In the matter of the petition of 

ROLLS-ROYCE pic 

for an exemption from § 33.27(f)(6) 

of Title 14, Code of Federal 

Regulations 

Regulatory Docket No. FAA-2014-0609 

GRANT OF EXEMPTION 

August 13,2014, Rolls-Royce pic, 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 CFR) § 33.27(f)(6), Amendment 33-31. 

The petitioner requests relief from the following regulation: 

Section 33.27(f)(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 overs peed that would result from a complete loss of load on a turbine rotor if the 

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 I 00 % rotor terminal speed 

resulting from HP shaft system failure . Petitioner's second point is that compliance with § 33.27 

cannot be shown, absent an exemption from§ 33.27(f)(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.F.R. § 33.27(f)(6) for the HP shaft system of the 

following Trent XWB engine models: 

ANE-14-425-E 
compliance with§ 33.27 for the HP shaft system will be unaffected by the granting of an 

exemption from§ 33.27(±)(6). Petitioner also states that granting this exemption will promote 

and serve the public interest, and petitioner identified those benefits in its package. 

Finally, petitioner also asserts that granting its exemption will have no adverse effect on safety 

and provided the following in support of that contention. 

1. The HP shaft system design utilized across the Trent engine family differs significantly 

from conventional shaft systems. A Failure Mode and Effect Analysis determined that 

no known or potential environmental threats to the integrity of the HP shaft system 

components exist that could lead to a loss of load failure. 

2. 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 than those of many other shaft systems in the past. 

3. The service experience of petitioner's three-shaft family of engines supports the 

theoretical assessment of points 1) and 2) above, and all incorporate the same design 

style for the HP shaft system as the engine models that are the subject of this petition. 

Operationally, over more than 250 million service flight hours have been recorded 

without failure of any reported HP shaft system torque path failures on the Trent family 

of engines. This performance translates to a failure rate ofless than 2.8 x 10-9 failures 

per flight hour. 

Federal Register Publication 

The FAA determined that publishing the petitioner's request for exemption in the Federal 

Register for public comment was necessary, per 14 CFR 11.87. A summary of the petition was 

published in the Federal Register on September 2, 2014 (79 FR 52104), for a 10 day period. The 

FAA received no comments. The complete petition is available at the Department of 

Transportation's Federal Docket Management System at http://regulations.gov under Docket No. 

FAA-2014-0609. 

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resulting from an HP shaft system failure. Based on what the petitioner requested and what the 

petitioner demonstrated in its supporting package, an exemption to § 33.27(f)(6) is not 

applicable. Rather, the petitioner's request is a petition for exemption to§ 33.27(b)(3), and the 

requirement that rotor burst margin be greater than 5 % of rotor terminal speed. Therefore, we 

interpret petitioner's request as a request for exemption to§ 33.27(b)(3). 

We reviewed the data provided by petitioner and find that the HP turbine rotor burst speeds for 

the Trent XWB engine models are greater than 100 % rotor terminal speed resulting from an HP 

shaft system failure. Based on our finding and the compensating factors that the petitioner 

demonstrated through good service experience of similar three-shaft design systems and 

evaluation of all known failure modes, we conclude that an exemption to§ 33.27(b)(3) for the 

Trent XWB-84, -79B, -79, and -75 engine models will have no adverse effect on safety. 

The FAA's Decision 

In consideration of the foregoing, and based on the FAA's interpretation of petitioner RollsRoyce's exemption request, I find that a grant of exemption will not adversely affect the level of 

safety provided by the regulations and is in the public interest. Therefore, pursuant to the 

authority contained in 49 U.S.C. §§ 106(f), 40113 and 44701, delegated to me by the 

Administrator, the petitioner, Rolls-Royce plc, is hereby granted an exemption to§ 33.27(b)(3) 

for HP shaft system failure with the condition that the HP turbine rotor burst speed be greater 

than 100 %rotor terminal speed resulting from HP shaft system failure. 

Issued in Burlington, Massachusetts, on October 3, 2014. 

fL c. ·?Vlttw--

j D' Alessandro Assistant Manager, Engine and Propeller Directorate 

Aircraft Certification Service 

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