[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