity must be 0.5 times the value obtained under
Sec. 25.341(a)(5)(i).
(6) The flight profile alleviation factor, Fg, must be
increased linearly from the sea level value to a value of 1.0 at the
maximum operating altitude defined in Sec. 25.1527. At sea level, the
flight profile alleviation factor is determined by the following
equation:
TP16SE94.015
Zmo = Maximum operating altitude defined in Sec. 25.1527.
(7) When a stability augmentation system is included in the
analysis, the effect of any significant system nonlinearities should be
accounted for when deriving limit loads from limit gust conditions.
(b) Continuous Gust Design Criteria. The dynamic response of the
airplane to vertical and lateral continuous turbulence must be taken
into account. The continuous gust design criteria of Appendix G of this
part must be used to establish the dynamic response unless more
rational criteria are shown.
8. By amending Sec. 25.343 by revising paragraph (b)(1)(ii) to read
as follows:
Sec. 25.343 Design fuel and oil loads.
(b) * * *
(1) * * *
(ii) The gust conditions of Sec. 25.341(a) but assuming 85% of the
design velocities prescribed in Sec. 25.341(a)(4).
* * * * *
9. By amending Sec. 25.345 by revising paragraphs (a) and (c) to
read as follows:
Sec. 25.345 High lift devices.
(a) If wing flaps are to be used during takeoff, approach, or
landing, at the design flap speeds established for these stages of
flight under Sec. 25.335(e) and with the wing flaps in the
corresponding positions, the airplane is assumed to be subjected to
symmetrical maneuvers and gusts. The resulting limit loads must
correspond to the conditions determined as follows:
(1) Maneuvering to a positive limit load factor of 2.0; and
(2) Positive and negative gusts of 25 ft/sec EAS acting normal to
the flight path in level flight. Gust loads resulting on each part of
the structure must be determined by rational analysis. The analysis
must take into account the unsteady aerodynamic characteristics and
rigid body motions of the aircraft. The shape of the gust must be as
described in Sec. 25.341(a)(2) except that--
Uds = 25 ft/sec EAS;
H = 12.5 c; and
c = mean geometric chord of the wing (feet).
* * * * *
(c) If flaps or other high lift devices are to be used in en route
conditions, and with flaps in the appropriate position at speeds up to
the flap design speed chosen for these conditions, the airplane is
assumed to be subjected to symmetrical maneuvers and gusts within the
range determined by--
(1) Maneuvering to a positive limit load factor as prescribed in
Sec. 25.337(b); and
(2) The discrete vertical gust criteria in Sec. 25.341(a).
* * * * *
10. By amending Sec. 25.349 by revising the introductory text and
paragraph (b) to read as follows:
Sec. 25.349 Rolling conditions.
The airplane must be designed for loads resulting from the rolling
conditions specified in paragraphs (a) and (b) of this section.
Unbalanced aerodynamic moments about the center of gravity must be
reacted in a rational or conservative manner, considering the principal
masses furnishing the reacting inertia forces.
* * * * *
(b) Unsymmetrical gusts. The airplane is assumed to be subjected to
unsymmetrical vertical gusts in level flight. The resulting limit loads
must be determined from either the wing maximum airload derived
directly from Sec. 25.341(a), or the wing maximum airload derived
indirectly from the vertical load factor calculated from
Sec. 25.341(a). It must be assumed that 100 percent of the wing air
load acts on one side of the airplane and 80 percent of the wing air
load acts on the other side.
11. By amending Sec. 25.351 by revising the introductory text and
by removing and reserving paragraph (b).
Sec. 25.351 Yawing Conditions.
The airplane must be designed for loads resulting from the
conditions specified in paragraph (a) of this section. Unbalanced
aerodynamic moments about the center of gravity must be reacted in a
rational or conservative manner considering the principal masses
furnishing the reacting inertia forces:
* * * * *
12. By revising Sec. 25.371 to read as follows:
Sec. 25.371 Gyroscopic loads.
The structure supporting the engines and the auxiliary power units
must be designed for the gyroscopic loads associated with the
conditions specified in Secs. 25.331, 25.341(a), 25.349 and 25.351 with
the engine or auxiliary power units at maximum continuous rpm.
13. By amending Sec. 25.373 by revising paragraph (a) to read as
follows:
Sec. 25.373 Speed control devices.
* * * * *
(a) The airplane must be designed for the symmetrical maneuvers
prescribed in Sec. 25.333 and Sec. 25.337, the yawing maneuvers
prescribed in Sec. 25.351, and the vertical and lateral gust conditions
prescribed in Sec. 25.341(a), at each setting and the maximum speed
associated with that setting; and
* * * * *
14. By amending Sec. 25.391 by revising the introductory text and
paragraph (e) to read as follows:
Sec. 25.391 Control surface loads: general.
The control surfaces must be designed for the limit loads resulting
from the flight conditions in Secs. 25.331, 25.341(a), 25.349 and
25.351 and the ground gust conditions in Sec. 25.415, considering the
requirements for--
* * * * *
(e) Auxiliary aerodynamic surfaces, in Sec. 25.445.
15. By revising Sec. 25.427 to read as follows:
Sec. 25.427 Unsymmetrical loads.
(a) In designing the airplane for lateral gust, yaw maneuver and
roll maneuver conditions, account must be taken of unsymmetrical loads
on the empennage arising from effects such as slipstream and
aerodynamic interference with the wing, vertical fin and other
aerodynamic surfaces.
(b) The horizontal tail must be assumed to be subjected to
unsymmetrical loading conditions determined as follows:
(1) 100 percent of the maximum loading from the symmetrical
maneuver conditions of Sec. 25.331 and the vertical gust conditions of
Sec. 25.341(a) acting separately on the surface on one side of the
plane of symmetry; and
(2) 80 percent of these loadings acting on the other side.
(c) For empennage arrangements where the horizontal tail surfaces
have dihedral angles greater than plus or minus 10 degrees, or are
supported by the vertical tail surfaces, the surfaces and the
supporting structure must be designed for gust velocities specified in
Sec. 25.341(a) acting in any orientation at right angles to the flight
path.
(d) Unsymmetrical loading on the empennage arising from buffet
conditions of Sec. 25.305(e) must be taken into account.
16. By amending Sec. 25.445 by changing the title and revising
paragraph (a) to read as follows:
Sec. 25.445 Auxiliary aerodynamic surfaces.
(a) When significant, the aerodynamic influence between auxiliary
aerodynamic surfaces, such as outboard fins and winglets, and their
supporting aerodynamic surfaces, must be taken into account for all
loading conditions including pitch, roll, and yaw maneuvers, and gusts
as specified in Sec. 25.341(a) acting at any orientation at right
angles to the flight path.
* * * * *
17. By amending Sec. 25.571 by revising paragraphs (b)(2) and (3)
to read as follows:
Sec. 25.571 Damage-tolerance and fatigue evaluation of structure.
* * * * *
(b) * * *
(2) The limit gust conditions specified in Sec. 25.341 at the
specified speeds up to VC and in Sec. 25.345.
(3) The limit rolling conditions specified in Sec. 25.349 and the
limit unsymmetrical conditions specified in Secs. 25.367 and 25.427 (a)
through (c), at speeds up to VC.
* * * * *
18. By adding a new Sec. 25.1517 to read as follows:
Sec. 25.1517 Rough air speed, VRA.
A rough air speed, VRA, for use as the recommended turbulence
penetration airspeed in Sec. 25.1585(a)(8), must be established,
which--
(1) is not greater than the design airspeed for maximum gust
intensity, selected for VB; and
(2) is not less than the minimum value of VB specified in
Sec. 25.335(d); and
(3) is sufficiently less than VMO to ensure that likely speed
variation during rough air encounters will not cause the overspeed
warning to operate too frequently. In the absence of a rational
investigation substantiating the use of other values, VRA must be
less than VMO--35 knots (TAS).
Issued in Washington, DC, on September 8, 1994.
Thomas E. McSweeny,
Director, Aircraft Certification Service.
[FR Doc. 94-22903 Filed 9-15-94; 8:45 am]
BILLING CODE 4910-13-M
