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 Assignment 
Federal Register I Vol. 59. No. 111 I Friday. fune 10. 1994 I Notices 30081 

T",ks 

The Loads and Dynamics 

Harmonization Working Group's tasks 

(

as follows: 

Task J-lnteroction a/Systems and 

tructure: Review existing special ________________ conditions for fly-by-wire airplanes and 

Aviation Rulemaking Advi50ry 

Committee; Transport Alrp~e and 

Engine Issues 

ACENCY: Federal Aviation 

Administration (FAA). DOT. 

ACT1ON: Notice of new task assigrunents 

for the Aviation Rulemaling AdviSOry 

Committee. 

SU .... ARY: Notice is given of new task 

assigrunents for the Loads and 

Dynamics Harmonization Working 

Group oftha Aviation Rulemaking 

Advisory Committee (ARAC). This 

notice inforrr.s the public of the 

activities of the AR.-\G. 

FOR FURTMER INFORMATlON CONTACT: 

Mich~el H. Borfitz. Assistant Executive 

Director. Aviation Rulemaking Advisory 

Committee. Transport Airplane and 

Engine Issues. FAA Engine" Propeller 

Directorate. 12 New England Executive 

Park. Burlington. Massachusetts 01803; 

telephone (617) 238-7110. fax (617) 

238-7 1 99. 

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\'ice and 

rt"commendations to the FAA 

Administra!or. through the Associate 

Administrator for Regulation and 

Certification. on the full range olthe 

FAA's rulerr.aking activitias with 

respect to aviation· related issues .. 

In order to develop such advice and 

recommendations. the hRAC may 

choose to establish working groups to 

which specific tasks are aSSigned. Such 

working groups are comprised of 

experts from those organizations having 

an interest in the aSSigned taJO:ks. A 

working group member need not be a 

representative of the full committee. 

One of the working groups established 

by the ARAC is the Loads and Dynamics 

Hannonizalion Working Group. 

The F.-\A announced at the Joint 

Aviation Authorities (JAA)·Federal 

Aviation Administration (FAA) 

Harmonization Conference in Toronto. 

Canrida June 2-5. 1992. that it would 

l:onsolidate within the ARAC struclure 

an ongoing objective to "hannonize" the 

Jo:nt Aviation Requirements (JAR) and 

the Federal Avio:tion Regulations (FAR). 

existing requirements for control 

systems. including automatic andlor 

power-operated systems, and 

recommend to the ARAC any new 

revised. general requirements needed for 

flight control systems and structures 

affected by those systems (§S 25.302. 

25.671.25.1329. part 25 appendix K). 

Task 2--Continuous Turbulence 

Loads: Review the requirement for the 

continuous turbulence standard in light 

of the ARAC proposal for a tuned 

discrete gust requirement in order to 

determine whether the continuous 

turbulence requirement should be 

revised or removed from the FAR/JAR 

for better consistency with the new 

proposed tuned discrete gust criteria 

, (§ 25.305(d)). 

Task 3-Strength ond Deformotion: 

Review the recent requirements adopted 

in the FAR by Amendment 25-71 (for 

the design of transport airplanes against 

buffet and forced structural vibrations) 

and consider appropriate changes for 

the JAR and FAR to harmonize these 

rules (§S 25.305 (e) and (01. 

Tosk 4-Design Flop Speeds: Review 

the current flap design loads 

requirements to resolve differences in 

interpretation between the FAA and 

JM concerning the structural design 

stall speeds on which the flap design 

speeds are based. Recenl measurements 

of gust speeds al low altitudes. where 

flaps are normally extended. indicate a 

more severe gust environment may be 

present. Review all aspects of the flap 

design load requirements. including the 

design airspeeds. vertical and head-on 

design gust criteria, and the effects of 

automatic retraction and load relief 

systems (§ 25 .335(e)). 

Task 5-ResiduaJ Strfmgth Loads for 

Damage Tolerance: Review the 

differences in residual strength design 

load requirements between the FAR and 

JAR and resolve differences to 

harmonize this rule. Prepare a Notice of 

Proposed Rulemaking or make 

recommendations to other ARAC efforts 

concerning FAR §25.571, so that they 

can be included in rulemaking that may 

be forthcoming from those efforts 

(§2S.571(b)). 

Tosk 6-Shock A.bsorption Tesfs: 

Review the changes recently introdured 

into the JAR that have resulted in 

differe'nces between the FAR and JAR in 

regard to the requirement for shock 

absorption tests. Review those r.banges 

in view of hannonizing the FAR and 

JAR (§ 25.723(a)). 

Task 7-Rough Air Speed: The AMC 

has proposed a new § 25.1517 

concerning rough air speed design 

standards in its proposal for a tuned 

discrete gust requirement. This action is 

hannonized with the current JAR 

25.1517; however. further changes in 

the rough air speed requirement may be 

needed in both the FAR and IAR. 

Review JAR 25.1517 and the new 

proposed FAR 25.1517 to determine if 

further changes are needed. If so. 

prepare a Notice of Proposed 

Rulemaking. or. if possible. comhine 

these changes with other rule'making 

efforts (§ 25.1517). 

Task s-Taxi. Takeoff. and wnding 

RoU: Prepare an advisory circular that 

establishes criteria 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-Broked Roll Conditions: 

Review the provisions of § 25.493 of the 

FAR and JAR concerning the braked roll 

condition and finalize a harmonized 

Notice nfProposed Rulemaking. 

Reports 

For each task listed. the Loads and 

Dynamics Harmonization Working 

Group should develop and present to 

the ARAC: 

1. A recommended work plan for 

completion of the task. including the 

rationale supporting such plan. for 

consideration at the meeting of the 

ARAC to consider transport airplane 

and engine issues held following 

publication of this notice; 

2. A detailed conceptual presenttltion 

on the proposed recommendalion(JO:). 

prior to proceeding with the work sttlted 

in item 3. below; 

3. A draft Notice of Proposed 

Rulemaking. with supporting econ omic 

and other required analyses. andlof any 

other related guidance malerial or 

collateral documents the working gfOUp 

determines to be appropriate; or. if new 

or revised requirements or compliance 

methods am not recommended. a draft 

report stating the rat ionale [or not 

making such recommendations; and 

4. A status report at each meeting of 

the ARAC held to consider transport 

airplane and engine issues. 

ParttcipatioD in Working Group Task 

An individual who hu expertise in . 

the subject maHer and wishes to become 

a member of the working group should 

write to the periOn listed under the 

caption FOR FURTHER INFOR .. ATION 

CONTACT expressing that desire. 

descrihing his or her interest in the 

task(~). and Slating the expertise he or 
she would bring to the working group. 

The request will be reviewed with the 

assistant chairman and working group 

leader. and the individual will be 

advised whether or not the request can 

be attornmodated. 

The Secretary of Transportation has 

determined that the inform.lion and use 

of the Aviation Rulemaking Advisory 

Committee are necessary in the public 

interest in connection with the 

performance of duties imposed on the 

FAA by law. Meetings ortha Aviation 

Rulemaking Advisory Committee will 

be open to the public. except as authorized by section 10(d} or the 

Federal Advisory Committee Act. 

Meetings of the working group will not 

be open to the public. except to the 

extent that individuals with 8n interest 

and expertise are selected to participate. 

No public announcement of working 

group meetings will be made. 

Issued in Washington. OC, on June 3, lw... 

0u-i5 A. ChrUI:ie. 

£JI«uliv~ DirK101". "'vialion Ru/~mo kinB 

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 

DEPARTMENT OF TRANSPORTATION 

Federal Aviation Administration 

[14 CFR part 25] 

[Docket No. ; Notice No. 

RIN: 

Revised Requirements for Gust and Continuous Turbulence Design Loads 

AGENCY: Federal Aviation Administration, DOT. 

ACTION: Notice of proposed rulemaking. 

SUMMARY: This notice proposes to revise the continuous turbulence design loads of the 

Federal Aviation Regulations (FAR) for transport category airplanes by incorporating changes 

developed in co-operation with the Joint Aviation Authorities (JAA) of Europe and the U.S., 

Canadian and European aviation industries through the Aviation Rulemaking Advisory Committee· 

(ARAC). This action is necessary because recent measurements of derived gust intensities in 

actual operation show that the current requirements do not accurately account for the distribution 

of turbulence in the atmosphere. Also, one of the 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 of aircraft mission. In addition to these issues regarding 

continuous turbulence, 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 continuous turbulence 

by revising the turbulence 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 or before [insert a date 120 days after the date of 

publication in the Federal Register] 

ADDRESSES: Comments on this notice may be mailed in triplicate to: Federal Aviation 

Administration (FAA), Office of the Chief Counsel, Attention: Rules Docket (AGC-10), Docket 

No. ,800 Independence Avenue SW., Washington, DC 20591; or delivered in triplicate to: 

Room 915G, 800 Independence Avenue SW., Washington, DC 20591. Comments delivered must 

be marked Docket No. . . Comments may also be submitted electronically to 
Revised 9/22/l9990raft Continuous 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 of comments 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. 

FOR FURTHER INFORMATION CONTACT: James Haynes, Airframe and Propulsion 

Branch, ANM-112, Transport Airplane Directorate, Aircraft Certification Service, FAA, 1601 

Lind Avenue, SW., Renton, WA 98055-4056; telephone (206) 227-213l. 

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 or notice number and submit comments in triplicate to the 

Rules Docket address above. All comments received on or before the closing date for comments 

will be considered by the Administrator before taking action on this proposed rulemaking. The 

proposals contained in this notice may be changed in light of comments received. All comments 

received will be available in the Rules Docket, both before and after the comment period Closing 

date, for examination by interested persons. A report summarizing each substantive public 

contact with FAA personnel concerning this rulemaking will be filed in the docket. Persons 

wishing the FAA to acknowledge receipt of their comments must submit with those comments a 

self-addressed, stamped postcard on which the following statement is made: "Comments to 

Docket No. ." The postcard will be date/time stamped and returned to the commenter. 

Availability of NPRM 

An electronic copy of this document may be downloaded using a modem and suitable 

communications software from the FAA regulations section of the F edworld electronic bulletin 

board service (telephone: 703-321-3339), the Fede~al Register's electronic bulletin board service 

(telephone: 202-512-1661), or the FAA's Aviation Rulemaking Advisory Committee Bulletin 

Board service (telephone: 202-267-5984). 

2 
Revised 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 or the Federal 

Register's web page at http://www.access.gpo/su_docs for access to recently published 

rulemaking documents. 

Any person may obtain a copy of this notice by submitting a request to the Federal Aviation 

Administration, Office Rulemaking, ARM-I, 800 Independence Avenue SW., Washington, DC 

20591; or by calling (202) 267-9680. Communications must identify the notice number of this 

NPRM. Persons interested in being placed on a mailing list for future rulemaking documents 

should also request a copy of Advisory Circular No. 11-2A, Notice of Proposed Rulemaking 

Distribution System, that describes the application procedures. 

Background 

The manufacturing, marketing and certification of transport airplanes is increasingly an 

international endeavor. In order for 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 of the countries to which the airplane is to be exported. 

The European countries have developed a common airworthiness code for transport 

airplanes that is administered by the Joint Aviation Authorities (JAA) of Europe. This code is the 

result of a European effort to harmonize the various airworthiness codes of the European 

countries and is called the Joint Aviation Requirements (JAR)-25. It was developed in a format 

similar to part 25. Many other countries have airworthiness codes that are aligned closely to part 

25 or to JAR-25, or they use these codes directly for their own certification purposes. Since 

1988, the FAA and JAA have been working toward complete harmonization ofJAR-25 and part 

25. 

The Aviation Rulemaking Advisory Committee (ARAC) was established by the FAA on 

February 15, 1991, with the purpose of providing information, advice, and recommendations to 

be considered in rulemaking activities. The FAA and JAA are continuing to work toward the 

harmonization of JAR-25 and part 25 by assigning ARAC specific tasks .. By notice in the Federal 

Register (59 FR 30081, June 10, 1994), the FAA assigned several new tasks to an ARAC 

working group of industry and government structural loads specialists from Europe, the United 

States, and Canada. Task 2 of this charter concerned the requirement to account for continuous 

3 
Revised 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 of recent revisions to the discrete gust requirement of 

Amendment 25-86 (61 FR 5218) in order to determine if the continuous turbulence requirement 

was still needed and if it was in need of revision 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 FAA by letter dated 

The current requirement to account for the loads produced by continuous turbulence 

(sometimes referred to as continuous gusts) was proposed by the FAA in Notice 68-18 (33 FR 

11913, August 22, 1968). This proposal was the culmination of a research effort by the U.S. 

aviation industry under a contract by the FAA to develop methods for treating loads resulting 

from flight in continuous turbulence. The rules in effect at that time required only the 

consideration of the response of the airplane to discrete gusts. The FAA stated in Notice 68-18 

that the discrete gust requirement accounted for the flexibility of the airplane but not necessarily 

the combination of elastic and rigid body motions. The basic objective of the FAA sponsored 

research effort was to develop methods of accounting for continuous turbulence loads by 

considering the statistical nature of turbulence in combination with both the elastic and rigid body 

modes of the airplane. The results of that effort were published in FAA Technical Reports 

ADS-53 and ADS-54 in 1966. Subsequently the FAA amended part 25 to require the 

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 of the 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 FR 24802, March 7, 1975). The FAA 

subsequently amended part 25 by adding appendix G (Amendment 25-54, 45 FR 60154, 

September 11, 1980) that set forth the two methodologies (design envelope and mission analysis) 

and specified the levels of required gust intensities for use in design. Section 25.305(d) was also 

4 
Revised 9122/1999Draft Continuous Turbulence NPRM June 24, 1999 

Revised 25.34l(c) number headings 9/3/99 

changed by 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 of contention and debate since the publication of the proposal for Appendix G. Several 

commenters to 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 

FAA recognized these shortcomings but, in the interest of safety, decided to go ahead with the 

requirement with the intention of refining the criteria as more information became available. The 

requirement provided a sea level value for gust intensity of 85 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 adequate by service experience. The phrase "dynamically similar 

model" has been subject to a wide range of interpretations and has resulted in non uniform 

application of the rule. In addition, the concept of adjusting the gust intensity based on dynamic 

similarity with another airplane is questionable since the need for a different gust intensity is 

related more to the intended operation of the airplane, rather than its dynamic characteristics. 

The alternative mission analysis method has also been the subject of considerable debate 

and controversy. With this method, the manufacturer must define a mission for the airplane which 

includes range, altitude, payload and other operational variables. Then, using a statistical model 

of the atmosphere, the manufacturer must show that 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 after the airplane goes into operation. Furthermore, the 

mission analysis design loads are sensitive to small changes in the definition of the aircraft mission. 

Therefore, small variations in approach can provide inconsistent results. 

Additional shortcomings in the current continuous turbulence requirement have been 

brought to light by experience in applying the current criteria, experience in service, and by the 

changing design features of transport airplanes. Many transport airplanes now incorporate 

automatic flight control systems and other features that can result in significant non-linearity's 

while the methodology normally employed for continuous turbulence is inherently linear. 

Efforts to better define the atmospheric model have continued since the adoption of 

Appendix G. Recent flight measurement programs conducted by FAA and the National 

5 
Revised 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 data recorders (DFDR) to derive gust load design information for airline 

transport airplanes. The Civil Aviation Authority (CAA) of the United Kingdom has conducted a 

comprehensive DFDR gust measurement program for transport airplanes in airline service. The 

program, called CAADRP (Civil Aircraft Airworthiness Data Recording Program), has resulted in 

an extensive collection of reliable gust data which has provided an improved insight into the 

distribution of gusts in the atmosphere. 

Recently, the regulatory authorities and the aviation industries of the U.S., Canada and 

Europe have engaged in studies with the aim of finding 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 to be sufficient, at this time, for 

treating both phenomena. The FAA believes that it is necessary to proceed with the improvement. 

and harmonization of the 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 FAA recently revised § 25.341 of the part 25 (Amendment 25-86,61 FR 5218, dated 

February 9, 1996 ) to provide a revised discrete gust methodology along with a refined gust 

distribution model of the atmosphere based on the 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 of part 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 current turbulence 

intensity model is inconsistent with the CAADRP data, and with the new atmospheric model 

prescribed for discrete gusts, and is in need of updating to accommodate modern transport 

airplanes. 

Discussion 

6 
Revised 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 continuous turbulence, elimination of the mission analysis method, 

provisions for treating non-linearities, and reorganization and clarification of the requirement. 

The FAA proposes to retain the design envelope criterion, but with a revised gust intensity 

distribution with altitude. The proposed gust intensities are based on analysis of gust 

measurements from the CAADRP program. The CAADRP data is the most recent gust 

information available and it represents measurements of gusts and turbulence on transport 

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 of the airplane. The FAA considers this to be a reliable means of 

accounting for airplane mission and it would be capable of being applied in a uniform manner. 

One member of the ARAC Working Group objected to the definition of a flight profile 

alleviation factor that changes the design turbulence intensity versus altitude based on selected 

aircraft design parameters. That member believed that the once in 70,000 hour gust represented 

an acceptable level of turbulence for design purposes. He accepted that the intensity of the 70,000 

hour gust properly varies with altitude; but he believed the probability of encountering a gust of 

that intensity at any point in time should be constant, regardless of the design parameters of a 

particular aircraft. 

The majority of the ARAC Working Group disagreed. In their view the proposal does not 

assume that atmospheric turbulence is dependent upon aircraft speed and altitude, or any other 

aircraft design parameter. The flight profile alleviation factor is simply a mathematical device that 

allows the expected operation of the 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. The flight 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 FR 5218, February 9, 1996). To support this proposal, an effort has been 

undertaken by the industries and airworthiness authorities of the United States, Canada and 

Europe to 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 

7 
Revised 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 to represent the best available measurements of the turbulence 

environment in which the airplane is likely to be operated. 

The mission analysis method for accounting for continuous turbulence loads would be 

eliminated as an option since the use of this method can provide inconsistent results depending on 

the assumptions made concerning the potential use of the airplane. The elimination of this method 

would not be significant since few manufacturers currently use 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 of advanced flight control systems into transport airplanes has presented 

special problems in the treatment of continuous turbulence. Some of these systems can exhibit 

significant non-linearities, while the standard mathematical approaches to continuous turbulence 

(i.e. frequency domain solutions) are valid only for linear systems. The current rule requires 

consideration of non-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 

itself could exhibit significant non-linearities. The proposed rule would require that any 

significant non-linearity be considered in a realistic or conservative manner, and it would provide 

additional criteria which can be used with other rational approaches that can account for nonlinearities (e.g. time domain solutions). 

The elimination of the mission analysis criterion would simplify the presentation of the 

continuous turbulence requirement so that the requirement can be conveniently presented directly 

in Subpart C rather than 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 on wing-mounted engines. Although the FAA believes that the existing 

8 
Revised 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 adequate for 

current and future configurations, there remains a possibility that a multi-axis gust encounter 

could produce higher loads under certain situations. To address the NTSB concern, the FAA 

contracted an independent organization to develop a method of performing multiaxis discrete gust 

analysis for wing mounted nacelles. The results of that 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). The current _ 

§ 25.445 already requires the effects of combined gust loading to be considered on auxiliary 

aerodynamic surfaces such as outboard fins and winglets. Furthermore, the current § 25.427(c) 

requires the effects of combined gust loading to be considered on some empennage arrangements 

such as T -tails. For airplanes with wing mounted engines, this proposal would extend the round 

the clock dynamic discrete gust criterion to wing 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 to the engine 

mounts, pylons and wing supporting structure. 

Section 25.571, "Damage tolerance and fatigue evaluation of structure", currently 

references the entire section 25.341 as one source of residual 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 FR 5218, 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 

9 
Revised 9/22/ I 999Draft 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 of the discrete gust in § 25.341(a) as amended (Amendment 25-86,61 

FR 5218, 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 of paragraph 25.341(a) and not the continuous turbulence of25.341(b). 

However, the FAA believes 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 of safety 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 of the discrete gust in § 25.341(a) as amended (Amendment 25-86,61 

FR 5218, 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 of paragraph 25.341(a) and not the continuous turbulence of25.341(b). However, 

the FAA believes 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 of safety 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 of the discrete gust in § 25.341(a) as amended (Amendment 25-86,61 

FR 5218, February 9, 1996), paragraph 25.371 "Gyroscopic loads" was amended as a conforming 

change so that gyroscopic loads were associated only with the discrete gust of paragraph 

25.341(a) and not the continuous turbulence of25.341(b). However, the FAA believes that in 

order to meet the level of safety intended by the revised continuous turbulence requirements it will 

be necessary to include gyroscopic effects, where appropriate, in calculation of total loads due to 

continuous turbulence. To this end a change is proposed to 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 Continuous Turbulence NPRM June 24, 1999 

Revised 25.341(c) number headings 9/3/99 

With the adoption of the 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 only the discrete 

gust of paragraph 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 order to 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, FAA believes that encounters with continuous turbulence can result in the activation of 

speed brakes 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. To this 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 of the 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 of paragraph 25.341(a) and not the continuous turbulence of25.341(b). However, 

the FAA believes 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 of safety 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, "the design 

speed for maximum gust intensity". The design turbulence intensities established for the gust 

design conditions at V c, "structural design cruising speed," and V D, "structural design diving 

speed," were developed in consideration of the 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 or continuous turbulence 

11 
Revised 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 speed based upon VB. 

Therefore, it is proposed to amend § 25.1517 to remove the link between VRA and VB. 

Paperwork Reduction 

In accordance with the Paperwork Reduction Act of 1995 (44 U.s.c. 3507(d)), there are 

no requirements for information collection associated with this proposed rule. 

International Compatibility 

The FAA reviewed the corresponding International Civil Aviation Organization regulations, 

where they exist, and has identified no differences in these proposed amendments and the foreign 

regulations. The FAA 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 or adopt a regulation only 

upon a reasoned determination that the benefits of the intended regulation justify its costs. 

Second, the Regulatory Flexibility Act of 1980 requires agencies to analyze the economic effect of 

regulatory changes on small entities. Third, the Office of Management and Budget directs 

agencies to assess the effects of regulatory changes on international trade. In conducting these 

analyses, the FAA has determined that this rule: (1) would generate benefits that justify its costs 

and is not a "significant regulatory action" as defined in the Executive Order; (2) is not significant 

as defined in DOT's Regulatory Policies and Procedures; (3) would not have a significant impact 

on a substantial number of small 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] 

Regulatory Flexibility Determination 

The Regulatory Flexibility Act of 1980 (RF A) was enacted by Congress to ensure that 

small entities are not unnecessarily and disproportionally burdened by Federal regulations. The 

12 
Revised 9/22/1 9990raft Continuous Turbulence NPRM June 24, 1999 

Revised 25.341(c) number headings 9/3/99 

RF A requires agencies to determine whether rules would have "a significant economic impact on 

a substantial number of small 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 RF A requirements in FAA rulemaking actions. The Order 

defines "small entities" in terms of size 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 of the affected small entities. 

The proposed rule would affect manufacturers of transport category airplanes produced 

under future new airplane type certifications. For airplane manufacturers, FAA Order 2100 .14A 

specifies a size threshold for classification as a small entity as 75 or fewer employees. Since no 

part 25 airplane manufacturer has 75 or fewer employees, the proposed rule would not have a 

significant economic impact on a substantial number of small 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 

the U.S. market. Instead, by harmonizing the standards of the FAR and the JAR. it would lessen 

restraints on trade . 

. Federalism Implications 

The regulations proposed herein would not have substantial direct effects on the states, on 

the relationship between the national government and the states, or on the distribution of power 

and responsibilities among the various levels of government. Thus, in accordance with Executive 

Order 12612, it is determined that this proposal does not have sufficient federalism implications to 

warrant the preparation of a Federalism Assessment. 

Conclusion 

Because the proposed changes to the continuous turbulence design load requirement are 

not expected to result in any substantial economic costs, the FAA has determined that this 

proposed regulation would not be significant under Executive Order 12866. Because this is an 

issue that has not prompted a great deal of public concern, the FAA has determined that this 

action is not significant under DOT Regulatory Policies and Procedures (44 FR 11034; February 

25, 1979). In addition, since there are no small entities affected by this rulemaking, the FAA 

13 
Revised 9/22/l9990raft Continuous Turbulence NPRM June 24, 1999 

Revised 25.341(c) number headings 9/3/99 

certifies that the rule, if promulgated, would not have a significant economic impact, positive or 

negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility 

Act, since none would be affected. A copy of the regulatory evaluation prepared for this project 

may be examined in the Rules Docket or obtained from the person identified under the caption 

"FOR FURTHER INFORMATION CONTACT." 

List of Subjects in 14 CFR part 2S 

_Air transportation, Aireraft, Aviation safety, Safety. 

The Proposed Amendments 

Accordingly, the Federal Aviation Administration (FAA) proposes to amend 14 CFR part 

25 of the Federal Aviation Regulations (FAR) as follows: 

PART 25 - AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY 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. By removing Appendix G to part 25, "Continuous Gust Design Criteria" and marking it 

"Reserved" . 

3. To amend Section 25.341 by revising paragraph 25.34 1 (a)(5)(i) to read as follows: 

(a) * * * * * 

(5) The following reference gust velocities apply: 

(i) At airplane speeds between VB and V c: 

Positive and negative gusts with reference gust velocities of 56.0 ft/sec EAS must be considered 

at sea level. The reference gust velocity may be reduced linearly from 56'0 ft/sec EAS at sea level 

to 44.0 ftlsec EAS at 15 000 feet. The reference gust velocity may be further reduced linearly 

from 44.0 ft/sec EAS at 15000 feet 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 of the airplane to 

vertical and lateral continuous turbulence must be taken into account. The dynamic analysis must 

14 
Revised 9/22/1999Draft Continuous Turbulence NPRM June 24, 1999 

Revised 25.341(c) number headings 9/3/99 

take into 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) of this section, the following 

equation must be used: 

PL = PL- tg ± Ucr A 

Where--

PL = limit load; 

PL-tg = steady I-g load for the condition; 

A = ratio of root-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 of A 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 structure to the atmospheric turbulence; and 

<1>(n) = normalized power spectral density of atmospheric turbulence given byL <1>(n) = -I 

Where1 + % (1.339Lny 

r; ,]~ r + (1.339Lny 

n = reduced frequency, radians per foot.; and 

L = scale of turbulence = 2,500 ft. 

(3) The limit turbulence intensities, Ucr, in feet per second true airspeed required for 

compliance with this paragraph are-

(i) At airplane speeds between VR and Ve: 

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

Vcr = Vcrref F g 

WhereRevised 9122/1 999Draft Continuous Turbulence NPRM June 24, 1999 

Revised 25.341(c) number headings 9/3/99 

Voref is the reference turbulence intensity that varies linearly with altitude from 90 fps 

(T AS) at sea level to 79 fps (T AS) at 24000 feet and is then constant at 79 fps 

(T AS) up to the altitude of 60000 feet. 

F g is the flight profile alleviation factor defined in paragraph (a)( 6) of this section; 

(ii) At speed VD: Vcr is equal to 112 the values obtained under subparagraph (3)(i) of this 

paragraph. 

(iii) At speeds between V C 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 of the airplane is included 

in the analysis, the effects of system non-linearities on loads at the limit load level must be taken 

into account in a realistic or conservative manner. 

(5) If necessary for the assessment of loads 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 

the Vcr values specified in subparagraph (3). The value oflimit load is that load with the same 

probability of exceedance in the turbulence field as A Vcr of the same load quantity in a linear 

approximated model. 

(c) Supplementary gllst conditions for wing mounted engines. For airplanes equipped 

with wing mounted engines, the engine mounts, pylons, and wing supporting structure must be 

designed for the maximum response at the nacelle center of gravity 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 of discrete gusts, one vertical and one lateral. The length of each of these gusts 

must be independentlyindependantly tuned to the maximum response in accordance with 

25.341(a). The penetration of the 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 

16 
Revised 9122/ I 999Draft Continuous Turbulence NPRM June 24, 1999 

Revised 25.341(c) number headings 9/3/99 

gust pair. In the absence of a more rational analysis, the following formula must be used for each 

of the maximum engine loads in all six degrees of freedom: 

deleted (3) ahead of below equation 

PL = PL- 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% of the gust 

velocities prescribed in § 25.341(a)(4) and 85% of the 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 power unit 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 or auxiliary power unit at the 

maximum rpm appropriate to the condition. For the purposes of compliance 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: 

17 
Revised 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: 

§ 25.1517 Rough 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 of perceptible buffeting. 

2) is sufficiently less than the maximum operating speed to ensure that likely speed 

variation during rough air encounters will not cause the overs peed warning to operate too 

frequently. 

In the absence of a rational investigation substantiating the use of other values, V RA must be less 

than VMo-35 KTAS. 

(b) At altitudes where V MO is limited by Mach number, a rough air Mach number MRA, for 

use as the recommended turbulence penetration Mach number, may be chosen to provide an 

optimum margin between low and high speed buffet boundaries." 

Issued in Washington D. C. on 

18 
u.s. Department 

of Transportation 

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 of compliance 

with the provisions of FAR Part 25 of the Federal Aviation Regulations (FAR) dealing with 

discrete gust and continuous turbulence dynamic loads. 

2. RELATED FAR SECTIONS. The contents of this AC are considered by the FAA in 

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 of structure 

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 of FAR Part 25. It provides some of the acceptable methods of 

modeling airplanes, airplane components, and configurations, and the validation of those modeling 

methods for the purpose of determining the response of the 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 of sophistication and detail required in the modeling depends on 

the complexity of the airplane and its systems. 

AC25341-2.DOC October 20,1999 1,20 

As submitted for technical editing and preliminary legal review. 
A TMOSPHERIC t----__ ~ 

MOTION 

AERODYNAMIC 14-_~ 

SYSTEM 

DYNAMICS 

AND 

STRUCTURE 

STATICFLIGHT I--__ ~ 

LOADS MODEL 

CONTROL 

SYSTEM 

Figure 1 Basic Elements of the Gust Response Analysis 

Design loads for encounters with gusts are a combination of the steady level I-g flight 

loads, and the gust incremental loads including the dynamic response of the 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 aero elastic 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 of complexity, 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-g flight conditions. 

• The discrete and continuous gust models of atmospheric turbulence. 

• Detailed representation of the airplane system including structural dynamics, 

aerodynamics, and control system modeling. 

AC25341-2.DOC October 20, 1999 

As submitted for technical editing and preliminary legal review. 
• Solution of the equations of motion and the extraction of response 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 of airplane configuration, weight, center of gravity, 

payload, fuel load, thrust, speed, and altitude. 

b. Steady Level I-g Flight Loads. The total design load is made up of static and 

dynamic load components. In calculating the static component, the airplane is assumed to be 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 if significant. 

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 of possible steady-state conditions that could be achieved in that flight condition. Typically, 

this would include consideration of effects 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 of speed brakes, flaps, or other drag or high lift devices, while they should be included in 

the steady-state condition, may be neglected in the calculation of incremental loads. 

d. Damage Tolerance Conditions. Limit gust loads, treated as ultimate, need to be 

developed for the structural failure conditions considered under Paragraph 25.571(b). Generally, 

for redundant structures, significant changes in stiffness or geometry do not occur for the types of 

damage under consideration. As a result, the limit gust load values obtained for the undamaged 

aircraft may be used and applied to the failed structure. However, when structural fllilures of the 

types considered under Paragraph 25.571(b) cause significant changes in stiffness or geometry, or 

both, these changes should be taken into account when calculating limit gust loads for the 

damaged structure. 

5. GUST MODEL CONSIDERATIONS. 

a. General. The gust criteria presented in Paragraph 25.341 consist of two models of 

atmospheric turbulence, a discrete model and a continuous turbulence model. It is beyond the 

scope of this AC to review the historical development of these models and their associated 

parameters. This information can be found in the preamble to FAR Part 25. This AC focuses on 

the application of those gust criteria to establish design limit loads. The discrete gust model is 

AC25341-2.DOC October 20,1999 3/20 

As submitted for technical editing and preliminary legal review. 
used. to represent single discrete extreme turbulence 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 of th(! airplane. 

b. Discrete Gust Model 

(1) Atmosphere. The atmosphere is assumed to be one dimensional with the gust 

velocity acting normal (either vertically or laterally) to the direction of airplane travd. The onedimensional assumption constrains the instantaneous vertical or lateral gust velocities to be the 

same at all points in planes normal to the direction of airplane 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 of the 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 of altitude in Paragraph 25.341(a)(5) and are given in terms 

offeet per second equivalent airspeed for a gust gradient distance, H, of350 feet. 

(B) Flight Profile Alleviation Factor, Fg - The reference gust velocity, UREF, is a 

measure of turbulence intensity as a function of altitude. In defining the value of Uref at each 

altitude, it is assumed that the aircraft is flown 100% of the time at that altitude. The factor F g is 

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 g is 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 of the 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 of the 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 = 50 Ft ~H=170Ft\ o 100 200 300 400 500 600 700 

Gust Penetration Distance, 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 of techniques. These include the explicit integration of the airplane 

equations of motion in the time domain, and frequency domain solutions utilizing Fourier 

transform techniques. These are discussed further in Section 7.0 of this AC. 

Maximum incremental loads, P1i . are identified by the peak values selected from time 

histories arising from a series of separate, 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, P1i for 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 of the load quantity, PLi, are 

calculated for the same instant in time using the expression: 

PLj = P(1-g)j ± PIj 

AC25341-2.DOC October 20,1999 

As submitted for technical editing and preliminary legal review. 
Note that in the case of a nonlinear aircraft, maximum positive inc~ementalloads may 

differ from maximum negative incremental loads. 

When calculating stresses which depend on a combination of external loads it may be 

necessary to consider time correlated load sets at time instants other than those which result in 

peaks for individual external load quantities. 

(3) Round-The-Clock Gust. When the effect of combined vertical and lateral gusts on 

airplane components is significant, then round-the-clock analysis should be conducted on these 

components and supporting structures. The vertical and lateral components of the 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, or horizontal tail surfaces having 

appreciable dihedral or anhedral (i.e., greater than 10°), or components supported by other lifting 

surfaces, for example T -tails, outboard fins and winglets. Whilst the round-the-clock load 

assessment may be limited to just 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 of the 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 of the load time histories induced 

from pure vertical and pure lateral gusts. The resultant incremental design value for a particular 

load of interest 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 of the 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) of this 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 of the combined gust pair is the same as that of a single discrete gust as 

defined by FAR paragraph 25.341(a) as described in Section 5(b)(I) of this AC. To achieve this 

equal-probability condition, in addition to the reductions in gust amplitudes that would be 

applicable if the input were a multi-axis Gaussian process, a further factor of 0.85 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 October 20, 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 - For a particular aircraft flight condition, the calculation 

of a specific response load requires that the amplitudes, and the time phasing, of the two gust 

components be 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) of this AC. In general, 

the vertical and lateral tuned gust lengths and the times to maximum response (measured from 

the onset of each 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 of the 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. The vertical 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 of load quantity i is at a maximum value, are 

added to yield the multi-axis incremental time-correlated value P1j for load quantity j. 

The set of time correlated design loads, PLj , corresponding to the peak value of the 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 of extemalloads it may be 

necessary to consider time 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 of continuous gust responses 

is assumed to be one dimensional with the gust velocity acting normal C either vertically or 

laterally) to the direction of airplane 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 of airplane 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 of turbulence, 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 V is 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. It is 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 of the associated gust field at each altitude. In defining the value OfUo-REF 

at each altitude, it is assumed that the airplane is flown 100% of the time at that altitude. The 

factor F g is then applied to account for the probability of the airplane flying at any given altitude 

during its service lifetime. 

It should be noted that the reference gust velocity is comprised of two 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. of this 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. For linear 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 of the turbulence 

and is defined in Section S.c. of this 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. of this 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 (or equiprobable) 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 of the complex function contained within the brackets. In 

this equation, the lowercase subscripts, i and j, 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 for technical 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 of the correlation coefficient method is required. For example, 

when the value of stress depends on two externally applied loads, such as torsion and shear, the 

equiprobable relationship between the two parameters forms an ellipse as illustrated in Figure 4. 

Design Value 

of Shear 

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 

of Torsion 

rEqual Probability 

Design Ellipse 

Figure-4 Equal Probability Design Ellipse 

In this figure, the points of tangency, T, correspond to the expressions for correlated load 

pairs given by the foregoing expressions. A practical additional set of equiprobable load pairs that 

should be considered to establish critical design stresses are given by the points of tangency 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): 

For tangents to lines AB and EF 

AC25341-2.DOC October 20,1999 11/20 

As submitted for technical editing and preliminary legal review. 
PLi = P(l-g)i +/- A iVa [(1 - pij)/2] 112 

and PLj = P(l-g)j -/+ A jVa [( 1 - P ij)/2] 1/2 

For tangents to lines CD and GH 

PLi = P(l-g)i ± A iVa [(1 + P ij)/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 of external load quantities is to evaluate directly the transfer function for the stress 

quantity of interest 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 MODELING CONSIDERATIONS 

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 of this AC for airplanes having properties and/or 

systems not reasonably or conservatively approximated by linear analysis methods. 

b. Structural Dynamic Model. The model should include both rigid body and flexible 

airplane degrees of freedom. If a modal approach is used, the structural dynamic model should 

include a sufficient number of flexible airplane modes to ensure both convergence of the modal 

superposition procedure and that responses from high frequency excitations are prolPerly 

represented. 

Most forms of structural 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 of deformation 

of the primary structure and control surfaces. Results from the models should be compared to test 

data as outlined in Section 9.b. of this AC in order to validate the accuracy of the model. 

c. Structural Damping. Structural dynamic models may include damping properties 

in addition to representations of mass and stiffness distributions. In the absence of better 

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 over the 0.03 value to be 

AC25341-2.DOC October 20, 1999 

As submitted for technical 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 of unsteady 

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 of the airplane. Generally, three-dimensional panel methods achieve bettc;:r modeling of 

the aerodynamic interference between lifting surfaces. The model should have a sufficient number 

of aerodynamic degrees of freedom to properly represent the steady and unsteady aerodynamic 

distributions under consideration. 

The buildup of unsteady aerodynamic forces should be represented. In two-dimensional 

unsteady analysis this may be achieved in either the frequency domain or the time domain through 

the application of oscillatory 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 of the unsteady 

aerodynamics should include the highest frequency of gust excitation and the highest structural 

frequency under consideration. Time lags representing the effect of the gradual penetration of the 

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. of this 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 of the aerodynamic forces should also ensure that the rigid 

body motion of the 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 power units should be designed for the gyroscopic loads induced by both 

discrete gusts and continuous turbulence. The gyroscopic loads for turbo propellers 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 of the solution process by including the gyroscopic terms in 

the equations of motion or the gyroscopic loads can be superimposed after the solUition of the 

equations of motion. Propeller and fan gyroscopic coupling forces (due to rotational direction) 

between symmetric and antisymmetric modes need not be taken into account if the coupling 

forces are shown to 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 of rotation. 

f Control Systems. Gust analyses of the basic configuration should include 

simulation of any control system for which interaction may exist with the rigid body response, 

structural dynamic response or external loads. If possible, 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 if the impact 

of the nonlinearity is negligible, or if it can be shown by analysis on a similar a.irplane/control 

system that a linear control law representation is conservative. If the control system is significantly 

nonlinear, and a conservative linear approximation to the control system cannot be developed, 

then the effect of the control system on the airplane responses should be evaluated in accordance 

with Section 8.0 of this AC. 

g. Stability. Solutions of the equations of motion for either discrete gusts or 

continuous turbulence 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. (Note that 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 or nonlinear system in question, and adjustments should be made to the dynamic 

model, as required, to achieve appropriate frequency and damping characteristics. 

If control system models are to be included in the gust analysis it is advisable to check that 

the following characteristics are acceptable and are representative of the airplane; 

• static margin of the unaugmented airplane 

• dynamic stability of the unaugmented airplane 

• the static aero elastic effectiveness of all control surfaces utilised by any feedback control system 

• gain and phase margins of any 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 of the 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 of motion and extracting dynamic loads from the airplane response. The airplane 

equations of motion are solved in either physical or modal 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. For the purposes of describing the solution of these equations in the n:mainder of 

this AC, modal coordinates will be assumed. A sufficient number of modal coordinates should be 

included to ensure that the loads extracted provide converged values. 

b. Solution of the Equations of Motion. Solution of the equations of motion can be ~ 

achieved through a number of techniques. For the continuous turbulence 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. of this AC) to generate the RMS value of the output response 

quantity. 

There are two primary approaches used to generate the output time histories for the 

discrete gust analysis; (1) by explicit integration of the airplane equations of moti.on in the time 

domain, and (2) by frequency domain solutions which can utilize Fourier transform techniques. 

c. Extraction of Loads 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 of the physical responses 

is given by a modal superposition of the displacements, velocities and accelerations of the rigid 

and elastic modes of vibration of the airplane structure. The number of modes carried in the 

summation should be sufficient to ensure converged results. 

A variety of methods may be used to obtain physical structural loads from a solution of the 

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 of providing a balanced set of airplane loads. If an infinite number of modes 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 of a given mode is equal to the product of the load associated with 

the normalized deformed shape of that mode and the value of the 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 of modal coordinates than the other two methods. 

In the Mode Acceleration method, the dynamic load response is composed of a 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 of the 

number of modes 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 of the number of normal 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 

of the 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 on the structure. 

If balanced 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 of this 

AC, if balanced 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 or continuous turbulence to become 

nonlinear with respect to intensity or shape should be represented realistically or conservatively in 

the calculation of loads. While many minor nonlinearities are amenable to a conservative linear 

solution, the effect of major nonlinearities cannot usually be quantified without expllicit 

calculation. 

The effect of nonlinearities 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 the unaugmented airplane elements of a loads calculation. Aerodynamic models may be refined 

to take account of minor nonlinear variation of aerodynamic distributions, due to local separation 

etc., through simple linear piecewise solution. Local or complete stall of a lifting surface would 

constitute a major nonlinearity and should not be represented without account being taken of the 

AC25341-2.DOC October 20, 1999 16/20 

As submitted for technical editing and preliminary legal review. 
influence of rate of change of incidence, 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 of nonlinear 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 of one-sided aerodynamic controls such as spoilers 

• hinge moment performance and saturation of aerodynamic control actuators 

The resulting influences on response will be airplane design dependent, and the manner in 

which they 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 or use of a 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. 

The influence of nonlinearities on one load quantity often runs contrary to the influence on 

other load quantities. For example, 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 state to another. 

d. Nonlinear Solution Methodology. Where explicit simulation ofnonlinearities is 

required, the loads response may be calculated through time domain integration of the equations 

of motion. 

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 of the airplane model 

excited by the discrete gust model described in Section S.b. of this AC. 

AC25341-2.DOC October 20,1999 17/20 

As submitted for technical editing and preliminary legal review. 
For time domain solution of the continuous turbulence conditions of Paragraph 25.341(b), 

a variety of approaches may be taken for the specification of the turbulence input time history and 

the mechanism for identifying limit loads from the resulting responses. 

It will normally be necessary to justify 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 of 0.4 times Ua (defined in Section S.C.l of this AC). The value of 

limit load is that load with the same probability of exceedance as A Ua of the same load quantity 

in a linear model. This is illustrated graphically in Figure 5. When using an analysis of this type, 

exceedance curves should be constructed using incremental load values up, or just beyond the 

limit load value. 

Linear Model 

Limit Load 

Exceedence Level 

Nonlinear Model 

Nonlinear 

Design 

Load 

Incremental Load 

Figure-5 Establishing Limit Lo~d for a Nonlinear Airplane 

AC25341-2.DOC October 20,1999 

As submitted for technical editing and preliminary legal review. 

18120 
The ,nonlinear simulation may also be performed in the frequency domain if the 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 V ALIDA TION 

a. General. The intent of analytical model validation is to establish that the analytical 

model is adequate for the prediction of gust response loads. The following sections discuss 

acceptable but not the only methods of validating 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 25.629-1 A 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 of better 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 over the 0.03 value to be consistent with the high 

structural response levels caused by extreme gust intensity, provided justification is given. 

d. Aerodynamic Model Validation. Aerodynamic modeling parameters fall into two 

categories: 

(i) steady or quasi steady aerodynamics governing static aeroelastic and flight 

dynamic airload distributions 

(ii) unsteady aerodynamics which interact with the flexible modes of the 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 25.629-1 A 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. If the airplane mathematical model used for gust 

analysis contains a representation of any feedback control system, then this segment of the model 

should be validated. The level of validation that should be performed depends on the complexity 

of the system and the particular airplane response parameter being controlled. Systf:~ms which 

control elastic modes of the airplane may require more validation than those which control the 

airplane rigi~ body response. Validation of elements of the 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 of the closed loop system. 

(1) System Rig or Airplane Ground Testing. Response of the system to artificial 

stimuli can be measured to verify the following: 

• The transfer functions of the sensors and any pre-control system anti-aliasing 

or other filtering. 

• The sampling delays of acquiring data into the control system. 

• The behavior of the control law itself 

• Any control system output delay and filter transfer function. 

• The transfer functions of the actuators, and any features of actuation system 

performance characteristics that may influence the actuator response to the 

maximum demands that might arise in turbulence; e.g. maximum rat(: of 

deployment, actuator hinge moment capability, etc. 

If this testing is performed, it is recommended that following any adaptation of the model 

to reflect this information, the complete feedback path be validated (open loop) against 

measurements taken from the rig or ground tests. 

(2) Flight Testing. The functionality and performance of any feedback control system 

can also be validated by direct comparison of the analytical model and measurement for input 

stimuli. If this testing is performed, input stimuli should be selected such that they exercise the 

features of the control system and the interaction with the airplane that are significant in the use 

of the 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