12.1 This chapter provides airworthiness design requirements for Defence aircraft structures. This chapter is applicable to all certified aircraft types, including crewed fixed-wing and rotary-wing aircraft and certified-category Uncrewed Aircraft Systems (UAS).
12.2 Within the context of the DASR and this chapter, DASA defines the aircraft structure as the aircraft components that are required to carry loads in order to perform their intended functions. The structure of an aircraft includes the fuselage, wing, empennage, landing gear, helicopter rotor and drive systems, control systems and surfaces, engine mounts, radome, nacelles, inlets, store mounts, structural operating mechanisms and other components that perform a structural function.
12.3 The airworthiness codes recognised by DASA (refer Section 1 Chapter 3) provide an appropriate benchmark for establishing and maintaining an adequate level of safety for aircraft structures. However, DASA has identified the following three circumstances where recognised airworthiness codes may require supplementation:
Recognised airworthiness codes not adequately covering the full scope of the proposed Defence Configuration, Role and Environment (CRE).
The nature of proposed military operations typically dictates the need for more complex and proactive in-service structural integrity management (over and above that contained in some airworthiness codes) to ensure risk of failure remains within that inherent in the Type Certification Basis (TCB).
Recognised airworthiness codes vary in terms of design requirements to address the threats associated with ageing of aircraft structures.
12.4 The requirements and guidance in this chapter are framed around these three circumstances. When developing a certification basis for a Military Type Certificate (MTC), Military Supplemental Type Certificate (MSTC) or Major Change, each applicant will need to assess the airworthiness design requirements inherent in the baseline Primary Certification Code (PCC) or standard and supplement or tailor, as required, using the essential requirements in this chapter. Applicants are highly encouraged to engage early with DASA as part of this process. Note that three likely scenarios exist for any given DASDRM requirement:
Intent of DASDRM requirement is not met by any requirement in the underlying PCC / standard. In this case, the applicant would need to supplement the PCC / standard to add a new requirement.
Intent of DASDRM requirement is partially met by an existing requirement in the underlying PCC / standard. In this case, the applicant may tailor the existing requirement so that it meets the full intent of the DASDRM requirement.
Intent of DASDRM requirement is fully met by an existing requirement in the PCC / standard. In this case, the applicant may propose to DASA that no supplementation or tailoring is required.
12.5 Separate to the DASA-prescribed airworthiness requirements under the DASR and DASDRM, it is Defence Policy that each Defence-registered aircraft type have an effective and efficient Aircraft Structural Integrity Program (ASIP) to:
ensure, so far as is reasonably practicable, that degradation of integrity does not present risks to health and safety
contribute to delivery and assurance of capability
support cost-effective sustainment of capability.
12.6 Under Defence Policy , the Defence-preferred approach for ASIPs is alignment with MIL-STD-1530; a cradle-to-grave management framework for aircraft structural integrity that encompasses system requirements, design, verification and sustainment. Despite the fact that the ASIP remit and objectives are broader than safety, an effective ASIP is key to supporting the essential design requirements set out in this chapter11.
12.7 The ASIP for each aircraft type should be documented in an Aircraft Structural Integrity Management Plan (ASIMP). ASIMPs are key documents that assist applicants and the regulated community by maintaining a record of relevant structural certification information and data, and demonstrate how the ASIP complies with the design requirements prescribed in this chapter and supports related DASR obligations22.
12.8 TCB Foundation. As outlined above, the airworthiness codes recognised by DASA provide an appropriate benchmark for developing a TCB for the aircraft structure, subject to supplementation and / or tailoring to address the requirements provided in this chapter and any unique Defence CRE aspects (as discussed below).
12.9 For cases where Defence is acquiring an aircraft where the baseline design requirements are not a recognised airworthiness code, then applicants are recommended to use the European Military Airworthiness Certification Criteria (EMACC) Handbook, in conjunction with this chapter, to develop and propose for DASA approval a comprehensive set of airworthiness design requirements for structural certification. Applicants are highly encouraged to engage early with DASA if this is the case.
12.10 TCB Supplementation and / or Tailoring. The requirements related to aircraft structures within the airworthiness codes recognised by DASA often provide enough flexibility to cover the full scope of the proposed Defence CRE.
12.11 Notwithstanding, recognised airworthiness codes (especially civil codes) sometimes do require supplementation and / or tailoring for structural aspects related to military-specific CRE. Examples of areas where structural-related supplementation and / or tailoring has been required in the past include: aerial delivery loads; runway conditions; military-specific system structures and structural loads aspects of crash protection (in accordance with DASDRM Section 2 Chapter 6).
12.12 For structural aspects, Defence’s type certification approach almost exclusively relies on fully or partially leveraging prior NAA/MAA certification. Performing a CRE Assessment (CREA) is an essential part of this process to ensure the prior certification is applicable for the Defence CRE and the design is safe for the intended Defence use.
12.13 Aircraft Original Equipment Manufacturers (OEMs) design analysis and verification will be conducted using a configuration baseline and usage, loading and environment assumptions that will not always be representative of Defence’s intended CRE. Furthermore, structural failure modes (in particular fatigue) can be highly sensitive to small changes in usage, loads and environment. Therefore, CREAs for aircraft structures usually require detailed and quantitative analysis. A particular focus of the CREA should be to quantitatively understand the impact of any CRE differences in the following areas:
structural loads, including means of verification (e.g. flight and ground loads testing)
design and fatigue / durability test spectra
fatigue / durability test interpretation
derivation of structural Airworthiness Limitations (AwLs) (i.e. structural and component life limits and structural inspection requirements)
12.14 The structural critical parts and associated AwLs, consistent with the TCB and the intended Defence CRE, must be identified during type certification (refer DASR AMC 21.A.41).
12.15 Variations in actual aircraft usage, environment and configuration can have a significant impact on when structural AwL actions (such as component retirement or inspections) need to be performed. This is especially the case for military operations where there is typically a higher variability in mission profiles compared to the civil sector.
12.16 The airworthiness codes recognised by DASA vary in terms of life tracking requirements. For example, civil airworthiness codes require AwLs to be identified33, however, they do not sufficiently address requirements for tracking life accrual necessary in the military context. DASA therefore prescribes the following essential airworthiness design requirement.
12.17 Design Requirement (Essential). The process for tracking life accrual of structural critical parts must be defined, commensurate with the complexity of the corresponding AwL requirements.
12.18 AwLs include, and will define, the lifing metric(s) and any algorithm, equation, factor or other engineering data which must be used to calculate life accrual. The complexity of the process for tracking life accrual of structural critical parts will vary based on the complexity of the AwL definition.
12.19 In many circumstances, the process for tracking life accrual of structural critical parts will require a dedicated system. In this context, the ‘system’ will typically comprise on-board and off-board hardware and software for data recording, processing and analysis, as well as the associated procedures44. The philosophy and design of the system must be based on the: certification basis; the aircraft type and role; the nature and definition for each AwL; anticipated operational variability (i.e. mission profile and mission mix variability) and uncertainty around ADF CRE. The following considerations should also be taken into account by applicants:
The system must ensure that the accomplishment of AwL actions for individual aircraft or components takes into account the applicable parameters that contribute to life degradation.
The system must have the ability to capture the necessary input data from all ground and flight operations (including development and / or acceptance testing) from the entry into service through to the retirement of each aircraft.
The system must have the ability to identify and appropriately account for any missing or invalid data.
The system must have the ability to track critical part life accrual per individual aircraft or per component as required. The level of component serialisation and tracking fidelity should ensure that individual critical parts do not exceed the underlying basis of AwLs.
Requirements must ensure the system will be validated prior to fielding. Validation should test the entire system (on-board and off-board elements) in an end-to-end fashion (i.e. from raw / input data through to the AwL lifing metric outputs). Validation should ensure that the specified system will produce reliable, repeatable and suitably-conservative outputs so that AwLs are not exceeded in-service. Also within the scope of validation is calibration of all instrumentation (e.g. strain sensors and accelerometers), as required.
12.20 Compliance with this design requirement should be demonstrated through system validation / test report(s), and documentation that defines the process and, where relevant, the dedicated system. These should also be included, either directly or via reference, in the ASIMP.
12.21 Once in-service, the CAMO must maintain a record of the current status of service life-limited components (i.e. track life accrual against AwLs to meet continuing airworthiness obligations - refer DASR M.A.305(d)(4)). Compliance with the essential design requirement above will ensure that the requirements and process for tracking life accrual for structural AwLs are clearly defined up-front as part of type certification.
12.22 Note that the system for Critical Part Life Tracking / IAT may be partially or fully common with the Usage Monitoring system (refer to requirement below). Whilst the systems themselves may have commonality, the outcomes required are separate; therefore, separate sets of requirements must be defined in line with these two essential design requirements.
12.23 The following references also provide useful information on tracking of life accrual for aircraft structures:
MIL-HDBK-516C Change Notice 255, section 5.7.3
MIL-STD-1530D Change 1 (Aircraft Structural Integrity Program), sections 5.4.5 and 5.5.2
JSSG-2006 (Joint Service Specification Guide, Aircraft Structures), sections A.3.15 / A.4.1566.
12.24 During design of an aircraft, OEMs demonstrate compliance with airworthiness requirements and develop critical part AwLs based on predicted operational profiles, material behaviour and environment. The OEM’s design assumptions are typically based on the intended aircraft role(s) and cannot always specifically account for each operator’s individual usage, environment and configuration throughout the entire life-cycle of an aircraft type. This is especially the case for military operations as roles, tactics and operating environments for a given aircraft type can vary significantly between operators and evolve over time. For military aircraft, monitoring accrual of lifing parameters against AwLs alone is insufficient to ensure that aircraft continue to operate within the risk level inherent in the TCB.
12.25 In order to ensure the continued integrity of the aircraft structure, systems and processes for Ongoing Monitoring and Periodic Assessment are required. The purpose of these two facets include:
ongoing monitoring of usage and structural condition in order to collect the data required for, and to determine the periodicity of, assessments
periodic assessment to ensure that the assumptions made during design and certification that could affect the integrity of structural critical parts remain valid for the Defence CRE.
12.26 The airworthiness codes recognised by DASA vary in terms of the requirements for Ongoing Monitoring and Periodic Assessment systems and processes. DASA therefore prescribes the three essential airworthiness design requirements outlined in this sub-section.
12.27 Once in-service, the MTCH must ensure the continued integrity of the aircraft structure through ongoing monitoring and periodic assessment (i.e. as a continued airworthiness obligation - refer DASR 21.A.44(c)). Compliance with the essential design requirements below will ensure that the relevant system and process requirements are clearly defined up-front as part of type certification.
12.28 Design Requirement (Essential). A Usage Monitoring system, as necessary to capture the operational data required to ensure the continued integrity of structural critical parts, must be defined.
12.29 The continued integrity of structural critical parts relies on ensuring the design assumptions (especially the design spectra) are continuously validated, or updated as required, based on actual operational data. Capturing the usage and loads information required to achieve this outcome requires a dedicated UM system. In this context, the ‘system’ will typically comprise on-board and off-board hardware and software for data recording, processing and analysis, as well as the associated procedures77.
12.30 The philosophy and design of the system must be based on the aircraft type and role, certification basis and structural design philosophy, anticipated operational variability (i.e. mission profile and mission mix variability) and uncertainty around ADF CRE. The following considerations should also be taken into account by applicants when defining UM system:
The system must have the ability to obtain the operational usage and measured or estimated loads data that will be required to update or confirm the design spectrum, identify when usage changes occur that warrant an update, and provide the data needed to establish or update a design / baseline spectrum.
The system must have the ability to capture all usage and operational factors which influence the integrity of structural critical parts. This should include, but is not limited to: all significant sources of repeated loads and other parameters that can influence fatigue cracking; operating location and environment parameters that can influence environmental degradation; and usage parameters that can influence flutter or other aeroelastic instability.
The system must have the ability to measure or estimate structural critical parts loads with sufficient fidelity. The aircraft type and role (and therefore loads that need to be monitored), anticipated operational variability and uncertainty around ADF CRE will dictate the type of monitoring required. For some aircraft types, estimation of structural loads via analytical methods88 may be sufficient; for others, direct loads measurement (i.e. using strain sensors) will be required. Examples of when a more-comprehensive system for direct loads measurement may be required include:
When there is a high degree of uncertainty with regards to the actual in-service loads that may be experienced by structural critical parts.
When analytical components of the UM system and / or system for tracking life accrual require ongoing validation with actual loads data.
Supporting a major modification or service life extension program.
The system must have the ability to capture the necessary input data from all ground and flight operations (including development and / or acceptance testing) from the entry into service through to the retirement of each aircraft type.
The system must have the ability to capture a representative quantity of valid operational data. An appropriate valid data capture rate should be defined for each aircraft type.
Requirements must ensure the system will be validated prior to fielding. Validation should test the entire system (on-board and off-board elements) in an end-to-end fashion (i.e. from raw / input data through to processed data outputs). Validation should ensure that the system produces reliable, repeatable and accurate outputs. Also within the scope of validation is calibration of all instrumentation (e.g. strain sensors and accelerometers), as required.
12.31 Compliance with this design requirement should be demonstrated through system validation / test report(s) and system definition document(s). These should also be included, either directly or via reference, in the ASIMP.
12.32 Note that the UM system may be partially or fully common with the system for Critical Part Life Tracking / IAT outlined above. Whilst the systems themselves may have commonality, the outcomes required are separate; therefore, separate sets of requirements must be defined in line with these two essential design requirements.
12.33 The following references also provide useful information on UM system requirements:
MIL-HDBK-516C Change Notice 299, section 5.7.2
MIL-STD-1530D Change 1 (Aircraft Structural Integrity Program), sections 5.4.4 and 5.5.1
JSSG-2006 (Joint Service Specification Guide, Aircraft Structures), sections A.3.15 / A.4.151010.
12.34 Design Requirement (Essential). A Structural Condition Monitoring system, as necessary to capture the structural condition data required to ensure the continued integrity of structural critical parts, must be defined.
12.35 Capturing and assessing data describing the actual condition of the aircraft structure is an essential aspect of ensuring the continued integrity of structural critical parts. This allows comparison of actual degradation / damage with that assumed and predicted during design. Capturing the data required to achieve this outcome requires a dedicated SCM system.
12.36 The system should be developed to suit each individual aircraft type, including the certification basis and design philosophy, structural maintenance regime and structure that must be monitored. The following considerations should be taken into account by applicants when defining SCM system:
The system must have the ability to capture the necessary data for all structural critical parts and other significant structure. The definition for what constitutes other significant structure will vary by aircraft type. Other significant structure should be included, as unanticipated degradation or unforeseen failure modes to such structure may warrant re-classification as critical parts.
The system must have the ability to capture the necessary data from production to disposal for each aircraft.
The system must have the ability to capture all structural discrepancies and structural inspection results1111 for the defined scope of structure.
The system must have the ability to capture all the information necessary to enable subsequent assessment to determine the root-cause of discrepancies and impact on continued structural integrity. This should be defined through detailed input data requirements, including data sources and the required fields. Types of information should include (at a minimum) details of the discrepancy, details of the asset, when and how the discrepancy was found, and disposition of discrepancy.
12.37 Compliance with this design requirement should be demonstrated through system definition document(s). These should also be included, either directly or via reference, in the ASIMP.
12.38 The following reference also provide useful information on SCM system requirements:
MIL-STD-1530D Change 1 (Aircraft Structural Integrity Program), sections 5.4.6 and 5.5.12.
12.39 Design Requirement (Essential). The process for periodic assessment, as necessary to ensure the continued integrity of structural critical parts, must be defined.
12.40 Periodic structural integrity assessments ensure the MTC (including type design, AwLs and operating limitations) and Instructions for Continuing Airworthiness (ICA) remain compliant with the TCB, and therefore that the risk of structural failure remains within that inherent in the TCB. This is primarily achieved by ensuring the assumptions made during design and certification that could affect the integrity of structural critical parts remain valid in light of actual operational usage and condition.
12.41 The periodic assessment philosophy and details will vary significantly based on the aircraft type and role, anticipated operational variability, certification basis and design philosophy. The following considerations should be taken into account by applicants when defining the periodic structural integrity assessment process:
The process must ensure that detailed and quantitative analysis will be conducted to compare actual ADF usage and condition to design assumptions to ensure that the MTC (including type design, AwLs and operating limitations) remain compliant with the TCB, and that all relevant ICA and monitoring provisions remain appropriate.
The process must define the necessary assessment types and associated frequencies and / or triggers.
12.42 Compliance with this design requirement should be demonstrated by documentation that defines the assessment process. This should also be included, either directly or via reference, in the ASIMP.
12.43 The following reference also provide useful information related to periodic structural integrity assessment:
MIL-STD-1530D Change 1 (Aircraft Structural Integrity Program), sections 5.5.3, 5.5.6, 5.5.7, 5.5.81212 and 5.5.13.
12.44 This is a placeholder for an anticipated Q4 2022 update to this chapter detailing DASA’s design requirements related to the threats associated with ageing of aircraft structures.