[Federal Register Volume 61, Number 28 (Friday, February 9, 1996)]
[Rules and Regulations]
[Pages 5218-5222]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 96-2633]
[[Page 5217]]
_______________________________________________________________________
Part V
Department of Transportation
_______________________________________________________________________
Federal Aviation Administration
_______________________________________________________________________
14 CFR Part 25
Revised Discrete Gust Load Design Requirements; Final Rule
Federal Register / Vol. 61, No. 28 / Friday, February 9, 1996 / Rules
and Regulations
[[Page 5218]]
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. 27902; Amdt. No. 25-86]
RIN 2120-AF27
Revised Discrete Gust Load Design Requirements
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Final rule.
-----------------------------------------------------------------------
SUMMARY: This amendment revises the gust load design requirements for
transport category airplanes. This amendment replaces the current
discrete gust requirement with a new requirement for a discrete tuned
gust; modifies the method of establishing the design airspeed for
maximum gust intensity; and provides for an operational rough air
speed. These changes are made in order to provide a more rational basis
of accounting for the aerodynamic and structural dynamic
characteristics of the airplane. These changes also provide for
harmonization of the discrete gust requirements with the Joint Aviation
Requirements (JAR) of Europe as recently amended.
EFFECTIVE DATE: March 11, 1996.
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:
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
[[Page 5219]]
government structural loads specialists of Europe, the United States,
and Canada was chartered by notice in the Federal Register (58 FR
13819, March 15, 1993) to harmonize certain specific sections of part
25, including the requirements related to discrete gusts. The
harmonization task concerning discrete gusts was completed by the
working group and recommendations were submitted to FAA by letter dated
October 15, 1993. The FAA concurred with the recommendations and
proposed them in Notice of Proposed Rulemaking (NPRM) No. 94-29 which
was published in the Federal Register on September 16, 1994, (59 FR
47756).
Discussion of Comments
Comments were received from domestic and foreign aviation
manufacturers and foreign airworthiness authorities. The majority of
the commenters agreed with the proposal and recommended its adoption.
However, some commenters disagreed substantially with the proposal
while providing alternative proposals that appeared to merit further
consideration by the Aviation Rulemaking Advisory Committee. Therefore
the FAA tasked the ARAC Loads and Dynamics Working Group by notice in
the Federal Register (60 FR 18874, April 13, 1995) 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 comments that follows is based on the recommendation
submitted to the FAA by ARAC on July 14, 1995.
One commenter suggests that the new method for calculating the
minimum VB results in lower values at altitude than the current
method provided in the Joint Aviation Requirements (JAR) and could
provide unrealistic margins above the stalling speed. The FAA
disagrees. The commenter provides no data or other information that
shows the new VB calculations to be unrealistic. The new method
for calculating the minimum VB is approximately the same as in the
current FAR and JAR; the main difference being that revised gust speeds
are used in the calculation. These gust speeds are based on actual
measurements in aircraft operation and are considered to result in a
realistic and conservative VB speed, even if it is somewhat lower
than the current requirements at some altitudes. In addition, a new
operational rough air speed, VRA, is provided in order to ensure
adequate stall margins while operating in rough air. As part of the
effort to harmonize the airworthiness requirements, the JAA is also
considering adopting this method of calculating the minimum VB
speeds. This commenter, along with several other, also points out an
error in the formula for the design speed for maximum gust intensity,
VB, in Sec. 25.335(d) and this error has been corrected.
One commenter suggests that the proposed tuned gust criteria do not
fully account for the dynamic response of the airplane and therefore
could produce unconservative results and seriously underpredict the
gust design loads. The commenter suggests that the proposal be replaced
by an entirely new method of accounting for discrete gusts. This method
is known in the industry as the statistical discrete gust method (SDG).
In response to the task defined in the Federal Register, the ARAC Loads
and Dynamics Working Group considered the commenters comments and the
alternate proposal in considerable detail. It is recognized by the
working group that the current proposed tuned gust criteria have some
limitations and that the suggested SDG method may have some promising
applications for predicting gust loads. However, the SDG method is in a
developmental stage, and there is currently no established industry
process for using this method in predicting gust design loads. The FAA
will retain the commenters proposal for possible consideration in
future rulemaking actions. In response to the commenters specific
concerns, neither ARAC nor the FAA agree that the tuned gust method
will result in unconservative design loads. In addition, for the
extreme gust gradient distances where the commenter questions the
adequacy of the tuned gust method to fully account for dynamic
response, the FAA considers that the additional continuous gust
criteria of Sec. 25.341(b) will compensate for any possible
deficiencies. The commenter provides some comparisons of loads produced
by the SDG method with the results of the proposed tuned gust method.
These results show no significant differences in overall load levels
when all factors are considered, and in some cases the SDG method
actually provided lower design loads. Therefore, except for an
editorial correction to the mathematical equation noted above, the
amendment is adopted as proposed.
Regulatory Evaluation Summary
Regulatory Evaluation, Regulatory Flexibility Determination, and Trade
Impact Assessment
Changes to federal regulations must undergo several economic
analyses. First, Executive Order 12866 directs Federal agencies to
promulgate new regulations or modify existing regulations only if the
potential benefits to society justify its costs. Second, the Regulatory
Flexibility Act of 1980 requires agencies to analyze the economic
impact of regulatory changes on small entities. Finally, the Office of
Management and Budget directs agencies to assess the effects of
regulatory changes on international trade. In conducting these
assessments, the FAA has determined that this rule: (1) will generate
benefits exceeding its costs and is not ``significant'' as defined in
Executive Order 12866; (2) is not ``significant'' as defined in DOT's
Policies and Procedures; (3) will not have a significant impact on a
substantial number of small entities; and (4) will not constitute a
barrier to international trade. These analyses, available in the
docket, are summarized below.
Costs and Benefits
The changes will have economic consequences. The costs will be the
incremental costs of meeting the tuned discrete gust requirements
rather than the current static discrete gust requirements. The benefits
will be the cost 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.
Industry sources provided information on the additional costs and
cost savings that would result from the 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 final rule.
Per Certification Costs and Savings Associated With Revised Discrete
Gust Load Requirements
[in thousands of dollars]
Current FAA certification requirement costs............. $29-$115
Current JAA certification requirement costs............. $70-$145
Current joint certification requirement costs........... $100-$150
Revised FAA certification requirement costs............. $70-$145
[[Page 5220]]
Revised joint certification requirement costs........... $70-$145
Savings (current joint certification costs minus revised
joint certification costs)............................. $5-$30
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
harmonized requirements will outweigh the expected incremental costs of
the rule. The FAA did not receive comments concerning this
quantification of costs during the comment period; therefore, the FAA
holds that these are representative costs and savings.
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
agencies to review rules which may 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 final rule will
not have a significant economic impact on a substantial number of small
entities.
International Trade Impact Assessment
The rule will 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 will harmonize
with those of the JAA and will, 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 level 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 promoted 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
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 fro 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 Amendments
In consideration of the foregoing, the Federal Aviation
Administration (FAA) amends 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 is revised to read as
follows:
Authority: 49 U.S.C. 106(g), 40113, 44701, 44702 and 44704.
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)
introductory text, by removing paragraphs (a) (1) and (2) and
redesignating paragraphs (a) (3) and (4) as (a) (1) and (2)
respectively and revising them to read as set forth below, and by
removing 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 paragraphs (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 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 envelope (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
[[Page 5221]]
[GRAPHIC] [TIFF OMMITTED] TR09FE96.016
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.
[GRAPHIC] [TIFF OMMITTED] TR09FE96.017
=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 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:
[GRAPHIC] [TIFF OMMITTED] TR09FE96.018
for 0 s 2H
where--
s=distance penetrated into the gust (feet);
Uds=the design gust velocity in equivalent airspeed specified in
paragraph (a)(4) of this section; 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:
[GRAPHIC] [TIFF OMMITTED] TR09FE96.019
where--
Uref=the reference gust velocity in equivalent airspeed defined in
paragraph (a)(5) of this section.
Fg=the flight profile alleviation factor defined in paragraph
(a)(6) of this section.
(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:
[GRAPHIC] [TIFF OMMITTED] TR09FE96.020
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.
(a) * * *
(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).
(b) * * *
(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:
[[Page 5222]]
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 reaching inertia fores.
(a) * * *
(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 later 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 revising 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
(b)(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 February 2, 1996.
David R. Hinson,
Administrator.
[FR Doc. 96-2633 Filed 2-8-96; 8:45 am]
BILLING CODE 4910-13-M
