# Exemption

No. 11075 UNITED

STATES

OF AMERICA DEPARTMENT

OF TRANSPORTATION FEDERAL AVIATION ADMINISTRATION BURLINGTON,

MA 01803-5229 In the matter

of the petition

of ROLLS-ROYCE

pic for

an exemption

from§ 33.27(£)(6) of Title

14, Code

of Federal Regulations

Regulatory

Docket

No. FAA-2014-0609 GRANT

OF EXEMPTION August 13,2014, Rolls-Royce

plc, ML-52,

PO Box

31, Derby,

DE24

8BJ, England, petitioned the Federal Aviation Administration

(FAA)

for

an exemption

from

Title

14, Code

of Federal Regulations

(14 CPR)§ 33.27(£)(6), Amendment

33-31. The petitioner

requests

relief

from

the following regulation: . Section 33.27(t)(6), Amendment

33-31, Turbine, compressor,

fan,

and turbosupercharger

rotor overspeed. "(f) 

# Failure

of

a shaft section

may

be excluded

from consideration

in determining

the highest overspeed

that would

result

from

a complete

loss ofload

on

a turbine

rotor

ifthe applicant: (6) Does

not exclude

the entire

shaft." Petitioner Rolls-Royce

requests

the FAA

grant

the exemption

based

on two points

. Petitioner's first point

is that

for each

of the Trent

XWB

engine

models

it identifies,

the petitioner

has shown that

the high-pressure

(HP) turbine

rotor

burst

speed

is greater

than

100

% 

# rotor terminal

speed resulting

from

HP shaft system

failure. Petitioner's

second

point

is that compliance

with§

33

. :?7 cannot

be shown

, absent

an exemption

from§

33

. 27(£)(6).

The petitioner

adds

, however

, that compliance

can

be shown without adversely

affecting

safety

, if the exemption

is granted

. Petitioner

requests exemption

from

14 C.P.R.

§ 33

. 27(£)(6)

for

the

HP shaft system

of the following

Trent

XWB engine

models

: ANE-14-066-

EModelTrentXWB-84TrentXWB-79BTrentXWB-79TrentXWB-75Ratingx1000lbs.84797975Petitionerassertsthatasdemonstratedinitssubstantiatingreport,thelevelofsafetyaffordedbycompliancewith§33.27fortheHPshaftsystemwillbeunaffectedbythegrantingofanexemptionfrom§33.27(±)(6).Petitioneralsostatesthatgrantingthisexemptionwillpromoteandservethepublicinterest,andpetitioneridentifiedthosebenefitsinitspackage.Finally,petitioneralsoassertsthatgrantingitsexemptionwillhavenoadverseeffectonsafetyandprovidedthefollowinginsupportofthatcontention.1.TheHPshaftsystemdesignutilizedacrosstheTrentenginefamilydifferssignificantlyfromconventionalshaftsystems.AFailureModeandEffectAnalysisdeterminedthatnoknownorpotentialenvironmentalthreatstotheintegrityoftheHPshaftsystemcomponentsexistthatcouldleadtoalossofloadfailure.2.Thespecialcontrolsappliedtothetorque-transmittingcomponents(i.e.,theHPcompressorreardrumandtheHPturbinediscoftheHPshaftsystem)areofahigherstandardthanthoseofmanyothershaftsystemsinthepast.3.Theserviceexperienceofpetitioner'sthree-shaftfamilyofenginessupportsthetheoreticalassessmentofpoints1)and2)above,andallincorporatethesamedesignstylefortheHPshaftsystemastheenginemodelsthatarethesubjectofthispetition.Operationally,overmorethan250millionserviceflighthourshavebeenrecordedwithoutfailureofanyreportedHPshaftsystemtorquepathfailuresontheTrentfamilyofengines.Thisperformancetranslatestoafailurerateoflessthan2.8x10-9failuresperflighthour.FederalRegisterPublicationTheFAAdeterminedthatpublishingthepetitioner'srequestforexemptionintheFederalRegisterforpubliccommentwasnecessary,per14CFR11.87.AsummaryofthepetitionwaspublishedintheFederalRegisteronSeptember2,2014(79FR52104),fora10dayperiod.TheFAAreceivednocomments.ThecompletepetitionisavailableattheDepartmentofTransportation'sFederalDocketManagementSystemathttp://regulations.govunderDocketNo.FAA-2014-0609.2TheFAA'sAnalysisOurreviewofthepetitioner'spetitionandsupportingdataindicatesthatwhatthepetitionerseeksisnotanexemptionto§33.27(f)(6),butto§33.27(b)(3).Section§33.27(b)(3)requiresarotorburstmarginbegreaterthan5%ofrotorterminalspeed.Section33.27(f)(6)prohibitsexclusionoftheentireshaftsystemfromfailureconsideration.Petitionerrequestedthattheexemptionfrom§33.27(f)(6)begrantedonthebasisofitsdemonstrationthatforeachofthelistedenginemodels,theHPturbinerotorburstspeedisgreaterthan100%rotorterminalspeedresultingfromanHPshaftsystemfailure.Basedonwhatthepetitionerrequestedandwhatthepetitionerdemonstratedinitssupportingpackage,anexemptionto§33.27(f)(6)isnotapplicable.Rather,thepetitioner'srequestisapetitionforexemptionto§33.27(b)(3),andtherequirementthatrotorburstmarginbegreaterthan5%ofrotorterminalspeed.Therefore,weinterpretpetitioner'srequestasarequestforexemptionto§33.27(b)(3).WereviewedthedataprovidedbypetitionerandfindthattheHPturbinerotorburstspeedsfortheTrentXWBenginemodelsaregreaterthan100%rotorterminalspeedresultingfromanHPshaftsystemfailure.Basedonourfindingandthecompensatingfactorsthatthepetitionerdemonstratedthroughgoodserviceexperienceofsimilarthree-shaftdesignsystemsandevaluationofallknownfailuremodes,weconcludethatanexemptionto§33.27(b)(3)fortheTrentXWB-84,-79B,-79,and-75enginemodelswillhavenoadverseeffectonsafety.TheFAA'sDecisionInconsiderationoftheforegoing,andbasedontheFAA'sinterpretationofpetitionerRollsRoyce'sexemptionrequest,Ifindthatagrantofexemptionwillnotadverselyaffectthelevelofsafetyprovidedbytheregulationsandisinthepublicinterest.Therefore,pursuanttotheauthoritycontainedin49 U.S.C.§§106(f),40113and44701,delegatedtomebytheAdministrator,thepetitioner,Rolls-Royceplc,isherebygrantedanexemptionto§33.27(b)(3)forHPshaftsystemfailurewiththeconditionthattheHPturbinerotorburstspeedbegreaterthan100%rotorterminalspeedresultingfromHPshaftsystemfailure.IssuedinBurlington,Massachusetts,onOctober3,2014.fLc.·?Vlttw--jD'AlessandroAssistantManager,EngineandPropellerDirectorateAircraftCertificationService3