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 Assignment 
Federal 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 of establishment of Loads 

and Dynamics Harmonization Working 

Group. -

SUMMARY: Notice is given of the 

establishment of the Loads and 

Dynamics Harmonization Working 

Group of the Aviation Rulemaking 

Advisory Committee (ARAC). This 

notice informs the public of the 

activities of the ARAC 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 

Federal Aviation Administration (FAA) 

has established an Aviation Rulemaking 

Advisory Committee (ARAC) (56 FR 

2190. January 22,1991; and 58 FR 9230. 

February 19. 1993). One area the ARAC 

deals with is transport airplane and 

engine issues (56 FR 31995; July 12, 

1991). These issues involve the 

airworthiness standards for transport 

airpla'les. engines and propellers in 

parts 25. 33 and 35 of the Federal 

Aviation Regulations (14 CFR parts 25. 

33 and 35) which are the responsibility 

of the FAA Director of Aircraft 

Certification. 

The FAA announced at the Joint 

A viation Authorities {JAA)-Federal 

A vfation Administration (FAA) 

Harmonization Conference in Toronto, 

Ontario. Canada. {June 2-5. 1992) that it 

would consolidate within the Aviation 

Rulemaking Advisory 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 were then in the . 

process of being {:O()rdinated between 

the JAA and the FAA. The 

harmonization process included the 

inteL.tion to pr&sent the results of JAAI 

FAA coordination to the public in the 

form of either a Notice of Proposed 

Rulemaking ar an adviSory circular-an 

objective comparable to and compatible 

with that assigned to the Aviation 

Rulemaking Advisory Committee. The 

Loads and Dynamics Harmonization 

Working Group is being formed to 

address loads and dynamics issues in 

JARIF AR parts 25 identified below. The 

Loads and Dynamics Harmonization 

Working Group \\'ill forward 

recommendations t1l the ARAC which 

will determi.ne whether to forward them 

1{)theF.AA 

Specifically. the Working Group', 

tasks are the following: The Loads and 

Dynamics Harmonization Worl~ing 

Group is charged with making 

recommendations to the ARAC 

concerning the FAA disposition oftha 

following subjects Tecently coordinated 

between the JAA and the FAA: 

Task l-General Design Loads 

Develop new or revised requirements. 

and associated advisory and guidanat 

material, for the general design loads for 

transport category airplanes (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). 

Task 2-Engine Torque and Gyroscopic 

Loads 

I Develop new or revised requirements. 

and associated advisory and guidan08 

material,-for determining the design 

loads for engine seizure conditions 

{FAR 25.361. 25.371 and other 

conforming changes). 

Task 3-Flutter, Deformation and FailSafe Criteria: 

Develop new or revised advisory and 

guidance material for flutter. 

defonnation and fail-safe criteria (FAR 

25.629). 

Reports 

A. Recommend time line{s) for 

completion of each task, including 

rationale. for consideration at the 

meeting of the ARAC to consider 

transport airplane and engine issues 

held following publication of this 

notice. 

B. Giva a detailed conceptual 

presentation on each task to the ARAC 

before proceeding with the work$tated 

under items C and D, below. If tasks 1 

and 2 require the development of more 

than one Notice of Proposed 

Rulemaking. identify what proposed 

amendments will be included in each 

notice. 

C. Draft one or more Notices of 

Proposed Rulemaking for Tasks 1 and 2 

proposing new or revised requirements, 

a supporting economic analysis and 

other required analysis. advisory and 

guidance material, and any other 

collateral documents the Working 

Group determines to be needed. 

D. Draft appropriate advisory and 

guidance material for Task 3. 

E. Give a status report on each task at 

each meeting of the ARAC held to 

consider transport airplane and engine 

issues. 

The Loads and Dynamics 

Harmonization Working Group will be 

comp"'sed of experts from those 

orgaruzatioos having an interest in the 

tasks assigned. A Working Group 

member need not necessarily be 8 

representative of one of the member 

organizations oftha ARAC. An 

individual who has expertise in the 

subject matter and wishes to become a 

member of the WOl'king Group should 

writ8 the person listed under the caption 

"FOR FURTHER *'FORMATION CONTACT" 

expNssing that deme. describing his or 

her interest in the task. and the 

expertise he or she would bring to the 

Wcding Group. The request will be 

reviewed with the Chairs of the ARAC 

Transport Airplane and Engine Interest 

Issues and the Loads and Dynamics 

Working Group. and the individual will 

be advised whether or not the request 

can be accommodated. 

The Secretary of Transportation has 

determined that the information and use 

of the ARAC is necessary in the public 

interest in connactian with the 

performance of duties of the FAA by 

law. Meetings of the ARAC will be open 

to the public except as authorized by 

section 10(d) of the Federal AdviSOry 

Committee Act. Meetings of the Laads 

and Dynamics Harmonization Working 

Group will not be open to the public 

except to the extent that individuals 

with an interest and expertise are 

~lected to participate. No public 

announcement of Working Group 

meetings will be made. 

Issued in Washington, DC, on March 8. 

1993. 

William J.Sullivan, 

Nsistant Executive Director for Transport 

Airplane and Engine Issues, Aviation 

Rulemaking Advisruy Committee. 

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

BlLUNG COOE 491o-1~ 
Recommendation Letter 
December 20, 1999 . 

Department of Transportation 

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 attached technical 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 of the following working group technical reports 

that you have submitted on behalf of the 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) (takeoff warning system); 25. 1333(b) (instru- / 

12/14/00 1,2.3 ment systems; and 25. 1423(bnpublic address 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 ind icating 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 (auxilI 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 aux12/20/00 2 iliary power unit load conditions) ~DHWG 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 of your July 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 

under the 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 of the 

Powerplant Installation Harmonization Working Group indicated that the working 

group members agreed that § 25.1103 was sufficiently harmonized and that any 

further action was beyond the scope of task 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 
Recommendation 
Proposed Text for NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.3611.362 

Developed from Recommendations of Working 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 of the U.S. and the Joint Aviation Requirements of Europe, without 

affecting current industry design practices. 

How Does This Proposed Regulation Relate to "Fast Track"? 

This proposed regulation results from the recommendations of ARAC submitted under the 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 by sudden engine stoppage due to malfunction or structural 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 of the 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 and its supporting structure must be designed for the effects of--

Docs # 11695 Page 13 0[20 
Proposed Text for NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.361/.362 

Developed from Recommendations of Working Group 

(1) A limit engine torque corresponding to takeoff power and propeller speed 

acting simultaneously with 75 percent of the limit loads from flight condition A of Sec. 

25. 333(b); 

(2) A limit torque corresponding to the maximum continuous power and propeller 

speed, acting simultaneously with the limit loads from flight condition A of Sec. 25.333(b); 

and 

(3) For turbopropeller installations, in addition to the conditions specified in 

paragraphs (a)(1) and (2) of this section, a limit engine torque corresponding to takeoff 

power and propeller speed, multiplied by a factor accounting for propeller control system 

malfunction, including quick feathering, acting simultaneously with 1 g level flight loads. In the 

absence of a rational analysis, a factor of 1.6 must be used. 

(b) For turbine engine installations, the engine mounts and supporting structure must be 

designed to withstand each of the following: 

(1) A limit engine torque load imposed by sudden engine stoppage due to 

malfunction or structural failure (such as compressor jamming). 

(2) A limit engine torque load imposed by the maximum acceleration of the 

engine. 

(c) The limit engine torque to be considered under paragraph (a) of this 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 with five or more cylinders; or 

(3) Two, three, or four, for engines with four, three, or two cylinders, respectively. 

What are the Current JAR standards: 

The current text of JAR-2S.361 is: 

JAR 25.361 Engine and APU torque. 

(a) Each engine mount and its supporting structure must be designed for engine torque 

effects combined with -- .. .... 

(1) A limit engine torque corresponding to takeoff power and propeller speed 

acting simultaneously with 75 percent of the limit loads from flight condition A of JAR 

25. 333(b); 

(2) A limit torque corresponding as specified in sub-paragraph (c) of this 

paragraph acting simultaneously with the limit loads from flight condition A of JAR 25.333(b); 

and 

(3) For turbopropeller installations, in addition to the conditions specified in subparagraphs (a)(1) and (2) of this paragraph, a limit engine torque corresponding to takeoff 

power and propeller speed, multiplied by a factor accounting for propeller control system 

Docs # 11695 Page 14 of20 
Proposed Text for NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.361/.362 

Developed from Recommendations of Working Group 

malfunction, including quick feathering, acting simultaneously with 19 level flight loads. In 

the absence of a rational analysis, a factor of 1.6 must be used. 

(b) For turbine engines and auxiliary power unit installations, the limit torque load 

imposed by sudden stoppage due to malfunction or structural failure (such as a compressor 

jamming) must be considered in the design of engine and auxiliary power unit mounts and 

supporting structure. In the absence of better information, a sudden stoppage must be 

assumed to occur in 3 seconds. 

(c) The limit engine torque to be considered under sub-paragraph (a)(2) of this 

paragraph is obtained by multiplying the mean torque by a factor of 1.25 for turbopropeller 

installations. 

(d) When applying JAR 25. 361 (a) to turbo-jet engines, the limit engine torque must be 

equal to 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 of the 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 of the 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 of jet 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 of foreign 

--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 of the 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 of20 
Proposed Text for NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.361/.362 

Developed from Recommendations of Working Group 

The FAA finds that, relative to the engine configurations that existed when the rule was first 

developed, these later generations of jet 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. Over the 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 of the 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 of the 

rotating system. These events are usually associated with temporary loss of power or thrust 

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 of20 
Proposed Text for NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.3611.362 

Developed from Recommendations of Working Group 

or combinations of these 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 of thrust-producing capability. Examples of these 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 of the rare occurrence of these 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 of safety. 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 It Address 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"). For the 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, or turbine 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 of the 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 of20 
Proposed Text for NPRM on Engine & Auxiliary Power Unit Load Conditions, § 25.361/.362 

Developed from Recommendations of Working 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 of the following: 

(1) sudden auxiliary power unit deceleration due to malfunction or structural failure; and 

(2) the maximum acceleration of the power unit. 

Add a new section 25.362, to read as 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, or turbine 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 of20 
O-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 CFR Part 25 

[Docket No. ; Notice No. ] 

RIN: 2120-

Engine and Auxiliary Power Unit Load Conditions. 

AGENCY: Federal Aviation Administration (FAA), DOT. 

ACTION: Notice of proposed rulemaking (NPRM). 

Revision 3 

8/19/99 

SUMMARY: This notice proposes to amend the engine loads design requirements for the 

engine mounts, auxiliary power unit mounts, engine pylons and adjacent supporting 

airframe structures on transport 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 subject between the U.S. regulations and the Joint 

Aviation Requirements (JAR) of Europe; doing so would relieve manufacturers of the 

current burden of certifying airplanes to two different sets of standards to achieve 

essentially the same intended safety benefit. 

DATES: Comments must be received on or before [insert date 90 days after date of 

publication in the Federal Register]. 

ADDRESSES: Comments on this document should be mailed or delivered, in duplicate, 

to: U.S. Department of Transportation Dockets, Docket No. __ ~, 400 Seventh Street 

SW., Room Plaza 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 Room Plaza 401 between 10:00 a.m. and 5:00 

p.m. weekdays, except Federal holidays. 

In addition, the FAA is maintaining an information docket of comments 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. 

FOR FURTHER INFORMATION 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 of the proposed action 

by submitting such written data, views, or arguments as they may desire. Comments 

relating to the 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 

or notice number and be submitted in duplicate to the DOT Rules Docket address 

specified above. 

All comments received, as well as a report summarizing each substantive public 

contact with FAA personnel concerning this proposed rulemaking, will be filed in the 

docket. The docket is available for public inspection before and after the comment closing 

date. 

All comments received on or before the closing date will be considered by the 

Administrator before taking action on this 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 of the comments received. 

Commenters wishing the FAA to acknowledge receipt of their comments 

submitted in response to this document must include a pre-addressed, stamped postcard 

with those comments on which the following statement is made: "Comments to Docket 

No. ." The postcard will be date stamped and mailed to the commenter. 

Availability of NPRMs 

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 Government Printing 

Office's (GPO) electronic bulletin board service (telephone: 202-512-1661), or, if 

applicable, the FAA' s Aviation Rulemaking Advisory Committee bulletin board service 

telephone: 800-322-2722 or 202-267-5948). 

Internet users may reach the FAA's web page at 

http://www.faa.gov/avr/armlnprmlnprm.htm or the GPO's webpage at 

http://www.access.gpo.gov/nara for access to recently published rulemaking documents. 

Any person may obtain a copy of this document by submitting a request to the 

Federal Aviation Administration, Office of Rule making, ARM-I, 800 Independence 

Avenue, SW., Washington, DC 20591; or by calling (202) 267-9680. Communications 

must identify the notice number or docket number of this NPRM. 

Persons interested in being placed on the mailing list for future rulemaking 

documents should request from the above office a copy of Advisory Circular No. 11-2A, 

«Notice of Proposed Rulemaking Distribution System," which describes the application 

procedure. 

3 
D-R-A-F-T 

(Incorporates Legal's input 

Background 

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The airworthiness standards for transport category airplanes are contained in 

14 CFR part 25 [commonly referred to as the Federal Aviation Regulations (FAR), 

part 25]. Manufacturers of transport category airplanes must show that each airplane they 

produce of a different type design complies with the relevant standards of part 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 to the US. 

under a bilateral airworthiness agreement. 

In Europe, the Joint Aviation Requirements (JAR) were developed by the Joint 

Aviation Authorities (JAA) to provide a common set of airworthiness standards for use 

within the European aviation community. The airworthiness standards for European type 

certification of transport category airplanes are contained in JAR-25, and are based on 14 

CFR part 25 [commonly referred to as part 25 of the Federal Aviation Regulations 

(FAR)].. Airplanes certificated to the JAR-25 standards, including airplanes manufactured 

in the US. for export to Europe, receive type certificates that are accepted by the aircraft 

certification authorities of 26 European member countries. 

Although part 25 and JAR-25 are very similar, they are not identical in every 

respect. Differences between the two sets of standards can result in substantial additional 

costs when airplanes are type certificated to both 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 of standards 

would not only economically benefit the aviation industry, but also would maintain the 

necessary high level of safety, the FAA and JAA consider "harmonization" of the two sets 

of standards to be a high priority. 

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In 1988, the FM in cooperation with the JAA and other organizations 

representing the American and European aerospace industries, began a process to 

harmonize the airworthiness requirements of the United States and the airworthiness 

requirements of Europe, especially in the areas of Flight Test and Structures. 

The Aviation Rulemaking Advisory Committee 

Later, in 1992, the FAA harmonization effort was undertaken by the Aviation 

Rulemaking Advisory Committee (ARAC). The ARAC was formally established by the 

FAA on January 22, 1991 (56 FR 2190), to provide advice and recommendations 

concerning the full range of the FAA's safety-related rulemaking activity. This advice was 

sought to develop better rules in less overall time using fewer FAA resources than are 

currently needed. The committee provides the opportunity for the FAA to obtain 

firsthand information and insight from interested parties regarding proposed new rules or 

revisions of existing rules. 

There are 64 member organizations on the committee, representing a wide range 

of interests within the aviation community. Meetings of the committee are open to the 

public, except as authorized by section 1 O( d) of the Federal Advisory Committee Act. 

The ARAC establishes working groups to develop proposals to recommend to the 

FAA for resolving specific issues. Tasks assigned to working groups are published in the 

Federal Register. Although working group meetings are not generally open to the 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 can be presented to the FAA as an advisory committee 

recommendation. 

The activities of the ARAC will not, however, circumvent the public rulemaking 

procedures. After an ARAC recommendation is received and found acceptable by the 

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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). The Working Group is made up 

of structural specialists from the aviation industry and government of Europe, the United 

States, and Canada. The task given to this Working Group was to harmonize the design 

loads section of Subpart C ("Structure") of 14 CFR part 25 with the counterpart 

requirements of the JAR. The Working Group developed specific recommendations for 

harmonizing the engine loading conditions. The ARAC approved those recommendations 

and recommended them to the FAA for rulemaking. The FAA has accepted ARAC's 

recommendation, and the proposed rulemaking contained in this notice follows from those 

recommendations and the activity ofthe Working Group. 

Issues Prompting This Proposal 

The current airworthiness standards contained in 14 CFR part 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 due to malfunction or 

structural failure (such as compressor jamming)." This was first made a specific 

requirement for U. S. transport category airplanes in 1957 by Civil Air Regulation (CAR) 

4b.216(a)(4). It was later carried forward in § 25.361(b)(I) of part 25 when the Federal 

Aviation Regulations were recodified. This same requirement is contained in section 

25.3610fJAR-25. 

Previous methods of complying with this requirement have entailed either: 

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

or 

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• designing to a torque level established by the polar moment of inertia of the 

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 of the particular engine, the engine manufacturers 

traditionally have provided the airframe manufacturers with the information necessary to 

install each engine. 

The size, configuration, and failure modes of jet 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 of foreign objects such as birds or ice. Whatever the 

source, those conditions could produce significant structural 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 of the 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. 

The FAA finds that, relative to the engine configurations that existed when the rule 

was first developed, these later generations of jet engines are sufficiently different and 

novel to justity amending the regulations to ensure that adequate design standards are 

available for the mounts and the structure supporting these newer engines. Therefore, in 

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order to maintain the level of safety intended by § 25.361(b)(1), the FAA considers 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 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 of the 

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. 

These terms are discussed in § 25.301 (''Loads''), § 25.303 (''Factor of safety"), 

and § 25.305 ("Strength and deformation"). 

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 of the rotating system. These events are usually associated with temporary loss of 

power or thrust 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 of these 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 of thrust-producing capability. Examples ofthese 

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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 of the rare occurrence of 

these 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 of safety. 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. 

Discussion of Proposal 

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 of event") and design criteria for those rare 

events resulting from structural failures (described above as the "second type of event"). 

For the 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, or turbine blade; and, for certain designs, 

any other engine structural failure that results in higher loads). 

Although the FAA recognizes that some engine configurations may exist in which 

the blade failure event is not the most critical load, the FAA expects that, for most 

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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 of the 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, the ultimate loads would be required 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 of part 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 of safety over that provided by the existing static engine torque 

criterion. 

With the addition of new § 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, or four 

cylinders, would be deleted. Transport category airplanes have not used these engines in 

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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 

particular requirements, since new engine failure ultimate load conditions would be 

contained in new § 25.362. 

6. Proposed revised § 25.361(c) would now require 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 of power or thrust, and 

• maximum engine acceleration. 

This proposal also would add a new § 25.361(d) that would contain similar design 

load requirements as those of proposed revised § 25.361(c). However, they would apply 

strictly to the power unit mounts and adjacent supporting airframe structure for auxiliary 

power unit (APU) installations. 

Finally, the title of § 25.361 would be changed from the current "Engine torque" 

to "Engine and auxiliary power unit torque." This change is necessary in order to provide 

a better indication of what the paragraph addresses. 

Relevant Advisory Materials 

The FAA is preparing to issue a new proposed Advisory Circular 25.362-1 to 

describe a means of compliance with the proposed regulation, which would meet the 

intended level of safety and promote consistent and effective application of the proposed 

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revised standards. Public comments concerning the proposed AC are invited by separate 

notice published elsewhere in this issue of the Federal Register 

Paperwork Reduction Act 

In accordance with the Paperwork Reduction Act of 1995 [44 U.S.c. 3507(d)], 

the FAA had determined there are no requirements for information collection associated 

with this proposed rule. 

Compatibility with ICAO Standards 

In keeping with U.S. obligations under the Convention on International Civil 

Aviation, it is FAA policy to comply with International Civil Aviation Organization 

(ICAO) Standards and Recommended Practices to the maximum extent practicable. The 

FAA determined that there are no ICAO Standards and Recommended Practices that 

correspond to this proposed regulation. 

Regulatory Evaluation Summary 

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. And fourth, the Unfunded Mandates Reform Act of 1995 (Pub. L. 

104-4) requires agencies to prepare a written assessment of the co§ts, benefits, and other 

effects of proposed or final rules that include a Federal mandate likely to result in the 

expenditure by State, local, or tribal governments, in the aggregate, or by the private 

sector, of$100 million or more annually (adjusted for inflation). In conducting these 

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

that justify its costs and would not be "a significant regulatory action" as defined in section 

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3(f) of Executive Order 12866 and, therefore, is not subject to review by the Office of 

Management and Budget; (2) would not have a significant impact on a substantial number 

of small entities; (3) would not constitute a barrier to international trade; and (4) would 

not contain a significant intergovernmental or private sector mandate. These analyses, 

available in the docket, are summarized below. The FAA invites the public to provide 

comments and supporting data on the 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 (RF A) establishes "as a principle of 

regulatory issuance that agencies shall endeavor, consistent with the objective of the rule 

and of applicable 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 consider flexible 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 

governmental jurisdictions. 

Agencies must perform a review to determine whether a proposed rule will have a 

significant economic impact on a substantial number of small entities. If the determination 

is that it will, the agency must prepare a regulatory flexibility analysis as described in the 

Act. 

However, if an agency determines that a proposed or final rule is not expected to 

have a significant economic impact on a substantial number of small entities, section 

605(b) ofthe 1980 Act provides that the head of the 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 add economic evaluation here. J 

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International Trade Impact Assessment 

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The provisions of this proposed rule would have little or no impact on trade 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 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. 

Therefore, in accordance with Executive Order 12612, it is determined that this proposal 

would not have sufficient federalism implications to warrant the preparation of a 

federalism assessment. 

U nfnnded Mandates Reform Act 

Title II ofthe Unfunded Mandates Reform Act of 1995 (the Act), codified in 

2 U.S.C. 1501-1571, requires each Federal agency, to the extent permitted by law, to 

prepare a written assessment ofthe effects of any Federal mandate in a proposed or final 

agency rule that may result in the expenditure by State, local, and tribal governments, in 

the aggregate, or by the private sector, of $100 million or more (adjusted annually for 

inflation) in anyone year. Section 204(a) ofthe Act, 2 U.S.C. 1534(a), requires the 

Federal agency to develop an effective process to permit timely input by elected officers 

(or their designees) of State, local, and tribal governments on a proposed "significant 

intergovernmental mandate." A "significant intergovernmental mandate" under the 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 anyone year. Section 203 of the Act, 2 U.S.c. 1533, which supplements 

section 204(a), provides that before establishing any regulatory requirements that might 

significantly or uniquely affect small governments, the agency shall have developed a plan 

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that, among other things, provides for notice to potentially affected small governments, if 

any, and for a meaningful and timely opportunity to provide input in the development of 

regulatory proposals. 

This proposed rule does not contain a Federal intergovernmental or private sector 

mandate that exceeds $100 million in anyone year. 

Environmental Analysis 

FAA Order 1 050.1D defines FAA actions that may be categorically excluded from 

preparation of a 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 of the 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). It has been determined that it is not a major regulatory action under the 

provisions of the EPCA. 

Regulations Affecting Intrastate Aviation in Alaska 

Section 1205 of the FAA Reauthorization Act of 1996 (110 Stat. 3213) requires 

the Administrator, when modifying regulations in Title 14 of the CFR in a manner 

affecting intrastate aviation in Alaska, to consider the extent to which Alaska is not served 

by transportation modes other than aviation, and to establish such regulatory distinctions 

as he or she considers appropriate. Because this proposed rule would apply to the 

certification offuture designs of transport category airplanes and their subsequent 

operation, it could, if adopted, affect intrastate aviation in Alaska. The FAA therefore 

specifically requests comments on whether there is justification for applying the proposed 

rule differently to intrastate operations in Alaska. 

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List of Subjects in 14 CFR Part 25: 

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Aircraft, Aviation safety, Reporting and record keeping requirements 

The Proposed Amendment 

In consideration of the foregoing, the Federal Aviation Administration proposes to 

amend part 25 of Title 14, Code of Federal Regulations, as follows: 

Part 25 - AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY 

AIRPLANES 

1. The authority citation for Part 25 continues to read as follows: 

Authority: 49 U.S.C. 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 power unit torque 

(a) * * * * * 

(b) The limit engine torque to be considered under paragraph (a) of this 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) For turbine 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 of the following: 

(1) sudden engine deceleration due to a malfunction which could result in a 

temporary loss of power or thrust; and 

(2) the maximum acceleration of the engine. 

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(d) For auxiliary power unit installations, the power unit 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 of the following: 

(1) sudden auxiliary power unit deceleration due to malfunction or structural 

failure; and 

(2) the maximum acceleration ofthe power unit. 

3. By adding a new section 25.362, to read as 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, or turbine 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. 

Issued in Washington, D. C., on 

Director, 

Aircraft Certification Service 

17 
US. Department 

of Transportation 

Federal AvIation 

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 Power Unit 

Load Conditions," of 14 CFR part 25 [commonly referred to as ef.part 25 of the Federal 

Aviation Regulations (FAR)], as it applies to transport category airplanes. These means 

are intended to provide guidance to supplement the engineering and operational judgment 

that must form the basis of any compliance findings relative to the design of engine 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 F ederal 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. It is issued to describe an acceptable means, but not the only 

means, for demonstrating compliance with the requirements for transport category 

airplanes. Terms such as "shall" and "must" are used only in the sense of ensuring 

applicability of this particular method of compliance when the acceptable method of 

compliance described in this document is used. 

2. RELA TED SECTIONS OF FEDERAL AVIATION REGULATIONS. 

a. Title 14, Code of Federal Regulations (CFR) Part 25: 

Section 25.361 "Engine and auxiliary power unit torque" 

Section 25.901 ''Powerplant installation" 

b. Title 14, Code of Federal 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 new terms have been defined for the transient engine 

failure conditions in order to present criteria in a precise and consistent manner in the 

following pages. In addition, some terms are employed from other fields and may not 

necessarily be in general use. For the purposes of this Advisory Circular, the following 

definitions should be used. 

a. Adjacent supporting airframe structure: Those parts of the primary 

airframe that are directly affected by loads arising within the engine. 

b. Blade loss: The loss of the most critical fan, compressor, or turbine blade. 

c. Factor of Safety: The ratio of ultimate load to limit load. 

~. Ground Vibration Test :(GVT): Ground resonance tests ofthe airplane 

normally conducted ffi-for compliance with § 25.629, "Aero elastic 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 of the engine 

structural failure, up to the time at which the engine stops rotating or achieves a steady 

windmilling rotational speed. 

g. Ultimate Load: The load that the structure must withstand vAthout failure. 

~. Windmilling engine rotational speed. The speed at which the rotating shaft 

systems of an unpowered engine will rotate due to the flow of air 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 Power Unit Load 

Conditions") requires that the engine mounts, pylons, and adjacent supporting airframe 

structure be designed to withstand 1 g flight loads combined with transient 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 

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failure conditions have-are usually been-caused by internal structural failures or ingestion 

offoreign objects, such as birds or ice. Whatever the source, these conditions may 

produce significant structural loads on the engine, engine mounts, pylon, and adjacent 

supporting airframe structure. With the development oflarger high-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 of the 

transient loads that can be produced during and following engine failures. Consequently, 

for engine failure events, it is considered necessary that the applicant t&-perform dynamic 

analysis that considers all load components. 

A dynamic model of the aircraft and engine configuration must be sufficiently detailed to 

characterize the transient 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. Of all the applicable engine structural 

failure conditions, -Dgesign and test experience have shown that, of aU the applieable 

engiRe struetural failure eORditioRS, the loss of a blade is likely to produce the most severe 

loads on the engine and airframe. Therefore, § 25.362 requires that the transient dynamic 

loads from these blade failure conditions be considered when evaluating structural 

integrity. However, service history shows examples of other severe engine structural 

failures where the engine thrust-producing capability is-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 of failures are 

applicable, and if they present a more critical load condition than blade loss. Examples of 

other engine structural failure conditions that should be considered in this respect are: 

• failure of a shaft, or 

• failure or loss of any shaft support bearing. 

5. EVALUATION OF TRANSIENT FAILURE CONDITIONS 

a. Objective. The applicant should show, by a combination of tests and 

analyses" that the airplane is capable of continued safe flight and landing after partial or 

eomplete loss ofa blade, including ensuing damage to other parts of the engine. 

The primary failure condition is expected to be blade release (refer to 14 CFR part 33, 

§ 33.94, ''Blade containment and rotor unbalance tests"). However, other structural 

failures may need to be considered as well, depending upon the engine configuration. 

The applicant also should give eoosideratioR toconsider the transient loads from the time 

of the engine structural failure, up to the time at which the engine stops rotating or 

achieves a steady windmilling rotational speed. [NOTE: The effects of continued 

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~&/1624/99 

rotation (windmilling) are described in AC 25-901-1, "System Safety Assessment of 

Powerplant Installations" -- cu"entiy in D-R-A-F-T form.] 

b. Evaluation. The applicant's evaluation should show that, from the moment 

of engine structural failure and during spool-down to the time of windmilling engine 

rotational speed, the engine-induced loads and vibrations will not cause failure of the 

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. For design of the engine mounts and pylon, the 

ultimate loads may be taken without any additional multiplying factors. At the same time, 

protection of the basic airframe is assured by using a multiplying factor of 1.25 on those 

ultimate loads for the design of the adjacent supporting airframe structure. 

c. Blade loss condition. The applicant should determine loads vrithin the 

eRgiHe, and on the engine mounts, pylon, and adjacent supporting airframe structure by 

dynamic analysis. The analysis should take into account all significant structural degrees 

of freedom. The transient engine loads should be determined for the fan blade failure 

condition and rotor speed, as specified in § 33.94, and over the full range of blade release 

angles to allow determination of the critical loads for all affected components. The 

amount of engine damage that develops during the failure event and, consequently, the 

loads produced, depends on material properties and temperature. Therefore, the analysis 

of transient engine loads should consider the effects of variations 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 to be 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 of incremental transient airframe loads should consider: 

• the effects ofthe engine mounting station on the 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). 

F or calculation of the combined ultimate airframe loads, the 1 g component may be 

associated with typical flight conditions. 

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d. Other failure conditions. If any other engine structural failure conditions, 

applicable to the specific engine design, are identified that present a more critical load 

condition than the 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 of the methodology. The objective of the analysis methodology 

is to develop acceptable analytical tools for conducting investigations of dynamic engine 

structural failure events. The goal of the analysis is to produce loads and accelerations 

suitable for evaluations of structural 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 of systems related to the engine installation 

that are essential for immediate flight safety (for example, fire bottles and fuel shut off 

valves). 

b. Scope of the 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 of the analysis. The eagiae struetural failure aaalysis should 

provide loads for all parts ofthe primary struoture that are signifieaatly affeeted by the 

failure. 

7. MATHEMATICAL MODELING AND VALIDATION. 

a. Components of the 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 of the airplane to 

the highest frequency needed to accurately represent the transient response. The 

integrated dynamic model consists of the following components that may be validated 

independently: 

• Airframe structural model. 

• Engine structural model. 

b. Airframe Structural Model and Validation. 

(1) An analytical model of the airframe is necessary in order to 

calculate the airframe responses due to the transient forces produced by the engine failure 

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event. The airframe manufacturers currently use reduced lumped mass finite element 

analytical models of the airframe for certification of aero elastic 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 

consider the whole aircraft response, the effects of automatic flight control systems, or 

unsteady aerodynamics. 

(2) A lumped mass beam model of the airframe, similar to that normally 

used for flutter analysis, is acceptable for frequency response analyses due to engine 

structural failure conditions. However, additional detail may be needed to ensure 

adequate fidelity for the engine structural failure frequency range. In particular, the engine 

structural failure analysis requires calculating the response of the airframe at higher 

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 be used as necessary. As far as possible, the meaSllred ground vibration tests 

(GVT) normally conducted for compliance with § 25.629 ("Aero elastic stability 

requirements") should be used to validate the analytical model. 

(3) Structural dynamic models include damping properties, as well as 

representations of mass and stiffuess distributions. In the absence of better information, it 

will normally be acceptable to assume a value of 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 

failure loads, provided it is justified. 

c. Engine Structural Model and Validation. 

(1) Engine manufacturers construct various types of dynamic 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 of these models can be rna fur either transieHt or steady state 

coaditioas. The guidaace provided ia paragraph 6.b of this advisory circular should be 

coasidered v/hea determimng the modeliag detail required. 

(2) Detailed FBM-finite element models typically include all major 

components of the propulsion system, such as: 

• the nacelle intake, 

• fan cowl doors, 

• thrust reverser, 

• common nozzle assembly, 

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• all structural casings, 

• frames, 

• bearing housings, 

• rotors, 

• gearbox, and 

• a representative pylon. 

Gyroscopic effects are included. The FEM-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 of the loss of support on engine structural 

response. 

(4) The model should be validated based on dedieated vibration tests 

and results of the blade loss test required for compliance with § 33.94, giving due 

allowance for the effects of the 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 of test, the 

model should be correlated to prior experience. For compliance with § 25.362, the engine 

model, once validated, should be modified to include the influence of representative 

adjacent supporting airframe structure. 

(5) Validation of the engine model static structure including the pylon 

is achieved by a combination of engine and component tests, which include structural tests 

on major load path components. The adequacy of the engine model to predict rotor 

critical speeds and forced response behavior is verified by measuring engine vibratory 

response when imbalances are added to the fan and other rotors. Vibration data are 

routinely monitored on a number of engines during the engine development cycle, thereby 

providing a solid basis for model correlation. 

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(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 correlation to be performed, instrumentation ofthe blade loss engine test 

should be used (e.g., use of high-speed cinema and video cameras, accelerometers, strain 

gauges, continuity wires, and shaft speed tachometers). This instrumentation should be 

capable of measuring loads on the adjacent support structure engine attachment structure. 

(7) The airframe and engine manufacturers should mutually agree upon 

+the definition of the model should be mutually agreed upon between the airframe and 

engine manufacturers, based on test and experience. 

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