Revised Discrete Gust Load Design Requirements; Proposed Rule DEPARTMENT OF TRANSPORTATION

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