Exemption No. 15079 

UNITED STATES OF AMERICA 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

BURLINGTON, MA 01803-5229 

In the matter of the petition of 

ROLLS-ROYCE pic Regulatory Docket No. FAA-2016-0072 

for an exemption from §33.27(±)(6) 

of Title 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) on behalf of Rolls-Royce plc for an exemption from § 3 3.2 7 (f)( 6), 

Amendment 33-31 of Title 14, Code of Federal Regulations (14 CFR). The proposed exemption, 

if granted, would allow some high-pressure (HP) shafts to be exempted from 14 CFR 

§33.27(±)(6). 

The petitioner requests relief from the following regulation: 

Section 33.27(±)(6), Amendment 33-31 prescribes that: 

"(f) Failure of a shaft section may be excluded from consideration in determining the 

highest overspeed that would result from a complete loss of load on a turbine rotor if the 

applicant: 

( 6) Does not exclude the entire shaft." 

The petitioner supports its request with the following information: 

The petitioner requests the FAA grant the exemption based on two points. The petitioner's first 

point is that the design of the 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 of safety that will meet the requirements of §33.27. 

ANE-16-000 1-E 
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The petitioner requests exemption from 14 C.P.R. §33.27(f)(6) for the HP shaft system of the 

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 of safety 

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 of that 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 

that there are no known or potential environmental threats to the integrity of the HP 

shaft system components that could lead to a failure from a loss of load: 

a. The HP shaft elements are life-limited-parts that comply with §33.70. 

b. A new nickel based superalloy material is used for the shaft which has been 

fully characterized and is controlled through a material specification. 

c. Design features are well understood, which are supported by service experience 

with parts of similar 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 chamber between the HP compressor and HP Turbine. 

f. No sources of oil fires exist that would threaten the HP shaft system. 

g. Abnormal shock loads, torsional overload and impact damage do not present a 

credible risk to integrity of the HP shaft system. 

h. In addition the detail assessment of the environment surrounding the shaft 

concluded that corrosion, wear, vibration, overheating and secondary effects 

from other failures or combination of failures are unlikely to cause a shaft 

failure. 

2. All assumptions regarding engine installation used in the assessment of the HP shaft 

system are recorded in the engine installation manual. 
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3. The special controls applied to the torque transmitting components (i.e., the HP 

Compressor Rear Drum and the HP Turbine disc of the HP shaft system) are of a higher 

standard to ensure a higher level of integrity than those of many other shaft systems in 

the past. 

4. The service experience of the Rolls-Royce three-shaft family of engines, all of which 

incorporate the similar design style for the HP shaft system as the Trent XWB engine 

models that are the subject of this petition, support the theoretical assessment of points 

1) and 3) above. Over the course of more than 256 million service flight hours, no HP 

shaft system torque paths have failed on the Trent family of engines. This performance 

reflects an extremely remote potential failure rate of less than 2. 7 x 1 0-9 failures per 

flight hour. 

Federal Register Publication 

A summary of the petition was published in the Federal Register on January 2, 2016 (81 FR 

3556). No comments were received. 

The FAA's analysis is as follows: 

The FAA finds that the data provided by 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 of similar three-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 on the torque path, which is different than the petitioner's 

experience. The LCF lives for the last stage of the HPC and HPT disks for the Trent XWB 

engines models are much lower than the previous experience. The new Trent XWB engines 

models are operating at higher stress conditions, and burst margins are lower than previous 

experience. 

Based on FAA findings, an additional compensating factor is needed to show an equivalent level 

of safety. Therefore, as a compensating factor, any feature on the HP shaft system that has a 

predicted safe cyclic life less than 150% of the declared safe cyclic life (DSCL) limit, must be 

eddy current inspected between 45% and 71% of the DSCL limit of the part. Based on the 

compensating factors the petitioner demonstrated, the additional compensating factor listed 

above, and evaluation of all 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 of the foregoing, the FAA finds that a partial grant of exemption is in the public 

interest. Therefore, pursuant to the authority contained in 49 U.S.C. §§106(f), 40113 and 44701, 
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delegated to me by the 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 on the HP shaft system that has a predicted safe cyclic life less than 

150% of the DSCL limit must be eddy-current inspected between 45% and 71% of the DSCL 

limit of the part. 

Issued in Burlington, Massachusetts, on February 29, 2016. 

Colleen M. D' Alessandro 

Manager, Engine and Propeller Directorate 

Aircraft Certification Service