ED Decision 2003/15/RM
(a) Strength requirements are specified in terms of limit loads (the maximum loads to be expected in service) and ultimate loads (limit loads multiplied by prescribed factors of safety). Unless otherwise provided, prescribed loads are limit loads.
(b) Unless otherwise provided, the specified air, ground, and water loads must be placed in equilibrium with inertia forces, considering each item of mass in the rotorcraft. These loads must be distributed to closely approximate or conservatively represent actual conditions.
(c) If deflections under load would significantly change the distribution of external or internal loads, this redistribution must be taken into account.
ED Decision 2003/15/RM
Unless otherwise provided, a factor of safety of 1.5 must be used. This factor applies to external and inertia loads unless its application to the resulting internal stresses is more conservative.
CS 27.305 Strength and deformation
ED Decision 2003/15/RM
(a) The structure must be able to support limit loads without detrimental or permanent deformation. At any load up to limit loads, the deformation may not interfere with safe operation.
(b) The structure must be able to support ultimate loads without failure. This must be shown by:
(1) Applying ultimate loads to the structure in a static test for at least 3 seconds; or
(2) Dynamic tests simulating actual load application.
ED Decision 2003/15/RM
(a) Compliance with the strength and deformation requirements of this Subpart must be shown for each critical loading condition accounting for the environment to which the structure will be exposed in operation. Structural analysis (static or fatigue) may be used only if the structure conforms to those structures for which experience has shown this method to be reliable.
In other cases, substantiating load tests must be made.
(b) Proof of compliance with the strength requirements of this Subpart must include:
(1) Dynamic and endurance tests of rotors, rotor drives, and rotor controls;
(2) Limit load tests of the control system, including control surfaces;
(3) Operation tests of the control system;
(4) Flight stress measurement tests;
(5) Landing gear drop tests; and
(6) Any additional tests required for new or unusual design features.
AMC1 27.307 Proof of structure
ED Decision 2023/001/R
(a) Purpose
This AMC provides guidance and acceptable means of compliance with CS 27.307, which specifies the requirements for proof of structure.
(b) Related Certification Specifications
CS 27.303 ‘Factor of safety’
CS 27.305 ‘Strength and deformation’
(c) Definitions
(1) Detail: a structural element of a more complex structural member (e.g. gear teeth, joints, splices, stringers, stringer run-outs, lugs, or access holes).
(2) Subcomponent: a major three-dimensional structure which can provide a complete structural representation of a section of the full structure (e.g. main gearbox housing, gears, section of a blade, rotor spherical bearing, tension-torsion (TT) strap beams, or frames).
(3) Component: a major section of the airframe structure or mechanical assembly (e.g. main gearbox assembly, blade, main rotor hub assembly, cabin, tailboom, fin, horizontal stabiliser or transmission/upper deck) which can be tested as a complete unit to qualify the structure.
(4) Full scale: the dimensions of the test article are the same as design; fully representative test specimen (not necessarily complete airframe or mechanical assembly).
(5) New structure: a structure for which the behaviour is not adequately predicted by analysis supported by previous test evidence. A structure that utilises significantly different structural design concepts such as details, geometry, structural arrangements, and load paths or materials from previously tested designs.
(6) Similar new structure: a structure that utilises similar or comparable structural design concepts such as details, geometry, structural arrangements, and load path concepts and materials to an existing tested design.
(7) Derivative/similar structure: a structure that uses structural design concepts such as details, geometry, structural arrangements, and load paths, stress levels and materials that are nearly identical to those on which the analytical methods have been validated.
(8) Previous test evidence: testing of the original structure that is sufficient to verify the structural behaviour in accordance with CS 27.305.
(d) Introduction
As required by sub-paragraph (a) of CS 27.307, the structure must be shown to comply with the strength and deformation requirements of Subpart C of CS-27. This means that the structure must be able to support:
(a) limit loads without detrimental permanent deformation; and
(b) ultimate loads without failure.
This implies the need of a comprehensive assessment of the external loads (addressed by CS 27.301), the resulting internal strains and stresses, and the structural allowables.
CS 27.307 requires compliance for each critical loading condition. Compliance may be shown by analysis supported by previous test evidence, analysis supported by new test evidence or by test only. As compliance by test only is impractical in most cases, a large portion of the substantiating data will be based on analysis.
There are a number of standard engineering methods and formulas which are known to produce acceptable, often conservative, results especially for structures where load paths are well defined.
Those standard methods and formulas, applied with a good understanding of their limitations, are considered to be reliable analyses when demonstrating compliance with CS 27.307. Conservative assumptions may be considered in assessing whether or not an analysis may be accepted without test substantiation.
The application of methods such as the finite element method or engineering formulas to complex structures in modern aircraft is considered to be reliable only when validated by full-scale tests (ground and/or flight tests). Experience relevant to the product in the utilisation of such methods should be considered.
(e) Classification of structure
(a) The structure of the product should be classified into one of the following three categories:
(1) new structure
(2) similar new structure
(3) derivative/similar structure
(b) Justifications should be provided for classifications other than new structure. Elements that should be considered are:
(1) the accuracy/conservatism of the analytical methods; and
(2) the comparison of the structure under investigation with a previously tested structure.
Considerations should include but are not limited to the following:
— external loads (bending moment, shear, torque, etc.);
— internal loads (strains, stresses, etc.);
— structural design concepts such as details, geometry, structural arrangements, load paths;
— materials;
— test experience (load levels achieved, lessons learned);
— deflections;
— deformations;
— extent of extrapolation from test stress levels.
(f) Need and extent of testing
The following factors should be considered in deciding the need for and the extent of testing including the load levels to be achieved:
(a) the classification of the structure (as above);
(b) the consequence of the failure of the structure in terms of the overall integrity of the rotorcraft;
(c) the consequence of the failure of interior items of mass and the supporting structure to the safety of the occupants.
Relevant service experience may be included in this evaluation.
(g) Certification approaches
The following certification approaches may be selected:
(a) Analysis, supported by new strength testing of the structure to limit and ultimate load. This is typically the case for a new structure.
Substantiation of the strength and deformation requirements up to limit and ultimate loads normally requires testing of subcomponents, full-scale components or full-scale tests of assembled components (such as a nearly complete airframe). The entire test programme should be considered in detail to ensure that the requirements for strength and deformation can be met up to limit load levels as well as ultimate load levels.
Sufficient limit load test conditions should be performed to verify that the structure meets the deformation requirements of CS 27.305(a) and to provide validation of internal load distribution and analysis predictions for all critical loading conditions.
Because ultimate load tests often result in significant permanent deformation, choices will have to be made with respect to the load conditions applied. This is usually based on the number of test specimens available, the analytical static strength margins of safety of the structure and the range of supporting detail or subcomponent tests. An envelope approach may be taken, where a combination of different load cases is applied, each one critical for a different section of the structure.
These limit and ultimate load tests may be supported by detail and subcomponent tests that verify the design allowables (tension, shear, compression) of the structure and often provide some degree of validation for ultimate strength.
(b) Analysis validated by previous test evidence and supported with additional limited testing. This is typically the case for a similar new structure.
The extent of additional limited testing (number of specimens, load levels, etc.) will depend upon the degree of change, relative to the elements of sub-paragraphs (e)(b)(1) and (2).
For example, if the changes to an existing design and analysis necessitate extensive changes to an existing test-validated finite element model (e.g. different rib spacing), additional testing may be needed. Previous test evidence can be relied upon whenever practical.
These additional limited tests may be further supported by detail and subcomponent tests that verify the design allowables (tension, shear, compression) of the structure and often provide some degree of validation for ultimate strength.
(c) Analysis, supported by previous test evidence. This is typically the case for a derivative/similar structure.
Justification should be provided for this approach by demonstrating how the previous static test evidence validates the analysis and supports showing compliance for the structure under investigation. Elements that need to be considered are those defined in sub-paragraphs (e)(b)(1) and (2).
For example, if the changes to the existing design and test-validated analysis are evaluated to assure that they are relatively minor, and the effects of the changes are well understood, the original tests may provide sufficient validation of the analysis and further testing may not be necessary. For example, if a weight increase results in higher loads along with a corresponding increase in some of the element thickness and fastener sizes, and materials and geometry (overall configuration, spacing of structural members, etc.) remain generally the same, the revised analysis could be considered to be reliable based on the previous validation.
(d) Test only
Sometimes no reliable analytical method exists, and testing must be used to show compliance with the strength and deformation requirements. In other cases, it may be elected to show compliance solely by tests even if there are acceptable analytical methods. In either case, testing by itself can be used to show compliance with the strength and deformation requirements of CS-27 Subpart C. In such cases, the test load conditions should be selected to assure that all critical design loads are encompassed.
If tests only are used to show compliance with the strength and deformation requirements for a single load path structure which carries flight loads, the test article should be of the minimum acceptable material quality or alternatively the test loads should be increased to account for variability in material properties. In lieu of a rational analysis, for metallic materials, a variability factor of 1.15 applied to the limit and ultimate flight loads may be used. If the structure has multiple load paths, no material correction factor is required.
(h) Interpretation of data
The interpretation of the substantiation analysis and test data requires an extensive review of:
— the representativeness of the loading;
— the instrumentation data;
— comparisons with analytical methods;
— the representativeness of the test article(s);
— the test set-up (fixture, load introductions);
— load levels and conditions tested;
— test results.
Testing is used to validate analytical methods except when showing compliance by test only. If the test results do not correlate with the analysis, the reasons should be identified, and appropriate action taken.
This should be accomplished whether or not a test article fails below ultimate load.
Should a failure occur below ultimate load, an investigation should be conducted for the product to reveal the cause of this failure. This investigation should include a review of the test specimen and loads, analytical loads, and the structural analysis. This may lead to adjustment in analysis/modelling techniques and/or part redesign and may result in the need for additional testing. The need for additional testing to ensure that ultimate load capability depends on the degree to which the failure is understood, and the analysis can be validated by the test.
The approach described above is valid for static justification. However, a similar approach can be extended for compliance with fatigue, dynamic and crashworthiness requirements. For these applications, the criteria and the classification have to be accepted by and agreed with the authority.
[Amdt 27/10]
AMC2 27.307 Proof of structure
ED Decision 2023/001/R
FAIRING SUBSTANTIATION
This AMC supplements FAA AC 27-1B, § AC 27.307 and should be used in conjunction with that AC when demonstrating compliance with CS 27.307.
Further to CS 27.301, the specified loads must be distributed appropriately or conservatively and significant changes in the distribution of the loads, as a result of deflection, must be taken into account. FAA AC 27-1B, § AC 27.307 refers to the need for flight test measurement in the scope of the fatigue and damage tolerance demonstration. The methods used to determine load intensities and distribution should be validated by flight load measurements unless the methods used for determining those loading conditions are shown to be reliable.
Each fairing, when appropriate, should be constructed and supported so that it can resist any vibration, inertia, and air load to which it may be subjected in operation. The vibrations level, the inertia and air loads should be validated by appropriately instrumented flight measurements as recommended in FAA AC 27-1B, § AC 27.307.
For the fairings and the associated supporting structure, the loads can be shown unreliably predicted and require a measurement during flight tests.
The loads derived from flight testing should be compared with those obtained from analytical methods.
Note: AMC No.2 to CS 25.301(b) provides an acceptable means of demonstrating compliance with the provisions of CS-25 related to the validation, by flight measurements, of the methods used for determination of flight load intensities and distributions, for large aeroplanes.
The methodology presented in the CS-25 AMC material may be adapted to CS-27, to provide further guidance to this AMC.
[Amdt 27/10]
AMC3 27.307 Proof of structure
ED Decision 2023/001/R
SEAT ADAPTER PLATES
(a) Purpose
This AMC provides an acceptable means of compliance for seat adapter plates. A seat adapter plate includes any other forms of new interface structure installed between the seat and the rotorcraft floor.
(b) Related Certification Specifications
— CS 27.307 ‘Proof of structure’
— CS 27.561 ‘General’
— CS 27.562 'Emergency landing dynamic conditions'
— CS 27.785 ‘Seats, berths, safety belts, and harnesses’
(c) Explanation
The requirements for seats under emergency landing dynamic conditions have been developed to prevent detachment of the seat under floor deformation and for the seat to help absorb the energy developed in crash conditions. This dynamic condition has been addressed with the 10° roll and 10° pitch deformation required by CS 27.562(b)(3) to ensure that the seat and the floor attachments will be designed to accommodate deformation. This objective should be maintained when a seat adapter plate is installed between the seat and the floor.
Introducing an adapter plate can move the problems created by floor deformation from the seat-to-track interface to the adapter-to-floor interface. The same level of safety is appropriate for the occupant of the seat whether it is installed in the rotorcraft with or without an adapter plate. The floor structure itself is not subject to the dynamic requirements of CS 27.562, therefore when additional structure such as an adapter plate is introduced to fix the seat to the floor, it is very important to determine whether that structure should be considered to be part of the seat or part of the floor. The installation of any interface between the existing floor and the seat should not create a weak element between the seat and the existing airframe. This has successfully been assured by testing the adapter with the seat according to the requirements of CS 27.562.
This AMC provides further guidance and acceptable means of compliance for classification of seat adapters, such as plinths or pallets, and supplements FAA § AC 25.562.
Plinths are subject to CS 27.562 compliance whereas pallets (traditionally defined as large adapters) are not, except for the attachment of the seat to the pallet.
FAA Policy Memo PS-ANM100-2000-00123 (which is applicable to CS-25 and can be extended to CS-27) suggests that it may also be possible to classify some smaller adapters as an integral part of the floor as follows:
‘Generally speaking, adapters of the size that contain a single row of seats (whether they are individual seat places or a common assembly), and mount into seat tracks, should be treated as part of the seat for purposes of certification in accordance with § 27/29.562. Larger, or more integrally mounted adapters, should be assessed to determine whether they should be treated as part of the floor for purposes of certification in accordance with § 27/29.561.’
To treat an adapter or other new interface structure as part of the floor when it does not appear to be similar to conventional floor structure, the applicant must substantiate that the adapter plate or any other structure installed between the existing floor and the seat attachment will not constitute a weak element under emergency landing conditions. The issue is whether the critical interface is between the seat and the adapter or between the adapter and the rotorcraft. No further detailed guidance is available to assist with the assessment required to make the classification of an adapter as part of the floor.
Where the proposed floor design utilises a plate above the existing floor or otherwise significantly differs in concept from the type design’s existing methods of floor construction, geometries and utilisation of load paths, it is not adequate to rely on compliance with CS 27.561 alone to determine whether the adapter plate may be considered to be part of the floor. This guidance does not intend to request a complete crash scenario evaluation, but asks for evidence that the adapter plate and associated new under floor structure will not degrade the level of protection compared to that offered by the seat if it were installed directly on the helicopter existing floor seat track and floor construction. For an adapter plate to be considered sufficiently integrated to be part of the floor, the adapter plate should be capable of accommodating floor deformation and be able to safely react and distribute the seat loads into the rotorcraft.
(d) Seat adapter plate definition and classification
(1) Definitions
The definitions of plinth and pallet that are available in AC 25.562(b) are valid.
In general, swivelling seat adapter plate systems are by definition considered to be plinths.
(2) Classification
There are three possible options for the seat-to-floor interface with corresponding means of compliance. In each case, the applicant is requested to show that any interface between the existing floor and the seat will not create a weaker element between the seat and the existing airframe than that that would exist for a CS 27.562-compliant seat attached directly to the standard floor, e.g. seat track.
Acceptable means of assessing seat installations using adapter plates:
Option 1
— The adapter is classified as a plinth following AC 25.562-1B.
— Compliance with CS 27.561 and CS 27.562 must be shown.
— The plinth must be tested as part of the seat according to CS 27.562(b)(1) and (b)(2) unless alternative compliance is agreed as per CS 27.562(d).
— The guidance of AC 25.562-1B and AMC 27.307 may be used to reduce the number of tests based on design similarity.
Option 2
— The adapter is classified as a pallet due to its size following AC 25.562-1B.
— The seat and its attachments to the pallet only are tested according to CS 27.562 and CS 27.561.
— The pallet is justified against CS 27.561 only.
Option 3
— If neither Option 1 nor 2 clearly apply, seat-to-floor interface structure may be proposed to be classified as an integral part of the floor based on one of the methods described below.
— If classification as part of the floor is agreed with the Agency, the seat and its attachments to the structure are tested according to CS 27.562 and compliance with CS 27.561 is shown for the whole installation.
Acceptable methods to be used in support of Option 3, allowing classification of the new seat-to-floor interface structure as an integral part of the floor structure:
Method 1
A design review showing the floor design for seat installation uses the same or an equivalent design principle as the current floor provided in the type design. If the pre-existing floor design used seats directly attached to seat track independently of the floor panel, then the introduction of a structural floor panel to which a seat is attached would represent a change in design philosophy, and a different method (e.g. Method 2) would need to be used to support Option 3.
Method 2
A detailed design review showing the level of integration of the plate to the floor, including the redundancy and strength of the attachments, that is acceptable to the Agency based on the experience of the applicant and the Agency with similar designs.
Any other alternative methods have to be agreed with the Agency.
Note:
When assessing the design, the following points should be considered by the applicant and the Agency, in particular for design change certification:
— The modified structure may be evaluated using AMC1 27.307 to categorise the structural elements as new, similar-new or similar. Comparison can be made with the existing type floor design (Method 1) or with designs that the applicant has previously substantiated according to Method 2.
— An adequate number of appropriately distributed attachments between the adapter plate and the rotorcraft floor structure should be provided to assure that the additional structure behaves as an integral part of the rotorcraft floor. The appropriate number, strength and degree of redundancy of the attachments will depend on the design of the adapter plate and positioning of the seats on the plate.
— A considerable degree of engineering judgement is required when making the classification of the structure; when there is any doubt about the capability of the proposed adapter design to act as an integral part of the floor, it will be classified as a plinth under Option1.
[Amdt 27/10]
ED Decision 2023/001/R
The following values and limitations must be established to show compliance with the structural requirements of this Subpart:
(a) The design maximum and design minimum weights.
(b) The main rotor rpm ranges power on and power off.
(c) The maximum forward speeds for each main rotor rpm within the ranges determined in sub-paragraph (b).
(d) The maximum rearward and sideward flight speeds.
(e) The centre of gravity limits corresponding to the limitations determined under sub-paragraphs (b), (c), and (d).
(f) The rotational speed ratios between each powerplant and each connected rotating component.
(g) The positive and negative limit manoeuvring load factors.
(h) The maximum and minimum density altitude and temperatures.