### Federal Aviation Administration Aviation Rulemaking Advisory Committee Transport Airplane and Engine Issue Area Loads and Dynamics Harmonization Working Group Task 2 – Engine Torque and Gyroscopic Loads

## Task AssignmentFederal RegIster I Vol. 58. No. 48 I Monday, March 

## 15, 1993 I Notices 13819

Aviation Rulemaking Advisory CommlHee; Loads and Dynamics Harmonization Working Group AGENCY: Federal Aviation Administration (FAA), DOT. AcnON: Notice ofestablishment ofLoads and Dynamics Harmonization Working Group.

- SUMMARY: Notice is given ofthe establishment oftheLoads and Dynamics Harmonization Working Group ofthe Aviation Rulemaking Advisory Committee (ARAC). This notice informs the public ofthe activities oftheARAC on transport airplane and engine issues. FOR FURTHER INFORMATION CONTACT: Mr. William]. Uoe) Sullivan. Assistant Executive Director, Aviation Rulemaking Advisory Committee. Aircraft Cenification Service (AIR-3l. aoo Independooce Avenue, SW., Washington. DC 20591. Telephone: (202) 267-9554; FAX.: (202) 267-5364. SUPPLEMENTARY INFORMATtON: The FederalAviation Administration (FAA) has established anAviation Rulemaking Advisory Committee (ARAC) (56 FR 2190.January 22,1991;and 58 FR 9230. February 19. 1993). One area the ARAC deals withis transport airplane and engine issues (56 FR 31995;July 12, 1991). These issues involve the airworthiness standards for transport airpla'les. engines andpropellers in parts 25. 33 and 35 ofthe Federal Aviation Regulations (14 CFR parts 25. 33 and 35) which are the responsibility oftheFAA Director ofAircraft Certification.

The FAA announced at theJoint Aviation Authorities {JAA)-Federal Avfation Administration (FAA) Harmonization Conference inToronto, Ontario.Canada. {June 2-5. 1992) that it would consolidate within the Aviation RulemakingAdvisory Committee structure an ongoing objective to "harmonize" the Joint Aviation Requirements (JAR) and the Federal Aviation Regulations (FAR). Coincident with that announcement. the FAA assigned to the ARAC those projects related to JAR1FAR 25.33 and 35 harmonization which werethen inthe

. process ofbeing{:O()rdinatedbetween the JAA and the FAA.The harmonization process included the inteL.tion to pr&sent the results ofJAAI FAA coordination to the public in the form ofeither a Notice ofProposed Rulemaking aran adviSory circular-an objective comparable to andcompatible with thatassigned to theAviation Rulemaking Advisory Committee. The Loads andDynamics Harmonization Working Group isbeing formed to address loads and dynamics issues in JARIF AR parts 25 identified below. The Loads andDynamicsHarmonization Working Group \\'ill forward recommendationst1l the ARAC which will determi.ne whetherto forward them 1{)theF.AA

Specifically. the WorkingGroup', tasks are the following: The Loads and Dynamics Harmonization**Worl~ing** Group ischargedwith making recommendations to the ARAC concerningthe FAA disposition oftha following subjectsTecently coordinated between the

### JAA and theFAA: Task l-GeneralDesign Loads

Develop new orrevised requirements. and associatedadvisory and guidanat material, forthegeneral designloadsfor transport categoryairplanes (FAR 25.331.15.335,25.341, %5.345. 25.351, 25.311.25.421,25.483.25.511.25.561 and 25.963 and other conforming changes). Task2-EngineTorque and Gyroscopic Loads

I Develop neworrevisedrequirements.

and associated advisoryand guidan08 material,-for determiningthedesign loads for engine seizure conditions {FAR 25.361. 25.371 and other conformingchanges). Task3-Flutter,Deformation andFailSafe Criteria:

Develop neworrevised advisory and guidance material for flutter. defonnationand fail-safecriteria (FAR 25.629). Reports

A. Recommend timeline{s) for completion ofeachtask, including rationale.forconsiderationatthe meeting oftheARAC to consider transportairplane and engine issues heldfollowing publicationofthis notice.

B. Giva a detailed conceptual presentation on eachtaskto theARAC beforeproceedingwith the work$tated underitemsC andD, below. Iftasks 1 and 2require the developmentofmore than one Notice ofProposed Rulemaking. identify whatproposed amendments willbeincludedin each notice.

C.Draft one ormoreNotices of ProposedRulemakingfor Tasks 1 and 2 proposingneworrevisedrequirements, a supportingeconomic analysis and otherrequired analysis. advisory and guidancematerial, andany other collateral documentstheWorking Group determinestobeneeded.

D. Draft appropriate advisory and guidancematerial forTask 3.

E. Give astatusreport oneach taskat each meetingoftheARAC heldto considertransport airplane and engine issues. The Loads andDynamics HarmonizationWorking Group willbe comp"'sedofexperts from those orgaruzatiooshaving an interestin the tasksassigned. A Working Group member neednotnecessarilybe 8 representative ofoneofthemember organizationsofthaARAC. An individualwho hasexpertise inthe subject matterand wishestobecome a memberoftheWOl'kingGroup should writ8the person listed underthecaption "FOR FURTHER *'FORMATION CONTACT" expNssingthatdeme. describinghisor herinterest inthetask. andthe expertise heorshewouldbring to the WcdingGroup.Therequestwill be reviewed withtheChairsofthe ARAC Transport Airplane andEngine Interest Issues and the Loads andDynamics Working Group. andtheindividual will be advised whetherornot the request can be accommodated.

The Secretary ofTransportation has determinedthattheinformation and use oftheARACisnecessary in the public interest inconnactianwiththe performance ofdutiesoftheFAAby law. Meetings ofthe ARAC willbe open to the public exceptas authorized by section 10(d) ofthe Federal AdviSOry Committee Act. Meetings ofthe Laads and Dynamics Harmonization Working Group willnotbe open to the public except to the extentthat individuals with an interest and expertise are**~lected** to participate. No public announcementofWorkingGroup meetings willbemade.

Issued inWashington, DC, on March 8. 1993. WilliamJ.Sullivan, NsistantExecutiveDirector for Transport AirplaneandEngineIssues,Aviation Rulemaking AdvisruyCommittee.

, [FR Doc. 93-5815Filed 3-12-93; 8:45 am]

BlLUNG COOE**491o-1~**

## Recommendation LetterDecember 20, 1999

.

Department ofTransportation Federal Aviation Administration 800 Independence Ave, SW Washington. D.C. 20591 Attention: Mr. Tom McSweeny, Associate Administrator for Regulation and CertifICation Reference: ARAC Tasking, Federal Register, November 26, 1999 Dear Tom, In accordance 

### with the reference tasking statement, the ARAC Transport Airplane and Engine Issues Group

# is pleased to forward the attachedtechnical report (in NPRMlAC format) which provides ARAC's

# ~:=a:isf~~~~~~~~~I~.~'i~iftr**~:~**

# k:'!:<!.r theTAEIG. ..'

. Sincerely,

C. R. Bolt Assistant Chair, TAEIG Phone: 860-565-9348. Fax 860-557-2277. MIS 162-24 Email: boltcr@pweh.com

cc: Dorenda Baker

- FAA-NWR'" Tony Fazio

- FAA. ARM-l'" Kristin Larson

- FAA-NWR Larry Hanson, Gulfstream* *letter only

## Acknowledgement Letter..**~**

## ~IjAR I 5 2000 

# f

Mr. Craig Bolt Assistant Chair, Transport Airplanes and Engines Issues Group 400 Main Street East Hartford, CT 06108 Dear Mr. Bolt: 

# f**:').~­ (~;** 

This letter acknowledges receipt ofthefollowing working group technical reports that you have submitted on behalfofthe Aviation Rulemaking Advisory Committee (ARAC) on Transport Airplane and Engine Issues (TAE): 

### Date of Task Description of Recommendation Working Letter No. Group Fast track reports addressing 

### §§ 25.703(a) thru (c) (takeoffwarning system); 25. 1333(b) (instru- / 

### 12/14/00 1,2.3 ment systems; and 25.1423(bnpublicaddress ASHWG system) Fast track reports addressing 

### §§ 25.111(c){4), 25.147, controllability in 1-engine inoperative condition; 25.161 (c) (2) and (4), and (e) (longitudinal trim and airplanes with 4 or more engines) 25.175(d) (static longitudinal stability; 25. 177(a)(b) (static lateral-directional stability); 25.253(a)(3) (high speed characteristics); 25. 1323(c) (airspeed indicating system); 25.1516 ,/ 

### 12/17/00 5 (landing gear speeds); 25.1527 (maximum oper- FTHWG ating altitude); 25.1583(c) and (f) operating limitations) 25.1585 (operating procedures); and 25.1587 (performance information) Fast track report addressing 

### § 25.903(e) (inflight /1 

### 12/17/00 7 engine failures) PPIHWG 

## /

/2

i Fast track reports addressing 

### §§ 25.1103 (auxil

# I iary power units); 25.933(a) (thrust reverers); 25.1189 (shutoff means); 25.1141 (powerplant controls); 25.1093 (air intake/induction systems); 25.1091 (air intake system icing protection; 25.943 (thrust reverser system tests); 25.934 (negative acceleration); 25.905(d) (propeller blade debris); 25.903(d)(1) (engine case burnthrough); 25.901(d) (auxiliary power unit installa- ../ 12/20/00 5 tion; and 1.1 (general definitions) PPIHWG Fast track report, category 2 format-NRRM ad12/20/00 4 dressing 

### § 25.302 and appendix K (interaction of LDHWG systems and structures /" Fast track report-(in NPRM/AC format) addressing 

### §§ 25.361 and 25.362 (engine and aux## ~DHWG 12/20/00 2 iliary power unit load conditions) Fast track report addressing 12/20/00 1 

### § 25.1438 (pressurization and low pressure MSHWG pneumatic systems) /' The above listed reports will be forwarded to the Transport Airplane Directorate for review. The Federal Aviation Administration's (FAA) progress will be reported at the TAE meetings. This letter also acknowledges receipt ofyourJuly 28, 1999, submittal which included proposed notices and advisory material addressing lightning protection. We apologize for the delay. Although the lightning protection task is not covered underthe fast track proposal, the FAA recognizes that technical agreement has been reached and we will process the package accordingly. The package has been sent to Aircraft Certification for review; the working group will be kept informed of its progress through the FAA representative assigned to the group. Lastly, at the December 8 - 9, 1999, TAE meeting, Mr. Phil Salee ofthe Powerplant Installation Harmonization Working Group indicated thatthe working group members agreed that 

### § 25.1103 was sufficiently harmonized and that any further action was beyond the scope oftask 8 assigned. We agreed with the TAE membership to close the task. This letter confirms the FAA's action to close the task to harmonize 

### § 25.1103. 

# i I

## RecommendationProposed Textfor NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.3611.362 Developed from Recommendations ofWorking Group


### Summary The Federal Aviation Administration proposes to amend the airworthiness standards for transport category airplanes concerning engine loads design requirements for the engine mounts, auxiliary power unit mounts, engine pylons, and adjacent supporting airframe structures. The proposed amendment would revise the regulations to further define the engine loading conditions that must be considered. The current regulations do not adequately define the engine loading conditions experienced in service. This proposal is intended to ensure that engine mounts and adjacent supporting structures are able to withstand the severest loads expected in service. Adopting this proposal would eliminate regulatory differences between the airworthiness standards ofthe U.S. and the Joint Aviation Requirements of Europe, without affecting current industry design practices.



### How Does This Proposed Regulation Relate to "FastTrack"? This proposed regulation results from the recommendations ofARAC submitted underthe FAA's Fast Track Harmonization Program. In this notice, the FAA proposes to amend 

### § 25.361, concerning engine and auxiliary power unit (APU) load conditions. The JAA plans a similar revision to the JAR.



### What is the Underlying Safety Issue Addressed by the Current Standards? The current airworthiness standards contained in 14 CFR part 25 require that turbine engine mounts and supporting structure must be designed to withstand "

. .. a limit engine torque load imposed bysudden engine stoppage due to malfunction orstructural failure (such as compressor jamming)." This was first made a specific requirement for U.S.-manufactured transport category airplanes in 1957 by Civil Air Regulation (CAR) 4b.216(a)(4). It was later carried forward in § 25.361 (b)(1) of part 25 when the Federal Aviation Regulations were recodified. This same requirement is contained in paragraph 25.361 of JAR-25. These standards were issued to ensure that engine mounts and adjacent structures are able to -withstandthe-loads--expected-to-ne imposed-un-them·during service.---Paijure ofthe engine

mount or supporting structure could lead to loss of an engine andlor damage to the airframe.



### What are the Current 14 CFR Standards? The current text of 14 CFR 

### § 25.361 is: 

### Sec. 25.361 

### Engine torque. (a) Each engine mount andits supporting structure mustbe designed for the effects of--

Docs # 11695 Page 13 0[20Proposed Textfor NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.361/.362 Developed from Recommendations ofWorking Group (1) A limit engine torque corresponding to takeoffpowerandpropellerspeed acting simultaneously with 75 percent of the limit loads from flight condition A ofSec. 

### 25. 333(b); (2) A limit torque corresponding to the maximum continuous powerandpropeller speed, acting simultaneously with the limit loads from flight condition A of Sec. 25.333(b); and (3) Forturbopropellerinstallations, in addition to the conditions specifiedin paragraphs (a)(1) and (2) ofthis section, a limit engine torque corresponding to takeoff powerandpropellerspeed, multipliedby

### a factoraccounting forpropellercontrol system malfunction, including quick feathering, acting simultaneously with 1 g level flight loads. In the absence of

### a rational analysis, 

### a factorof 1.6 mustbe used. (b) Forturbine engine installations, the engine mounts andsupporting structure must be designed to withstand each ofthe following: (1) A limit engine torque load imposed bysudden engine stoppage due to malfunction orstructural failure (such as compressor jamming). (2) A limit engine torque load imposed by the maximum acceleration ofthe engine. (c) The limit engine torque to be considered underparagraph (a) ofthis section mustbe obtainedbymultiplying mean torque for the specifiedpowerandspeedby

### a factorof- 

### (1) 1.25 for turbopropellerinstallations; (2) 1.33 forreciprocating engines with five ormore cylinders; or (3) Two, three, orfour, for engines with four, three, ortwo cylinders, respectively.



### What are the CurrentJAR standards: The current text ofJAR-2S.361 is: JAR 25.361 Engine andAPU torque. (a) Each engine mount andits supporting structure must be designed for engine torque effects combined with
-- .. .... (1) A limit engine torque corresponding to takeoffpowerandpropellerspeed acting simultaneously with 75 percent of the limit loads from flight condition A ofJAR 

### 25. 333(b); (2) A limit torque corresponding as specifiedin sub-paragraph (c) ofthis paragraph acting simultaneously with the limit loads from flight condition 

### A ofJAR 25.333(b); and (3) Forturbopropellerinstallations, in addition to the conditions specifiedin subparagraphs (a)(1) and(2) ofthis paragraph, a limit engine torque corresponding to takeoff powerandpropellerspeed, multipliedby

### a factoraccounting forpropellercontrolsystem

Docs # 11695 Page 

### 14 of20Proposed Text for NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.361/.362 Developed from Recommendations ofWorking Group
malfunction, including quick feathering, acting simultaneously with 19 level flight loads. In the absence ofa rational analysis, a factorof 

### 1.6 mustbe used. (b) For turbine engines andauxiliarypowerunit installations, the limit torque load imposedbysudden stoppage due to malfunction orstructural failure (such as a compressor jamming) must be considered in the design ofengine andauxiliarypowerunit mounts and supporting structure. In the absence ofbetterinformation, 

### a sudden stoppage must be assumedto occurin 

### 3 seconds. (c) The limit engine torque to be considered undersub-paragraph (a)(2) ofthis paragraph is obtainedbymultiplying the mean torque by

### a factorof 

### 1.25 for turbopropeller installations. (d) When applying JAR 

### 25.361(a) to turbo-jet engines, the limit engine torque mustbe equalto the maximum accelerating torque for the case considered. [See ACJ

### 25. 301 (b).]



### How Have the Standards Been Applied? Previous methods of complying with the requirements of § 25.361 have entailed either: • designing to a specific torque value prescribed by the engine manufacturer, or • designing to a torque level established by the polar moment of inertia ofthe rotating sections and the time required to stop the rotation, as defined by the engine manufacturer. Since the circumstances and the events from which these loads are generated are dependent on the characteristics ofthe particular engine, the engine manufacturers traditionally have provided the airframe manufacturers with the information necessary to install each engine. 

### Why is a Revision to the Current Standards Needed? The size, configuration, and failure modes ofjet engines have changed considerably since 

### § 25.361 (b) was first adopted. The original requirement addressed primarily turbine engine failure conditions that resulted in sudden engine deceleration and, in some cases, seizures. Those failure conditions were usually caused by internal structural failures or ingestion offoreign --objects such**as·birds-Or---ice~RateveI"-tRe-sGlJrcei** thoseconditioRS -Goula-proauce sigflifte-aAtstructural loads on the engine, engine mounts, pylon, and adjacent supporting airframe structure. With the development of larger high-bypass ratio turbofan engines, however, it has become apparent that engine seizure torque loads alone do not adequately define the full loading imposed on the engine mounts, pylons, and their adjacent supporting airframe structure. The progression to high-bypass ratio turbofan engines of larger diameter and fewer blades with larger chords has increased the magnitude ofthe transient loads that can be produced during and following engine failures. As engines have grown much larger, their fans are capable of producing much higher torque loads when subjected to sudden deceleration.

Docs # 11695 Page 

### 15 of20Proposed Text for NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.361/.362 Developed from Recommendations ofWorking Group

The FAA finds that, relative to the engine configurations that existed when the rule was first developed, these later generations ofjet engines are sufficiently different and novel to justify amending the regulations to ensure that adequate design standards are available for the mounts and the structure supporting these newer engines. Overthe last several years, manufacturers have applied for, and the FAA has granted, numerous Special Conditions (under the provisions of 14 CFR § 21.16, "Special Conditions") for use of new design standards applicable to engine load conditions airplane models incorporating new-technology engines. Such Special Conditions have been approved recently for Boeing Models: • 737-600/700/800 (62 FR 50494, September 26, 1997), • 757-300 (64 FR 32011, January 21,1999), and • 767-400ER (64 FR 27478, May 20, 1999). Likewise, the JAA has granted "exceptions" to manufacturers who have applied to use similar new engine design standards. In order to maintain the level of safety intended by § 25.361 (b)(1), and to address the newertechnology engine designs, the FAA considers that the addition to the regulations of a more comprehensive criterion is necessary -- one that considers all load components when designing to address engine failure events.



### What are "Engine Loads"? When addressing loads and their effects on aircraft engines and structure, the term limit load is generally used to specify the maximum load to be expected in service. The structure must support the limit load without detrimental permanent deformation. Further, at any load up to limit loads, the deformation may not interfere with safe operation ofthe airplane. The term ultimate load is used to specify the limit load multiplied by a prescribed factor of safety. The structure must be able to support this ultimate load without failure. Loads arising from very infrequent events (discussed below) are sometimes prescribed directly as ultimate loads without any additional factor of safety.**~-=r-hese-terms ar-e-diSG\.lSsed-iR-§-2-5.30~-"l--Qads"j,§** **2S.~ctorof-safety"-jjand§ 25.305~-~-------­**  ("Strength and deformation").



### What are the Types of "Engine Events" that This Proposed Rule Addresses? Studies made by the engine and the airframe manufacturers have shown that large turbofan engines exhibit two distinct classes of sudden deceleration events: The first type of event involves transient deceleration conditions involving rapid slowing ofthe rotating system. These events are usually associated with temporary loss of power orthrust capability, and often result in some engine distress, such as blade and/or wear strip damage. Examples are high power compressor surges, blade tip rub during maneuvers, bird encounters,

Docs # 11695 Page 

### 16 of20Proposed Text for NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.3611.362 Developed from Recommendations ofWorking Group

or combinations ofthese events. Based on the frequency of occurrence, the FAA considers these events to be limit load conditions that require the 1.5 factor of safety prescribed in 

### § 25.303 to obtain ultimate loads. The second type of event involves major engine failures that result in extensive engine damage and permanent loss ofthrust-producing capability. Examples ofthese types of events are fan blade failures, bearing failures, and shaft failures. It is evident from service history that these most severe sudden engine failure events are sufficiently infrequent to be considered ultimate load conditions. Because ofthe rare occurrence ofthese events and the conservative rational method in which the loads are to be obtained, the FAA proposes that these ultimate loads be applied to engine mounts and pylon structure without an additional factor ofsafety. At the same time, to provide additional protection for the more critical airframe structure, the FAA proposes that these ultimate loads be multiplied by an additional factor of 1.25 when applied to the adjacent supporting airframe structure.



### What Is the Proposed Action and How Does ItAddress the Underlying Safety Issue? This proposal would add a new § 25.362 addressing engine failure loads, which would distinguish between design criteria for the more common failure events (described above as the "first type of event") and design criteria for those rare events resulting from structural failures (described above as the "second type of event"). Forthe more rare but severe engine failure events, the proposed criteria would allow deformation in the engine supporting structure in order to absorb the higher energy associated with high-bypass turbofan engines. At the same time, the proposed criteria would protect the adjacent primary structure in the wing and fuselage by providing an additional safety margin. Specifically, new § 25.362 would require that the engine mounts, pylons, and adjacent supporting airframe structure be designed to withstand 1 g flight loads combined with transient dynamic loads that could result from various engine structural failure conditions (i.e., .the loss of any fan, compressor, orturbine blade; and, for certain designs, any other engine structural failure that results in higher loads).**~1!bolJghJhe** FMrecogoizes that**some~ngine** configurations**~_~xistjIlJllhi(;tl tI1E~J:)I~g~u**  _______ _ failure event is not the most critical load, the FAA expects that, for most conventional engines, the blade failure event will be the most severe event that needs to be investigated. Such a failure event, in which the most critical blade is assumed to fail at the maximum permissible rotational speed, is a required test under the certification standards of 

### § 33.94, "Blade containment and rotor unbalance tests." In addition to these certification tests, the engine manufacturers normally conduct additional developmental tests for each engine design. These tests, taken as a whole, allow a very reliable estimate ofthe transient engine loads resulting from failure events. Because the loads are supported by actual tests conducted in the most critical conditions of operation, the proposed rule would allow the loads developed from these conditions to be used directly as ultimate loads, with no additional factor when applied to engine mounts and pylons. However,

Docs # 11695 Page 

### 17 of20Proposed Textfor NPRM on Engine &Auxiliary Power Unit Load Conditions, § 25.361/.362 Developed from Recommendations ofWorking Group (d) For auxiliary power unit installations, the power unit mounts and adjacent supporting airframe structure must be designed to withstand 1 g level flight loads acting simultaneously with the maximum limit torque loads imposed by each ofthe following: (1) sudden auxiliary power unit deceleration due to malfunction or structural failure; and (2) the maximum acceleration ofthe power unit.

Adda newsection 

### 25.362, to readas follows: § 25.362 Engine failure loads (a) For engine supporting structure, an ultimate loading condition must be considered that combines 

### 19 flight loads with the transient dynamic loads resulting from: (1) the loss of any fan, compressor, orturbine blade; and (2) separately, where applicable to a specific engine design, any other engine structural failure that results in higher loads. (b) The ultimate loads developed from the conditions specified in paragraph (a) are to be: (1) multiplied by a factor of 1.0 when applied to engine mounts and pylons; and (2) multiplied by a factor of 1.25 when applied to adjacent supporting airframe structure.

Docs # 11695 Page

### 20 of20O-R-A-F-T (Incorporates Legal's input as signed off in Naples 9/21/99) [4910-13] DEPARTMENT OF TRANSPORTATION Federal Aviation Administration 14 CFRPart 25 [DocketNo. ;NoticeNo. ] RIN: 2120- Engine and Auxiliary PowerUnit Load Conditions. AGENCY: Federal Aviation Administration (FAA), DOT. ACTION: Notice ofproposed rulemaking (NPRM). Revision 3 8/19/99

SUMMARY: This notice proposes to amend the engine loads design requirements for the engine mounts, auxiliary powerunit mounts, engine pylons and adjacent supporting airframe structures ontransport category airplanes by further defining the engine loading conditions to be considered. The current regulation does not adequately define the engine loading conditions experienced in service. This proposal is intended to ensure that engine mounts and adjacent supporting structures are able to withstand the severest loads expected in service. This proposal also is intended to achieve common design and certification requirements for this subjectbetweentheU.S. regulations and the Joint Aviation Requirements (JAR) ofEurope; doing so would relieve manufacturers ofthe currentburden ofcertifying airplanes totwo different sets ofstandardsto achieve essentially the same intended safety benefit. DATES: Comments must bereceived on orbefore [insert date 90 days after date of publication in the Federal Register]. ADDRESSES: Comments onthis document should be mailed or delivered, in duplicate, to: U.S. Department ofTransportationDockets, DocketNo.

__**~,** 400 Seventh Street SW., RoomPlaza 401, Washington, DC 20590. Comments also may be sent electronically to the following Internet address: 9-NPRM-CMTS@faa.dot.gov.

## O-R-A-F-T (Incorporates Legal's input Revision 3 

### 8/19/99 Comments may be filed and examined in RoomPlaza 401 between 10:00 a.m. and 5:00 p.m. weekdays, except Federal holidays. In addition, the FAAis maintaining an information docket ofcomments in the FAA, Transport Airplane Directorate, Aircraft Certification Service, 1601 Lind Avenue S.W., Renton, Washington 98055-4056. Comments in the information docket may be inspected between 7:30 a.m. and 4:00 p.m. weekdays, except Federal holidays. FORFURTHERINFORMATION CONTACT: James Haynes, FAA, Airframe and Cabin Safety Branch (ANM-115), Transport Airplane Directorate, Aircraft Certification Service, 1601 Lind Ave S.W., Renton, Washington 98055-4056; telephone (425) 227- 2131; facsimile (425) 227-1320. SUPPLEMENTARY INFORMATION: 

### Comments Invited Interested persons are invited to participate in the making ofthe proposed action by submitting such written data, views, or arguments as they may desire. Comments relating tothe environmental, energy, federalism, or economic impact that might result from adopting the proposals in this document are also invited. Substantive comments should be accompanied by cost estimates. Comments must identify the regulatory docket

ornotice number and be submitted in duplicate tothe DOT Rules Docket address specified above. All comments received, as well as a report summarizing each substantive public contact with FAApersonnel concerning this proposed rulemaking, will be filed in the docket. The docket is available for public inspectionbefore and afterthe comment closing date. All comments received on orbefore the closing date will be considered by the Administrator before taking action onthis proposed rulemaking. Comments filed late will



### 2

## O-R-A-F-T (Incorporates Legal's input Revision 3 

### 8/19/99 be considered as far as possible without incurring expense or delay. The proposals in this document may be changed in light ofthe comments received. Commenters wishing theFAAto acknowledge receipt oftheir comments submitted in response to this document must include a pre-addressed, stamped postcard with those comments on which the following statement is made: "Commentsto Docket No. ." The postcard will be date stamped and mailed to the commenter. Availability ofNPRMs An electronic copy ofthis document may be downloaded using a modem and suitable communications software from the FAAregulations section ofthe Fedworld electronicbulletinboard service (telephone: 703-321-3339), the Government Printing Office's (GPO) electronic bulletin board service (telephone: 202-512-1661), or, if applicable, the FAA's Aviation Rulemaking Advisory Committeebulletin board service telephone: 800-322-2722 or202-267-5948). Internet users may reach the FAA'sweb page at http://www.faa.gov/avr/armlnprmlnprm.htm orthe GPO's webpage at http://www.access.gpo.gov/nara for access to recently published rulemaking documents. Any person may obtain a copy ofthis document by submitting a request to the Federal Aviation Administration, Office ofRulemaking, ARM-I, 800 Independence Avenue, SW., Washington, DC 20591; orby calling (202) 267-9680. Communications must identify the notice number or docket number ofthis NPRM. Persons interested in being placed on the mailing list for future rulemaking documents should request from the above office a copy ofAdvisory CircularNo. 11-2A, «Notice ofProposed Rulemaking Distribution System," which describes the application procedure.

3

## D-R-A-F-T (Incorporates Legal's input Background 

### Revision 3 

### 8/19/99

The airworthiness standards for transport category airplanes are contained in 14 CFRpart 25 [commonly referred to as the Federal Aviation Regulations (FAR), part 25]. Manufacturers oftransport category airplanes must showthat each airplane they produce ofa different type design complies with the relevant standards ofpart 25. These standards apply to airplanes manufactured within the US. for use by US.-registered operators, and to airplanes manufactured in other countries and imported tothe US. under a bilateral airworthiness agreement. InEurope, theJoint Aviation Requirements (JAR) were developed bytheJoint Aviation Authorities (JAA) to provide a common set ofairworthiness standards for use within the European aviation community. The airworthiness standards for European type certification oftransport category airplanes are contained in JAR-25, and are based on 14 CFRpart 25 [commonly referred to as part 25 ofthe Federal Aviation Regulations (FAR)].. Airplanes certificated to the JAR-25 standards, including airplanes manufactured in the US. for exportto Europe, receive type certificates that are acceptedbythe aircraft certification authorities of26 European member countries. Although part 25 and JAR-25 are very similar, they are not identical in every respect. Differences between the two sets ofstandards can result in substantial additional costs when airplanes are type certificated toboth standards. These additional costs, however, frequently do not bring about an increase in safety. For example, part 25 and JAR-25 may use different means to accomplish the same safety intent. In this case, the manufacturer is usually burdened with meeting both requirements, although the level of safety is not increased correspondingly. Recognizing that a common set ofstandards would not only economically benefit the aviation industry, but also would maintain the necessary high level ofsafety, the FAA and JAA consider "harmonization" ofthetwo sets ofstandards to be a high priority. 4

## O-R-A-F-T (Incorporates Legal's input Revision 3 8/19/99
In 1988, the FMin cooperation with the JAA and other organizations representing the American and European aerospace industries, began a process to harmonize the airworthiness requirements ofthe United States and the airworthiness requirements ofEurope, especially in the areas ofFlight Test and Structures. The Aviation Rulemaking Advisory Committee Later, in 1992, the FAAharmonization effort was undertakenby the Aviation Rulemaking Advisory Committee (ARAC). The ARAC was formally established bythe FAA on January 22, 1991 (56 FR2190), to provide advice and recommendations concerning the full range ofthe FAA's safety-related rulemaking activity. This advice was sought to develop betterrules in less overall time using fewer FAAresources than are currently needed. The committee provides the opportunity for the FAAto obtain firsthand information and insight from interested parties regarding proposed new rules or revisions ofexisting rules. There are 64 member organizations onthe committee, representing a wide range ofinterests within the aviation community. Meetings ofthe committee are opentothe public, except as authorized by section 1 O(d) ofthe Federal Advisory Committee Act. The ARAC establishes working groups to develop proposalsto recommend tothe FAAfor resolving specific issues. Tasks assigned to working groups are published in the Federal Register. Although working group meetings are not generally open tothe public, all interested parties are invited to participate as working group members. Working groups report directly to the ARAC, and the ARAC must accept a working group proposal before that proposal canbe presented tothe FAA as an advisory committee recommendation. The activities ofthe ARAC will not, however, circumvent the public rulemaking procedures. After an ARAC recommendation is received and found acceptablebythe

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### ---------------


## O-R-A-F-T (Incorporates Legal's input Revision 3 8/19/99 FAA, the agency proceeds with the normal public rulemaking procedures. Any ARAC participation in a rulemaking package will be fully disclosed in the public docket. Harmonization Working Group The Loads and Dynamics Harmonization Working Group was chartered by notice in the Federal Register (58 FR 13819, March 15, 1993). TheWorking Group is made up ofstructural specialists from the aviation industry and government ofEurope, the United States, and Canada. The task given to this Working Group was to harmonize the design loads section ofSubpart C ("Structure") of 14 CFRpart 25 with the counterpart requirements ofthe JAR. The Working Group developed specific recommendations for harmonizing the engine loading conditions. The ARAC approved those recommendations and recommended them to the FAAfor rulemaking. The FAA has accepted ARAC's recommendation, and the proposed rulemaking contained in this notice follows from those recommendations and the activity oftheWorking Group. Issues Prompting This Proposal The current airworthiness standards contained in 14 CFRpart 25 and JAR-25 require that turbine engine mounts and supporting structure must be designed to withstand "

... A limit engine torque load imposed by sudden engine stoppage dueto malfunction or structural failure (such as compressorjamming)." This was first made a specific requirement for U. S. transport category airplanes in 1957 by Civil Air Regulation (CAR) 4b.216(a)(4). Itwas later carried forward in 

### § 25.361(b)(I) ofpart 25 whenthe Federal Aviation Regulations were recodified. This same requirement is contained in section 25.3610fJAR-25. Previous methods ofcomplying with this requirement have entailed either:

• designing to a specific torque value prescribed by the engine manufacturer,

or

6**~~-----~ .~---------~--------**  

## O-R-A-F-T (Incorporates Legal's input Revision 3 8/19/99

• designing to a torque level established bythe polarmoment ofinertia ofthe rotating sections and thetime required to stop the rotation, as defined by the engine manufacturer. Since the circumstances and the events from which these loads are generated are dependent onthe characteristics ofthe particular engine, the engine manufacturers traditionally have provided the airframe manufacturers with the information necessaryto install each engine. The size, configuration, and failure modes of jetengines have changed considerably since 

### § 25.361(b) was first adopted. The original requirement addressed primarily turbine engine failure conditions that resulted in sudden engine deceleration and, in some cases, seizures. Those failure conditions were usually caused by internal structural failures oringestion offoreign objects such as birds orice. Whateverthe source, those conditions could produce significant structural loads onthe engine, engine mounts, pylon, and adjacent supporting airframe structure. With the development oflarger high-bypass ratio turbofan engines, however, it has become apparent that engine seizure torque loads alone do not adequately define the full loading imposed onthe engine mounts, pylons, and their adjacent supporting airframe structure. The progression to high-bypass ratio turbofan engines oflarger diameter and fewer blades with larger chords has increased the magnitude ofthe transient loads that can be produced during and following engine failures. As engines have grown much larger, their fans are capable ofproducing much higher torque loads when subjected to sudden deceleration. The FAAfinds that, relative to the engine configurationsthat existed whenthe rule

was first developed, these later generations of jet engines are sufficiently different and novel tojustity amending the regulations to ensurethat adequate design standards are available for the mounts and the structure supporting these newer engines. Therefore, in 7

## D-R-A-F-T (Incorporates Legal's input Revision 3 8/19/99 orderto maintain the level ofsafety intended by 

### § 25.361(b)(1), the FAAconsiders that a
more comprehensive criterion is necessary -- one that considers all load components when designing to address engine failure events. Engine Loads and Events When addressing loads and their effects on aircraft engines and structure, the term limit load is generally used to specify the maximum load tobe expected in service. The structure must support the limit load without detrimental permanent deformation. Further, at any load up to limit loads, the deformation may not interfere with safe operation ofthe airplane. The term ultimate load is used to specify the limit load multiplied by a prescribed factor ofsafety. The structure mustbe able to support this ultimate load without failure. Loads arising from very infrequent events (discussed below) are sometimes prescribed directly as ultimate loads without any additional factor ofsafety. Theseterms are discussed in 

### § 25.301 (''Loads''), 

### § 25.303 (''Factor ofsafety"), and 

### § 25.305 ("Strength and deformation"). Studies made bythe engine and the airframe manufacturers have shownthat large turbofan engines exhibit two distinct classes ofsudden deceleration events: The first type ofevent involves transient deceleration conditions involving rapid slowing ofthe rotating system. These events are usually associated withtemporaryloss of power orthrust capability, and often result in some engine distress, such as blade and/or

wear strip damage. Examples are high power compressor surges, blade tip rub during maneuvers, bird encounters, or combinations ofthese events. Based onthe frequency of occurrence, the FAA considersthese events to be limit load conditions that require the 1.5 factor ofsafety prescribed in 

### § 25.303 to obtain ultimate loads. The second type ofevent involves major engine failures that result in extensive engine damage and permanent loss ofthrust-producing capability. Examples ofthese 8O-R-A-F-T 

### (Incorporates Legal's input Revision 3 8/19/99 types ofevents are fan blade failures, bearing failures, and shaft failures. It is evident from service history that these most severe sudden engine failure events are sufficiently infrequent tobe considered ultimate load conditions. Because ofthe rare occurrence of these events and the conservative rational method in which the loads areto be obtained, the FAAproposes that these ultimate loadsbe applied to engine mounts and pylon structure without an additional factor ofsafety. At the same time, to provide additional protection for the more critical airframe structure, the FAAproposes thatthese ultimate loads be multiplied by an additional factor of 1.25 when applied tothe adjacent supporting airframe structure. Discussion ofProposal Accordingly, this proposal would add a new 

### § 25.362 addressing engine failure loads, which would distinguish between design criteria for the more common failure events (described above as the "first type ofevent") and design criteria for those rare events resulting from structural failures (described above as the "secondtype ofevent"). Forthe more rare but severe engine failure events, the proposed criteria would allow deformation in the engine supporting structure in orderto absorb the higher energy associated with high-bypass turbofan engines. At the same time, the proposed criteria would protect the adjacent primary structure in the wing and fuselage by providing an additional safety margin. Specifically, new 

### § 25.362 would require that the engine mounts, pylons, and adjacent supporting airframe structurebe designed to withstand 1 g flight loads combined with transient dynamic loads that could result from various engine structural failure conditions (i.e., .the loss ofany fan, compressor, orturbine blade; and, for certain designs, any other engine structural failure that results in higher loads). Although the FAArecognizes that some engine configurations may exist in which theblade failure event is not the most critical load, the FAA expectsthat, for most 9

## O-R-A-F-T (Incorporates Legal's input Revision 3 8/19/99 conventional engines, the blade failure event will be the most severe event that needs to be investigated. Such a failure event, in which the most critical blade is assumed to fail at the maximum permissible rotational speed, is a required test under the certification standards of

### § 33.94, "Blade containment and rotorunbalance tests." In addition to these certificationtests, the engine manufacturers normally conduct additional developmental tests for each engine design. These tests, taken as a whole, allow a very reliable estimate ofthe transient engine loads resulting from failure events. Because the loads are supported by actual tests conducted in the most critical conditions ofoperation, the proposed rule would allow the loads developed from these conditionsto be used directly as ultimate loads, with no additional factor when applied to engine mounts and pylons. However, the ultimate loads would berequired to be multiplied by a factor of 1.25 when applied to adjacent supporting airframe structure. Further, the proposed new 

### § 25.362 would address only the transient engine failure load condition, since the sustained loads resulting from continued windmilling after failure currently are addressed by 

### §§ 25.901 and 25.903 ofpart 25. The proposed new conditions addressed in 

### § 25.362 are more rationally determined, and will be treated as dynamic conditions including all significant input and response loads. The FAA has determined that designing for these new conditions would achieve an improved level ofsafety overthat provided by the existing static engine torque criterion. With the addition ofnew 

### § 25.362, the current requirements of

### § 25.361 would be revised as follows: 1. Current 

### § 25.361(a) would remain unchanged. 2. Current 

### § 25.361(c) would be redesignated as new 

### § 25.361(b). 3. Current 

### § 25.361(c)(3), which refers to engines with two, three, orfour cylinders, would be deleted. Transport category airplanes have not used these engines in 10O-R-A-F-T (Incorporates Legal's input Revision 3 8/19/99 the past, nor are they expected to use them in the future. Therefore, the references serve
no purpose in the rule. 4. Current 

### § 25.361(b) would be redesignated as 

### § 25.361(c). 5. Current 

### § 25.361(b)(1) includes a sudden engine stoppage event as a limit load condition. This condition was addressed by considering only engine torque as a static load condition. This proposal would remove the sudden engine stoppage condition from these particularrequirements, since new engine failure ultimate load conditions would be contained in new 

### § 25.362. 6. Proposed revised 

### § 25.361(c) would nowrequire that the engine mounts, pylons, and adjacent supporting structure be designed to withstand 19 level flight loads acting simultaneously with the maximum limit torque loads imposed by:

• sudden engine deceleration due to a malfunction which could result in a temporary loss ofpower orthrust, and

• maximum engine acceleration. This proposal also would add a new 

### § 25.361(d) that would contain similar design load requirements as those ofproposed revised 

### § 25.361(c). However, they would apply strictly to the powerunit mounts and adjacent supporting airframe structure for auxiliary powerunit (APU) installations. Finally, the title of

### § 25.361 would be changed from the current "Engine torque" to "Engine and auxiliary powerunit torque." This change is necessary in orderto provide

a betterindication ofwhat the paragraph addresses. Relevant Advisory Materials The FAAis preparingto issue a new proposed Advisory Circular 25.362-1 to describe a means ofcompliance with the proposed regulation, which would meet the intended level ofsafety and promote consistent and effective application ofthe proposed

11

## D-R-A-F-T (Incorporates Legal's input Revision 3 8/19/99 revised standards. Public comments concerning the proposed AC are invited by separate notice published elsewhere in this issue ofthe Federal Register 

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

### Compatibility with ICAO Standards In keeping with U.S. obligations underthe Convention onInternational Civil Aviation, it is FAApolicy to comply with International Civil Aviation Organization (ICAO) Standards and Recommended Practices to the maximum extent practicable. The FAAdetermined thatthere are no ICAO Standards and Recommended Practices that correspond to this proposed regulation. 

### Regulatory Evaluation Summary Changes toFederal 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 thebenefits ofthe intended regulation justify its costs. Second, the Regulatory Flexibility Act of 1980 requires agencies to analyze the economic effect ofregulatory changes on small entities. Third, the Office of Management and Budget directs agencies to assess the effects ofregulatory changes on international trade. And fourth, the Unfunded MandatesReform Act of 1995 (Pub. 

### L. 104-4) requires agencies to prepare a written assessment ofthe co§ts, benefits, and other effects ofproposed orfinal rules that include a Federal mandate likely to result in the expenditureby State, local, ortribal governments, in the aggregate, orby the private sector, of$100 million ormore annually (adjusted for inflation). In conductingthese analyses, the FAAhas determined that this proposed rule: (1) would generate benefits thatjustify its costs and would notbe "a significant regulatory action" as defined 

### in section 12

## O-R-A-F-T (Incorporates Legal's input Revision 3 8/19/99 3(f) ofExecutive Order 12866 and, therefore, is not subject to review by the Office of Management and Budget; (2) would nothave a significant impact on a substantial number ofsmall entities; 

### (3) would not constitute a barrierto international trade; and 

### (4) would not contain a significant intergovernmental orprivate sector mandate. These analyses, available in the docket, are summarizedbelow. The FAAinvites the public to provide comments and supporting data onthe assumptions made in this evaluation. All comments received will be considered in the final regulatory evaluation. 

### Initial Regulatory Flexibility Determination The Regulatory Flexibility Act of 1980 

### (RFA) establishes "as a principle of regulatory issuance that agencies shall endeavor, consistent withthe objective oftherule and ofapplicable statutes, to fit regulatory and informational requirements to the scale of the business, organizations, and governmental jurisdictions subject to regulation." To achieve that principle, the Act requires agencies to solicit and considerflexible regulatory proposals and to explain the rationale for their actions. The Act covers a wide-range of small entities, including small businesses, not-for-profit organizations, and small governmentaljurisdictions. Agencies must perform a review to determine whether a proposed rule will have a significant economic impact on a substantial number ofsmall entities. Ifthe determination is that it will, the agency must prepare a regulatory flexibility analysis as described in the Act. However, ifan agency determines that a proposed orfinal rule is not expected to have a significant economic impact on a substantial number ofsmall entities, section 605(b) ofthe 1980 Act provides that the head ofthe agency may so certify and a regulatory flexibility analysis is not required. The certification must include a statement providing the factual basis for this determination, and the reasoning should be clear. 

### [APO to addeconomic evaluation here.

## J 13O-R-A-F-T 

### (Incorporates Legal's input International TradeImpactAssessment 

### Revision 3 8/19/99

The provisions ofthis proposed rule would have little orno impact ontrade for U.S. firms doing business in foreign countries and foreign firms doing business in the United States. 

### Federalism Implications The regulation proposed herein would not have a substantial direct effect onthe States, onthe relationship betweenthe national Government and the states, oronthe distribution ofpower and responsibilities among the various levels ofgovernment. Therefore, in accordance withExecutive Order 12612, it is determined thatthis proposal would not have sufficient federalism implications to warrant the preparation ofa federalism assessment. 

### Unfnnded Mandates Reform Act Title II oftheUnfunded Mandates Reform Act of 1995 (the Act), codified in 2 U.S.C. 1501-1571, requires each Federal agency, tothe extent permitted by law, to prepare a written assessment ofthe effects ofany Federal mandate in a proposed orfinal agency rule that may result in the expenditure by State, local, and tribal governments, in the aggregate, orbythe private sector, of$100 million ormore (adjusted annually for inflation) in anyoneyear. Section 204(a) oftheAct, 2 U.S.C. 1534(a), requires the Federal agencyto develop an effective process to permit timely input by elected officers (ortheir designees) ofState, local, and tribal governments on a proposed "significant intergovernmental mandate." A "significant intergovernmental mandate" underthe Act is any provision in a Federal agency regulation that will impose an enforceable duty upon State, local, and tribal governments, in the aggregate, of$100 million (adjusted annually for inflation) in anyoneyear. Section 203 ofthe Act, 2 U.S.c. 1533, which supplements section 204(a), provides thatbefore establishing any regulatory requirements that might significantly oruniquely affect small governments, the agency shall have developed a plan 14

## D-R-A-F-T (Incorporates Legal's input Revision 3 8/19/99 that, among otherthings, provides for notice to potentially affected small governments, if any, and for a meaningful and timely opportunityto provide input in the development of regulatory proposals. This proposed rule does not contain a Federal intergovernmental orprivate sector mandate that exceeds $100 million in anyoneyear. 

### Environmental Analysis FAA Order 1 050.1D defines FAA actions that may be categorically excluded from preparation ofa National Environmental Policy Act (NEPA) environmental assessment or environmental impact statement. In accordance with FAA Order 1050.1D, appendix 4, paragraph 4(j), this rulemaking qualifies for a categorical exclusion. 

### Energy Impact The energy impact ofthe proposed rule has been assessed in accordance with the Energy Policy and Conservation Act (EPCA) and Public Law 94-163, as amended (42 U.S.c. 6362). Ithas been determined that it is not a major regulatory action underthe provisions ofthe EPCA. 

### Regulations Affecting IntrastateAviation in Alaska Section 1205 ofthe FAAReauthorization Act of 1996 (110 Stat. 3213) requires the Administrator, when modifying regulations in Title 14 ofthe CFRin a manner affecting intrastate aviation in Alaska, to considerthe extent to which Alaska is not served by transportation modes otherthan aviation, and to establish such regulatory distinctions

as he or she considers appropriate. Becausethis proposed rule would apply to the certification offuture designs oftransport category airplanes and their subsequent operation, it could, ifadopted, affect intrastate aviation in Alaska. The FAAtherefore specifically requests comments on whether there isjustification for applying the proposed rule differently to intrastate operations in Alaska. 15

## O-R-A-F-T (Incorporates Legal's input List ofSubjects in 14 CFRPart 25: 

### Revision 3 8/19/99

Aircraft, Aviation safety, Reporting and record keeping requirements 

### The Proposed Amendment In consideration ofthe foregoing, the Federal Aviation Administration proposes to amend part 25 ofTitle 14, Code ofFederal Regulations, as follows: 

### Part25

- AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES 1. The authority citation for Part 25 continues to read as follows: Authority: 49 U.S.C. 106(g), 40113, 44701-44702, and 44704. 2. Amend 

### § 25.361 by revising the title, by revising paragraphs (b) and (c), and by adding new paragraph (d), to read as follows: 

### § 25.361 Engine and auxiliary powerunit torque 

### (a) 

## * 

## * * * * (b) The limit engine torque to be considered underparagraph (a) ofthis section must be obtained by multiplying mean torque for the specified power and speed by a factor of- (1) 1.25 for turbopropeller installations; (2) 1.33 for reciprocating engines. (c) Forturbine engine installations, the engine mounts, pylons, and adjacent supporting airframe structure must be designed to withstand 1 g level flight loads acting simultaneously with the maximum limit torque loads imposed by each ofthe following: (1) sudden engine deceleration due to a malfunction which could result in a temporary loss ofpower orthrust; and (2) the maximum acceleration ofthe engine.

16O-R-A-F-T (Incorporates Legal's input Revision 3 8/19/99 (d) For auxiliary powerunit installations, the powerunit mounts and adjacent supporting airframe structure must be designed to withstand 19 level flight loads acting simultaneously with the maximum limit torque loads imposed by each ofthe following: (1) sudden auxiliary powerunit deceleration due to malfunction or structural failure; and (2) the maximum acceleration ofthe powerunit. 3. By adding a new section 25.362, to read as follows: 

### § 25.362 Engine failure loads (a) Forengine supporting structure, an ultimate loading condition must be considered that combines 19 flight loads with the transient dynamic loads resulting from: (1) the loss ofany fan, compressor, orturbine blade; and (2) separately, where applicable to a specific engine design, any other engine structural failure that results in higher loads. (b) The ultimate loads developed from the conditions specified in paragraph (a)

are tobe: (1) multiplied by a factor of 1.0 when applied to engine mounts and pylons; and (2) multiplied by a factor of 1.25 when applied to adjacent supporting airframe structure. Issued in Washington, D.C., on

Director, Aircraft Certification Service



### 17US. Department ofTransportation FederalAvIation Administration

Subject: ENGINE FAILURE LOADS 

# Advisory 

# Circular

Date: DRAFT 

### 8/13169121/99 Initiated By: ANM-115 AC No: 25.362-1 Change: Rev**J~** w/Legal input WORKING DRAFT -- NOT FOR PUBLIC RELEASE. 1. 

### PURPOSE. This Advisory Circular (AC) describes an acceptable means for showing compliance with the requirements of§25.362, "Engine and Auxiliary PowerUnit Load Conditions," of 14 CFRpart 25 [commonly referred to as ef.part 25 oftheFederal Aviation Regulations (FAR)], as it applies to transport category airplanes. These means are intended to provide guidanceto supplement the engineering and operationaljudgment that must form the basis ofany compliance findings relative to the design ofengine mounts and their supporting structures for loads developed from the engine failure conditions described in § 25.362. The guidance provided in this document is directed to airplane and engine manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation Administration transport airplane type certification engineers and their designees. Like all advisory circular material, this AC is not, in itself, mandatory, and does not constitute a regulation. Itis issued to describe an acceptable means, but not the only means, for demonstrating compliance with the requirements fortransport category airplanes. Terms such as "shall" and "must" are used only in the sense ofensuring applicability ofthis particular method ofcompliance when the acceptable method of compliance described in this document is used.



### 2. RELATED SECTIONS OFFEDERALAVIATION REGULATIONS.

a. Title 14, Code ofFederal Regulations (CFR) Part 25: Section 25.361 "Engine and auxiliary powerunit torque" Section 25.901 ''Powerplant installation" b. Title 14, Code ofFederal Regulations (CFR) Part 33:Draft AC No. 25.362-1 RJD Rev §6/1624/99

Section 33.23 ''Engine mounting attachments and structure" Section 33.65 "Surge and stall characteristics" Section 33.94 ''Blade containment and rotor unbalance tests" 3. DEFINITIONS. Some newterms have been defined for thetransient engine failure conditions in orderto present criteria in a precise and consistent manner in the following pages. In addition, some terms are employed from otherfields and may not necessarily bein general use. Forthe purposes ofthis Advisory Circular, the following definitions should beused.

a. Adjacent supporting airframe structure: Those parts ofthe primary airframe that are directly affected by loads arising within the engine. b. Blade loss: The loss ofthe most critical fan, compressor, orturbineblade.

c. Factor ofSafety: The ratio ofultimate load to limit load.**~.** Ground Vibration Test :(GVT): Ground resonance tests ofthe airplane normally conductedffi-for compliance with 

### § 25.629, "Aeroelastic stability requirements."

e. Limit Load: The maximum load that the structure is designed to carry without detrimental permanent defurmation. 

### fg.. Transient failure loads: Those loads occurring from the time ofthe engine structural failure, up to the time at which the engine stopsrotating or achieves a steady windmilling rotational speed.
g. Ultimate Load: Theloadthat the structure must withstand vAthout failure.**~.** Windmilling engine rotational speed. The speed at whichthe rotating shaft systems ofan unpowered engine will rotate dueto the flow ofair into the engine as a result ofthe forward motion ofthe airplane. The vrlndmilling engine rotational speed 

### will vary as a function ofaireraft speed.

4. BACKGROUND.

a. Requirements. Section 25.362 (''Engine and Auxiliary PowerUnit Load Conditions") requires that the engine mounts, pylons, and adjacent supporting airframe structurebe designed to withstand 1 g flight loads combined withtransient dynamic loads resulting from each engine structural failure condition. b. Engine failure loads. Turbine engines have experienced failure conditions that have resulted in sudden engine deceleration and, in some cases, seizures. These

Page 2Draft AC No. 25.362-1 RJD Rev**~/1624199** failure conditionshave-are usuallybeen-caused by internal structural failures oringestion offoreign objects, such as birds orice. Whateverthe source, these conditions may produce significant structural loads onthe engine, engine mounts, pylon, and adjacent supporting airframe structure. With the development oflargerhigh-bypass ratio turbine engines, it became apparent that engine seizure torque loads alone did not adequately define the full loading imposed on the engine mounts, pylons, and their adjacent supporting airframe structure. The progression to high-bypass ratio turbine engines of larger diameter and fewer blades with larger chords has increased the magnitude ofthe transient loads that can be produced during and following engine failures. Consequently, for engine failure events, it is considered necessarythat the applicant t&-perform dynamic analysis that considers all load components. A dynamic model ofthe aircraft and engine configuration must be sufficiently detailed to characterizethetransient and steady state loads for the engine mounts, pylons, and adjacent supporting airframe structure during the failure event and subsequent run down.

c. Engine structural failure conditions. Ofall the applicable engine structural failure conditions, -Dgesign and test experience have shown that, ofaU the applieable engiRe struetural failure eORditioRS, the loss ofa blade is likely to produce the most severe loads onthe engine and airframe. Therefore, 

### § 25.362 requires thatthe transient dynamic loads from theseblade failure conditions be considered when evaluating structural integrity. However, service history shows examples ofother severe engine structural failures where the engine thrust-producing capabilityis-was lost, and the engine has experienced extensive internal damage. For each specific engine design, the applicant should OORsideratioR should be gillen as to consider whether these types offailures are applicable, and ifthey present a more critical load conditionthanblade loss. Examples of other engine structural failure conditions that should be considered in this respect are: • failure ofa shaft, or • failure orloss ofany shaft support bearing.



### 5. EVALUATION OFTRANSIENT FAILURE CONDITIONS

a. Objective. The applicant should show, by a combination oftests and analyses" that the airplane is capable ofcontinued safe flight and landing afterpartial or eomplete loss ofablade, including ensuing damage to other parts ofthe engine. The primary failure condition is expected to be blade release (refer to 14 CFRpart 33, 

### § 33.94, ''Blade containment and rotorunbalance tests"). However, other structural failures may need to be considered as well, depending uponthe engine configuration. The applicant also should give eoosideratioR toconsider thetransient loads from thetime ofthe engine structural failure, up tothe time at which the engine stops rotating or achieves a steady windmilling rotational speed. 

### [NOTE: The effects ofcontinued

Page 3Draft AC No. 25.362-1 RJD Rev**~&/1624/99** rotation (windmilling) are described in AC 25-901-1, "System Safety Assessment of Powerplant Installations"

-- 

### cu"entiyin D-R-A-F-T 

### form.] b. Evaluation. The applicant's evaluation should show that, from the moment ofengine structural failure and during spool-downto the time ofwindmilling engine rotational speed, the engine-induced loads and vibrations will not cause failure ofthe engine mounts, pylon, and adjacent supporting airframe structure. Major engine structural failure events are considered as ultimate load conditions, since they occur at a sufficiently infrequent rate. Fordesign ofthe engine mounts and pylon, the ultimate loads may be taken without any additional multiplying factors. At the same time, protection ofthebasic airframe is assured by using a multiplying factor of 1.25 onthose ultimate loads for the design ofthe adjacent supporting airframe structure.

c. Blade loss condition. The applicant should determine loadsvrithin the 

### eRgiHe, and onthe engine mounts, pylon, and adjacent supporting airframe structure by dynamic analysis. The analysis should take into account all significant structural degrees offreedom. The transient engine loads should be determined for the fan blade failure condition and rotor speed, as specified in 

### § 33.94, and overthe full range ofblade release angles to allow determination ofthe critical loads for all affected components. The amount ofengine damage that develops during the failure event and, consequently, the loads produced, depends onmaterial properties and temperature. Therefore, the analysis oftransient engine loads should consider the effects ofvariations in engine material properties and temperature. This req\:liremeftt step in the analysis may be satisfied by analyzing:
• the engine stiffness characteristics at typical flight temperatures, and
• the engine strength and deflection characteristics at maximum design temperatures. The forcing function tobe applied to the pylon and airframe is normally generated and validated by the engine manufacturer, including those changes needed to represent the critical flight conditions. The analysis ofincremental transient airframe loads should consider:
• the effects ofthe engine mounting station onthe airplane (i.e., right side, left side, inboard position, etc.); and
• the most critical airplane mass distribution (i.e., fuel loading for wingmounted engines and payload distribution for fuselage-mounted engines). Forcalculation ofthe combined ultimate airframe loads, the 1 g component may be associated with typical flight conditions.

Page 4Draft AC No. 25.362-1 RJD Rev !!&/1624/99 d. Other failure conditions. Ifany other engine structural failure conditions, applicable tothe specific engine design, are identified that present a more critical load condition thanthe blade loss condition, they should be evaluated by dynamic analysis to a similar standard and using similar assumptions to those described in paragraph 5.c., above.



### 6. ANALYSIS METHODOLOGY.

a. Objective ofthe methodology. The objective ofthe analysis methodology is to develop acceptable analytical tools for conducting investigations ofdynamic engine structural failure events. The goal ofthe analysis is to produce loads and accelerations suitable for evaluations ofstructural integrity. However, where required for compliance with 

### § 25.901 ("Powerplant installation"), loads and accelerations may also need to be produced for evaluating the continued function ofsystems related tothe engine installation that are essential for immediate flight safety (for example, fire bottles and fuel shut off valves).b. Scope ofthe analysis. The analysis ofthe aircraft and engine configuration should be sufficiently detailed to determine the transient and steady-state loads for the engine mounts, pylon, and adjacent supporting airframe structure during the engine failure event and subsequent run-down.

o. Results ofthe analysis. The eagiae struetural failure aaalysis should provide loads for all parts oftheprimary struoture that are signifieaatly affeeted by the failure.

7. 

### MATHEMATICAL MODELINGAND VALIDATION.

a. Components ofthe integrated dynamic model. The applicant should calculate**A~irframe** dynamic responses should be ealeulated with an integrated model of the engine, pylon, and adjaeem supporting airframe structure. The integrated dynamic model used for engine structural failure analyses should be representative ofthe airplane to the highest frequency needed to accurately represent the transient response. The integrated dynamic model consists ofthe following components that may be validated independently:

• Airframe structural model.

• Engine structural model. b. Airframe Structural Model and Validation. (1) An analytical model ofthe airframe is necessary in orderto calculate the airframe responses due to the transient forces produced by the engine failure

Page 5Draft AC No. 25.362-1 RJD Rev**~/1624/99** event. The airframe manufacturers currently use reduced lumped mass finite element analytical models ofthe airframe for certification ofaeroelastic stability (flutter) and dynamic loads, iaeluding gust, landiag impact, and taxi. A typical model consists of relatively few lumped masses connected by weightless beams. A full airplane model is not usually necessary for the engine failure analysis, and it is normally not necessary to considerthe whole aircraft response, the effects ofautomatic flight control systems, or unsteady aerodynamics. (2) A lumped mass beam model ofthe airframe, similar to that normally used for flutter analysis, is acceptable for frequency response analyses dueto engine structural failure conditions. However, additional detail may be needed to ensure adequate fidelity for the engine structural failure frequency range. Inparticular, the engine structural failure analysis requires calculating theresponse ofthe airframe athigher frequencies than are usually needed to obtain accurate results for the other loads analyses, such as dynamic gust and landing impact. The applicant should use Ffinite element models should beused as necessary. As far as possible, the meaSllred ground vibration tests (GVT) normally conducted for compliance with 

### § 25.629 ("Aeroelastic stability requirements") should beused to validate the analytical model. (3) Structural dynamic models include damping properties, as well as representations ofmass and stiffuess distributions. Inthe absence ofbetterinformation, it will normally be acceptable to assume a value of0.03 (i.e., 1.5% equivalent critical viscous damping) for all flexible modes. Structural damping may be increased overthe 0.03 value tobe consistent with the high structural response levels caused byextreme failure loads, provided it isjustified.

c. Engine Structural Model and Validation. (1) Engine manufacturers construct various types ofdynamic models to determine loads and to perform dynamic analyses on the engine rotating components, static structures, mounts, and nacelle components. Dynamic engine models can range from a centerline two-dimensional (2D) model, to a centerline model with appropriate three-dimensional (3D) features, such as mount and pylon, up to a full 3D finite element**model~** (3D¥EM). 

### Any ofthese models canbe rna fur eithertransieHt or steady state coaditioas. The guidaace provided iaparagraph 6.b ofthis advisory circular shouldbe coasidered v/hea determimng the modeliag detail required. (2) DetailedFBM-finite element models typically include all major components ofthe propulsion system, such as: • the nacelle intake,
• fan cowl doors,
• thrust reverser, • common nozzle assembly,

Page 6--------

-- ---------

Draft AC No. 25.362-1 RJD Rev §i/1624/99

• all structural casings,
• frames,
• bearing housings,
• rotors, • gearbox, and
• a representative pylon. Gyroscopic effects are included. TheFEM-finite element models provide for representative connections at the engine-to-pylon interfaces, as well as all interfaces between components (e.g., inlet-to-engine and engine-to-thrust reverser). (3) Features modeled specifically for blade loss analysis typically include:
• fan imbalance,
• component failure,
• rubs (blade-to-casing, and intershaft),
• resulting stiffuess changes, and
• aerodynamic effects, such as thrust loss and engine surge. Manufacturers whose engines fail the rotor support structure by design during the blade loss event should also evaluate the effect ofthe loss ofsupport on engine structural response. (4) The model should be validated based ondedieated vibration tests and results ofthe blade loss test required for compliance with 

### § 33.94, giving due allowance for the effects ofthe test mount structure. The model should be capable of accurately predicting the transient loads from blade release through run-down to steady state. In cases where compliance with § 33.94 is granted by similarity instead oftest, the model should be correlated to prior experience. Forcompliance with § 25.362, the engine model, once validated, should be modified to include the influence ofrepresentative adjacent supporting airframe structure. (5) Validation ofthe engine model static structure including the pylon is achieved by a combination ofengine and component tests, which include structural tests on major load path components. The adequacy ofthe engine model to predict rotor critical speeds and forced response behavioris verified by measuring engine vibratory response when imbalances are added to the fan and otherrotors. Vibration data are routinely monitored on a number ofengines during the engine development cycle, thereby providing a solid basis for model correlation.

Page 7Draft AC No. 25.362-1 RJD Rev**~/1624/99** (6) Correlation ofthe model against the 

### § 33.94 blade loss engine test is a demonstration for which the model accurately predicts:
• initial blade release event loads,
• any rundown resonant response behavior,
• frequencies,
• potential structural failure sequences, and
• general engine movements and displacements. To enable this correlationto be performed, instrumentation oftheblade loss engine test should be used (e.g., use ofhigh-speed cinema and video cameras, accelerometers, strain gauges, continuity wires, and shaft speed tachometers). This instrumentation should be capable ofmeasuring loads onthe adjacent support structure engine attachment structure. (7) The airframe and engine manufacturers should mutually agree upon +the definition ofthe model should be mutually agreed upon between the airframe and engine manufacturers, based on test and experience.

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