[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