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MOC VTOL.2240 (a) and (b) Structural durability

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1.      Introduction

VTOL.2240 (a) and (b) requests the applicant to perform all necessary evaluations and actions (inspection, procedures) “to prevent structural failures due to strength degradation, which could result in serious or fatal injuries, or extended periods of operation with reduced safety margins.”

For the category Basic, this comprises of any relevant inspections or other procedures to prevent structural failure (e.g. replacement time for safe life evaluation).

For the category Enhanced, this includes any relevant inspections or other procedures to detect structural damages before failure (Damage Tolerance evaluation).

A distinction is thus made between categories Basic and Enhanced concerning durability: while both categories have the same objective to prevent structural failures due to strength degradation, for Enhanced category the detection of structural damage is added to VTOL.2240(a).

Table 1 summarises the accepted means to demonstrate compliance with VTOL.2240 (a) and (b) regarding structural durability and the associated guidance material additionally applicable:

Table 1: Summary of the means of compliance for categories basic and enhanced

Type of Structure

Category Basic

Category Enhanced

Metallic

Sections 7 and 8 in this MOC, which include the adaptation of CS 27.571 (Amdt. 6) “Fatigue evaluation of flight structure” and of AC 27.571

 

Instead, it is also accepted to use Sections 3 and 4 in this MOC which include an adaptation of CS 29.571 “Fatigue evaluation of metallic structure” (Amdt. 6) and of AC 29.571

Sections 3 and 4 in this MOC, which include the adaptation of CS 29.571 (Amdt. 6) “Fatigue evaluation of metallic structure” and of AC 29.571 (flaw tolerance and crack growth method)

Composite

Sections 5 and 6 in this MOC, which include the adaptation of CS 27.573 (Amdt. 6) “Fatigue evaluation of composite rotorcraft structures” and of AC 27.573 and AMC 20-29.

Design precaution for metallic and composite

CS 23.627 (Amdt 4) “Fatigue strength” is accepted as means of compliance

 

2.      Selected Structural Elements (SSE)

Selected Structural Elements (SSE) are parts which carry flight or ground loads, or parts loaded in fatigue the failure of which would reduce the structural integrity of the aircraft.

The following is a non-exhaustive list of SSE examples:

(a)      Wing and empennage.

(1)     Control surfaces, slats, flaps, and their mechanical systems and attachments (hinges, tracks, and fittings);

(2)     Integrally stiffened plates;

(3)     Primary fittings;

(4)     Principal splices;

(5)     Skin or reinforcement around cutouts or discontinuities;

(6)     Skin-stringer combinations;

(7)     Spar caps; and

(8)     Spar webs.

(b)     Fuselage.

(1)     Frames and adjacent skin;

(2)     Door frames;

(3)     Pilot-window posts;

(4)     Structural bulkheads;

(5)     Skin and any single frame or stiffener element around a cutout;

(6)     Skin or skin splices, or both,

(7)     Door skins, frames, and latches; and

(8)     Window frames.

(c)      Landing gear and their attachments.

(d)     Engine mount/supports

(e)     Lift Thrust Units

(1)     Rotors including blades, propeller, hubs

(2)     Rotor drive systems between the engines and the rotor hubs,

(3)     Transmission mounting

(f)      Fixed and rotating control system

3.      Means of Compliance for structural durability of metallic structures in the category Enhanced:

(a)      Each Selected Structural Element (SSE) should be identified, as defined in Section 2 of this MOC.

(b)     A fatigue tolerance evaluation of each SSE should be performed, and appropriate inspections and retirement time or approved equivalent means should be established to avoid failure during the operational life of the VTOL.

(c)      Each fatigue tolerance evaluation should include:

(1)     In-flight measurements to determine the fatigue loads or stresses for the SSEs identified in (b) in all critical conditions throughout the range of design limitations required in MOC VTOL.2200 (including altitude effects), except that manoeuvring load factors need not exceed the maximum values expected in operations.

(2)     The loading spectra as severe as those expected in operations based on loads or stresses determined under (c)(1), including external load operations, if applicable, and other high frequency power-cycle operations.

(3)     Take-off, landing, and taxi loads when evaluating the landing gear (including skis and floats) and other affected SSEs.

(4)     For each SSE identified in (b), a threat assessment, which includes a determination of the probable locations, types, and sizes of damage taking into account fatigue, environmental effects, intrinsic and discrete flaws, or accidental damage that may occur during manufacture or operation.

(5)     A determination of the fatigue tolerance characteristics for the SSE with the damage identified in (c)(4) that supports the inspection and retirement times, or other approved equivalent means.

(6)     Analyses supported by test evidence and, if available, service experience.

(d)     A residual strength determination should be performed that substantiates the maximum damage size assumed in the fatigue tolerance evaluation. In determining inspection intervals based on damage growth, the residual strength evaluation should show that the remaining structure, after damage growth, is able to withstand design limit loads without failure.

(e)     The effect of damage on stiffness, dynamic behaviour, loads and functional performance should be considered.

(f)      The inspection and retirement times or approved equivalent means established under this Section should be included in the Airworthiness Limitation Section of the Instructions for Continued Airworthiness required by VTOL.2625

(g)      If inspections for any of the damage types identified in (c)(4) cannot be established within the limitations of geometry, inspectability, or good design practice, then supplemental procedures, in conjunction with the SSE retirement time, should be established to minimize the risk of occurrence of these types of damage that could result in a failure during the operational life of the VTOL capable aircraft.

(h)     Discrete source damage tolerance evaluation. The aircraft should be capable of successfully completing a flight during which likely structural damage occurs as a result of

(1)     Uncontained High-Energy Fragments and Sustained Imbalance as specified in VTOL.2240 (d)

(2)     Bird impact as specified in VTOL.2250

4.      Additional guidance for structural durability of metallic structures in the category Enhanced:

Table 2 below provides the necessary adaptations to use AC 29.571 A and B as additional guidance for the fatigue of metallic structures in the category Enhanced:

Table 2: Adaptations to AC 29.571 A and B for the fatigue of metallic structures in the category Enhanced

AC 29.571A. § 29.571 (Amendment 29-28)  FATIGUE TOLERANCE EVALUATION OF STRUCTURE

AC 29.571B. § 29.571 (Amendment 29-55) FATIGUE TOLERANCE EVALUATION OF METALLIC STRUCTURE

Original Text or reference

General Changes/Adaptations

 

“rotorcraft” and “helicopter”

To be replaced by “VTOL capable aircraft”

“the FAA” and “the Administrator”

To be replaced by “EASA”

“Principal Structural Element” or “PSE”

To be replaced by “Selected Structural element” or “SSE”

“§ 29.571”

To be replaced by “VTOL.2240 (a) and (b)”

“Catastrophic failure”

Concept not applicable to the VTOL durability objective.

To be replaced by “failure”.

“§ 29.309”

To be replaced by “VTOL.2200”

“§ 29.1529”

To be replaced by “VTOL.2625 Instructions for Continued Airworthiness”

AC 29.571A. § 29.571 (Amendment 29-28)  FATIGUE TOLERANCE EVALUATION OF STRUCTURE

Paragraph

Changes/ Adaptations in addition to the “General changes/adaptations” above

 

Accepted without additional changes

AC 29.571B. § 29.571 (Amendment 29-55) FATIGUE TOLERANCE EVALUATION OF METALLIC STRUCTURE

Paragraph

Changes/ Adaptation in addition to the “General changes/adaptations” above

a.          Purpose

To be replaced by the paragraph below:

“This advisory material provides additional guidance with the provisions of VTOL 2240 (a) and (b) dealing with the fatigue tolerance evaluation of VTOL metallic structure. This guidance applies to conventional metallic materials. (Corresponding guidance for composite structure can be found in AC 27.573. The fatigue evaluation procedures outlined in this advisory material are for guidance purposes only and are neither mandatory nor regulatory in nature. Although a uniform approach to fatigue tolerance evaluation is desirable, it is recognized that in such a complex area, new design features and methods of fabrication, new approaches to fatigue tolerance evaluation, and new configurations may require variations and deviations from the procedures described herein.”

d.(1) Definitions

Applicable except PSE (xvi), which should be replaced by the definition of SSE provided in Section 2 of MOC VTOL.2240 (a) and (b)

d.(2).(ii)

The sentence below should be removed:

“Further mitigation of the sources of damage may be achieved by adoption of a critical parts plan to help ensure that the condition of the part remains as envisaged by the designer throughout its life cycle (see § 29.602). “

d.(3).(i) Selection of PSE Selected Structural Elements

 

The text in (i) should be replaced as follows:

“Selection of SSE : All SSE, as defined in Section 2 of MOC VTOL.2240(a) and (b), should be identified. Specific areas of interest within the SSE that may require particular attention include the following:”

The text in (A) to (G) remains unchanged.

d.(3).(ii)

“§ 29.309” should be replaced by “VTOL.2215”

(f).(2).Identification of PSE SSE

 

The first sentence is deleted and should be replaced by:

“The fatigue tolerance evaluation should first consider all airframe structure and structural elements, and assemblies susceptible to fatigue loading or fatigue originated from damage.”

(f).(2).(i)

The first sentence is deleted, since the Failure Mode and Effects Analysis is not required for VTOL durability.

(f).(4).(i) Rotorcraft VTOL Usage Spectrum.

 

The following is added at the end:

“The existing guidance available for flight spectrum determination are based on aeroplane/rotorcraft usage. However, considering the limited experience available on VTOL the applicant should anticipate a realistic and conservative spectrum addressing all flight phases and flight configurations conservatively. The principle to establish a VTOL spectrum can be derived from the existing guidance material”

(f).(4).(iv)

To  be fully replaced by

“The usage spectrum should be presented to the FAA EASA for their concurrence. It should include normal operation over the range of rotorcraft VTOL configurations including a percent time under ‘external load’ conditions, in all flight phases and configurations.

These should be distributed conservatively.”

(f).(4).(v)

To be replaced by: “AC 27-1B MG 11, provides further detail for the development of the rotorcraft usage spectrums used in the fatigue tolerance evaluations.

(f).(5).(ii).(F) 

Should be modified as follows:

“Credit may be given to manufacturing, transport, handling, installation, and maintenance instructions finalized to minimize or avoid damages. Examples of these processes or instructions could be: "frozen manufacturing processes," Flight Critical Parts programs, material selection to mitigate intrinsic flaws like inclusions and defects, procedures to reduce deviations from nominal structures, etc.”

(f).(6). Inspectability and Inspection Methods.

“§ 29.1529 of the regulatory requirements.” should be replaced by  “VTOL.2625 Instructions for Continued Airworthiness”

The reference to “§ 29.571” should be replaced by “Section 3 (f) in  MOC VTOL.2240(a) and (b)”.

(f).(6).(ii).(D)

“§ 29.1529 of the regulatory requirements.” should be replaced by  “VTOL.2625 Instructions for Continued Airworthiness”

The reference to “§ 29.571” should be replaced by “Section 3 (e) in this MOC VTOL.2240(”.

(f).(7).(i) Retirement Times

To remove : “(as required by § 29.571(d)(iii))”

 

5.      Means of Compliance for structural durability of composite structures in the categories Basic and Enhanced:

(a)      Composite aircraft structure should be evaluated under the damage tolerance requirements (d) unless the applicant establishes that a damage tolerance evaluation is impractical within the limits of geometry, inspectability, and good design practice. In such a case, the composite aircraft structure should undergo a fatigue evaluation in accordance with (c).

(b)     Damage Tolerance Evaluation:

(1)     Damage tolerance evaluations of composite structures should show that failure due to static and fatigue loads is avoided throughout the operational life or prescribed inspection intervals of the VTOL capable aircraft.

(2)     The damage tolerance evaluation should include all SSEs, as defined in Section 2 of this MOC.

(3)     Each damage tolerance evaluation should include:

(i)      The identification of the structure being evaluated;

(ii)     A determination of the structural loads or stresses for all critical conditions throughout the range of limits in VTOL.2215 (including altitude effects), supported by in-flight and ground measurements, except that manoeuvring load factors need not exceed the maximum values expected in service;

(iii)     The loading spectra as severe as those expected in service based on loads or stresses determined under (b)(3)(ii), including external load operations, if applicable, and other operations including high torque events. The occurrence distribution of all flight phases and flight configurations should be conservatively selected.

(iv)     A Threat Assessment for all structure being evaluated that specifies the locations, types, and sizes of damage, considering fatigue, environmental effects, intrinsic and discrete flaws, and impact or other accidental damage (including the discrete source of the accidental damage) that may occur during manufacture or operation;

(v)     An assessment of the residual strength and fatigue characteristics of all structure being evaluated that supports the replacement times and inspection intervals established under (b)(4); and

(vi)     allowances for the detrimental effects of material, fabrication techniques, and process variability.

(4)     Replacement times, inspections, or other procedures should be established to require the repair or replacement of damaged parts to prevent failure. These replacement times, inspections, or other procedures should be included in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness required by VTOL.2625 Instructions for Continued Airworthiness.

(vii)    Replacement times should be determined by tests, or by analysis supported by tests to show that throughout its life the structure is able to withstand the repeated loads of variable magnitude expected in-service. In establishing these replacement times, the following items should be considered:

(A)     Damage identified in the Threat Assessment required by (b)(3)(iv);

(B)     Maximum acceptable manufacturing defects and in-service damage (i.e., those that do not lower the residual strength below ultimate design loads and those that can be repaired to restore ultimate strength); and

(C)     Ultimate load strength capability after applying repeated loads.

(viii)   Inspection intervals should be established to reveal any damage identified in the Threat Assessment required by (b)(3)(iv) that may occur from fatigue or other in-service causes before such damage has grown to the extent that the component cannot sustain the required residual strength capability. In establishing these inspection intervals, the following items should be considered:

(A)     The growth rate, including no-growth, of the damage under the repeated loads expected in-service determined by tests or analysis supported by tests; and

(B)     The required residual strength for the assumed damage established after considering the damage type, inspection interval, detectability of damage, and the techniques adopted for damage detection. The minimum required residual strength is the limit load.

(5)     The effects of damage on stiffness, dynamic behaviour, loads and functional performance should be taken into account when substantiating the maximum assumed damage size and inspection interval.

(c)      Fatigue Evaluation:

If an applicant establishes that the damage tolerance evaluation described in (b) is impractical within the limits of geometry, inspectability, or good design practice, the applicant should conduct a fatigue evaluation of the particular composite aircraft structure and:

(1)     Identify structure considered in the fatigue evaluation;

(2)     Identify the types of damage considered in the fatigue evaluation;

(3)     Establish supplemental procedures to minimise the risk of failure associated with damage identified in (c)(2); and

(4)     Include these supplemental procedures in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness required by VTOL.2625 Instructions for Continued Airworthiness

(d)     Discrete damage source evaluation. The aircraft should be capable of continued safe flight and landing (for Category Enhanced) or controlled emergency landing (for Category Basic) with the likely structural damage occurring as a result of

(1)     Uncontained High-Energy Fragments and Sustained Imbalance as specified in VTOL.2240 (d)

(2)     Bird impact as specified in VTOL.2250

 

6.      Additional guidance for structural durability of composite structures in the categories Basic and Enhanced:

Table 3 below provides the necessary adaptations to use AC 27.573 as additional guidance for fatigue of composite structures in the categories Basic and Enhanced:

Table 3: Adaptations to AC 27.573 for the fatigue of composite structures in the categories Basic and Enhanced

§ 27.573 (Amendment 27-47) DAMAGE TOLERANCE AND FATIGUE EVALUATION OF COMPOSITE ROTORCRAFT STRUCTURES

Original Text or reference

General Changes/Adaptations

 

“rotorcraft” and “helicopter”

To be replaced by “VTOL capable aircraft”

“the FAA”, “the Administrator”, “the Rotorcraft Directorate”

To be replaced by “EASA”

“Principal Structural Element” or “PSE”

To be replaced by “Selected Structural element” or “SSE”

“§ 27.573”

To be replaced by “VTOL.2240 (a) and (b)”

“Catastrophic failure”

Concept not applicable to the VTOL durability objective.

To be replaced by “failure”.

“§ 27.309”

To be replaced by “VTOL.2200”

“§ 27.1529”

To be replaced by “VTOL.2625 Instructions for Continued Airworthiness”

“AC 20-107”

To be replaced by “AMC 20-29”

Paragraph

Changes/ Adaptation in addition to the “General changes/adaptations” above

d (20) Design Limit loads

“§ 27.301(a)” should be replaced by “VTOL.2230”

d (46) Principal Structural Element (PSE).

“PSE” should be replaced by the definition of “SSE” provided in Section 2 of MOC VTOL.2240 (a) and (b)

f. Procedures for Substantiation of Rotorcraft Composite Structure.

This paragraph is modified as follows:

“The composite structures evaluation has been divided into eight basic regulatory areas to provide focus on relevant regulatory requirements. These eight areas are: fabrication requirements; basic constituent, pre-preg and laminate material acceptance requirements, and material property determination requirements; protection of structure; lightning protection; static strength evaluation; damage tolerance and fatigue evaluation; dynamic loading and response evaluation; and special repair and continued airworthiness requirements. Original as well as alternate or substitute material system constituents (e.g., fibers, resins), material systems (combinations of constituents and adhesives), and composite designs (e.g., laminates, cocured assemblies, bonded assemblies) should be qualified in accordance with the methodology presented in the following paragraphs.

 

Each regulatory area will be addressed in turn. It is important to remember that proper certification of a composite structure is an incremental, building block process, which involves phased”

 

 FAA/AUTHORITY EASA involvement and incremental approval in each of the various areas outlined herein. It is recommended that an  FAA/AUTHORITY certification team approach be used for composite structural substantiation. The team should consist of FAA/AUTHORITY and cognizant members of the applicant’s organization. Personnel who are composites specialists (or are otherwise knowledgeable in the subject) should be primary team member candidates. Once selected, it is recommended that team meetings be held periodically (possibly in conjunction with type boards) during certification to ensure the building block certification process is accomplished as intended. The team should assure that permanent documentation in the form of reports or other FAA/AUTHORITY acceptable documents are included in the certification data package.

 

The documentation includes but is not limited to the structural substantiation reports (both analysis and test), manufacturing processes and quality control, and Instructions for Continued Airworthiness (maintenance, overhaul, and repair manuals). The Airworthiness Limitations Section of the Instructions for Continued Airworthiness is approved by EASA FAA engineering. Engineering practices for many of the areas identified below are available in CMH-17.”

f.(1).(v).

This paragraph is modified as follows:

“Alternate fabrication and process specifications should be approved and must comply with § 27.605 VTOL.2260. Any alternate specifications should provide at least the same level of quality and safety as the original specification. Any changes should be presented for FAA EASA approval well in advance of the effective date of the production change.”

f.(2). (i) to (vi)

The first sentence is modified as follows:

“The second area is the basic raw constituent, pre-preg and laminate material acceptance requirements, and material property determination requirements of §§ 27.603 and 27.613 VTOL.2260.”

f.(3)

The first sentence is modified as follows:

“The third area is the protection of structure as required by § 27.609 VTOL.2255”

f.(4)

This paragraph is modified as follows:

“The fourth area is the lightning protection requirements of § 27.610 VTOL.2335 Lightning Protection. Protection should be provided and substantiated in accordance with analysis and with tests such as those of AC 20-53, “Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition Caused by Lightning” and FAA Report DOT/FAA/CT-86/8 paragraph 17.1 of ASTM F3061/F3061M-19 “Standard Specification for Systems and Equipment in Small Aircraft”. For composite structure projects involving rotorcraft certificated to earlier certification bases (which do not automatically include the lightning protection requirements of § 27.610), these requirements should be imposed as special conditions. The design should be reviewed early in the certification process to ensure proper protection is present. The substantiation test program should also be established, reviewed and approved early to ensure proper substantiation.”

f(5)

The first sentence is modified as follows:

“The fifth area is the static strength evaluation requirements of §§ 27.305 and

27.307  VTOL.2235 for composite structure.”

f(5).(iii)

The first sentence is modified as follows:

“Allowables should be evaluated and used as specified in § 27.613 VTOL 2260”

f(5).(v)

The following sentence is modified as indicated:

”If sufficient process and quality controls cannot be achieved, it may be necessary to account for greater variability with special factors (§ 27.619)  VTOL.2265 applied to the design”

f(6).(i). Background.

The first sentence is modified as follows:

“The static strength determination required by §§ 27.305 and 27.307 VTOL.2235 establishes the ultimate load capability for composite structures that are manufactured, operated, and maintained with established procedures and conditions. The damage tolerance and fatigue evaluation required by § 27.573 section 5 of the MOC VTOL.2240 (a) and (b) implies procedures that allow the composite structure to retain the intended ultimate load capability when subjected to expected fatigue loads and conditions during its operational life.”

f(6).(iii) Means of compliance

The following sentences are modified as indicated:

“For each PSE SSE, inspections, replacement times, or other procedures must be established as necessary to avoid catastrophic failure. Compliance with the Following requirementsof § 27.573(d b) and (e)Section 5 (b) and (c) of MOC VTOL.2240(a) and (b) should be shown by one, or a combination of, the methods described subsequently….”

“In that case, supplemental procedures must be established and submitted to the FAA EASA for approval acceptance. In any case, the FAA EASA must approveagree with the methodology used for compliance to § 27.573 in accordance with Section 4 of MOC VTOL.2240(a) and (b)”

f(6).(iii) (D) Other Procedures.

The first sentence is modified as follows:

“Other procedures are allowed according to § 27.573(d) Section 5 (b) of MOC 2240(a) and (b).“

f(6).(iii) (E) Supplemental Procedures

 

This paragraph is modified as indicated below:

“If the damage tolerant evaluations as described previously cannot be achieved within the limitations of geometry, inspectability, or good design practice, a fatigue evaluation using supplemental procedures may be proposed to the FAA/AUTHORITY EASA per § 27.573(e) Section 5 (c) of MOC VTOL.2240(a) and (b).

The applicant must establish that the damage tolerance criteria are impracticable and cannot be satisfied for the specific PSE SSE, locations, and threats considered. In addition, the types of damage considered in the evaluations must be identified. Finally, supplemental procedures must be established to minimize the risk of catastrophic failure with the damages considered.”

f(6).(iv) (B)(1) Identification of Principal Structural Elements Selected Structural Elements.

The complete subparagraph (1) is replaced with Section 2 of MOC VTOL.2240(a) and (b).

f(6).(iv) (B)(1) (i)

This sentence is modified as follows:

“Rotor blades, propellers and attachment fittings.”

f(6).(v).(B).(1)

The final sentences in this paragraph are modified as follows:

“The distribution and number of strain gauges should cover the load spectrum adequately for each part essential to the safe operation of the rotorcraft as identified in § 27.573(d)(1) Section 5 (b)(1) of MOC VTOL.2240(a) and (b). Other devices such as accelerometers may be used as appropriate.”

f(6).(v).(C)

This paragraph is replaced by the following:

“Parts to be Strain-Gauged: Fatigue critical sections of the Selected Structural Elements (SSE), as defined in Section 2 of MOC VTOL.2240(a) and (b), should be strain-gauged.”

f(6).(vi).(A).(4)

The last sentence is modified as follows:

“A note to this effect should also appear in the rotorcraft VTOL airworthiness limitations section of the maintenance manual prepared in accordance with §§ 27.573 and 27.1529 VTOL.2240 and VTOL.2625”

f(7)

The first sentence is modified as follows:

“The seventh major area is the dynamic loading and response requirements of §§ 27.241, 27.251, and 27.629 VTOL.2160, for vibration and resonance frequency determination and separation for aeroelastic stability and stability margin determination for dynamically critical flight structure.”

f(8)

The first sentence is modified as follows:

“The eighth area is the special repair and continued airworthiness requirements of §§ 27.611, 27.1529, VTOL.2625 and 14 CFR part 27 Appendix A, for composite structures.”

 

7.      Means of Compliance for structural durability of metallic structures in the category Basic

(a)      General. Each SSE, as defined in Section 2 of this MOC, should be identified and evaluated under (b), (c), (d), or (e). The following applies to each fatigue evaluation:

(1)     The procedure for the evaluation should be approved.

(2)     The locations of probable failure should be determined.

(3)     In-flight measurement should be included in determining the following:

(i)      Loads or stresses in all critical conditions throughout the range of limitations in VTOL.2200, except that manoeuvring load factors need not exceed the maximum values expected in operation.

(ii)     The effect of altitude upon these loads or stresses.

(4)     The loading spectra should be as severe as those expected in operation. The loading spectra should be based on loads or stresses determined under (a)(3).

(b)     Fatigue tolerance evaluation. It should be shown that the fatigue tolerance of the structure ensures that the probability of fatigue failure is extremely remote without establishing replacement times, inspection intervals or other procedures under MOC VTOL.2625.

(c)      Replacement time evaluation. It should be shown that the probability of fatigue failure is extremely remote within a replacement time furnished under MOC VTOL.2625.

(d)     Fail-safe evaluation. The following apply to fail-safe evaluation:

(1)     It should be shown that all partial failures will become readily detectable under inspection procedures furnished under MOC VTOL.2625.

(2)     The interval between the time when any partial failure becomes readily detectable under (d)(1), and the time when any such failure is expected to reduce the remaining strength of the structure to limit or maximum attainable loads (whichever is less), should be determined.

(3)     It should be shown that the interval determined under (d)(2) is long enough, in relation to the inspection intervals and related procedures furnished under MOC VTOL.2625, to provide a probability of detection great enough to ensure that the probability of failure is extremely remote.

(e)     Combination of replacement time and fail-safe evaluations. A component may be evaluated under a combination of (c) and (d). For such component it should be shown that the probability of failure is extremely remote with an approved combination of replacement time, inspection intervals, and related procedures furnished under MOC VTOL.2625.

(f)      Fatigue strength: The structure should be designed, as far as practicable, to avoid points of stress concentration where variable stresses above the fatigue limit are likely to occur in normal service.

(g)      Discrete source damage evaluation. The aircraft should be capable of successfully completing a flight during which likely structural damage occurs as a result of

(1)     Uncontained High-Energy Fragments and Sustained Imbalance as specified in VTOL.2240 (d)

(2)     Bird impact as specified in VTOL.2250

8.      Additional guidance for structural durability of metallic structures in the category Basic:

Table 4 below provides the necessary adaptations to use AC 27.571 as additional guidance for fatigue of metallic structures in the category Basic

Table 4: Adaptations to AC 27.571 for the fatigue of metallic structures in the category Basic

AC 27.571 FATIGUE EVALUATION OF FLIGHT STRUCTURE

AC 27.571. § 27.571 (Amendment 27-26) FATIGUE EVALUATION OF FLIGHT STRUCTURE

Original Text or reference

General Changes/Adaptations

 

“rotorcraft” and “helicopter”

To be replaced by “VTOL capable aircraft”

“the FAA” and “the Administrator”

To be replaced by “EASA”

“Principal Structural Element” or “PSE”

To be replaced by “Selected Structural element” or “SSE”

“§ 27.573”

To be replaced by “VTOL.2240 (a) and (b)”

“Catastrophic failure”

Concept not applicable to the VTOL durability objective.

To be replaced by “failure”.

“§ 27.309”

To be replaced by “VTOL.2200”

“§ 29.1529”

To be replaced by “VTOL.2625 Instructions for Continued Airworthiness”

AC 27.571. § 27.571 (Amendment 27-26) FATIGUE EVALUATION OF FLIGHT STRUCTURE

Paragraph

Changes/ Adaptation in addition to the “General changes/adaptations” above

a. (2)

The last sentence is modified as follows:

“See Appendix A of FAR Part 27, paragraphs A27.4 and paragraph AC 27.1529 for information VTOL.2625 Instructions for Continued Airworthiness”

a. (3)

To be removed

a. (4)

To be removed

b.(2) (i)

The following sentence should be deleted:

“(An FAA/AUTHORITY letter for specific test authorization would ordinarily be required.)“

b.(5)

This paragraph should be completely replaced by the following sentence:

“The Applicant should propose a conservative spectrum that conservatively covers all intended operations of the VTOL capable aircraft. The occurrence distribution of all flight phases and flight configurations should be conservatively selected .”

AC 27.571A. §27.571 (Amendment 27-33) FATIGUE EVALUATION OF FLIGHT STRUCTURE FOR CATEGORY A CERTIFICATION

Paragraph

Changes/ Adaptation in addition to the “General changes/adaptations” above

a. Explanation

To be removed

b. Procedures

The following introduction text is to be removed:

“For Category A certification, the tests specified in paragraph AC 29.571A are required for fatigue tolerance evaluation.

Paragraph AC 29.571A is repeated in this section”

b(7) bearing

New section to be added with the following content:

“Additional guidance for bearings is provided under MOC VTOL 2250(c)”

 
Details
Source-Title: MOC VTOL.2240 (a) and (b) Structural durability View latest amendments
Erulesid: ERULES-1963177438-22501 Compare versions
Domain: Initial airworthiness
Amendedby: n/a
Applicabilitydate: n/a
Entryintoforcedate: n/a
Regulatorysource: n/a Navigate
Regulatorysubject: SC-VTOL
Typeofcontent: MoC (Means of Compliance)
Origin: EASA
Category: EAR