### FINAL **REGULATORY EVALUATION**

**FATIGUE TOLERANCE EVALUATION OF METALLIC STRUCTURES (14 CFR Part 29**  **.571 )** 

OFFICE OF AVIAT ION POLICY AND PLANS AIRCRAFT REGULATORY ANALYSIS BRANCH, APO
- 3O 0

George A. Euring, Jr.
March 16 , 2011

### TABLE OF CONTENTS
Page

I. Introduction...................................................................................... 1 II. Background...................................................................................... 2 III. Benefits........................................................................................... 4 IV
. Costs............................................................................................. 10 V. Bene fit
- Cost Comparison..................................................................... 17

APPENDIX A: Existing and Final Rule Comparison List of Sources1



### REGULATORY EVALUATION OF **FATIGUE TOLERANCE EVALUATION OF METALLIC STRUCTURES** 

**I. INTRODUCTION**  This regulatory

evaluation assesses the economic impact of a final rule amending Part 29 .571 of Title 14, Code of Federal Regulations (14 CFR) to modify the regulations applicable to transport category metallic rotorcraft structure. This final rule will revise the fatigue tolerance evaluation (FTE) safety requirem ents to address advances in structural fatigue substantiation technology

for metallic rotorcraft structure
. The FAA received 3 comments from Transport Canada on the Notice of Proposed Rulemaking (NPRM) . None of these comments required a change from the r egulatory evaluation conclusions. Therefore, the initial regulatory evaluation becomes the final regulatory evaluation. This final regulation is the result of information gathered from a review of catastrophic fatigue f ailures, and is intended to i mprov e the level of safety. The final rule will ensure that , should serious accidental damage occur during manufacturing or within the operational life of the rotorcraft, the remaining structure can withstand fatigue loads that are likely to occur, without fai lure, until the damage is detected or a part is replaced. In addition to improving the level of safety for FTE of all principa l structural elements (PSE), this final rule will more closel y harmonize Federal Aviation Administration (FAA ) standards with re quirements of the European Aviation Safety Agency (EASA ).2

**II. BACKGROUND** 
**A. Introduction**  Fatigue of rotorcraft dyn amic components was first addressed in the 1950’s by means of safe
- life methodology . In the 1980’s, it was recognized that higher levels of reliability might be realized by

taking into account the fatigue strength reducing effects of damage that can occur during manufacture or in operational service. The introduction of composites led the manufacturers and regulatory authorities to develo p a robust safe
- life methodology by taking into account the reducing effects of aging, temperature, moisture absorption, impact damage, and recognition of a minimum quality standard on the specific static and fatigue strength . Furthermore, where clearly v isible damages resulted from impact or other sources, inspection programs were developed to maintain safety. In parallel, crack growth methodology has been successfull y used for solving short
- term airworthiness problems in metallic structures of rotorcraf t and as the certification basis for civil and military

transport aircraft applications. These advances in design, anal yt ical methods, and industry practices made it feasible to address certain t ypes of damage that could result in fatigue failure. The g eneral understanding of rotorcraft fatigue tolerance evaluation has developed considerably in the past several ye ars, making possible the more advanced techn iques described in this final rule
. **B. Rulemaking Prior To This Proposal**  The most recent rulemaki ng , prior to this final rule, started in 1982 and resulted in the current rule, Amendment 29
- 28. There has not been an y other rulemaking activity in this area since 1989 when Amendment 29
- 28 became effective (54 FR 43930, October 27, 1989. There3

have bee n no ADs issued as a result of this rule. There has been onl y one aircraft certificated under Amendment 29
- 28, the Sikorsky S
- 92. **C .**  **The Aviation Rulemaking Advisory Committee (ARAC -**  **2000 ):**  The FAA requested that the Aviation Rulemaking Advisory Committe e (ARAC) study the need to revise the applicable regulations on fatigue evaluation in light of advancements in technology

and operational procedures and to develop regulatory recommendations. Following a notice published in the Federal Register on April 5 , 2000 (65 FR 17936), an ARAC Working Group was chartered to study and make appropriate recommendations concerning whether new
or rev ised airworthiness standards were appropriate regarding fatigue evaluation of transport rotorcraft metallic structure. Th e ARAC is composed of 76 member organizations with a wide range of interests in the aviation community

and provides the FAA with firsthand information and insight regarding proposed new or revised rules. The ARAC established working groups to develop
reco mmendations for resolving specific airworthiness issues. In response to the FAA’s tasking of April 5, 2000, ARAC’s Rotorcraft Metallic Structure Damage Tolerance Working Group developed recommendations addressing changes to the fatigue evaluation require ments for transport rotorcraft found in 14 CFR §29.571 to improve currency and understanding. The ARAC accepted those recommendations and presented them to the FAA. This final rule is based on those recommendations.4



### II **I .**  **BENEFITS** A. **Introduction** In the absence of a new rule, it is likely that future metallic fatigue
- related accidents will continue to occur on p art 29 rotorcraft in a manner similar to what has happened in the past. A key benefit of this final rule will be the avoidance of these accident s. This chapter discusses the number of metallic fatigue related accidents in part 29 helicopters for the historical 27 year study period 1982 through 2008 . The costs of these accidents are then estimated and projec ted into the future for 27 ye ars. Thi s study period was chosen because the NTSB Accident Database starts in 1982 and 2008 was the latest y ear for which, at least, some data was available when this study was prepared. The costs of these preventable accidents that will occur without this final rule are the benefits of this final rule because we believe that this final rule will prevent at least some of these future avertable accidents. The benefits of this final rule were also estimated for the shorter, ten year period 1998 to 2008. **B. Hist**  **orical Accidents** The FAA reviewed historical metallic fatigue related accidents for part 29 helicopters for the 27 ye ar period 1982 through 2008. In the review of the accident history , the FAA considered only accidents that were relevant to metallic rotor craft structural fatigue problems and that might have been prevented if this rule had been in effect . In addition, the FAA did not consider events in which externally

aggravating circumstances existed, such as operation of the aircraft outside of its weight and balance limitations. To obtain this history we examined the NTSB Aviation5

Accident Database & S ynopses and the Aviation Safety Information Analy sis and Sharing Sy stem (ASIAS) database s . Table B 1 shows that between 1982 and 2008, there were 9 meta llic structural fatigue related Transport Category Rotorcraft accidents
. The FAA believes that these 9 accidents could have been prevented if this rule had been in effect. T hese accidents resulted in 7 fatalities, 3 serious injuries and 3 minor injuries . In addition, all of the aircraft involved in the se accidents were either destro yedor received substantial damage. 1 Table B1A shows the same information for the time period 1998 to 2008. During this period there were 2 metallic structural fatigue related Transport Category Rotorcraft accidents. The FAA believes that both these accidents could have been prevented if this rule had been in effect. These accidents resulted in 3 fatalities and one serious injury . In addition, both of the aircraft involv ed in these accidents were destro yed
.

1 National Transportation Safety Board (NTSB) Accident Reports, F AA Aviation Safety Information Analysis and Sharing (ASIAS).67

**C. Current Dollar**  **Market Value of Historical Accidents**  We calculate the costs of a future averted accident in order to quantify future benefits. The minimum value of a statistical aviation fatality avoided i s set at $5.8 million, that of a serious injury

(assumed to be the average of a severe, serious, and moderate injury) at $1,122,700 , and that of a minor injury at $82,800. The associated medical and legal costs are included in the injury

costs. Inadditi on, the average replacement cost of a destroyed turbine rotorcraft greater than or equal to 7,000 pounds equals $2,303,00 0 , the restoration costs are estimated to be $470,000 and NTSB accident i nvestigation costs about $45,000

. 2

2 “Economic Values for FAA Investment and Regulatory Decisions, A Guide.” (GRA Incorporated, Final Report, Revised October 3, 2007); “Aviation Accident Investigation Costs,” (FAA APO Bulletin, August 2000).8

Table B2 shows the econom ic value assigned to the historical losses listed in Table B1. The total cost of these 9 accidents is estimated to be approximately $58 million and the average cost of one of these accidents is estimated to be approximately $6.4 million.

Table B2A s hows the same information for the two accidents that occurred between 1998 and 2008. The total cost of these 2 accidents is estimated to be approximately $23.2 million and the average cost of one of these accidents is estimated to be approximately $11.6 m illion.9

**D. Present Value Benefits**  The present value benefit estimate assumes that the probability of an accident is equa lly likely in an y ye ar of the 27
- ye ar study period (Uniform Distribution)
. If the final rule were to be 100% effective, 9 accide nts will be avoided over the next 27 ye ars and, the present value bene fit of avoiding these accidents will be approximately $25.4 million. If the final rule were to be 56% effective, 5 accidents will be avoided over the next 27 ye ars and, the present val ue benefit of avoiding these accidents will be approximately $14.1 million. If the final rule were to be 22% effective, 2 accidents will be avoided over the next 27 ye ars and, the present value benefit of avoiding these accidents will be approximately $5 .6 million. If the final rule were to be 11% effective, 1 accident will be avoided over the next 27 ye ars and, the present value benefit of avoiding this accident will be approximately $2.8 million. See Table B3.10

Table B3A presents the same infor mation for the 10 year period 1998 to 2008. The present value benefit estimate assumes that the probability of an accident is equally likely in an y ye ar of the 10
- ye ar study period (Uniform Distribution). If the final rule were to be 100% effective, 2 accidents will be avoided over the next 10 ye ars and, the present value benefit of avoiding these accidents will be approximately $16.3 million. If the final rule were to be 50% effective, 1 accident will be avoided over the next 10 ye ars and, the present value benefit of avoiding this accident will be approximately $8.2 million.1112

**IV .**  **COSTS**
**A. Introduction**  This final rule will add costs to rotorcraft manufacturers and operators. R otorcraft manufacturers will be r equire d to conduct additional a ctions in the certification process . The final rule will require additional inspections for rotorcraft operators . These incremental costs will be estimated in the remainder of this chapter. We assign certification costs in the first year for each new d ecade. The operator inspection costs increase as the new production helicopter fleet increases. We did not estimate the life cycle costs (and benefits) because we do not have reliable information on helicopter end
of
- service time. For this anal ys is we l et the fleet accumulate without retiring any of the new production helicopters. **B. Rotorcraft Manufacturer Incremental Costs**  **1. Incremental Certification Actions**  The final rule will require rotorcraft manufacturers to take the following additional action s in the certification process : 1) perform a more thorough threat assessment, and submit a threat assessment methodology report to the FAA for approval; 2) perform a more rigorous residu al strength assessment ; 3) include both retirement times **an d**  inspection intervals in the Airworthiness L imitation Section (AL S) of the Instructions for Continued Airworthiness (ICA). The rotorcraft manufacturers currentl y perform a threat assessment and a resid ual strength assessment in the certification p rocess. However, the se assessments will become more robust under this final rule and consequently add costs
.13

The current rule mandates that manufacturers must establish inspection intervals **or**
r etirement times. This final rule mandates that both retirem ent times **and** inspection intervals must be included in the airworthiness limitation section of the Instructions for Continued Airworthiness , thereby

increasing costs . Appendix A provides a chart comparing the current rule to this final rule. 2. **Number of Certifications**  There are currently

three transport category

helicopter manufacturers in the United States, Bell (A wholl y owned subsidiary of Textron, Inc.) , Sikorsky (A subsidiary of United Technologies Corporation), and Erickson Air
- Crane , Inc. . Two of these manufacturers, Bell and Sikorsky , expect to obtain new transport category helicopter t ype certificates in the future. Representatives from Bell and Sikorsky estimate there will be one new t yp e certificate every 10 years for each of their compan ies. These estimates result in a total of 2 new transport category helicopter t ype certificates every 10 years. The study period for this analy sis is 27 ye ars. We front
- load certification to the first year of each new decade. Therefore, it is assumed that there will be two new transport category helicopter t ype certificates in the first, eleventh, and twenty - first years of the study period, resulting in a total of six new transport category helicopter ty pe certificates during the 27 year study period. **3. Costs**  **Per Certification** Based on information from industry representatives on the ARAC Working Group, we estimate that the average additional cost to perform a more thorough threat assessment will be $87,000 per certification; the average additional co st to perform the more rigorous residual strength assessment required by this rule will be an additional $43,5 00; and putting both14

retirement times and inspection intervals in the airworthiness limitation section of the Instructions for Continued Airworthi ness will c ost , on average , an additional $46,980

. Based on information received from industry representatives, we also estimate that over the next 27 years, Part 29 rotorcraft structures will be comprised of approximately 50% metallic parts and 50% comp osite parts. Hence, the additional per certification cost under this final rule will be $43,500 for a threat assessment ($87,000 * 0.5 = $43,500), $21,750 for a r esidual strength assessment ($43,500 * 0.5 = $21,750), and $23,49 0 for putting both inspectio n intervals and retirement times in the airwo rthiness limitation section ($46 , 980 * 0.5 = $23,49 0). Therefore, we estimate that the total certification cost per new ty pe certification will be $88,740 ($43,500 + $21,750 + $23,49 0 = $88,740 ). Table C 1 belo w shows a summary of the average cost per certification.15

**C. Rotorcraft Operators**  **Costs**
In addition to the incremental costs to rotorcraft manufacturers for aircraft certification, this final rule will increase the rotorcraft operator ’ s costs by re quiring additional inspections. Industry representatives on the ARAC Working Group estimated that approximately 30 components will require additional inspection as a result of this final rule , and that it will take a mechanic one ho ur to inspect each component. Therefore , an inspection will take 30 man
- hours. Based on an aviation mechanic’s wage rate of $53 per hour, each inspection will cost approximately

$1,6 00 (30 man
- hours X $53 per hour = $1,590 ). Based on information received from industry repre sentatives, we estimate that inspections will occur , on average , approximately every 1 , 250 flight hours. Inorder to estimate the number of ye ars between inspections, it is necessary to determine the approximate number of hours that a transport category h elicopter might fly

in a year. Table C2 shows that, f rom 2000
- 2007 , turbine rotorcraft flew an average of 387 flight hours annually . (FAA Aerosp ace Forecasts, Fiscal Years 2008
- 2025, p. VI
- 3; p. 86 ) Therefore , inspections will occur , on average , about once every

3 years (1 , 250 / 387 = 3.22 ) during the 27 ye ar study period
.16

**D. Total Costs** 
**1. Introduction** The total costs of the final rule consist of certification costs and inspection costs. Certification costs are a one time cost borne b y the manufacturer for each new ty pe certificate. Inspection costs are a recurring cost (every three years) borne by

the operators.17

**2. Total**  **Certification Costs** For cost estimation purposes, we assume that the new models will be certificated in years 1, 11 , an d 21 , during th e 27
- ye ar anal ys is period, and that each future aircraft certification will have a production run of 10 years. The total undiscounted cost of six certifications during the 27 ye ar anal ys is period is approximately

$532,000. The total prese nt value cost, at a 7% discount rate, for six certifications during the 27 ye ar anal ys is period is about $293 ,000. **3. Total**  **Inspections Costs**  The total inspections cost depends upon the cost per inspection and the number of aircraft produced. The cost per inspection has been estimated above at $1,600. The forecasted production rates for a new Sikorsky model is based on the first four years of production of the Sikorsky S
- 92 (2004
- 2007). In the fourth year of production of the S
- 92, Sikorsky produced 29 S
- 92’s. This number is used as a forecast for the fifth through tenth ye ar of production
. During the 2004
- 2007 period Bell’s production of transport category rotorcraft was approximately

the same as Sikorsky ’s. Therefore, we assume that Bell’s produ ction rate for a new model will be equal to Sikorsky 's. We estimate that the tot al inspections costs over the 27
- year analy sis peri od will be approximately

$8.5 million in un disc ounted costs or about $2.6 million in present value costs
over 27 years . **D .**  **Total Incremental**  **Costs** T he total cost of this final rule over 27 ye ars is estimated to be approximately $9.0 million in undiscounted costs or about $2 .9 million in discounted costs. Table C3 shows the calculation of the total costs of this final rul e.1819

**V. BENEFIT COST COMPARISON**  **A. Benefits**  The benefits of the final rule are the value of the accidents that may be averted by

the final rule during the next 27 years. The estimated present value benefits range from a minimum of $ 5.6 million if one accident is prevented during the next 27 years to a maximum of $25.4 million if 9 accidents are averted. **B. Costs**  The costs of the final rule are the additional costs for manufacturers to prepare a more thorough certification process and the costs for op erators to conduct additional inspections. The present value costs of this final rule are estimated to be $2.9 million. **C. Benefit Cost Comparison**  **s** Table BC1 shows a comparison of the benefits and costs of the final rule with both the benefits and the costs estimated for the 27 ye ar study period
.

Table BC1 shows that the present value benefits of the final rule exceed the present value costs of the final rule if the final rule averts as few as two accidents in the next 27 years . The FAA Rotorcra ft Directorate estimates that the final rule will be at least 5 0 percent (approximatel y 5 accidents) effective at preventing the identified past 9 accidents which could20

happen in the future. However, the final rule is still cost beneficial if it is as little as 22% effective. Therefore, the present value benefits of the final rule exceed the present value costs of the final rule
. Table BC2 shows a comparison of the benefits and costs of the final rule with the benefits estimated for 10 ye ars and the cos ts estimated for the 27 year study period.

Table BC2 shows that the present value benefits of the final rule exceed the present value costs of the final rule even when the benefits are estimated for ten years and the costs are estimated for the full 27 year study period. The FAA Rotorcraft Directorate estimates that the final rule will be between 50 percent (1 accident) and 100 percent (2 accidents) effective at preventing the identified past 2 accidents which could happen in the future. Therefore, the present value benefits of the final rule exceed the present value costs of the final rule.1 **Appendix A: Existing and**  **Final Rule Comparison**  **Existing Rule 29.571: Fatigue Evaluation of Structure** (Part 29 Subpart C

– Strength Requirements) **Final Rule 29.571 Fatigue Tolerance Evaluation of Metallic**  **Structure.** (a) **General .**  An evaluation of the strength of principal elements, detail design points, and fabrication techniques must show that catastrophic failure due to fatigue, considering the effects of environment, intrinsic/discrete flaws, or accidental damage will be avoided. Parts to be evaluated include, but are not limited to, rotors, rotor drive sy stems between the engines and rotor hubs, controls, fuselage, fixed and movable control surfaces, en gine and transmission mountings, landing gear, and their related primary

attachments. In addition, the following apply : (a) **General** . A fatigue tolerance evaluation of the principal structural elements (PSEs) identified in paragraph (c) below must be pe rformed and appropriate actions established, as defined in paragraph (g), to avoid catastrophic failure during the operational life of the rotorcraft. This evaluation must consider the effects of both fatigue and the damage determined in paragraph (d)(3) below. Parts to be evaluated include, but are not limited to, rotors, rotor drive s ys tems between the engines and rotor hubs, controls, fuselage, fixed and movable control surfaces, engine and transmission mountings, landing gear, and their related primary attachments. (1) Each evaluation required b y this section must include

– (i) The identification of principal structural elements, the failure of which could result in catastrophic failure of the rotorcraft. (ii) In
- flight measurement in determining the loads or stresses for items in paragraph (a)(1)(i) of this section in all critical conditions throughout the range of limitations in Sec. 29.309 (including altitude effects), except that maneuvering load factors need not exceed the maximum values expected in operations; and (iii) Loading spectra as severe as those expected in operation based on loads or stresses determined under paragraph (a)(1)(ii) of this section, including external load operations, if applicable, and other high frequency power cycle operations. (b) The co mpliance methodology must be submitted to the regulatory

authorit y for approval.

(2) Based on the evaluations required b y this section, inspections, replacement times, combinations thereof, or other procedures must (c) Considering all structure and/or structural elements and assemblies, the PSEs must be identified. PSEs are2

be established as necessary to avoid catastrophic failure. These inspections, replacement times, combinations thereof, or other procedures must be included in the airworthiness limitations section of the Instructions for Continued Airworthiness required by Sec. 29.1529 and Section A29.4 of Appe ndix A of this part. structural elements that contribute significantl y to the carryi ng of flight or ground loads and the fatigue failure of which can lead to catastrophic failure of the rotorcraft. (b) **Fatigue tolerance evaluation (including tolerance to flaws).** The structure must be shown by anal ys is supported by

test evidence and, if available, service experience to be of fatigue tolerant design. The fatigue tolerance evaluation must include the requirements of either paragraph (b)(1), (2), or (3) of this section, or a combination thereof, and also must include a determination of the probable locations and modes of damage caused by fatigue, conside ring environmental effects, intrinsic/discrete flaws, or accidental damage. Compliance with the flaw tolerance requirements of paragraph (b)(1) or (2) of this section is required unless the applicant establishes that these fatigue flaw tolerant methods for a particular structure cannot be achieved within the limitations of geometry , inspectability , or good design practice. Under these circumstances, the safe
- life evaluation of paragraph (b)(3) of this section is required. (d) Each evaluation required b y th is section must include: (1) In
- flight measurement in determining the fatigue loads or stresses for the PSEs identified in paragraph (c) above in all critical conditions throughout the range of limitations in 29.309 (including altitude effects), except that maneuvering load factors need not exceed the maximum values expected in operations. (2) Loading spectra as severe as those expected in operation based on loads or stresses determined under paragraph (d)(1) of this section, including external load operations, if applicable, and other high
- frequency power - cycle operations. (3) A determination for the PSEs identified in paragraph (c) above of the probable locations, ty pes, and sizes of damage considering fatigue, environmental effects, intrinsic/discrete flaws, or accid ental damage that may occur during manufacture or operation. (4) A determination of the fatigue tolerance characteristics for the PSEs with the damages identified in paragraph (d)(3) above to support accomplishment of paragraph (g) below. (5) Analy ses supported by test evidence and/or service experience. (1) **Flaw tolerant safe -** **life evaluation**

. Itmust be shown that the structure, with flaws present, is able to withstand repeated loads of variable magnitude without detectable flaw growth for the following time inte rvals
-- (i) L ife of the rotorcraft; or (e) A residual strength determination is required to establish the allowable damage size. For inspection interval determination based on damage growth, the residual strength requirement is limit load considered as ultimate.3

(ii) Within a replacement time furnished under Section A29.4 of Appendix A to this part. (2) **Fail -** **safe (residual strength after flaw growth) evaluation**

. It must be shown that the structure remaining after a partial failure is able to withstand de sign limit loads without failure within an inspection period furnished under Section A29.4 of Appendix A to this part. L imit loads are defined in Sec. 29.301(a). (i) The residual strength evaluation must show that the remaining structure after flaw growth is able to withstand design limit loads without failure within its operational life. (ii) Inspection intervals and methods must be established as necessary to ensure that failures are detected prior to residual strength conditions being reached. (iii) If s ignificant changes in structural stiffness or geometry , or both, follow from a structural failure or partial failure, the effect on flaw tolerance must be further investigated. (f) The effect of damage on stiffness, dy namic behavior, loads and functional p erformance must also be considered.

(3) **Safe -** **life evaluation**

. Itmust be shown that the structure is able to withstand repeated loads of variable magnitude without detectable cracks for the following time intervals
-- (i) L ife of the rotorcraft; or (ii) W ithin a replacement time furnished under Section A29.4 of Appendix A to this part. **]** (g) Based on the evaluations required b y this section, inspections and retirement times or approved equivalent means must be established to avoid catastrophic failure. Th e inspections and retirement times or other means as applicable must be included in the airworthiness limitation section of the Instructions for Continued Airworthiness required by Section 29.1529 and Section A29.4 of Appendix A of this part. (h) If insp ections for an y of the damage t yp es identified in part d(3) above, cannot be established within the limitations of geometry , inspectability , or good design practice, then supplemental procedures, when available, must be established that will minimize the risk of each of these t ypes of damage being present or leading to catastrophic failure.1 

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Sy mposium, Federal Aviation Administration. 5.
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– Fiscal Years 2008
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Helicopter Statistics”, HAI 2009 Helicopter Annual 10
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. ---------- , National Transport Safet y Board (NTSB) Accident Reports , Various Years. 13
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## List of Sources **(Cont.)**

No. Source 16
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### www.Rotor.com/portals/12/10%20yr.pdf , Accessed 08/08/2008. 17
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