FINAL REGULATORY EVALUATION

DAMAGE TOLERANCE AND FATIGUE EVALUATION 

OF 

COMPOSITE ROTORCRAFT STRUCTURE

(14 CFR Parts 27.573 and 29.573)

OFFICE OF AVIATION POLICY AND PLANS

Economic Analysis Division

George A. Euring, Jr.

June, 2011
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TABLE OF CONTENTS

 Page

I. Introduction......................................................................................1

II. Background.......................................................................................2

III. Benefits...........................................................................................4

IV. Costs..............................................................................................5

V. Benefit Cost Comparison......................................................................11
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DAMAGE TOLERANCE AND FATIGUE EVALUATION OF COMPOSITE 

ROTORCRAFT STRUCTURE

I. INTRODUCTION

This regulatory evaluation estimates the benefits and costs, $392,000 in present value, of 

a new Federal Aviation Regulation. The new rule will amend Parts 27 and 29 of Title 14, Code 

of Federal Regulations (14 CFR) by adding new parts 27.573 and 29.573 Damage Tolerance and 

Fatigue Evaluation for Composite Structure. 

The current regulations were written for metallic structure and do not provide adequate 

certification standards for composite materials and structures. 

This new rule will add airworthiness standards to evaluate the fatigue strength and 

residual static strength of composite rotorcraft structures using a damage tolerance evaluation 

and, if that is impractical, a fatigue evaluation. This rule will also add safety requirements for 

composite rotorcraft structure that address advances in composite structures technology. In 

addition to improving the level of safety for all principal structural elements (PSEs), the final 

rule will also increase safety by leading to a more harmonized international standard with the 

European Aviation Safety Agency (EASA). 

The proposed rule was published as an NPRM (Notice of Proposed Rulemaking) on 

January 6, 2009. One economic comment was received during the comment period. As a result 

of the comment, we provide an upper bound for the compliance costs. 
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II. BACKGROUND

A. Statement of the Problem

The current regulations do not provide adequate certification standards for composite 

materials and structures. Certification has been based on a broad interpretation of the metallic 

fatigue substantiation and the design and construction airworthiness standards. The Type 

Certification solutions sought for composite and metal structures are different. As a result, 

special conditions were necessary for some certification projects using composite structures. 

The increased use of composites in rotorcraft structures, certification, and service 

experiences for composite rotorcraft structures over the last twenty-five years has created a need 

to reconsider the current regulations for damage tolerance and fatigue evaluation. To this end, an 

Aviation Rulemaking Advisory Committee (ARAC) working group (WG) was chartered to study 

the problems and offer recommendations for rotorcraft composite structure minimum safety

standards. 

B. History

The international ARAC WG that assembled in 2000 consisted of industry practitioners, 

regulatory officials, and technical specialists, each with unique experiences with composite 

applications. The two co-chairs, one from a U.S. manufacturer and the other from a European 

manufacturer, each had many years of rotorcraft industry experience with composite structures. 

The team included members familiar with the different rotorcraft structures, including airframe, 

main and tail rotor drive systems, main and tail rotor blades and hubs, rotor controls, fixed and 

movable control surfaces, and transmission mountings. Both Part 27 (normal) and 29 (transport) 

category rotorcraft manufacturers had representatives on the team. Such a balanced team was 

needed to develop recommendations that consider the unique technical issues and design 
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characteristics for composites used in the different rotorcraft structures. 

The ARAC WG considered alternatives and determined that rulemaking action should be 

recommended. The WG felt that taking no action would result in continued confusion among 

and within manufacturers and authorities. Relying on advisory material only, as in the past, did 

not always remove the need for special conditions, and advisory material was not enforceable. 

The only existing rule for damage tolerance and fatigue evaluation of structures using composite 

materials is § 23.573 (a). Much of this Part 23 rule has details for composite airframe structures 

subjected to relatively low numbers of fatigue cycles. Because some critical rotorcraft structures 

experience a high number of fatigue cycles, the ARAC WG felt the Part 23 airworthiness 

standards were not acceptable for the complete range of composite rotorcraft structures. 

The ARAC WG’s effort resulted in recommendations for new regulatory standards and 

standardized means of compliance that allows for innovation and changes in technology. To 

carry out the recommendations, the WG proposed regulations, §§ 27.573 and 29.573, specific to 

composite rotorcraft structures. The WG concurrently developed advisory material to support 

the proposed regulations. 

In reviewing experiences with composite rotorcraft structures, it became clear that 

different safety concerns existed, depending on the specific rotorcraft application. Many of the 

differences related to fatigue and accidental damage threats, design detail, and critical load cases. 

For example, rotor blades, which are dynamically loaded by predominantly centrifugal forces, 

are different from an airframe structure with significant compressive loads. Because of this 

variability, the ARAC WG developed a general rule applicable to all types of composite 

rotorcraft structures.
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C. The Need for Regulatory Change

New composite rotorcraft structure standards are needed because:

 Existing regulations, §§ 27.571 and 29.571, do not adequately describe the fatigue 

certification requirements for composite rotorcraft structures. The lack of regulatory

requirements has led to inconsistent interpretations from one rotorcraft certification project to 

another, resulting in different burdens on industry to substantiate the safety of their composite 

rotorcraft structures. As discussed previously, some special conditions have been required to 

establish adequate and appropriate safety standards.

 Some applicants state that the lack of a separate composite rotorcraft structure rule has 

resulted in confusion, because neither the current rules nor the various advisory materials provide 

clear and complete guidance for certification of composite rotorcraft structures. New rules and 

AC material are intended to alleviate this problem.

 The new rules and advisory materials will integrate and reflect the rotorcraft industry’s

historical experience in carrying out certification of composite rotorcraft structures.

 There have been significant changes in composite technology since the current rule 

and the original advisory material were written. In recent years there have been changes in 

design, analysis, testing, manufacturing, maintenance techniques, and maintenance procedures. 

The new regulations and advisory materials will account for these significant changes.

III. Benefits

This final rule systematizes past FAA and industry practice regarding the use of 

composites on rotorcraft, including special conditions and advisory circulars. Although we 

anticipate both cost savings and improved safety in consequence of the proposed rationalization 

of inspection and replacement schedules, we are unable to quantify these benefits. Nevertheless, 
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we believe that the qualitatively estimated benefits are real and significant and exceed the rule’s 

costs. 

We presented this benefit argument in the NPRM, asked for comments, received none. 

Therefore, we did not change the determination that benefits exceed costs.

IV. COSTS 

A. Introduction

The final rule will require rotorcraft manufacturers and operators to take additional 

actions. Manufacturers will be required to:

1) perform a more thorough threat assessment, 

2) perform a more rigorous residual strength assessment, 

and 

3) establish inspection intervals. 

Operators will be required to conduct additional inspections if the final rule results in a 

more rigorous inspection schedule than the operator is currently following. However, we believe

that the schedule of any inspections required would largely coincide with that of metallic 

structure inspections. 

B. Manufacturers Costs

The rotorcraft manufacturers currently perform a threat assessment and a residual 

strength assessment, but these would become more robust under the proposed rule. The current 

metallics requirements mandate the establishment by manufacturers of inspection intervals or

retirement times. This final composites rule mandates both retirement times and inspection 

intervals that must be included in the airworthiness limitation section of the Instructions for 

Continued Airworthiness (ICA). 
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It is current practice for the rotorcraft manufacturers to establish retirement times for 

composite structures. Therefore, this final rule does not impose any incremental costs associated 

with establishing retirement times. It will, however, impose the costs of establishing inspection 

intervals. 

Relying on information from industry representatives on the ARAC Working Group, the 

FAA estimates that the average additional cost for Part 29 aircraft manufacturers to perform a 

more thorough threat assessment will be $100,000 per certification; the average additional cost to 

perform the more rigorous residual strength assessment proposed by this rule will be an 

additional $50,000 per certification; and putting both retirement times and inspection intervals in 

the airworthiness limitation section of the Instructions for Continued Airworthiness will cost, on 

average, an additional $54,000 per certification. These costs are associated with certification of 

rotorcraft using all-composite parts in the certified categories and to vary in proportion to the 

share of composite parts.

Using information received from industry representatives, the FAA also estimates that 

over the next 27 years, Part 29 rotorcraft structures will comprise approximately 50% metallic 

parts and 50% composite parts. Hence, the additional Part 29 certification cost under this final 

rule will be $50,000 for a threat assessment ($100,000 * 0.5 = $50,000), $25,000 for a residual 

strength assessment ($50,000 * 0.5 = $25,000), and $27,000 for putting both inspection intervals 

and retirement times in the airworthiness limitation section ($54,000 * 0.5 = $27,000). 

Therefore, the FAA estimates that the average total certification cost per new Part 29 type 

certification will be $102,000 ($50,000 + $25,000 + $27,000 = $102,000). 

Using information received from industry representatives, the FAA estimates that over 

the next 27 years, part 27 rotorcraft structures will comprise approximately 80% metallic parts 
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and 20% composite parts. Using information from industry representatives, the FAA estimates 

that there are no additional costs to Part 27 manufacturers for the threat assessment and the 

residual strength assessment. Putting both retirement times and inspection intervals in the 

airworthiness limitation section will cost, on average, an additional $48,000 per Part 27 

certification. Hence, the average total certification cost per new Part 27 type certification under 

this proposed rule would be $9,600 ($48,000 * 0.2 = $9,600). Table C1 below shows a summary

of the average cost per certification.

Using information from industry representatives, the FAA estimates that there would be 2 

new Part 29 type certifications every 10 years, and 3.5 new Part 27 type certifications every 10 

years. We front-load certification to the first year of each new decade. Therefore, we assume

there will be 2 new part 29 and 3.5 new part 27 helicopter certificates in the first, eleventh, and 

twenty-first years of the analysis period. This results in a total of 6 new part 29 and 10.5 part 27 

helicopter type certificates during the 27 year analysis period. 

At an average Part 29 new certification cost of $102,000, the expected Part 29 cost is 

$612,000 (6*$102,000). With the Part 27 new certification average cost increase of $9,600, the 

expected Part 27 compliance cost is $100,800 (10.5*$9,600). The sum of Part 27 and Part 29 

compliance cost equals the total compliance cost of $712,800 or $392,000 in present value. 
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C. Operators Costs

Operators costs will consist of costs for any inspections in excess of what they are 

currently performing. It is anticipated that there will be no additional inspection costs imposed 

as a result of this final rule. The minutes of the Rotorcraft Issues Group of the Aviation 

Rulemaking Advisory Committee for the meeting of February 15, 2002, held at Orlando, Florida,

contained a report of the Working Group on Damage Tolerance and Fatigue Evaluation of 

Composite Rotorcraft Structure. Part of this report states that “Industry has indicated there 

should be no cost impact as a result of the new rule and new advisory circular material. They

have also stated they have read the document and understand it and know what to do.” 

Operators have always performed inspections on their helicopters. We do not expect 

composite parts to require more inspections, unless the composite part has a longer life than the 

metallic part. For a part with a longer life, the initial inspection is likely to occur later than the 

same part made of metal. 

A review of proprietary information from a helicopter manufacturer that produces both 

metal and composite helicopters indicates that composite parts have maintenance requirements 

the same as or lower than those of metal parts. Of special interest is that metal rotor blades 

typically are life-limited (needing to be replaced at specified intervals), while composite blades 

have no life limit and are typically replaced because of condition. This difference can result in 

major savings to helicopter operators. 

We received no comments regarding this conclusion. Therefore we accept our finding of 

no additional costs to operators. 
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D. Total Costs

The total costs of this final rule are the total certification costs of the final rule over the 27 

year analysis period and are estimated to be about $713,000 in undiscounted costs or about 

$392,000 in discounted costs as shown in Table C2. Based on a commenter cost estimate of 

12,000 hours and an estimated $86, the burdened hourly rate for an aeronautical engineer, we 

estimate the total nominal dollar estimate to be $1,032,000 (12,000*$86), or ($567,000 in present 

value). The originally-provided ARAC data provides the lower bound cost estimate and by

accepting the commenter’s information we have an upper bound cost estimate. Over 27 years 

the present value difference between these estimates is $175,000. Based on this information we 

estimate the nominal total compliance costs of this final rule to be between $713,000 and 

$1,032,000 and the present value total compliance costs to be between $392,000 and $567,000. 
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V. Benefit-Cost Comparison

This final rule systematizes past FAA and industry practice regarding the use of 

composites on rotorcraft, including special conditions and advisory circulars Although we 

anticipate both cost savings and improved safety in consequence of the proposed rationalization 

of testing, inspection and replacement schedules, we are unable to quantify these benefits. 
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Nevertheless, based on industry-provided data we believe that this final rule, will yield benefits 

exceeding the estimated costs.