DRAFT 

DRAFT: This document does not represent final agency action on this matter and should not be 

viewed as a guarantee that any final action will follow in this or any other form. 

Subject: Turbine Rotor Strength Requirements of 

14 CFR § 33.27. 

Date: DRAFT

Initiated By: ANE-111 

AC No: 33.27-

1A[DRAFT]

Change: 

1. Purpose. This advisory circular (AC) provides guidance and acceptable methods, but not 

the only methods, for demonstrating compliance with the rotor strength (overspeed) 

requirements of 14 CFR § 33.27. 

2. Applicability. 

 a. The guidance provided in this document is directed to engine manufacturers, modifiers, 

foreign regulatory authorities, and Federal Aviation Administration (FAA) engine type 

certification engineers and their designees. 

 b. This material is neither mandatory nor regulatory in nature and does not constitute a 

regulation. It describes acceptable means, but not the only means, for demonstrating compliance 

with the applicable regulations. The FAA will consider other methods of demonstrating 

compliance that an applicant may elect to present. Terms such as “should,” “shall,” “may,” and 

“must” are used only in the sense of ensuring applicability of this particular method of 

compliance when the acceptable method of compliance in this document is used. While these 

guidelines are not mandatory, they are derived from extensive FAA and industry experience in 

determining compliance with the relevant regulations. On the other hand, if the FAA becomes 

aware of circumstances that convince us that following this AC would not result in compliance 

with the applicable regulations, we will not be bound by the terms of this AC, and we may 

require additional substantiation as the basis for finding compliance. 

 c. This material does not change, create any additional, authorize changes in, or permit 

deviations from existing regulatory requirements. 

3. Related Regulations. 14 CFR §§ 33.4, 33.27, 33.64 and 33.70. 

4. Related Documents. Please check the FAA’s website at 

http://www.faa.gov/regulations_policies/ for the latest revision of the following documents: 

 a. AC 33-2B, Aircraft Engine Type Certification Handbook, September 10, 1993. 
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DRAFT: This document does not represent final agency action on this matter and should not be 

viewed as a guarantee that any final action will follow in this or any other form. 

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 b. AC 33-3, Turbine and Compressor Rotors Type Certification Substantiation Procedures, 

September 9, 1968. 

 c. AC 33.64-1, Guidance for Pressurized Static Parts, September 13, 2010. 

 d. AC 33.70-1, Guidance for Aircraft Engine Life-Limited Parts Requirements, July 31, 

2009. 

5. Definitions. For the purposes of this AC, the following definitions apply: 

 a. Approved Dimensional Limits. The maximum allowable dimensional growth at various 

disk locations that do not result in a hazardous condition. These approved dimensional limits, 

also known as “acceptable growth limits,” support the selection of the actual serviceable limits 

versus rotor overspeed level established within the Instructions for Continued Airworthiness 

(ICA) under § 33.4. 

 b. Burst Margin. The predicted burst speed divided by the maximum permissible speed. 

This ratio is expressed as a percentage. 

 c. Extremely Remote. Failure conditions having probability of occurrence in the range of 

1.0E-7 to 1.0E-9 per engine flight hour. 

 d. Maximum Permissible Rotor Speed. The maximum approved rotor speed, including 

transients, for the maximum approved rating, including emergency ratings. 

 e. Rotor. An individual stage of a fan, compressor, or turbine assembly (some assemblies 

may consist of only one stage), including but not limited to bolted or welded assemblies. 

 f. Sample Rotor. A test rotor or rotor assembly (including, for example, coverplates, 

spacers, or blade retention devices) that meets the following criteria: 

 (1) Represents the standard to be certified; and 

 (2) Has known material properties and dimensions. 

 g. Shaft System. The system of components that transmits torque between the disk driving 

flange or shaft attachment members of the system that produce power (for example, the turbine) 

and the system that uses this power (for example, the compressor, fan or propeller). Although 

the shaft system transmits mainly torsional loads, it consists of all rotating parts necessary for the 

rotor(s) to perform their function, whether or not such parts transmit torque. Therefore, the shaft 

system may include, but may not be limited to, the following: 

 (1) Drive shafts; 

 (2) Gears; 
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DRAFT: This document does not represent final agency action on this matter and should not be 

viewed as a guarantee that any final action will follow in this or any other form. 

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 (3) Gearboxes; 

 (4) Rotor wing arms; 

 (5) Stub shafts; and 

 (6) Rotor hubs. 

6. Objectives. The safety objectives of the overspeed (strength) requirements ensure that the 

rotors: 

 a. Possess sufficient strength with a margin to burst above certified operating conditions 

and above failure conditions leading to rotor overspeed; and 

 b. Do not exhibit a level of growth or damage that could lead to a hazardous condition, 

such as fire, uncontainment, or loads greater than the ultimate loads of the engine mounts. 

7. General. 

 a. The applicant may comply with the overspeed (strength) requirements by using the 

required tests and/or necessary analysis to meet the objectives identified in paragraph 6 of this 

AC. 

 b. The demonstration of compliance with §§ 33.27(a) and (d) may be made separately, or 

be combined, as described in this AC. 

 c. Sections 33.27(a) and (d) allow various means of compliance (“tests, analysis or a 

combination”) in order to meet the objectives identified. The applicant is responsible for 

proposing the appropriate means of compliance, in accordance with the guidance defined in this 

AC. 

 d. The applicant should define and validate the analysis techniques necessary to support 

compliance with § 33.27 before use. The applicant should base the calibration and validation of 

the analytical techniques on prior overspeed tests which may include the current rotor being 

certified. An analysis is considered validated if it can accurately: 

 (1) Predict rotor burst speed; 

 (2) Identify the rotor stage with the lowest burst margin within each engine module; 

and 

 (3) Predict the dimensional growth versus rotor speed at critical rotor locations. 

 e. Any assumptions made during the testing or analysis should be conservative. 
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DRAFT: This document does not represent final agency action on this matter and should not be 

viewed as a guarantee that any final action will follow in this or any other form. 

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 f. Any analysis approach allowed under § 33.27 should be defined and validated before 

usage. 

 g. The applicant should submit the appropriate substantiation to determine which of the 

overspeed conditions listed in § 33.27(b) is the most critical for each individual rotor stage with 

respect to the requirements of § 33.27(a). This substantiation should consider the entire flight 

envelope. A similar analysis should be submitted with respect to the requirements of § 33.27(d). 

 h. If deliberate blade shedding limits the peak overspeed: 

 (1) The conditions of §§ 33.27(b)(3) and (b)(4) apply to a fully bladed rotor at the 

shedding speed; and 

 (2) The analysis to determine the most critical speed with respect to rotor integrity 

should consider blade shedding throughout the flight envelope. The blade failure speed should 

consider an adverse combination of dimensional tolerances, temperatures, and material 

properties of the blades and rotor. Consequently, the most critical speed with respect to rotor 

integrity might not coincide with the highest achievable blade shedding speed. 

 i. If blade shedding or turbine blade/vane meshing and clashing is used to limit the 

maximum attainable overspeed, the applicant must demonstrate the following by test, analysis, 

or a combination of both: 

 (1) The maximum attainable overspeed; and 

 (2) That a hazardous engine condition does not occur at such speed. 

 j. While considering the most adverse combination of dimensional tolerances and material 

properties, as required in §§ 33.27(a) and (d), the applicant should also consider the tolerances 

and material properties of other components that adversely influence stress levels in the rotor, 

such as the overspeed limiter or blades. 

 k. For each individual rotor stage, the applicant must consider the most adverse 

combination of temperature and temperature gradients possible throughout the entire operating 

envelope. 

 l. The appropriate percentage rotor speed of § 33.27(b) should be applied after making the 

necessary speed adjustments for temperatures, material properties and tolerance effects. The 

necessary speed adjustments for temperature and material properties will normally be established 

on the basis of appropriate ratios of material properties. 

 m. Failure conditions which are sudden and transient (reference §§ 33.27(a) and (d)) are 

typified by loss of load failures, i.e., characterized by high rates of acceleration and deceleration 

with no dwell period at the highest overspeed attained. The applicant should examine all 

possible failure conditions to determine if any case exists which would result in a dwell period at 
DRAFT 

DRAFT: This document does not represent final agency action on this matter and should not be 

viewed as a guarantee that any final action will follow in this or any other form. 

5

speeds close to that of the transient short duration failure condition. If such a case exists, the 

applicant should determine which condition is the most critical with respect to rotor integrity. 

 n. When several rotors are of similar design, are made of materials to the same 

specification, and subjected to similar stress conditions, temperature levels and gradients, 

applicants may demonstrate compliance to § 33.27(a) by testing only the most critical rotor with 

respect to burst. This requires determination of the burst speed for each rotor to select the most 

critical rotor, which has the smallest margin to burst above the speeds specified in § 33.27(b). 

For multistage rotors in which the rotors do not meet the conditions of similarity as described 

above, applicants should substantiate compliance of each rotor stage with § 33.27 by using 

representative test data. 

 o. Appropriate tests or analysis based on tests should establish the burst speed of each fan, 

compressor, and turbine rotor stage in relation to the most critical condition prescribed in 

§ 33.27(b). The certification documentation should report these speeds. These burst speeds 

should be based on the most adverse combination of dimensional tolerances, temperature, and 

material properties. 

 p. The most critical rotor with respect to burst might not be the most critical with respect 

to growth. To determine the most critical rotor with respect to growth for compliance with 

§ 33.27(d), the applicant should consider the components surrounding each rotor. 

 q. The consequences of rotor growth sufficient to cause significant contact or 

displacement between engine components should be assessed to determine if the requirements of 

§ 33.27(d)(1) can be met. 

 r. When determining compliance with the requirements of § 33.27(d)(2), the applicant 

should consider whether the rotor would exhibit any condition likely to prevent the safe 

operation of the engine in service following any failure or combination of failures considered 

under §§ 33.27(b)(3) or (b)(4). This period of time might equal that required to recognize the 

event and shut the engine down or to that required for continued safe flight and landing. The 

length of time might also depend upon the operating instructions for an overspeed event. 

8. Acceptable Means of Compliance. The following is an acceptable method, but not the only 

method, for demonstrating compliance with the rotor strength (overspeed) requirements of 

§ 33.27. The applicant may propose a different way of meeting the regulatory requirement. 

However, if the applicant chooses to use the method in this AC, the applicant must conform to 

the AC in all important respects. 

 a. The applicant must test the most critical stage of each rotor module (fan, LPC, HPC, 

HPT, LPT) on a rig or engine at the conditions necessary to demonstrate that a minimum 

strength rotor would meet the requirements of §§ 33.27(a) and (d). 

 b. The pass/fail criteria following the test for compliance to § 33.27(d) are that the test 

rotor must not exhibit conditions such as cracking or growth which preclude continued safe 

operation. 
DRAFT 

DRAFT: This document does not represent final agency action on this matter and should not be 

viewed as a guarantee that any final action will follow in this or any other form. 

6

 c. Where the conditions of §§ 33.27(b)(1) or (b)(2) are the most critical, testing a sample 

rotor for the required period of time in an engine at 96% or greater of the speed necessary to 

demonstrate that a minimum strength rotor would meet the requirements of §§ 33.27(a) and (d) 

provided: 

 (1) this resultant reduced test condition is not less severe than that required to 

demonstrate compliance with §§ 33.27(b)(3) and (b)(4); and, 

 (2) it is shown from a validated method of burst prediction that burst would not have 

occurred at the conditions of §§ 33.27(b)(1) or (b)(2). 

 d. An analytical modeling method based on representative test data may be acceptable 

provided that: 

 (1) the model has been validated by comparison with results from specimen and rotor 

tests; 

 (2) its use is limited to rotors with material, geometry, stress, and temperature 

conditions encompassed by those used to construct the model; and 

 (3) its predictions show that the certification standard rotor is not more critical, with 

respect to burst and growth, than any similar rotor for which substantiation has been 

demonstrated both by rotor test and model prediction. 

 e. Any test may be continued to rotor burst after the required time duration by increasing 

the rotor speed until the rotor bursts. If the applicant chooses this method, then it should be 

shown that: 

 (1) the sample rotor was initially run at conditions not less severe than those required 

for compliance with § 33.27(a) and 

 (2) Section 33.27(d) can be complied with using an approved analytical modeling 

method. 

 f. The engine control devices, systems, and instruments referred to in § 33.27(e) are 

usually provided in modern engines by overspeed protection or circuits which, although they 

may be independent devices, are generally part of the electronic engine control system. One 

acceptable method for demonstrating reasonable assurance of the functionality of the protection 

systems or circuits is to test them with a built-in test equipment (BITE) test or a periodic 

functional test. For the overspeed protection system, the BITE test should test 100% of the 

electrical/electronic part of the protection system. The applicant may use a periodic functional 

test for the mechanical or actuating part of the overspeed system. 
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DRAFT: This document does not represent final agency action on this matter and should not be 

viewed as a guarantee that any final action will follow in this or any other form. 

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9. Factors to be Considered in Determining Test Conditions. 

 a. Temperature. The applicant should establish temperatures and temperature gradients by 

temperature surveys on an engine or by deriving them from a validated analysis. Adjustments of 

test speed or blade mass or both should be applied to compensate for any deviation from the 

required temperatures and temperature gradients. The rotor temperatures required by § 33.27(b) 

are as follows: 

 (1) For §§ 33.27(b)(1) and (b)(2), the material temperatures and temperature gradients 

equal to the most adverse that could be achieved when operating in the engine at the required 

rating condition. 

 (2) For §§ 33.27(b)(3) and (b)(4), the material temperatures and temperature gradients 

equal to the most adverse that could be achieved when operating in the engine at the required 

rating condition immediately prior to the failure(s). 

 b. Sample Rotor Material Properties. 

 (1) The applicant may determine the material properties of the sample rotor from 

attached test rings/bars when the correlation of their properties has been established by a 

validated method using coupons obtained from forgings/castings of the type to be approved. 

 (2) When attached test rings/bars are not available to determine the material properties 

of the sample rotor, the applicant may establish a value for the material properties by assuming 

the sample rotor possesses material properties equal to known average properties of similar 

rotors from the same manufacturing process lot. If using this method, the applicant must also 

show that this assumption regarding material properties is valid within acceptable confidence 

limits. Such limits should be based on the same statistical methods used to define the average 

and minimum material properties used in the design of the rotor. 

 c. Dimensional Tolerances. Analysis of dimensional tolerances should be made to 

identify the most adverse combination with respect to the integrity of the rotors. 

10. Failure Cases. 

 a. Introduction. Sections 33.27(b)(3) and (b)(4) state the test speeds at which the rotor 

structural integrity requirements must be demonstrated for all engine ratings during single or 

dual engine failure conditions. Examples of a single engine failure condition are a failure of a 

control system or shaft failure that can cause an overspeed condition. With respect to 

combinations of failures, at any rating, if the likelihood of a combination is not greater than 

extremely remote (probability range of 1.0E-7 to 1.0E-9 per engine flight hour) the case need not 

be considered. 

 b. Loss of Load Events. 
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DRAFT: This document does not represent final agency action on this matter and should not be 

viewed as a guarantee that any final action will follow in this or any other form. 

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 (1) To determine the highest overspeed resulting from a loss of load event to be 

considered under § 33.27(c), applicants should consider, for all possible failure locations, factors 

such as system inertia, available gas energy, whether the rotor is held in plane, and overspeed 

protection devices. 

 (2) Applicants cannot exclude the failure of the shaft system when considering single 

failure events, but can provide engineering assessments supporting their finding that certain 

locations on the shaft system are not required for failure assessment when complying with 

§ 33.27(c). 

 (3) When addressing loss-of-load events (that is, any failure of the shaft system that 

effectively uncouples the turbine from its load), the applicant, according to § 33.27(c), must 

determine the highest terminal speed of the rotor resulting from the shaft failure of any shaft 

element throughout its flight envelope. Once the highest terminal speed is determined, the 

applicant should design and demonstrate that disks have 5% margin above that speed. 

 (4) The following guidance describes two acceptable approaches for determining the 

maximum terminal rotor speed for shaft failure cases when complying with §§ 33.27(b)(3) and 

(b)(4). 

 

 (a) For any component or element that forms part of the rotor shaft system, 

identify all single point failures that can lead to an overspeed condition and determine by test the 

resulting terminal rotor speed. The applicant may use analysis if it has sufficient validation to 

prior testing or service events. 

 (b) If the applicant proposes to exclude certain shaft element(s) from failure 

consideration in determining the terminal rotor speed, the applicant must show by engineering 

assessment that failure of the excluded shaft element(s) cannot be expected to occur during the 

life of the engine. The applicant and the FAA must establish and agree upon the structural 

integrity criteria used for this assessment on a case-by-case basis. In general, the assessment 

should: 

 1 Identify and consider all failures that can lead to an overspeed condition. 

Examples of shaft element failure modes include, but are not limited to: fatigue (LCF/HCF), 

torsional overload; bearing failure; shaft rub; overtemperature; and loss of rotor centerline. The 

assessment should also include failure or combinations of failures in the surrounding 

environment (including mating or adjacent hardware) that could result in failure of the shaft 

element(s). The assessment of the environment should take into account possible wear, 

corrosion, fire, and contact with adjacent components or structure that could lead to the failure of 

the shaft element(s). 

 2 Show that the shaft element(s) complies with the requirements of § 33.70. 

 3 Show that the material and design features of the shaft element(s) are well 

understood, as evidenced by significant test and service experience with parts of similar design. 
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DRAFT: This document does not represent final agency action on this matter and should not be 

viewed as a guarantee that any final action will follow in this or any other form. 

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 4 Show that any stress and failure analysis methods used are adequately 

validated. 

 5 Identify the effect on the shaft of loads which could be transmitted by shock 

loading resulting from, for example, bird strikes or blade failure. 

 6 Identify any assumptions regarding engine installation. Installation 

assumptions should be noted in the Installation Instructions required by § 33.5. 

 (5) Shaft system elements meeting the agreed-upon criteria can be excluded from 

consideration. The applicant would then determine the maximum terminal rotor speed from the 

failure of the remaining shaft system elements.