ExemptionNo. 15079 UNITED STATES OF AMERICA DEPARTMENT OF TRANSPORTATION FEDERAL AVIATION ADMINISTRATION BURLINGTON, MA 01803-5229

Inthe matter ofthe petition of ROLLS-ROYCE pic Regulatory Docket No. FAA-2016-0072 for an exemption from §33.27(±)(6) ofTitle 14, Code ofFederal Regulations

PARTIAL GRANT OF EXEMPTION
On December 30, 2015, Mr. Richard Shepherd, Airworthiness Specialist, Rolls-Royce plc (Rolls-Royce), ML-52, PO Box 31, Derby, DE24 8BJ, England, petitioned the Federal Aviation Administration (FAA) onbehalfofRolls-Royce plc for an exemption from § 33.27(f)(6), Amendment 33-31 ofTitle 14, Code ofFederal Regulations (14 CFR). The proposed exemption, ifgranted, would allow some high-pressure (HP) shafts to be exempted from 14 CFR §33.27(±)(6). The petitioner requests relieffrom the following regulation: Section 33.27(±)(6), Amendment 33-31 prescribes that: "(f) Failure ofa shaft section maybe excluded from considerationin 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." The petitioner supports its request with the following information: The petitionerrequests the FAA grant the exemption based ontwo points. The petitioner's first point is that the design ofthe Trent XWB HP shaft system is not amenable to analysis by shaft section. The petitioner's second point is that they are not able to show compliance with §33.27 without an exemption from §33.27(±)(6). However, the petitioner claims they can show an equal level ofsafety thatwill meetthe requirements of§33.27.

ANE-16-0001-E2 The petitionerrequests exemption from 14 C.P.R. §33.27(f)(6) for the HP shaft system ofthe following Trent XWB engine models: Model Rating x 1000 lbs.
Trent XWB-84 Trent XWB-79B Trent XWB-79 Trent XWB-75 84 79 79 75 The petitioner states that, as demonstrated in its substantiating reports, the level ofsafety afforded by compliance with §33.27 for the HP shaft system will be unaffected by granting an exemption from §33.27(f)(6). The petitioner also states that granting this exemption will promote and serve the public interest, and petitioner identified those benefits in its package. Finally, petitioner asserts that granting its petition will have no adverse effect on safety. The petitioner provided the following in support ofthat contention.
1. The HP shaft system design used across the Trent engine family differs significantly from conventional shaft systems. A Failure Mode and Effect Analysis has determined thatthere are no known orpotential environmental threats to the integrity ofthe HP shaft system components that could lead to a failure from a loss ofload:

a. The HP shaft elements are life-limited-parts that comply with §33.70. b. A newnickel based superalloy material is used for the shaft whichhas been fully characterized and is controlled through a material specification.

c. Design features are well understood, which are supported by service experience with parts ofsimilar design. d. The Trent XWB HP Shaft system has a large diameter relative to its length and is radially separated from the low-pressure (LP) and intermediate-pressure (IP) shafts which therefore mitigates the potential for shaft rubs.

e. There is no bearing chamberbetweenthe HP compressor and HP Turbine. f. No sources ofoil fires exist that would threatenthe HP shaft system. g. Abnormal shock loads, torsional overload and impact damage do not present a credible risk to integrity ofthe HP shaft system. h. In additionthe detail assessment ofthe environment surrounding the shaft concluded that corrosion, wear, vibration, overheating and secondary effects from other failures or combination offailures are unlikely to cause a shaft failure. 2. All assumptions regarding engine installation used inthe assessment ofthe HP shaft system are recorded inthe engine installation manual.3 3. The special controls applied to the torque transmitting components (i.e., the HP Compressor Rear Drum and the HP Turbine disc ofthe HP shaft system) are ofa higher standard to ensure a higher level ofintegrity thanthose ofmany other shaft systems in the past.
4. The service experience ofthe Rolls-Royce three-shaft family ofengines, all ofwhich incorporate the similar design style for the HP shaft system as the TrentXWB engine models that are the subject ofthis petition, support the theoretical assessment ofpoints 1) and 3) above. Overthe course ofmore than 256 million service flight hours, no HP shaft system torque paths have failed onthe Trent family ofengines. This performance reflects an extremely remote potential failure rate ofless than 2.7 x 1 0-9 failures per flight hour.


### FederalRegister Publication A summary ofthe petition was published inthe Federal Register onJanuary 2, 2016 (81 FR 3556). No comments were received. 

### TheFAA's analysis is as follows: The FAA finds that the dataprovidedby the petitioner identified and considered all known failure modes that can lead to an overspeed condition. The compensating factors the petitioner demonstrated through good service experience ofsimilarthree-shaft design systems are acceptable except one. The historical data (good experience) regarding the low-cycle fatigue (LCF) failure mechanism is not applicable to this design. The life-limited location for some of the Trent XWB engine models is onthe torque path, which is different than the petitioner's experience. The LCF lives for the last stage ofthe HPC and HPT disks for the Trent XWB engines models are much lowerthan the previous experience. The new TrentXWB engines models are operating at higher stress conditions, and burst margins are lowerthan previous experience. Based on FAA findings, an additional compensating factor is needed to show an equivalent level ofsafety. Therefore, as a compensating factor, any feature onthe HP shaft system that has a predicted safe cyclic life less than 150% ofthe declared safe cyclic life (DSCL) limit, mustbe eddy current inspected between45% and 71% ofthe DSCL limit ofthe part. Based onthe compensating factors the petitioner demonstrated, the additional compensating factor listed above, and evaluation ofall known failure modes, the FAA concludes that a partial exemption to §33.27(±)(6) for the Trent XWB-84, Trent XWB-79B, Trent XWB-79, and Trent XWB-75 engine models will have no adverse effect on safety. 

### The FAA's Decision In consideration ofthe foregoing, the FAA finds that a partial grant ofexemption is in the public interest. Therefore, pursuant to the authority contained in49 U.S.C. §§106(f), 40113 and 44701,4 delegated to me bythe Administrator, Rolls-Royce plc, is granted an exemption from 14 CFR §33.27(±)(6), subject to the conditions and limitations listed below. Conditions and Limitations 1. Any feature onthe HP shaft system that has a predicted safe cyclic life less than 150% ofthe DSCL limit must be eddy-current inspected between 45% and 71% ofthe DSCL limit ofthe part. Issued in Burlington, Massachusetts, on February 29, 2016.

Colleen M. D'Alessandro Manager, Engine and Propeller Directorate Aircraft Certification Service