### Federal Aviation Administration Aviation Rulemaking Advisory Committee Transport Airplane and Engine Issue Area Loads and Dynamics Harmonization Working Group Task 5 – Continuous Turbulence Loads

## Task AssignmentFederal Register I Vol. 59. No. 111 I Friday. fune 10. 1994 I Notices 30081 T",ks

TheLoads and Dynamics Harmonization Working Group's tasks ( as follows: Task J-lnteroction a/Systemsand tructure: Review existing special

________________ conditionsforfly

-by

-wire airplanes and Aviation Rulemaking Advi50ry Committee; Transport**Alrp~e** and Engine Issues
ACENCY: Federal Aviation Administration (FAA)

.DOT. ACT1ON: Notice ofnewtask assigrunents for the Aviation RulemalingAdviSOry Committee. SU

.... ARY: Notice is given ofnewtask assigrunents for theLoadsand Dynamics Harmonization Working Group ofthaAviation Rulemaking Advisory Committee (ARAC). This notice inforrr.s the public ofthe activities oftheAR.-\G. FOR FURTMERINFORMATlON CONTACT:**Mich~el** H. Borfitz. Assistant Executive Director. Aviation Rulemaking Advisory Committee.TransportAirplane and Engine Issues. FAA Engine" Propeller Directorate. 12NewEngland Executive Park. Burlington.Massachusetts01803; telephone (617) 238-7110. fax (617) 238-7 199. SUPPLEMENTARY IHFORMATIOH: On January 22. 1991 (56 FR 2190). the Federal Aviation Administrnlion (FAA)**e~tahlished** the Aviation Rulemaking Addsory Committee (AR.o\C). The comrr.ittee provides ad\'iceand rt"commendationsto theFAA Administra!or.through the Associate Administrator for Regulation and Certification. onthefull range olthe FAA's rulerr.aking activitias with respect to aviation·related issues

..

In orderto develop such advice and recommendations.thehRAC may choose to establish working groups to which specifictasksare aSSigned. Such working groups are comprisedof expertsfrom those organizations having an interest in the aSSigned taJO:ks. A workinggroup member need not bea representative ofthefull committee. One ofthe working groupsestablished by the ARAC is theLoads and Dynamics HannonizalionWorking Group.

TheF.-\A announced at theJoint Aviation Authorities (JAA)·Federal Aviation Administration (FAA) Harmonization Conferencein Toronto. Canrida June 2-5. 1992

.that it would l:onsolidate within the ARAC struclure an ongoingobjectiveto "hannonize" the Jo:nt Aviation Requirements (JAR) and the Federal Avio:tion Regulations (FAR). existingrequirements forcontrol systems. includingautomatic andlor power-operated systems, and recommend to theARACany new revised. generalrequirements needed for flight control systems and structures affected by those systems (§S 25.302. 25.671.25.1329.part 25 appendix K).

Task2--Continuous Turbulence Loads: Review therequirement for the continuousturbulence standard in light ofthe ARAC proposal for a tuned discrete gust requirement inorderto determinewhetherthecontinuous turbulencerequirement shouldbe revised orremoved from theFAR/JAR for betterconsistencywiththe new proposedtuned discrete gust criteria

, (§ 25.305(d)). Task 3-Strength ondDeformotion: Review the recent requirements adopted in theFAR byAmendment 25-71 (for the design oftransportairplanesagainst buffet and forced structuralvibrations) and consider appropriate changes for theJAR and FAR to harmonizethese rules (§S25.305 (e) and (01.

Tosk 4-DesignFlop Speeds: Review the current flap design loads requirementstoresolve differences in interpretationbetween theFAA and JMconcerningthe structuraldesign stall speedson whichtheflap design speedsarebased. Recenl measurements ofgust speedsal lowaltitudes. where flaps are normally extended. indicate a more severe gustenvironment may be present. Review all aspects oftheflap design load requirements.includingthe design airspeeds. vertical andhead-on design gust criteria, and the effects of automatic retraction and load relief systems (§25 .335(e)). Task5-ResiduaJStrfmgth Loadsfor Damage Tolerance:Review the differences inresidual strength design load requirementsbetween theFAR and JAR and resolvedifferences to harmonizethisrule. Preparea Notice of Proposed Rulemakingor make recommendationsto otherARAC efforts concerningFAR §25.571, so that they can be included inrulemakingthat may be forthcoming from thoseefforts (§2S.571(b)).

Tosk 6-ShockA.bsorption Tesfs: Review thechanges recently introdured intotheJAR thathave resulted in differe' ncesbetween theFAR andJAR in regard to therequirementfor shock absorption tests. Review those r.banges in view ofhannonizingthe FAR and JAR (§ 25.723(a)).

Task 7-RoughAirSpeed: TheAMC has proposed a new §25.1517 concerningrough air speeddesign standardsin its proposal for a tuned discretegust requirement. This action is hannonized with thecurrentJAR 25.1517; however. further changes in therough air speedrequirement may be needed in boththeFARand IAR

. ReviewJAR 25.1517 andthe new proposed FAR 25.1517 to determine if furtherchangesare needed. Ifso. preparea Notice ofProposed Rulemaking. or. ifpossible. comhine thesechangeswith otherrule'making efforts (§ 25.1517).

Task s-T axi. Takeoff. andwnding RoU:Prepare an advisory circularthat establishescriteria that may be used to r.alculate rough runway and taxiway loads. as required by

## s§ 25.491. 2S.235. and 25.305.

Task 9-BrokedRoll Conditions: Reviewthe provisions of§ 25.493 ofthe FAR andJAR concerningthebraked roll condition and finalize a harmonized NoticenfProposed Rulemaking. Reports

Foreach task listed.theLoads and Dynamics Harmonization Working Group should develop and present to the ARAC: 1.A recommended workplan for completion ofthetask. includingthe rationale supporting such plan. for consideration at the meeting ofthe ARAC toconsidertransport airplane and engineissuesheld following publication ofthis notice;

2. A detailed conceptual presenttltion on theproposedrecommendalion(JO:). priorto proceeding with the work sttlted in item 3. below;

3. A draft NoticeofProposed Rulemaking. with supportingeconomic and otherrequired analyses. andlofany other related guidance malerial or collateral documentsthe workinggfOUp determinesto be appropriate; or. ifnew orrevised requirementsorcompliance methods am notrecommended. a draft report statingthe rationale [or not makingsuch recommendations;and

4. A statusreport at each meetingof the ARAC held to considertransport airplane and engine issues. ParttcipatioD in WorkingGroupTask

An individual whohuexpertise in

. the subject maHerand wishes tobecome a memberofthe working group should write to the periOn listed underthe caption FOR FURTHER INFOR

.. ATION CONTACT expressingthat desire. descrihing his orherinterest in the**task(~).** and Slating the expertise he orshe would bringtothe workinggroup. The request will be reviewed with the assistant chairman and workinggroup leader. and the individual willbe advised whether or not the request can be attornmodated.

TheSecretaryofTransportation has determinedthat theinform.lion anduse oftheAviation Rulemaking Advisory Committee are necessary in thepublic interest in connection with the performanceofduties imposed onthe FAA by law. Meetings orthaAviation Rulemaking Advisory Committee will be open tothepublic. except as authorizedby section 10(d} orthe Federal Advisory Committee Act. Meetings ofthe workinggroup will not be open tothe public. except tothe extentthat individualswith 8n interest and expertise are selected to participate

. No publicannouncement ofworking group meetings will be made.

Issued in Washington. OC,on J une 3, lw..

. 0u-i5A. ChrUI:ie.**£JI«uliv~**DirK101". "'vialion**Ru/~mo ki** **nB** Advisor}'Commitlee-. IFR Doc. 94-14147 Filed**6-~94 ;**  8:45 am)

....

U«i COOl**"'~ll-111**

## Recommendation[4910-13] Revised 9/2211999Draft Continuous Turbulence NPRM June 24, 1999 Revised 25.341(c) number headings 9/3/99

DEPARTMENTOFTRANSPORTATION FederalAviation Administration [14 CFR

### part 25] [Docket No. ; Notice No. RIN: Revised Requirements for Gust and Continuous Turbulence DesignLoads AGENCY: Federal Aviation Administration, DOT. ACTION: Notice ofproposed rulemaking. SUMMARY: This notice proposes to revise the continuousturbulence design loads ofthe Federal Aviation Regulations (FAR) for transport category airplanes by incorporating changes developed in co-operationwith the Joint Aviation Authorities (JAA) ofEurope and the U.S., Canadian and European aviation industries through the Aviation Rulemaking Advisory Committee· (ARAC). This action is necessary because recent measurements ofderived gust intensities in actual operation showthat the current requirements do not accurately account for the distribution ofturbulence in the atmosphere. Also, one ofthe optional methodologies for treating continuous turbulence (i.e. mission analysis) in the current rule is eliminated since it is overly sensitive to small changes in the definition ofaircraft mission. In addition to these issues regarding continuousturbulence, The National Transportation Safety Board (NTSB) has provided a Safety Recommentation, A-93-137 which raises concerns the potential for combined vertical and lateral discrete gusts. This proposal is intended to improve the requirements for continuousturbulence by revising theturbulence intensity criteria, eliminating the mission analysis method, providing a multi-axis discrete gust criterion, and reorganizing and clarifying the rule. DATES: Comments must be received on orbefore [insert a date 120 days afterthe date of publication in theFederal Register] ADDRESSES: Comments onthis notice may be mailed in triplicate to: Federal Aviation Administration (FAA), Office ofthe ChiefCounsel, Attention: Rules Docket (AGC-10), Docket No. ,800Independence Avenue SW., Washington, DC 20591; ordelivered in triplicate to: Room 915G, 800 Independence Avenue SW., Washington, DC 20591. Comments delivered must bemarked DocketNo.

. . Comments may also be submitted electronically toRevised 9/22/l9990raftContinuous Turbulence NPRM June 24, 1999 Revised 25.341(c) number headings 9/3/99 

### nprmcmts@mail.hq.faa.gov. Comments may be examined in Room 915G

### weekdays, except Federal holidays, between 8:30 

### a.m. and 5:00 

### p.m. In addition, the FAA is maintaining an information docket ofcomments in the Transport Airplane Directorate (ANM-100), 

### FAA, 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 4:00 

### p.m. 

### FORFURTHERINFORMATION 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) 227-213l. 

### 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 ornotice number and submit comments in triplicate to the Rules Docket address above. All comments received on orbefore the closing date for comments will be considered by the Administratorbefore taking action on this proposed rulemaking. The proposals contained in this notice may be changed in light ofcomments 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. Areport summarizing each substantive public contact with FAA personnel concerning this rulemaking will be filed in the docket. Persons wishing the FAA to acknowledge receipt oftheir comments must submit with those comments a self-addressed, stamped postcard on which the following statement is made: "Comments to DocketNo. ." The postcard will be date/time stamped and returned to the commenter. 

### Availability ofNPRM An electronic copy ofthis document may be downloaded using amodem and suitable communications software from the FAA regulations section ofthe Fedworld electronic bulletin board service (telephone: 703-321-3339), 

### the**Fede~al** 

### Register's electronic bulletin board service (telephone: 202-512-1661), 

### orthe FAA's Aviation Rulemaking Advisory CommitteeBulletin Board service (telephone: 202-267-5984).

2Revised 9122/l999Draft Continuous Turbulence NPRM June 24, 1999 Revised 25.341(c) number headings 9/3/99 Internet users may reach the FAA's web page at http://www.faa.govv orthe Federal Register's web page at http://www.access.gpo/su_docs for access to recently published rulemaking documents. Any personmay obtain a copy ofthis notice by submitting a request to the Federal Aviation Administration, Office Rulemaking, ARM-I, 800 Independence Avenue SW., Washington, DC 20591; orby calling (202) 267-9680. Communications must identify the notice number ofthis NPRM. Persons interested in being placed on a mailing list for future rulemaking documents should also request a copy ofAdvisory CircularNo. 11-2A, Notice ofProposed Rulemaking Distribution System, that describes the application procedures. Background The manufacturing, marketing and certification oftransport airplanes is increasingly an international endeavor. In orderfor U. S. Manufacturers to export transport airplanes to other countries the airplane must be designed to comply, not only with the U.S. airworthiness requirements for transport airplanes (14 CFR part 25), but also with the airworthiness requirements ofthe countries to which the airplane is to be exported. TheEuropean countries have developed a common airworthiness code for transport airplanes that is administered by theJoint Aviation Authorities (JAA) ofEurope. This code is the result ofa European effort to harmonize the various airworthiness codes oftheEuropean countries and is called theJoint Aviation Requirements (JAR)-25. Itwas developed in a format similarto part 25. Many other countries have airworthiness codesthat are aligned closely to part 25 orto JAR-25, orthey use these codes directly for their own certification purposes. Since 1988, theFAAand JAA have been working toward complete harmonization ofJAR-25 and part 25. TheAviation Rulemaking Advisory Committee (ARAC) was established by theFAAon February 15, 1991, withthe purpose ofproviding information, advice, and recommendations to beconsidered in rulemaking activities. The FAA and JAA are continuing to worktoward the harmonization ofJAR-25 and part 25 by assigning ARAC specific tasks

.. Bynotice in theFederal Register (59FR30081, June 10, 1994), the FAA assigned several newtasks to an ARAC workinggroup ofindustry and government structural loads specialists from Europe, the United States, and Canada. Task 2 ofthis charter concerned the requirement to account for continuous

3Revised 9122/1999Draft Continuous Turbulence NPRM June 24, 1999 Revised 25.341(c) number headings 9/3/99 turbulence loads. The assigned task was to review the current requirement for continuous turbulence in part 25 and JAR-25 in light ofrecent revisions to the discrete gust requirement of Amendment 25-86 (61 FR5218) in orderto determine ifthe continuous turbulence requirement
was still needed and ifit was in need ofrevision to be consistent with the new discrete gust requirement of§ 25.341(a). The ARAC Loads and Dynamics Harmonization working group has completed its work for this task and has made recommendations to the FAAby letter dated

The current requirement to account for the loads producedby continuousturbulence (sometimes referred to as continuous gusts) was proposedby theFAAin Notice 68-18 (33 FR 11913, August 22, 1968). This proposal was the culmination ofa research effort by theU.S. aviation industryunder a contract by the FAAto develop methods for treating loads resulting from flight in continuous turbulence. The rules in effect at that time required only the consideration ofthe response ofthe airplane to discrete gusts. The FAA stated in Notice 68-18 that the discrete gust requirement accounted for the flexibility ofthe airplane but not necessarily the combination ofelastic and rigid body motions. The basic objective ofthe FAA sponsored research effort was to develop methods ofaccounting for continuous turbulence loads by considering the statistical nature ofturbulence in combination with boththe elastic and rigid body modes ofthe airplane. The results ofthat effort were published in FAATechnical Reports ADS-53 and ADS-54 in 1966. Subsequently the FAA amended part 25 to requirethe consideration ofloads arising from continuous turbulence (Amendment 25-23, 35 FR 5665, April 8, 1970). Amendment 25-23 added a new paragraph, 

### § 25.305(d), that required the dynamic response ofthe airplane to continuous turbulence be taken into account. No methodology or advisory material were provided for showing compliance, however, FAA Reports ADS-53 and ADS-54 suggested two methods in use by aircraft manufacturers. These methods were considered acceptable by FAA. Later, in 1975, the FAA proposed these methods as means of compliance in an Appendix to part 25 (Notice 75-27, 40 FR24802, March 7, 1975). TheFAA subsequently amended part 25 by adding appendix G (Amendment 25-54, 45 FR60154, September 11, 1980) that set forth the two methodologies (design envelope and mission analysis) and specified the levels ofrequired gust intensities for use in design. Section 25.305(d) was also

4Revised 9122/1999Draft Continuous Turbulence NPRM June 24, 1999 Revised 25.34l(c) number headings 9/3/99 changedby amendment 25-54 to require that the criteria presented in Appendix G be used unless

more rational criteria were shown. The gust intensities provided for use with the design envelope method have been the subject ofcontention and debate since the publication ofthe proposal for Appendix G. Several commentersto that proposal objected to the proposed Appendix G, stating that the atmospheric model was not yet sufficiently defined and that the analyses techniques were still developing. The FAArecognized these shortcomings but, in the interest ofsafety, decided to go ahead with the requirement with the intention ofrefining the criteria as more information became available. The requirement provided a sea level value for gust intensity of85 fps for the design envelope method, however, this could be reduced to 75 fps by using a comparison with a dynamically similar model in which 75 fps is shown to be adequateby service experience. Thephrase "dynamically similar model" has been subject to a wide range ofinterpretations and has resulted in non uniform application ofthe rule. 

### In addition, the concept ofadjustingthe gust intensitybased on dynamic similarity with another airplane is questionable since the need for a different gust intensity is related more to the intended operation ofthe airplane, ratherthan its dynamic characteristics. The alternative mission analysis method has also been the subject ofconsiderable debate and controversy. With this method, the manufacturer must define a mission for the airplane which includes range, altitude, payload and otheroperational variables. Then, using a statistical model ofthe atmosphere, the manufacturer must showthat the design strength will not be exceeded, within a certain probability, during the airplane operational life. Predicting the mission is not always reliable since missions can change afterthe airplane goes into operation. Furthermore, the mission analysis design loads are sensitive to small changes in the definition ofthe aircraft mission. Therefore, small variations in approach can provide inconsistent results. Additional shortcomings in the current continuous turbulence requirement have been broughtto light by experience in applying the current criteria, experience in service, and by the changing design features oftransport airplanes. Manytransport airplanes now incorporate automatic flight control systems and otherfeatures that can result in significant non-linearity's while the methodology normally employed for continuous turbulence is inherently linear. Efforts to betterdefine the atmospheric model have continued since the adoption of Appendix G. Recent flight measurement programs conducted by FAA and theNational

5Revised 912211999Draft Continuous Turbulence NPRM June 24, 1999 Revised 25.34l(c) number headings 9/3/99 Aeronautics and Space Administration (NASA) have been aimed at utilizing measurements from the digital flight datarecorders (DFDR) to derive gust load design information for airline transport airplanes. The Civil Aviation Authority (CAA) ofthe United Kingdom has conducted a comprehensive DFDRgust measurement programfor transport airplanes in airline service. The program, called CAADRP (Civil Aircraft Airworthiness DataRecording Program), has resulted in
an extensive collection ofreliable gust data which has provided an improved insight into the distribution ofgusts in the atmosphere. Recently, the regulatory authorities and the aviation industries ofthe U.S., Canada and Europe have engaged in studies with the aim offinding a single gust design methodology that would account for both discrete gust and continuous turbulence. Although several promising methods are still under study, no single method is considered tobe sufficient, at this time, for treating both phenomena. The FAAbelieves that it is necessaryto proceed with the improvement. and harmonization ofthe current gust criteria for both safety and economic reasons. Therefore, ARAC has proceeded with developing harmonized improvements to the continuous turbulence and discrete gust design load conditions as separate requirements. The FAArecently revised 

### §25.341 ofthe part 25 (Amendment 25-86,61 FR5218, dated February 9, 1996 ) to provide a revised discrete gust methodology along with a refined gust distribution model ofthe atmosphere based onthe CAADRP data. These criteria were set forth in paragraph (a) of

### § 25.341. The continuous turbulence requirement was moved, without change, from 

### §25.305(d) to 

### §25.341(b) so that all the gust design criteria, including continuous turbulence, would be specified in the same section ofpart 25. ARAC believes, and the FAA agrees, that a continuous turbulence criterion is still needed in addition to the discrete gust criterion since it accounts for the response to totally different, but still realistic, atmospheric characteristics. However, it is recognized that the currentturbulence intensity model is inconsistent with the CAADRP data, and with the new atmospheric model prescribed for discrete gusts, and is in need ofupdating to accommodate modern transport airplanes. Discussion

6Revised 9/22/19990raft Continuous Turbulence NPRM June 24, 1999 Revised 25.34I(c) number headings 9/3/99 The proposed requirement includes a revision to the gust intensity model used in the design envelope method for continuousturbulence, elimination ofthe mission analysis method, provisions for treating non-linearities, and reorganization and clarification ofthe requirement. TheFAAproposesto retain the design envelope criterion, but with a revised gust intensity distribution with altitude. The proposed gust intensities are based on analysis ofgust measurements from the CAADRP program. The CAADRP data is the most recent gust information available and it represents measurements ofgusts and turbulence ontransport airplanes in actual operation. In addition, the flight profile alleviation factor already defined for the discrete gust in 

### §25.341(a) as amended (Amendment 25-86,61 FR 5218, February 9, 1996) would be used to adjust the gust intensity distribution according to certain aircraft parameters that relate to the intended use ofthe airplane. The FAA considers this to be a reliable means of accounting for airplane mission and it would be capable ofbeing applied in a uniform manner. One member ofthe ARAC Working Group objected to the definition ofa flight profile alleviation factor that changes the design turbulence intensity versus altitude based on selected aircraft design parameters. That memberbelieved that the once in 70,000 hourgust represented
an acceptable level ofturbulence for design purposes. He accepted that the intensity ofthe 70,000 hourgust properly varies with altitude; but he believed the probability ofencountering a gust of that intensity at any point in time should be constant, regardless ofthe design parameters ofa particular aircraft. The majority ofthe ARAC Working Group disagreed. In their view the proposal does not
assumethat atmospheric turbulence is dependent upon aircraft speed and altitude, orany other aircraft design parameter. The flight profile alleviation factor is simply a mathematical devicethat allows the expected operation ofthe airplane to be taken into account by introducing multiplying factors, based on fuel loading and maximum operating altitude, that adjust the required design turbulence intensities. Theflight profile alleviation factor in this proposal is identical in magnitude and effect to that used in the discrete gust requirements of

### §25.341(a) (as amended by Amendment 25-86,61 FR5218, February 9, 1996). To supportthis proposal, an effort has been undertakenby the industries and airworthiness authorities ofthe United States, Canada and Europeto evaluate the new proposed criteria and ensure that they are adequate for current conventional transport airplanes as well as for new technology airplanes that may include systems

7Revised 9122/l999Draft Continuous Turbulence NPRM June 24, 1999 Revised 25.341(c) number headings 9/3/99 that react in a non-linear manner. Furthermore, the proposed design turbulence intensity distributions are believed torepresent the best available measurements ofthe turbulence environment in which the airplane is likely tobe operated. The mission analysis method for accounting for continuous turbulence loads would be eliminated as an option since the use ofthis method can provide inconsistent results depending on the assumptions made concerningthe potential use ofthe airplane. The elimination ofthis method would not be significant since few manufacturers currentlyuse it as the primary means of addressing continuous turbulence. In addition, the mission would be taken into account in the proposed design envelope criterion, since a flight profile alleviation factor is provided as discussed above. The introduction ofadvanced flight control systems into transport airplanes has presented special problems in the treatment ofcontinuous turbulence. Some ofthese systems can exhibit significant non-linearities, while the standard mathematical approaches to continuousturbulence (i.e. frequency domain solutions) are valid only for linear systems. The current rule requires consideration ofnon-linearities only in relation to stability augmentation systems, however, with modern transport airplanes it is possible that the primary flight control systems and the airplane itselfcould exhibit significant non-linearities. The proposed rule would require that any significant non-linearity be considered in a realistic orconservative manner, and it would provide additional criteria which canbe used with otherrational approaches that can account for nonlinearities (e.g. time domain solutions). The elimination ofthe mission analysis criterion would simplify the presentation ofthe continuous turbulence requirement so that the requirement can be conveniently presented directly in Subpart C ratherthan in Appendix G. Appendix G would be eliminated and the continuous turbulence requirement would be set forth, with some reorganization and clarification, in paragraph (b) of§ 25.341 "Gust and turbulence loads". Following an accident in which an airplane shed a large wing mounted nacelle, the National Transportation Safety Board (NTSB) recommended (Safety Recommendation A-93- 137, November 15, 1993) that the FAA should amend the design load requirements to consider multiple axis loads encountered during severe turbulence. This recommendation was specifically addressed at gust loads onwing-mounted engines. Although the FAAbelieves that the existing

8Revised 9/221l999Draft Continuous Turbulence NPRM June 24. 1999 Revised 25.34l(c) number headings 9/3/99 designs are adequate and that the current existing discrete gust and the proposed continuous tl:lrbl:llenee**~criteria** have already been improved to the point that they should be adequatefor current and future configurations, there remains a possibility that a multi-axis gust encounter could producehigher loads under certain situations. To address theNTSB concern, the FAA contracted an independent organization to develop a method ofperforming multiaxis discrete gust analysis for wing mounted nacelles. The results ofthat study were reported to FAA in Stirling Dynamics Labratories Report No SDL -571-TR-2 dated May 1999. The recommendations of that report were accepted by ARAC and the FAA and are set forth in this proposal. The proposal addresses the NTSB recommendation by prescribing two dynamic gust criteria for airplanes with wing mounted engines. These are a round-the-clock discrete gust criterion and a multi-axis dual discrete gust criterion. These criteria are set forth in a new paragraph 25.341(c). Thecurrent

_ 

### §25.445 already requires the effects ofcombined gust loading to be considered on auxiliary aerodynamic surfaces such as outboard fins and winglets. Furthermore, the current 

### § 25.427(c) requiresthe effects ofcombined gust loading to be considered on some empennage arrangements such as T-tails. Forairplanes with wing mounted engines, this proposal would extend the round the clock dynamic discrete gust criterion towing mounted nacelles and provide an additional multi-axis dynamic discrete gust criterion. These criteria, set forth in § 25.341(c), would be applied as airplane dynamic conditions although the assessment would be limited tothe engine mounts, pylons and wing supporting structure. Section 25.571, "Damagetolerance and fatigue evaluation ofstructure", currently references the entire section 25.341 as one source ofresidual strength loads for the damage tolerance assessment. No changes are proposed for this reference to 

### §25.341, so the additional gust loads derived from the new 

### § 25.341(c) would be included in the damage tolerance assessment required by 

### §25.571. Some current part 25 airplanes have maximum certified operating altitudes up to 51,000 feet. To be fully applicable to these, and future part 25 airplanes, this proposal defines gust intensities for all altitudes up to 60,000 feet. This is inconsistent with the discrete gust requirements of

### §25.341(a) (as amended by Amendment 25-86,61 FR5218, February 9, 1996), that define the discrete gust velocities at altitudes up to 50,000 feet only. Therefore, as a conforming change, it is proposed to amend 

### § 25.341(a)(5)(i) to define discrete gust velocities up

9Revised 9/22/I999Draft Continuous Turbulence NPRM June 24. 1999 Revised 25.341(c) number headings 9/3/99
to 60,000 feet, thereby achieving consistency between discrete gust and continuous turbulence criteria.With the adoption ofthe discrete gust in 

### § 25.341(a) as amended (Amendment 25-86,61 FR5218, February 9, 1996), paragraph 25.343 "Design fuel and oil loads" was amended as a conforming change so that the design criterion for the structural reserve fuel condition included only the discrete gust ofparagraph 25.341(a) and not the continuous turbulence of25.341(b). However, the FAAbelieves that both a continuous turbulence criterion and a discrete gust criterion are needed since they account for the response to totally different, but still realistic, atmospheric characteristics. Therefore, to meet the level ofsafety intended by the structural
reserve fuel requirements it was deemed necessary to include a continuous turbulence loads criterion in paragraph (b)(l)(ii) of

### § 25.343. With the adoption ofthe discrete gust in 

### §25.341(a) as amended (Amendment 25-86,61 FR5218, February 9, 1996), paragraph 25.345 "High lift devices" was amended as a conforming change so that the design criterion for en-route conditions with flaps deployed included only the discrete gust ofparagraph 25.341(a) and not the continuous turbulence of25.341(b). However, the FAAbelieves that both a continuous turbulence criterion and a discrete gust criterion are needed since they account for the response to totally different, but still realistic, atmospheric characteristics. Therefore, to meet the level ofsafety intended by the en-route requirements it
was deemed necessary to include a continuous turbulence loads criterion in paragraph (c)(2) of 

### §25.345. With the adoption ofthe discrete gust in 

### §25.341(a) as amended (Amendment 25-86,61 FR5218, February 9, 1996), paragraph 25.371 "Gyroscopicloads" was amended as a conforming change so that gyroscopic loads were associated only with the discrete gust ofparagraph 25.341(a) and not the continuous turbulence of25.341(b). However, the FAAbelieves that in orderto meet the level ofsafety intended by the revised continuous turbulence requirements it will be necessary to include gyroscopic effects, where appropriate, in calculation oftotal loads due to continuous turbulence. To this end a change is proposedto Section 25.371 so that it would reference the entire section 25.341 and include both continuous turbulence loads as well as discrete gust loads.

10**-~ ---~-----** 

Revised 9/22/1999Draft ContinuousTurbulence NPRM June 24, 1999 Revised 25.341(c) numberheadings 9/3/99 With the adoption ofthe discrete gust in 

### § 25.341(a) as amended (Amendment 25-86, 61 FR 5218, February 9, 1996), paragraph 25.373 "Speed Control Devices" was amended as a conforming change so that the design requirement for these devices referenced onlythe discrete gust ofparagraph 25.341(a) and not the continuous turbulence of25.341(b). The continuous turbulence paragraph was moved from 25.305(d) to 25.341(b) only as an organizational change, and in orderto not impose additional requirements on speed control devices, such as speed brakes, it was necessary to change the reference so that it only referred to 25.341(a). Now, however, FAAbelieves that encounters with continuous turbulence can result in the activation of speedbrakes to slow the airplane to the recommended turbulence penetration speeds, and so the loads induced by turbulence should be considered while these devices are deployed. Tothis end,
a change is proposed to Section 25.373 so that it would reference the entire section 25.341 and include both continuous turbulence loads as well as discrete gust loads. With the adoption ofthe discrete gust in 

### § 25.341(a) as amended (Amendment 25-86,61 FR 5218, February 9, 1996), paragraph 25.391 "Control surface loads: general" was amended as
a conforming change so that the design load criterion for control surfaces included only the discrete gust ofparagraph 25.341(a) and not the continuous turbulence of25.341(b). However, the FAAbelieves that both a continuous turbulence criterion and a discrete gust criterion are needed since they account for the response to totally different, but still realistic, atmospheric characteristics. Therefore, to meet the level ofsafety intended for the aircraft as a whole it was deemed necessary to design control surfaces for limit loads resulting from the continuous turbulence conditions. To this end a change is proposed to Section 25.391 so that it would include 2S.341(a) and 25.341(b) for discrete gust as well as continuous turbulence loads. The proposal does not include a continuous turbulence design condition at 

### VB, "thedesign speed for maximum gust intensity". The design turbulence intensities established for the gust design conditions at Vc, "structural design cruising speed," and VD, "structural design diving speed," were developed in consideration ofthe full operational envelope so that a specific continuous turbulence design condition at 

### VB is not considered necessary, provided the current practices for operating in severe turbulence are continued. Since Amendment 25-86 (61 FR 5221, February 9, 1996) the discrete gust requirements of

### § 25.341 have not contained a specific discrete gust design condition at 

### VB. Without any specific discrete gust orcontinuousturbulence

11Revised 9122/l999Draft Continuous Turbulence NPRM June 24, 1999 Revised 25.341(c) number headings 9/3/99 design criteria at VB there is no technical reason to prescribe a rough air speedbased upon VB. Therefore, it is proposed to amend 

### §25.1517 to remove the link between VRA and VB. 

### PaperworkReduction In accordance with the PaperworkReduction Act of 1995 (44 U.s.c. 3507(d)), there are
no requirements for information collection associated with this proposed rule. 

### International Compatibility TheFAAreviewed the corresponding International Civil Aviation Organization regulations, where they exist, and has identified no differences in these proposed amendments and the foreign regulations. TheFAA has also reviewed the Joint Airworthiness Authorities Regulations and has discussed similarities and differences in these proposed amendments and the foreign regulations. 

### 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 oradopt a regulation only upon a reasoned determination that the benefits ofthe intended regulationjustify 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 ofManagement and Budget directs agencies to assess the effects ofregulatory changes on international trade. In conductingthese analyses, the FAAhas determined that this rule: (1) would generate benefits thatjustify its costs and is not a "significant regulatory action" as defined in the Executive Order; (2) is not significant
as defined in DOT's RegulatoryPolicies and Procedures; (3) would not have a significant impact
ona substantial number ofsmall entities; and (4) would not constitute a barrier to international trade. These analyses, available in the docket, are summarized below. Regulatory Evaluation Summary [To be completed]
RegulatoryFlexibilityDetermination TheRegulatory Flexibility Act of1980 (RFA) was enacted by Congressto ensure that small entities are not unnecessarily and disproportionallyburdened by Federal regulations. The

12Revised 9/22/19990raft Continuous Turbulence NPRM June 24, 1999 Revised 25.341(c) number headings 9/3/99 RF

### Arequires agencies to determine whether rules would have "a significant economic impact on
a substantial number ofsmall entities," and, in cases where they would, to conduct a regulatory flexibility analysis. "FAA Order 2100.1 4A, Regulatory Flexibility Criteria and Guidance, prescribes standards for complying with RFA requirements in FAArulemaking actions. The Order defines "small entities" in terms ofsize thresholds, "significant economic impact" in terms of annualized cost thresholds, and "substantial number" as a number which is not less than eleven and which is more than one-third ofthe affected small entities. The proposed rule would affect manufacturers oftransport category airplanes produced under future new airplane type certifications. Forairplane manufacturers, FAA Order 2100.14A specifies a size threshold for classification as a small entity as 75 orfewer employees. Since no part 25 airplane manufacturer has 75 orfewer employees, the proposed rule would not have a significant economic impact on a substantial number ofsmall airplane manufacturers. International Trade Impact Assessment The proposed rule would have no adverse impact on trade opportunities for U.S. manufacturers selling airplanes in foreign markets and foreign manufacturers selling airplanes in theU.S. market. Instead, by harmonizing the standards ofthe FAR and the

### JAR. it would lessen restraints ontrade

.

.Federalism Implications The regulations proposed herein would not have substantial direct effects onthe states, on the relationship between the national government and the states, oron the distribution ofpower and responsibilities among the various levels ofgovernment. Thus, in accordance with Executive Order 12612, it is determined that this proposal does not have sufficient federalism implications to warrant the preparation ofa Federalism Assessment. Conclusion Becausethe proposed changes to the continuous turbulence design load requirement are not expected to result in any substantial economic costs, the FAAhas determined that this proposed regulation would not be significant underExecutive Order 12866. Becausethis is an issue that has not prompted a great deal ofpublic concern, the FAAhas determined that this action is not significant underDOT Regulatory Policies and Procedures (44 FR 11034; February 25, 1979). In addition, since there are no small entities affected by this rulemaking, the FAA

13Revised 9/22/l9990raft Continuous Turbulence NPRM June 24, 1999 Revised 25.341(c) number headings 9/3/99 certifies that the rule, ifpromulgated, would not have a significant economic impact, positive or negative, on a substantial number ofsmall entities under the criteria ofthe Regulatory Flexibility Act, since none would be affected. A copy ofthe regulatory evaluation prepared for this project may be examined in the Rules Docket orobtained from the person identified underthe caption "FORFURTHER INFORMATION CONTACT." 

### List ofSubjects in 14 CFR part 2S _Air transportation, Aireraft, Aviation safety, Safety. 

### The Proposed Amendments Accordingly, the Federal Aviation Administration (FAA) proposesto amend 14 CFR part 25 oftheFederal Aviation Regulations (FAR) as follows: PART 25

- AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AlRPLANES 1. The authority citation for Part 25 continues to read as follows: Authority: 49 U.S.c. app. 1347, 1348, I354(a), 1357 (d)(2), 1372, 1421 through 1430, 1432, 1442,1443,1472,1510,1522, 1652(e), 1655(c), 1657(t), 49 U.S.c. I06(g) 2. Byremoving Appendix G to part 25, "Continuous Gust Design Criteria" and marking it "Reserved"

. 3. To amend Section 25.341 by revising paragraph 25.341 (a)(5)(i) to read as follows: 

### (a) 

### * * 

## * * * (5) Thefollowing reference gust velocities apply: (i) At airplane speeds between VB and Vc: Positive and negative gusts with reference gust velocities of56.0 ft/sec EAS must be considered at sealevel. Thereference gust velocity may be reduced linearly from 56'0 ft/sec EAS at sea level to 44.0 ftlsec EAS at 15 000 feet. The reference gust velocity may be further reduced linearly from 44.0 ft/sec EAS at 15000feet to 20.86 ft/sec EAS at 60 000 feet. 

### * * * * * * * * * * 4. To amend Section 25.341 by revising paragraph 25.341(b) and adding a new paragraph 25.341(c) to read as follows: (b) Continuolls Turbulence Design Criteria. The dynamic response ofthe airplane to vertical and lateral continuous turbulence must be taken into account. The dynamic analysis must

14Revised 9/22/1999Draft Continuous Turbulence NPRM June 24, 1999 Revised 25.341(c) number headings 9/3/99 takeinto account unsteady aerodynamic characteristics and all significant structural degrees of freedom including rigid body motions. The limit loads must be determined for all critical altitudes, weights, and weight distributions as specified in 

### §25.321(b), and all critical speeds within the ranges indicated in paragraph (b)(3). (I) Except as provided in paragraphs (b)(4) and (b)(5) ofthis section, the following equation must beused: PL 

### =PL-tg 

### ± UcrA Where-- PL 

### =limit load; PL-tg 

### =steady I-g load for the condition; A 

### =ratio ofroot-mean-square incremental load for the condition to root-mean-square turbulence velocity; and Ucr 

### =limit turbulence intensity in true airspeed, specified in paragraph (b)(3) of this section. (2) Values ofA must be determined according to the following formula:

Where--- H(n) = the frequency response function, determined by dynamic analysis, that relates the loads in the aircraft structureto the atmospheric turbulence; and 

## <1>(n) =normalized power spectral density ofatmospheric turbulence given byL 

## <1>(n)

## = - I
Where

### 1

### +

## % (1.339Lny 

## r;**,]~** 

# r

+(1.339Lny



## n =reduced frequency, radians perfoot.; and L 

### =scale ofturbulence

### =2,500 ft. (3) Thelimit turbulence intensities, Ucr, in feet per secondtrue airspeed required for compliance with this paragraph are- (i) At airplane speeds between VR and Ve:

15----------_
..

## _- ---.---

Vcr 

### = Vcrref F gWhereRevised 9122/1999Draft Continuous Turbulence NPRM June 24, 1999 Revised 25.341(c) numberheadings 9/3/99

Voref is the reference turbulence intensity that varies linearly with altitude from 90 fps (TAS) at sea level to 79 fps (TAS) at 24000 feet and is then constant at 79 fps (TAS) up to the altitude of60000 feet. F g is the flight profile alleviation factor defined in paragraph (a)(6) ofthis section; (ii) At speed VD: Vcr is equal to 112 the values obtained under subparagraph (3)(i) ofthis paragraph. (iii) At speeds between VC and VD: Vcr is equal to a value obtained by linear interpolation. (iv) At all speeds both positive and negative continuous turbulence must be considered.

_ (4) When an automatic system affecting the dynamic response ofthe airplane is included in the analysis, the effects ofsystem non-linearities onloads at the limit load level must betaken into account in a realistic orconservative manner. (5) Ifnecessary for the assessment ofloads on airplanes with significant non-linearities, it must be assumed that the turbulence field has a root-mean-square velocity equal to 40 percent of theVcr values specified in subparagraph (3). The value oflimit load is that load with the same probability ofexceedance in the turbulence field as AVcr ofthe same load quantity in a linear approximated model. (c) 

### Supplementarygllst conditionsforwingmountedengines. Forairplanes equipped with wing mounted engines, the engine mounts, pylons, and wing supporting structure must be designed for the maximum response at the nacelle center ofgravity derived from the following dynamic gust conditions applied to the airplane:

________ (>,...:1'"'-)A discrete gust determined in accordance with 25.341(a) at each angle normal to the flight path, and separately,

__ ---'(

..... 2'+.-)A pair ofdiscrete gusts, one vertical and one lateral. The length ofeach ofthese gusts must be independentlyindependantly tuned to the maximum response in accordance with 25.341(a). The penetration ofthe airplane in the combined gust field and the phasing of the vertical and lateral component gusts must be established to develop the maximum response to the

16Revised 9122/I999Draft ContinuousTurbulence NPRM June 24, 1999 Revised 25.341(c) number headings 9/3/99 gust pair. In the absence ofa more rational analysis, the following formula must be used for each ofthe maximum engine loads in all six degrees offreedom: deleted (3) ahead ofbelow equation 

### P

L 

## =P L- 1g +

## O.85~L~ +

## L~

Where--

PL 

### =limit load; PL-1g 

### =steady 1-g load for the condition; Lv 

### =Peak incremental response load due to a vertical gust according to 

### § 25.341(a); and LL 

### =Peak incremental response load due to a lateral gust according to 

### § 25.341(a).

-
5. To amend Section 25.343 by revising paragraph 25.343(b)(1)(ii) to read as follows: (b) 

### (1) 

## * * * * * * 

## * * * * (ii) The gust and turbulence conditions of

### § 25.341, but assuming 85% ofthe gust velocities prescribed in 

### § 25.341(a)(4) and 85% ofthe turbulence intensities prescribed in 

### § 25.341(b)(3). 6. To amend Section 25.345 by revising paragraph 25.345(c)(2) to read as follows: 

### (c) 

### * * * * * (2) The vertical gust and turbulence conditions prescribed in § 25.34l. 7. To amend Section 25.371 to read as follows: 

### §25.371 Gyroscopic loads. The structure supporting any engine or auxiliary powerunit must be designed for the loads, including gyroscopic loads, arising from the conditions specified in 

### §§ 25.331, 25.341, 25.349,25.351,25.473,25.479, and 25.481, with the engine orauxiliary powerunit at the maximum rpm appropriate to the condition. Forthe purposes ofcompliance with this paragraph, the pitch maneuver in 

### §25.331(c)(1) must be carried out until the positive limit maneuvering load factor (point A2 in 

### § 25.333(b» is reached. 9. To amend Section 25.373 by revising paragraph 25.373(a) to read as follows:

17Revised 9122/l999Draft Continuous Turbulence NPRM June 24, 1999 Revised 25.34l(c) numbelr headings 9/3/99 (a) The airplane must be designed for the symmetrical maneuvers and gusts prescribed in §§ 25.333, 25.337, the yawing maneuvers in §25.351, and the vertical and lateral gust and turbulence conditions prescribed in 

### § 25.341(a) and (b) at each setting and the maximum speed associated with that setting; and; 

## * * * * * 10. To amend Section 25.391 to read as follows: § 25.391 Control surface loads: general The control surfaces must be designed for the limit loads resulting from the tlight conditions in §§ 25.331, 25.341(a) and (b), 25.349 and 25.351 and the ground gust conditions in § 25.415, considering the requirements for

------ 

## * * * * 

## * * * * * * 11. To amend Section 25.1517 to read as follows: § 25.1517Rough air speed VRA (a) At altitudes where VMO is not limited by Mach number, a rough air speed VRA, for use
as the recommended turbulence penetration air speed, must be established which: 1) is not less than a speed allowing a positive maneuvering load factor of 1.4 before the onset ofperceptiblebuffeting. 2) is sufficiently less than the maximum operating speed to ensure that likely speed variation during rough air encounters will not cause the overspeed warning to operatetoo frequently. In the absence ofa rational investigation substantiating the use ofothervalues, VRA must be less than VMo-35 KTAS. (b) At altitudes where VMO is limited by Mach number, a rough airMach number MRA, for
use as the recommended turbulence penetrationMach number, may be chosento provide an optimum margin betweenlow and high speed buffet boundaries."

Issued in Washington D.C. on

18

# u.s. Department ofTransportation Federal Aviation Administration 

# Draft Advisory Circular

Subject: DYNAMIC GUST LOADS Date: 

### 10/20/99 AC No. 25.341 Initiated by: ANM-110 Change:
1. PURPOSE. This advisory circular (AC) sets forth an acceptable means ofcompliance withthe provisions ofFARPart 25 ofthe Federal Aviation Regulations (FAR) dealing with discrete gust and continuous turbulence dynamic loads. 2. RELATED FAR SECTIONS. The contents ofthis AC are considered by theFAAin determining compliance with the discrete gust and continuous turbulence criteria defined in Paragraph 25.341. Related sections are: 25.343 25.345 25.349 25.371 25.373 25.391 25.427 25445 25.571 Design fuel and oil loads High lift devices Rolling conditions Gyroscopic loads Speed control devices Control surface loads Unsymmetrical loads Auxiliary aerodynamic surfaces Damage-tolerance and fatigue evaluation ofstructure Reference should also be made to Paragraphs, 25.301, 25.302, 25.303, 25.305, 25.321, 25.335, 25.1517. 3. OVERVIEW. This AC addresses both discrete gust and continuous turbulence (or continuous gust) requirements ofFARPart 25. It provides some ofthe acceptable methods of modeling airplanes, airplane components, and configurations, and the validation ofthose modeling methods for the purpose ofdetermining the response ofthe airplane to encounters with gusts. How the various airplane modeling parameters are treated in the dynamic analysis can have a large influence on design load levels. The basic elements to be modeled in the analysis are the elastic, inertial, aerodynamic and control system characteristics of the complete,.coupled airplane (Figure 1). The degree ofsophistication and detail required in the modeling depends on the complexity ofthe airplane and its systems.

AC25341-2.DOC October 20,1999 

### 1,20 As submitted fortechnical editing and preliminary legal review.ATMOSPHERIC t----

__**~** MOTION AERODYNAMIC**14-_~** SYSTEM

DYNAMICS AND STRUCTURE

STATICFLIGHT I--

__**~** LOADS MODEL CONTROL SYSTEM



## Figure 1 Basic Elements ofthe Gust Response Analysis Design loads for encounters with gusts are a combination ofthe steady level I-g flight loads, and the gust incremental loads including the dynamic response ofthe airplane. The steady I-g flight loads can be realistically defined by the basic external parameters such as speed, altitude, weight and fuel load. They can be determined using static aeroelastic methods. The gust incremental loads result from the interaction of atmospheric turbulence and airplane rigid body and elastic motions. They may be calculated using linear analysis methods when the airplane and its flight control systems are reasonably or conservatively approximated by linear analysis models. Nonlinear solution methods are necessary for airplane and flight control systems that are not reasonably or conservatively represented by linear analysis models. Nonlinear features generally raise the level ofcomplexity, particularly for the continuous turbulence analysis, because they often require that the solutions be carried out in the time domain. The modeling parameters discussed in the following sections include: • Design conditions and associated steady, level I-gflight conditions. • The discrete and continuous gust models ofatmospheric turbulence. • Detailed representation ofthe airplane system including structural dynamics, aerodynamics, and control system modeling.

AC25341-2.DOC October20, 1999 As submitted for technical editing and preliminary legal review.• Solution ofthe equations ofmotion and the extraction ofresponse loads. • Considerations for nonlinear airplane systems. • Analytical model validation techniques. 4. DESIGN CONDITIONS.

a. General. Analyses should be conducted to determine gust response loads for the airplane throughout its design envelope, where the design envelope is taken to include, for example, all appropriate combinations ofairplane configuration, weight, center ofgravity, payload, fuel load, thrust, speed, and altitude. b. Steady Level I-gFlight Loads. The total design load is made up ofstatic and dynamic load components. In calculating the static component, the airplane is assumed tobe in trimmed steady level flight, either as the initial condition for the discrete gust evaluation or as the mean flight condition for the continuous turbulence evaluation. Static aeroelastic effiects should be taken into account ifsignificant.
To ensure that the maximum total load on each part of the airplane is obtained, the associated steady-state conditions should be chosen in such a way as to reasonably envelope the range ofpossible steady-state conditions that could be achieved in that flight condition. Typically, this would include consideration ofeffects such as speed brakes, power settings between zero thrust and the maximum for the flight condition, etc.

c. Dynamic Response Loads. The incremental loads from the dynamic gust solution are superimposed on the associated steady level flight I-g loads. Load responses in both positive and negative senses should be assumed in calculating total gust response loads. Generally the effects ofspeed brakes, flaps, orotherdrag orhigh lift devices, while they should be included in the steady-state condition, may be neglected in the calculation ofincremental loads. d. Damage Tolerance Conditions. Limit gust loads, treated as ultimate, need tobe developed for the structural failure conditions considered underParagraph 25.571(b). Generally, for redundant structures, significant changes in stiffness orgeometry do not occurfor the types of damage under consideration. As a result, the limit gust load values obtained for the undamaged aircraft may beused and applied to the failed structure. However, when structural fllilures ofthe types considered under Paragraph 25.571(b) cause significant changes in stiffness orgeometry, or both, these changes should be taken into account when calculating limit gust loads for the damaged structure. 5. GUSTMODEL CONSIDERATIONS.

a. General. The gust criteria presented in Paragraph 25.341 consist oftwo models of atmospheric turbulence, a discrete model and a continuous turbulence model. 

### It is beyond the scope ofthis AC to review the historical development ofthese models and their associated parameters. This information canbe found in the preamble to FARPart 25. This AC focuses on the application ofthose gust criteria to establish design limit loads. The discrete gust model is

AC25341-2.DOC October 20,1999 

### 3/20 As submitted fortechnical editing and preliminary legal review.used.to represent single discrete extremeturbulence events. The continuous turbule:nce model represents longer duration turbulence encounters which excite lightly damped modes. Dynamic loads for both atmospheric models must be considered in the structural design ofth(! airplane.
b. Discrete Gust Model (1) Atmosphere. The atmosphere is assumed to be one dimensional with the gust velocity acting normal (either vertically orlaterally) to the direction ofairplane travd. The onedimensional assumption constrains the instantaneous vertical orlateral gust velocities to be the same at all points in planes normal to the direction ofairplane travel. Design level discrete gusts are assumed to have I-cosine velocity profiles. The maximum velocity for a discrete gust is calculated using a reference gust velocity, UREF, a flight profile alleviation factor, Fg , and an expression which modifies the maximum velocity as a function ofthe gust gradient distance, H. These parameters are discussed further below. (A) Reference Gust Velocity, UREF - Derived effective gust velocities representing guS!S occurring once in 70,000 flight hours are the basis for design gust velocities. These reference velocities are specified as a function ofaltitude in Paragraph 25.341(a)(5) and are given in terms offeet per second equivalent airspeed for a gust gradient distance, H, of350feet. (B) Flight Profile Alleviation Factor, Fg

- The reference gust velocity, UREF, is a measure ofturbulence intensity as a function of altitude. In defining the value of Uref at each altitude, it is assumed that the aircraft is flown 100% ofthe time at that altitude. The factor F gis then applied to account for the expected service experience in terms of the probability of the airplane flying at any given altitude within its certification altitude range. F gis a minimum value at sea level, linearly increasing to 1.0 at the certified maximum altitude. The expression for Fg is given in Paragraph 25.34I(a)(6). (C) Gust Gradient Distance, H - The gust gradient distance is that distance over which the gust velocity increases to a maximum value. Its value is specified as ranging from 30 to 350 ft. (It should be noted that if 12.5 times the mean geometric chord ofthe airplane's wing exceeds 350 feet, consideration should be given to covering increased maximum gust gradient distances.) (D) Design Gust Velocity, Uds

- Maximum velocities for design gusts are proportional to the sixth root ofthe gust gradient distance, H. The maximum gust velocity for a given gust is then defined as:

The maximum design gust velocity envelope, Uds, and example design gust velocity profiles are illustrated in Figure 2.

AC25341-2.DOC October 20,1999 

### 4/20 As submitted for technical editing and preliminary legal review.--------------- --- --- -**-----~------~--**

Gust Gradient Distance, H

### =290 Ft

/

### ',>(~ 

# // \\ u=

# U;-(l-CO{'::)]

# ~H=170Ft\ H = 50 Ft



## o 100 200 300 400 500 600 700 Gust PenetrationDistance, S - Feet Figure-2 Typical (I-cosine) Design Gust Velocity Profiles (2) Discrete Gust Response. The solution for discrete gust response time histories can be achieved by a number oftechniques. These include the explicit integration ofthe airplane equations ofmotion in the time domain, and frequency domain solutions utilizing Fourier transform techniques. These are discussed further in Section 7.0 ofthis AC. Maximum incremental loads, P1i
. are identified by the peak values selected from time histories arising from a series ofseparate, I-cosine shaped gusts having gradient distances ranging from 30 to 350 feet. Input gust profiles should cover this gradient distance range in sufficiently small increments to determine peak loads and responses. Historically 10 to 20 gradient distances have been found to be acceptable. Both positive and negative gust velocities should be assumed in calculating total gust response loads. It should be noted that in some cases, the peak incremental loads can occur well after the prescribed gust velocity has returned to zero. In such cases, the gust response calculation should be run for sufficient additional time to ensure that the critical incremental loads are achieved. The design limit load, PLi,corresponding to the maximum incremental load, P1ifor a given load quantity is then defined as: PLi 

### =P(l-g)i 

### ±P1i Where P(l-gji is the I-g steady load for the load quantity under consideration. The set of time correlated design loads, PLj ,corresponding to the peak value ofthe load quantity, PLi, are calculated for the same instant in time using the expression:
PLj 

### =P(1-g)j 

### ±PIj

AC25341-2.DOC October20,1999 As submitted for technical editing and preliminary legal review.Notethat in the case ofa nonlinear aircraft, maximum positive

### inc~ementalloads may differ from maximum negative incremental loads.
When calculating stresses which depend on a combination ofexternal loads it may be necessary to consider time correlated load sets at time instants otherthan those which result in peaks for individual external load quantities. (3) Round-The-Clock Gust. When the effect ofcombined vertical and lateral gusts on airplane components is significant, then round-the-clock analysis should be conducted onthese components and supporting structures. The vertical and lateral components ofthe gust are assumed to have the same gust gradient distance, H and to start at the same time. Components that should be considered include wing mounted engines, orhorizontal tail surfaces having appreciable dihedral or anhedral (i.e., greaterthan 10°), orcomponents supported by other lifting surfaces, for example T-tails, outboard fins and winglets. Whilst the round-the-clock load assessment may be limited tojust the components under consideration, the loads themselves should be calculated from a whole airplane dynamic analysis. The round-the-clock gust model assumes that discrete gusts may act at any angle normal to the flight path ofthe airplane. Lateral and vertical gust components are correlated since the round-the-clock gust is a single discrete event. For a linear airplane system, the loads due to a gust applied from a direction intermediate to the vertical and lateral directions

- the round-theclock gust loads

- can be obtained using a linear combination ofthe load time histories induced from pure vertical and pure lateral gusts. The resultant incremental design value for a particular load ofinterest is obtained by determining the round-the-clock gust angle and gust length giving the largest (tuned) response value for that load. The design limit load is then obtained using the expression for PL given above in section 5.b.2. (4) Supplementary Gust Conditons for Wing Mounted Engines. (A) Atmosphere

- For aircraft equipped with wing mounted engines, FAR paragraph 2S.341(c) requires that engine mounts, pylons and wing supporting structure be designed to meet a round-the-clock discrete gust requirement and a multi-axis discrete gust requirem(:flt. The model ofthe atmosphere and the method for calculating response loads for the roundthe-clock gust requirement is the same as that described in Section 5(b)(3) ofthis AC.
For the multi-axis gust requirement, the model of the atmosphere consists of two independent discrete gust components, one vertical and one lateral, having amplitudes such that the overall probability ofthe combined gust pair is the same as that ofa single discrete gust as defined by FAR paragraph 25.341(a) as described in Section 5(b)(I) ofthis AC. To achieve this equal-probability condition, in addition to the reductions in gust amplitudes that would be applicable ifthe input were a multi-axis Gaussian process, a further factor of0.85 is incorporated into the gust amplitudes to account for non-Gaussian properties of severe discrete gusts. This factor was derived from severe gust data obtained by a research aircraft specially instrumented to

AC25341-2.DOC October20, 1999 As submitted for technical editing and preliminary legal review.measure vertical and lateral gust components. This information is contained in Stirling Dynamics Labratories Report No SOL -S71-TR-2 dated May 1999. (B) Multi-Axis Gust Response

- Fora particular aircraft flight condition, the calculation ofa specific response load requires that the amplitudes, and the time phasing, ofthe two gust componentsbe chosen, subject to the condition on overall probability specified in (A) above, such that the resulting combined load is maximized. For loads calculated using a linear aircraft model, the response load may be based upon the separately tuned vertical and lateral discrete gust responses for that load, each calculated as described in Section S(b)(2) ofthis AC. In general, the vertical and lateral tuned gust lengths and the times to maximum response (measured from the onset ofeach gust) will not be the same. Denote the independently tuned vertical and lateral incremental responses for a particular aircraft flight condition and load quantity i by LVi and LLi, respectively.The associated multi-axis gust input is obtained by multiplying the amplitudes of the independently-tuned vertical and lateral discrete gusts, obtained as described in the previous paragraph, by O.85*Lv

## /-J (LVi2+LLi2) and O.85*Ld..J (LVi2+LLi2) respectively. The time-phasing ofthe two scaled gust components is such that their associated peak loads occur at the same instant.
The combined incremental response load is given by:

and the design limit load, PLi.corresponding to the maximum incremental load, P[j, for the given load quantity is then given by:
PLi = P(I-g)i 

### ±PIi
where P(I-g)i is the 1-g steady load for the load quantity under consideration. The incremental, time correlated loads corresponding to the specific flight condition under consideration are obtained from the independently-tuned vertical and lateral gust inputs for load quantity i. Thevertical and lateral gust amplitudes are factored by O.85*Lv;/..J (LVi2+LLi2) and O.85*Ld..J(Lvi2+LLi2) respectively. Loads LVj and LLj resulting from these reduced**vl~rtical** and lateral gust inputs, at the time when the amplitude ofload quantity iis at a maximum value, are added toyield the multi-axis incremental time-correlated value P1j for load quantityj.
The set oftime correlated design loads, PLj ,corresponding to the peakvalue ofthe load quantity, PLi, are obtained using the expression: PLj 

### =P(I-g)j 

### ±PIj

AC25341-2.DOC October 20,1999 

### 7,20 As submitted for technical editing and preliminary legal review.

### Note that with significant nonlinearities, maximum positive incremental loads may differ from maximum negative incremental loads. 

### ".rhen calculating stresses which depend on a combination ofextemalloads it may be necessary to considertime correlated load sets at time instants other than those which result in peaks for individual m(temalload quantities.

c. Continuous Turbulence Model. (1) Atmosphere. The atmosphere for the determination ofcontinuous gust responses is assumed to be one dimensional with the gust velocity acting normal C 

### eithervertically or laterally) to the direction ofairplane travel. The one-dimensional assumption constrains the instantaneous vertical or lateral gust velocities to be the same at all points in planes normal to the direction ofairplane travel. The random atmosphere is assumed to have a Gaussian distribution of gust velocity intensities and a von Karman power spectral density with a scale ofturbulence, L, equal to 2500 feet. The expression for the von Karman spectrum for unit, root-mean-squar,e (RMS) gust intensity, <I>\CO), 

### is given below. In this expression 0 

### =ro/V 

### where, ro 

### is the circular frequency in radians per second, and Vis the airplane velocity in feet per second true airspeed.

The von Karman power spectrum for unit RMS gust intensity is illustrated in Figure 3.

AC25341-2.DOC October 20, 1999 

### 8,20 As submitted for technical editing and preliminary legal review..- 

## ....**~** "'CI C';I**~** 

# -- -- N.-

U Cl) r/)**~ ~**  

# --
.- 

# c: 

# -- eo l.E+3

l.E+2

l.E+1

1.E+0
L 

### =2500 Feet 1.E-1

l.E-2 

### +---..-------,,.....---..----.----'--....., 1.0E-5 1.0E-4 1.0E-3 1.0E-2 1.0E-1 1.0E+0 Reduced Frequency, 

## n- RadlFt Figure-3 The von Karman Power Spectral Density Function, 

### <1>,(n)
The design gust velocity, Uo- I applied in the analysis is given by the product of the reference gust velocity, Uo-REF ' and the profile alleviation factor, Fg, as follows:

where values for Uo-REF, are specified in Paragraph 25.341(b)(3) in feet per second true airspeed and Fg is defined in Paragraph 25.341(a)(6). The value of Fg is based on airplane design parameters and is a minimum value at sea level, linearly increasing to 1.0 at the certified maximum design altitude. Itis identical to that used in the discrete gust analysis.
As for the discrete gust analysis, the reference continuous turbulence gust intensity, Uo-REF' defines the design value ofthe associated gust field at each altitude. In defining the value OfUo-REF at each altitude, it is assumed that the airplane is flown 100% ofthe time at that altitude. The factor F g is then applied to account for the probability ofthe airplane flying at any given altitude during its service lifetime. It should be noted that the reference gust velocity is comprised oftwo components, a root-mean-square (RMS) gust intensity and a peak to RMS ratio. The separation of these components is not defined and is not required for the linear airplane analysis. Guidance is provided in Section 8.d. ofthis AC for generating a RMS gust intensity for a nonlinear simulation.

AC25341-2.DOC October 20,1999 

### 9,20 As submitted for technical editing and preliminary legal review.(2) Continuous Turbulence Response. Forlinear airplane systems, the solution for the response to continuous turbulence may be performed entirely in the frequency domain, using the RMS response. A is defined in paragraph 2S.341(b)(2) and is repeated here in modified notation for load quantity i, where:

or

In the above expression ¢1 (n) is the input von Karman power spectrum ofthe turbulence and is defined in Section S.c. ofthis AC, 

### hi (in) is the transfer function relating the output load quantity, i, to a unit, harmonically oscillating, one-dimensional gust field, and the asterisk superscript denotes the complex conjugate. When evaluating Ai, the integration should be continued until a converged value is achieved since, realistically, the integration to infinity may be impractical. The design limit load, PLi, is then defined as:
PLi 

### =P(I'g)i 

### ±p/i



### =P(l'g)i 

### ±Ua Ai
where 

### Va is defined in Section S.c. ofthis AC, and P(I'g)i is the I-g steady state value for the load quantity, i, under consideration. As indicated by the formula, both positive and negative load responses should be considered when calculating limit loads. Correlated (orequiprobable) loads can be developed using cross-correlation coefficients, Pij, computed as follows:

<Xl 

# f

¢/CO)real[h;CiO)h";CiO)]dO
o

where, 'real[

...]' denotes the real part ofthe complex function contained within the brackets. In this equation, the lowercase subscripts, i andj, denote the responses being correlated. A set of design loads, PLj, correlated to the design limit load PLi, are then calculated as follows:

AC2S341-2.DOC October 20,1999 As submitted fortechnical editing and preliminary legal review.The correlated load sets calculated in the foregoing manner provide balanced load distributions corresponding to the maximum value of the response for each external load quantity, i, calculated. When calculating stresses, the foregoing load distributions may not yield critical design values because critical stress values may depend on a combination of external lloads. In these cases, a more general application ofthe correlation coefficient method is required. For example, when the value ofstress depends on two externally applied loads, such as torsion and shear, the equiprobable relationship between the two parameters forms an ellipse as illustrated in Figure4.

Design Value ofShear

T

G T F 1 1 1 1 1 1 1 1 1**/~One-g** Load 1/ 

## -r'--------

1 1 1 1 T Design Value ofTorsion

rEqualProbability Design Ellipse

Figure-4 Equal Probability Design Ellipse
Inthis figure, the points oftangency, T, correspond to the expressions for correlated load pairs given by the foregoing expressions. A practical additional set ofequiprobable load pairs that should be considered to establish critical design stresses are given by the points oftangency to the ellipse by lines AB, CD, EF and GH. These additional load pairs are given by the following expressions (where i = torsion and

### j = shear):
Fortangents to lines AB and EF

AC25341-2.DOC October 20,1999 

### 11/20 As submitted fortechnical editing and preliminary legal review.PLi 

### = P(l-g)i 

### +/- 

## AiVa [(1

- pij)/2] 112 and PLj 

### = P(l-g)j -/+ 

## A jVa [( 1
- Pij)/2] 1/2 Fortangents to lines CD and GH PLi 

### = P(l-g)i 

### ± 

## AiVa [(1 

### + Pij)/2] 112 and
All correlated or equiprobable loads developed using correlation coefficients will provide balanced load distributions.
A more comprehensive approach for calculating critical design stresses that depend on a combination ofexternal load quantities is to evaluate directly the transfer function for the stress quantity ofinterest from which can be calculated the gust response function, the value for RMS response, 

## A, and the design stress values p(\-g)

### ± Va

## A

. 6. AIRPLANE MODELINGCONSIDERATIONS

a. General. The procedures presented in this section generally apply for airplanes having aerodynamic and structural properties and flight control systems that may be reasonably or conservatively approximated using linear analysis methods for calculating limit load. Additional guidance material is presented in Section 8 ofthis AC for airplanes having properties and/or systems not reasonably orconservatively approximated by linear analysis methods. b. Structural Dynamic Model. The model should include both rigid body and flexible airplane degrees offreedom. Ifa modal approach is used, the structural dynamic model should include a sufficient number offlexible airplane modes to ensure both convergence ofthe modal superpositionprocedure and that responses from high frequency excitations are prolPerly represented.
Most forms ofstructural modeling can be classified into two main**categori(~s:** (1) the socalled "stick model" characterised by beams with lumped masses distributed along their lengths, and (2) finite element models in which all major structural components (frames, ribs, stringers, skins) are represented with mass properties defined at grid points. Regardless of the approach taken for the structural modeling, a minimum acceptable level of sophistication, consistent with configuration complexity, is necessary to represent satisfactorily the critical modes ofdeformation ofthe primary structure and control surfaces. Results from the models should be compared to test data as outlined in Section 9.b. ofthis AC in orderto validate the accuracy ofthe model.

c. Structural Damping. Structural dynamic models may include damping properties in addition to representations ofmass and stiffness distributions. In the absence ofbetter information it will normally be acceptable to assume 0.03 (i.e. 1.5% equivalent critical viscous damping) for all flexible modes. Structural damping may be increased overthe 0.03 value to be

AC25341-2.DOC October 20, 1999 As submitted fortechnical editing and preliminary legal review.consistent with the high structural response levels caused by extreme gust intensity, provided justification is given. d. Gust and Motion Response Aerodynamic Modeling. Aerodynamic f()rces included in the analysis are produced by both the gust velocity directly, and by the airplane response. Aerodynamic modeling for dynamic gust response analyses requires the use ofunsteady two-dimensional or three-dimensional panel theory methods for incompressible or compressible flow. The choice of the appropriate technique depends on the complexity of th(! aerodynamic configuration, the dynamic motion of the surfaces under investigation and the flight speed envelope ofthe airplane. Generally, three-dimensional panel methods achieve bettc;:r modeling of the aerodynamic interference between lifting surfaces. The model should have a sufficient number ofaerodynamic degrees offreedom to properly represent the steady and unsteady aerodynamic distributions under consideration. The buildup ofunsteady aerodynamic forces should be represented. In two-dimensional unsteady analysis this may be achieved in either the frequency domain orthe time domain through the application ofoscillatory or indiciallift functions, respectively. Where three-dimensional panel aerodynamic theories are to be applied in the time domain (e.g. for nonlinear gust solutions), an approach such as the 'rational function approximation' method may be employed to transform frequency domain aerodynamics into the time domain. Oscillatory lift functions due to gust velocity or airplane response depend on the reduced frequency parameter, k. The maximum reduced frequency used in the generation ofthe unsteady aerodynamics should include the highest frequency ofgust excitation and the highest structural frequency under consideration. Time lags representing the effect ofthe gradual penetration ofthe gust field by the airplane should also be accounted for in the buildup oflift due to gust velocity.

The aerodynamic modeling should be supported by tests or previous experience as indicated in Section 9.d. ofthis AC. Primary lifting and control surface distributed aerodynamic data are commonly adjusted by weighting factors in the dynamic gust response analyses. The weighting factors for steady flow (k 

### = 0) may be obtained by comparing wind tunnel test results with theoretical data. The correction ofthe aerodynamic forces should also ensure that the rigid body motion ofthe airplane is accurately represented in order to provide satisfactory short period and Dutch roll frequencies and damping ratios. Corrections to primary surface aerodynamic loading due to control surface deflection should be considered. Special attention s.hould also be given to control surface hinge moments and to fuselage and nacelle aerodynamics be:cause viscous and other effects may require more extensive adjustments to the theoretical coefficients. Aerodynamic gust forces should reflect weighting factor adjustments performed on the steady or unsteady motion response aerodynamics.

e. Gyroscopic Loads. As specified in Paragraph 25.371, the structure supporting the engines and the auxiliary powerunits should be designed for the gyroscopic loads induced by both discrete gusts and continuousturbulence. The gyroscopic loads for turbopropellers and turbofans

AC25341-2.DOC October 20,1999 

### 13/20 As submitted for technical editing and preliminary legal review.may be calculated as an integral part ofthe solution process by including the gyroscopic terms in the equations ofmotion orthe gyroscopic loads can be superimposed after the solUition ofthe equations ofmotion. Propeller and fan gyroscopic coupling forces (due to rotational direction) between symmetric and antisymmetric modes need not be taken into account ifthe coupling forces are shownto be negligible.
The gyroscopic loads used in this analysis should be determined with the engine or auxiliary power units at maximum continuous rpm. The mass polar moment of inertia used in calculating gyroscopic inertia terms should include the mass polar moments of inertia of all significant rotating parts taking into account their respective rotational gearing ratios and directions ofrotation. 

### f Control Systems. Gust analyses ofthe basic configuration should include simulation ofany control system for which interaction may exist with the rigid body response, structural dynamic response or external loads. Ifpossible, these control systems should be uncoupled such that the systems which affect "symmetric flight" are included in the vertical gust analysis and those which affect "antisymmetric flight" are included in the lateral gust analysis.
The control systems considered should include all relevant modes of operation. Failure. conditions should also be analyzed for any control system which influences the design loads in accordance with Paragraph 25.302, Appendix 

### K. The control systems included in the gust analysis may be assumed to be linear ifthe impact ofthe nonlinearity is negligible, or ifit can be shown by analysis on a similar a.irplane/control systemthat a linear control law representation is conservative. Ifthe control system is significantly nonlinear, and a conservative linear approximation to the control system cannot be developed, then the effect ofthe control system on the airplane responses should be evaluated in accordance with Section 8.0 ofthis AC.
g. Stability. Solutions ofthe equations ofmotion for either discrete gusts or continuousturbulence require the dynamic model be stable. This applies for all modes, except possibly for very low frequency modes which do not affect load responses, such as the phugoid mode. (Notethat the short period and Dutch roll modes do affect load responses). A stability check should be performed for the dynamic model using conventional stability criteria appropriate for the linear ornonlinear system in question, and adjustments should be made to the dynamic model, as required, to achieve appropriate frequency and damping characteristics.
Ifcontrol system models are to be included in the gust analysis it is advisable to check that the following characteristics are acceptable and are representative ofthe airplane; • static margin ofthe unaugmented airplane • dynamic stability ofthe unaugmented airplane • the static aeroelastic effectiveness ofall control surfaces utilised by any feedback control system • gain and phase margins ofany feedback control system coupled with the: airplane rigid body and flexible modes

AC25341-2.DOC October 20, 1999 As submitted for technical editing and preliminary legal review. 

### 14/20• the aeroelastic flutter and divergence margins ofthe unaugmented airplane, and also for any feedback control system coupled with the airplane.

7. DYNAMIC LOADS

a. General. This section describes methods for formulating and solving the airplane equations ofmotion and extracting dynamic loads from the airplane response. The airplane equations ofmotion are solved in either physical ormodal coordinates and include all terms important in the loads calculation including stiffness, damping, mass, and aerodynamic forces due to both airplane motions and gust excitation. Generally the aircraft equations are solved in modal coordinates. Forthe purposes ofdescribing the solution ofthese equations in the n:mainder of this AC, modal coordinates will be assumed. A sufficient number ofmodal coordinates should be included to ensure that the loads extracted provide converged values. b. Solution ofthe Equations ofMotion. Solution ofthe equations ofmotion canbe**~** achieved through a number oftechniques. For the continuousturbulence analysis, the equations of motion are generally solved in the frequency domain. Transfer functions which relate the output response quantity to an input harmonically oscillating gust field are generated and these transfer functions are used (in Section S.c. ofthis AC) to generate the RMS value ofthe output response quantity.
There are two primary approaches used to generate the output time histories for the discrete gust analysis; (1) by explicit integration ofthe airplane equations ofmoti.on in the time domain, and (2)by frequency domain solutions which can utilize Fouriertransform techniques.

c. Extraction ofLoads and Responses. The output quantities that may be extracted from a gust response analysis include displacements, velocities and accelerations at structural locations; load quantities such as shears, bending moments and torques on structural components; and stresses and shear flows in structural components. The calculation ofthe physical responses is given by a modal superposition ofthe displacements, velocities and accelerations ofthe rigid and elastic modes ofvibration ofthe airplane structure. The number ofmodes carried in the summation should be sufficient to ensure converged results.
A variety ofmethods may be used to obtain physical structural loads from a solution ofthe modal equations of motion governing gust response. These include the Mode Displacement method, the Mode Acceleration method, and the Force Summation method. All three methods are capable ofproviding a balanced set ofairplane loads. Ifan infinite number ofmodes can be considered in the analysis, the three will lead to essentially identical results. The Mode Displacement method is the simplest. In this method, total dynamic loads are calculated from the structural deformations produced by the gust using modal superposition. Specifically, the contribution ofa given mode is equal to the product ofthe load associated with the normalized deformed shape ofthat mode and the value ofthe displacement response given by

AC2S341-2.DOC October 20,1999 

### 15/20 As submitted for technical editing and preliminary legal review.the associated modal coordinate. For converged results, the Mode**D~splacement** method may need a significantly larger number ofmodal coordinates than the othertwo methods.
In the Mode Acceleration method, the dynamic load response is composed ofa static part and a dynamic part. The static part is determined by conventional static analysis (including rigid body "inertia relief'), with the externally applied gust loads treated as static loads. The dynamic part is computed by the superposition of appropriate modal quantities, and is a function ofthe number ofmodes carried in the solution. The quantities to be superimposed involve both motion response forces and acceleration responses (thus giving this method its name). Since the static part is determined completely and independently ofthe number ofnormal modes ca.rried, adequate accuracy may be achieved with fewer modes than would be needed in the**Modj~** Displacement method.
The Force Summation method is the most laborious and the most intuitive. In this method, physical displacements, velocities and accelerations are first computed by superposition ofthe modal responses. These are then used to determine the physical inertia forces and other motion dependent forces. Finally, -these forces are added to the externally applied forces to give the total dynamic loads acting onthe structure. Ifbalanced airplane load distributions are needed from the discrete gust analysis, they may be determined using time correlated solution results. Similarly, as explained in Section 5.c ofthis AC, ifbalanced airplane load distributions are needed from the continuous turbulence analysis, they may be determined from equiprobable solution results obtained using cross-correlation coefficients.
8. NONLINEAR CONSIDERA TrONS

a. General. Any structural, aerodynamic or automatic control system characteristic which may cause airplane response to discrete gusts orcontinuousturbulence to become nonlinear with respect to intensity or shape should be represented realistically or conservatively in the calculation ofloads. While many minor nonlinearities are amenable to a conservative linear solution, the effect ofmajor nonlinearities cannot usually be quantified without expllicit calculation. The effect ofnonlinearities should be investigated above limit conditions to assure that the system presents no anomaly compared to behaviour below limit conditions, in accordance with Paragraph 25.302 Appendix K(b)(2).
b. Structural and Aerodynamic Nonlinearity. A linear elastic structural model, and a linear (unstalled) aerodynamic model are normally recommended as conservative and acceptable for theunaugmented airplane elements ofa loads calculation. Aerodynamic models may be refined to take account ofminor nonlinear variation ofaerodynamic distributions, due to local separation etc., through simple linear piecewise solution. Local or complete stall ofa lifting surface would constitute a major nonlinearity and should not be represented without account being taken ofthe

AC25341-2.DOC October 20, 1999 

### 16/20 As submitted for technical editing and preliminary legal review.influence ofrate ofchange ofincidence, i.e., the so-called 'dynamic stall' in which the range of linear incremental aerodynamics may extend significantly beyond the static stall incidence.

c. Automatic Control System Nonlinearity. autopilots, stability control systems and load alleviation source ofnonlinear response. For example, Automatic flight control systems, systems often constitute the primary

• non-proportional feedback gains • rate and amplitude limiters • changes in the control laws, or control law switching • hysteresis • use ofone-sided aerodynamic controls such as spoilers • hinge moment performance and saturation ofaerodynamic control actuators The resulting influences on response will be airplane design dependent, and the manner in whichthey are to be considered will normally have to be assessed for each design. Minor influences such as occasional clipping of response due to rate or amplitude limitations, where it is symmetric about the stabilized I-g condition, can often be. represented through quasi-linear modeling techniques such as describing functions oruse ofa linear equivalent gam. Major, and unsymmetrical influences such as application of spoilers for load alleviation, normally require explicit simulation, and therefore adoption of an appropriate solution based in the time domain.
Theinfluence ofnonlinearities on one load quantity often runs contrary to the influence on otherload quantities. Forexample, an aileron used for load alleviation may simultaneously relieve wing bending moment whilst increasing wing torsion. Since it may not be possible to represent such features conservatively with a single airplane model, it may be conservatively acceptable to consider loads computed for two (possibly linear) representations which bound the realistic condition. Another example of this approach would be separate representation of continuous turbulence response for the two control law states to cover a situation where the airplane may occasionally switch from one stateto another. d. Nonlinear Solution Methodology. Where explicit simulation ofnonlinearities is required, the loads response may becalculated through time domain integration ofthe equations ofmotion.
For the tuned discrete gust conditions of Paragraph 2S.341(a), limit loads should be identified by peak values in the nonlinear time domain simulation response ofthe airplane model excited by the discrete gust model described in Section S.b. ofthis AC.

AC25341-2.DOC October 20,1999 

### 17/20 As submitted fortechnical editing and preliminary legal review.Fortime domain solution ofthe continuous turbulence conditions ofParagraph 25.341(b), a variety ofapproaches may be taken for the specification ofthe turbulence input time history and the mechanism for identifying limit loads from the resulting responses. It will normally be necessary tojustify that the selected approach provides an equivalent level of safety as a conventional linear analysis and is appropriate to handle: the types of nonlinearity on the aircraft. This should inciude verification that the approach provides adequate statistical significance in the loads results.
A methodology based upon stochastic simulation has been found to be acceptable for load alleviation and flight control system nonlinearities. In this simulation, the input is a long, Gaussian, pseudo-random turbulence stream conforming to a von Karman spectrum with a root-meansquare (RMS) amplitude of0.4 times Ua (defined in Section S.C.l ofthis AC). The value of limit load is that load with the same probability ofexceedance as AUa ofthe same load quantity in a linear model. This is illustrated graphically in Figure 5. When using an analysis ofthis type, exceedance curves should be constructed using incremental load values up, orjust beyond the limit load value.

LinearModel

LimitLoad Exceedence Level NonlinearModel

Nonlinear

Design Load



# Incremental Load Figure-5 Establishing Limit**Lo~d** for a Nonlinear Airplane

AC25341-2.DOC October 20,1999 Assubmitted fortechnical editing and preliminary legal review. 

### 18120The,nonlinear simulation may also be performed in the frequency domain ifthe frequency domain method is shown to produce conservative results. Frequency domain methods include, but are not limited to, Matched Filter Theory and Equivalent Linearization.
9. ANALYTICAL MODEL VALIDATION

a. General. The intent ofanalytical model validation is to establish that the analytical model is adequate for the prediction ofgust response loads. The following sections discuss acceptable but not the only methods ofvalidating the analytical model. In general, it is not intended that specific testing be required to validate the dynamic gust loads model. b. Structural Dynamic Model Validation. The methods and test data used to validate the flutter analysis models presented in AC 25.629-1A should also be applied to validate the gust analysis models. These procedures are addressed in the AC 25.629-1A.

c. Damping Model Validation. In the absence ofbetterinformation it will normally

- be acceptable to assume 0.03 (i.e. 1.5% equivalent critical viscous damping) for all flexible modes. Structural damping may be increased overthe 0.03 value tobe consistent with the high structural response levels causedby extreme gust intensity, providedjustification is given. d. Aerodynamic Model Validation. Aerodynamic modeling parameters fall into two categories: (i) steady orquasisteady aerodynamics governing static aeroelastic and flight dynamic airload distributions (ii) unsteady aerodynamics which interact with the flexible modes ofthe airplane. Flight stability aerodynamic distributions and derivatives may be validated by wind tunnel tests, detailed aerodynamic modeling methods (such as CFD) or flight test data. If detailed analysis or testing reveals that flight dynamic characteristics of the airplane**dim~r** significantly from those to which the gust response model have been matched, then the implications on gust loads should be investigated. The analytical and experimental methods presented in AC 25.629-1A for flutter analyses provide acceptable means for establishing reliable unsteady aerodynamic characteristics both for motion response and gust excitation aerodynamic force distributions. The aeroelastic implications on airplane flight dynamic stability should also be assessed.

e. Control System Validation. Ifthe airplane mathematical model used for gust analysis contains a representation ofany feedback control system, then this segment ofthe model should be validated. Thelevel ofvalidation that should be performed depends onthe complexity ofthe system and the particular airplane response parameterbeing controlled.**Systf:~ms** which control elastic modes ofthe airplane may require more validation than thosewhich control the airplane**rigi~** body response. Validation ofelements ofthe control system (sensors, actuators,

AC25341-2.DOC October 20, 1999 

### 19/20 As submitted for technical editing and preliminary legal review.anti-aliasing filters, control laws, etc.) which have a minimal effect on the output load and response quantities under consideration can be neglected. 

### It will normally be more convenient to substantiate elements of the control system independently, i.e. open loop, before undertaking the validation ofthe closed loop system. (1) System Rig orAirplane Ground Testing. Response ofthe system to artificial stimuli canbe measured to verify the following: • The transfer functions ofthe sensors and any pre-control system anti-aliasing orotherfiltering. • The sampling delays ofacquiring data into the control system. • Thebehavior ofthe control law itself • Any control system output delay and filter transfer function. • The transfer functions ofthe actuators, and any features ofactuation system performance characteristics that may influence the actuatorresponse to the maximum demands that might arise in turbulence; e.g. maximum rat(: of deployment, actuator hinge moment capability, etc. Ifthis testing is performed, it is recommended that following any adaptation ofthe model to reflect this information, the complete feedback path be validated (open loop) against measurements taken from the rig orground tests. (2) Flight Testing. Thefunctionality and performance ofany feedback control system can also be validated by direct comparison ofthe analytical model and measurement for input stimuli. Ifthis testing is performed, input stimuli should be selected such that they exercise the features ofthe control system and the interaction with the airplane that are significant in the use ofthe mathematical model for gust load analysis. These might include: • Airplane response to pitching and yawing maneuver demands. • Control system and airplane response to sudden artificially introduced demands such as pulses and steps. • Gain and phase margins determined using data acquired within the flutter test program. These gain and phase margins can be generated by passing known signals through the open loop system during flight test.

AC25341-2.DOC October 20,1999 As submitted for technical editing and preliminary legal review. 

### 20,20