(a) There currently are very few industry standards that outline the critical damage threats for particular composite structural applications with enough detail to establish the necessary design criteria or test and analysis protocol for complete damage tolerance evaluation.
In the absence of standards, it is the responsibility of individual applicants to perform the necessary development tasks to establish such data in support of product substantiation. Some factors to consider in development of a damage threat assessment for a particular composite structure include part function, location on the airplane, past service data, accidental damage threats, environmental exposure, impact damage resistance, durability of assembled structural details (e.g., long-term durability of bolted and bonded joints), adjacent system interface (e.g., potential overheating or other threats associated with system failure), and anomalous service or maintenance handling events that can overload or damage the part. As related to the damage threat assessment and maintenance procedures for a given structure, the damage tolerance capability and ability to inspect for known damage threats should be developed.
(b) Foreign object impact is a concern for most composite structures, requiring attention in the damage threat assessment. This is needed to identify impact damage severity and detectability for design and maintenance. It should include any available damage data collected from service plus an impact survey. An impact survey consists of impact tests performed with representative structure, which is subjected to boundary conditions characteristic of the real structure. Many different impact scenarios and locations should be considered in the survey, which has a goal of identifying the most critical impacts possible (i.e., those causing the most serious damage but are least detectable). Wh en simulating accidental impact damage at representative energy levels, blunt or sharp impactors of different sizes and shapes should be selected to cause the most critical and least detectable damage, according to the load conditions
(e.g., tension, compression or shear). Until sufficient service experience exists to make good engineering judgments on energy and impactor variables, impact surveys should consider a wide range of conceivable impacts, including runway or ground debris, hail, tool drops, and vehicle collisions. This consideration is important to the assumptions needed for use of probabilistic damage threat assessments in defining design criteria, inspection methods, and repeat inspection intervals for maintenance. Service data collected over time can better define impact surveys and design criteria for subsequent products, as well as establish more rational inspection intervals and maintenance practice. In review of such information, it should be realized that the most severe and critical impact damages, which are still possible, may not be part of the service database.
(c) Once a damage threat assessment is completed, various damage types can be classified into five categories of damage as described below (refer to figure 3). These categories of damage are used for communication purposes in this AC. Other categories of damage, which help outline a specific path to fatigue and damage tolerance substantiation, may be used by applicants in agreement with the regulatory authorities.
(i) Category 1: Allowable damage that may go undetected by scheduled or directed field inspection and allowable manufacturing defects. Structural substantiation for Category 1 damage includes demonstration of a reliable service life, while retaining ultimate load capability. By definition, such damage is subjected to the requirements and guidance associated with paragraph 7 of this AC. Some examples of Category 1 damage include BVID and allowable defects caused in manufacturing or service (e.g., small delamination, porosity, Page 12
______ I small scratches, gouges, and minor environmental damage) that have substantiation data showing ultimate load is retained for the life of an aircraft structure.
Category 1 Damage:
BVID, Allowed Mfg. damage
Category 2 Damage:
Design Load Level Ultimate ~ Maximum load per lifetime Continued safe flight Limit Critical Damage Threshold (CDT)
1.5 Factor of Safety VID, damage requiring repair per normal inspection process Category 3 Damage:
Obvious damage found within a few flights of occurrence, requiring immediate repair Category 4 Damage:
Discrete source damage, obvious to flight crew, requiring repair after flight Allowable
Damage Limit Category 5 Damage:
Anomalous damage not covered in design but known to operations, Increasing Damage Severity requiring immediate repair (ADL) Figure 3 - Schematic diagram of design load levels versus categories of damage severity
(ii) Category 2: Damage that can be reliably detected by scheduled or directed field inspections performed at specified intervals. Structural substantiation for Category 2 damage includes demonstration of a reliable inspection method and interval while retaining loads above limit load capability. The residual strength for a given Category 2 damage may depend on the chosen inspection interval and method of inspection. Some examples of Category 2 damage include visible impact damage (VID), VID (ranging in size from small to large), deep gouges or scratches, manufacturing mistakes not evident in the factory, detectable delamination or debonding, and major local heat or environmental degradation that will sustain sufficient residual strength until found. This type of damage should not grow or, if slow or arrested growth occurs, the level of residual strength retained for the inspection interval is sufficiently above limit load capability.
(iii) Category 3: Damage that can be reliably detected within a few flights of occurrence by operations or ramp maintenance personnel without special skills in composite inspection. Such damage must be in a location su ch that it is obvious by clearly visible evidence or cause other indications of potential damage that becomes obvious in a short time interval because of loss of the part form, fit or function. Both indications of significant damage warrant an expanded inspection to identify the full extent of damage to the part and surrounding structural areas. In practice, structural design features may be needed to provide sufficient large damage capability to ensure limit or near limit load is maintained with easily detectable, Category 3 damage. Structural substantiation for Category 3 damage includes demonstration of a reliable and quick detection, while retaining limit or near limit load capability. The primary difference between Category 2 and 3 damages are the demonstration of large damage capability Page 13
at limit or near limit load for the latter after a regular interval of time which is much shorter than the former. The residual strength demonstration for Category 3 damage may be dependent on the reliable short time detection interval. Some examples of Category 3 damage include large VID or other obvious damage that will be caught during walk-around inspection or during the normal course of operations (e.g., fuel leaks, system malfunctions or cabin noise).
(iv) Category 4: Discrete source damage from a known incident such that flight maneuvers are limited. Structural substantiation for Category 4 damage includes a demonstration of residual strength for loads specified in the regulations. It should be noted that pressurized structure will generally have Category 4 residual strength requirements at a level higher than shown in figure 3. Some examples of Category 4 damage include rotor burst, birdstrikes (as specified in the regulations), tire bursts, and severe in-flight hail.
(v) Category 5: Severe damage created by anomalous ground or flight events, which is not covered by design criteria or structural substantiation procedures. This damage is in the current guidance to ensure the engineers responsible for composite aircraft structure design and the FAA work with maintenance organizations in making operations personnel aware of possible damage from Category 5 events and the essential need for immediate reporting to responsible maintenance personnel. It is also the responsibility of structural engineers to design-in sufficient damage resistance such that Category 5 events are self-evident to the operations personnel involved. An interface is needed with engineering to properly define a suitable conditional inspection based on available information from the anomalous event. Such action will facilitate the damage characterization needed prior to repair.
Some examples of Category 5 damage include severe service vehicle collisions with aircraft, anomalous flight overload conditions, abnormally hard landings, maintenance jacking errors, and loss of aircraft parts in flight, including possible subsequent high-energy, wide-area (blunt) impact with adjacent structure. Some Category 5 damage scenarios will not have clearly visual indications of damage, particularly in composite structures. However, there should be knowledge of other evidence from the related events that ensure safety is protected, starting with a complete report of possible damage by operations.
(d) The five categories of damage will be used as examples in subsequent discussion in this paragraph and in paragraphs 9 and 10. Note that Category 2, 3, 4, and 5 damages all have associated repair scenarios.
(2) Structure details, elements, and subcomponents of critical structural areas should be tested under repeated loads to define the sensitivity of the structure to damage growth. This testing can form the basis for validating a no-growth approach to the damage tolerance requirements. The testing should assess the effect of the environment on the flaw and damage growth characteristics and the no-growth validation. The environment used should be appropriate to the expected service usage. Residual stresses will develop at the interfaces between composite and metal structural elements in a design due to differences in thermal expansion. This component of stress will depe nd on the service temperature during repeated load cycling and is considered in the damage tolerance evaluation. Inspection intervals should be established, considering both the likelihood of a particular damage and the residual strength capability associated with this damage. The intent of this is to assure that structure is not Page 14
Residual strength Damage initiation
• or occurrence H Slow-growth approach * Ultimate loads r - - - -------- I "' I I No-growth approach ** I Limit loads * Repair to Restore Ultimate St rength **No growth without repair is n ot acceptable Time ---
- - - - - - - Shows Acceptable Interval at reduced RS before being repaired (No-growth case).
--- Shows Unacceptable Interval at reduced RS before being repaired (No-growth case).
9/8/09 AC 20-107B exposed to an excessive period of time with residual strength less than ultimate, providing a lower safety level than in the typical slow growth situation, as illustrated in figure 4.
Conservative assumptions for capability with large damage sizes that would be detected within a few flights may be needed when probabilistic data on the likelihood of given damage sizes does not exist. Once the damage is detected, the compone nt is either repaired to restore ultimate load capability or replaced.
Figure 4 - Schematic diagram of residual strength illustrating that significant accidental damage with “no-growth” should not be left in the structure without repair for a long time
(a) The traditional slow growth approach may be appropriate for certain damage types found in composites if the growth rate can be shown to be slow, stable and predictable.
Slow growth characterization should yield conservative and reliable results. As part of the slow growth approach, an inspection program should be developed consisting of the frequency, extent, and methods of inspection for inclusion in the maintenance plan. Inspection intervals should be established such that the damage will have a very high probability of detection between the time it becomes initially inspectable and the time at which the extent of the damage reduces the residual static strength to limit load (considered as ultimate), including the effects of environment. For any detected damage size that reduces the load capability below ultimate, the component is either repaired to restore ultimate load capability or replaced. Should functional impairment (such as unacceptable loss of stiffness) occur before the damage becomes otherwise critical, part repair or replacement will also be necessary.
(b) Another approach involving growth may be appropriate for certain damage types and design features adopted for composites if the growth can reliably be shown to be predictable and arrested before it becomes critical. Figure 5 shows schematic diagrams for all three damage growth approaches applied to composite structure. The arrested growth method is applicable when the damage growth is mechanically arrested or terminated before becoming critical (residual static strength reduced to limit load), as illustrated in figure 5. Arrested growth may occur due to design features such as a geometry change, reinforcement, thickness change, or a structural joint. This approach is appropriate for damage growth that is inspectable and found to be reliably arrested, including all appropriate dynamic effects. Structural details, elements, and Page 15
See Figure 4 for an example residual strength curve for the "No Growth" Case.
Ultimat~!==r===============-==:----------~ :.:_ - - - - + Static Strength Limit --1-------'l:.:-=-=-==Ar=--re=st-=e=d=--G=row=-=t=h=--=-=-=-==--=-=------------"-i..---- Damage Size Threshold of Detectability_ -+-- ·· - ·· - ·· - • ..... --------- ~ + ------ ..
_ _J Arrested Growth -+-- ·· - ·· - ·· - ·· - ·· + I _J I No G~~th .. - .. - .. - . ~low Growth ---- -- I - Slow Growth Inspection Interval Basis Acceptable "No-growth" Inspection Interval for Damage Shown Acceptable "Arrested Growth" Inspection Interval for Damage Shown time
9/8/09 AC 20-107B subcomponents of critical structural areas, components or full-scale structures, should be tested under repeated loads for validating an arrested growth approach. As was the case for a “nogrowth” approach to damage tolerance, inspection intervals should be established, considering the residual strength capability associated with the arrested growth damage size (refer to the dashed lines added to figure 5 to conceptually show inspection intervals consistent with the slow growth basis). Again, this is intended to ensure that the structure does not remain in a damaged condition with residual strength capability close to limit load for long periods of time before repair. For any damage size that reduces load capability below ultimate, the component is either repaired to restore ultimate load capability or replaced.
Figure 5 - Illustrations of residual strength and damage size relationships for three different approaches to composite structural damage tolerance substantiation
(c) The repeated loading should be representative of anticipated service usage. The repeated load testing should include damage levels (including impact damage) typical of those that may occur during fabrication, assembly, and in-service, consistent with the inspection techniques employed. The damage tolerance test articles should be fabricated and assembled in accordance with production specifications and processes so that the test articles are representative of production structure.
(3) The extent of initially detectable damage should be established and be consistent with the inspection techniques employed during manufacture and in service. This information will naturally establish the transition between Category 1 and 2 damage types (i.e., inspection methods used by trained inspectors in scheduled maintenance). For damage that is clearly detectable to an extent that it will likely be found before scheduled maintenance (i.e., allowing classification as Category 3 damage), detection over shorter intervals and by untrained personnel Page 16
may be permitted. Flaw/damage growth data should be obtained by repeated load cycling of intrinsic flaws or mechanically introduced damage. The number of cycles applied to validate both growth and no-growth concepts should be statistically significant, and may be determined by load and/or life considerations and a function of damage size. The growth or no growth evaluation should be performed by analysis supported by test evidence or by tests at the coupon, element, or subcomponent level.
(4) The extent of damage for residual strength assessments should be established, including considerations for the probability of detection using selected field inspection procedures. The first four cat egories of damage should be considered based on the damage threat assessment. In addition, Category 3 damage should be detected in a walk-around inspection or through the normal course of operations. Residual strength evaluation by component or subcomponent testing or by analysis supported by test evidence should be performed considering that damage. The evaluation should demonstrate that the residual strength of the structure will reliably be equal to or greater than the strength required for the specified design loads (considered as ultimate), including environmental effects. The statistical significance of reliable subcomponent and detail residual strength assessments may include conservative methods and engineering judgment. It should be shown that stiffness properties have not changed beyond acceptable levels.
(a) For the no-growth, slow growth, arrested growth approaches, residual strength testing should be performed after repeated load cycling. All probabilistic analyses applied for residual strength assessments should properly account for the complex nature of damage defined from a thorough damage threat assessment. Conservative damage metrics are permitted in such analyses assuming sufficient test data on repeated load and environmental exposure exists.
(b) Composite designs should afford the same level of fail-safe, multiple load path structure assurance as conventional metals design. Such is also the expectation in justifying the use of static strength allowables with a statistical basis of 90 percent probability with 95 percent confidence.
(c) Some special residual strength considerations for bonded structure are given in paragraph 6.c.(3).
(5) The repeated load spectrum developed for fatigue testing and analysis purposes should be representative of the anticipated service usage. Low amplitude load levels that can be shown not to contribute to damage growth may be omitted (truncated). Reducing maximum load levels (clipping) is generally not accepted. Variability in repeated load behavior should be covered by appropriate load enhancement or life scatter factors and these factors should take into account the number of specimens tested. The use of such factors to demonstrate reliability in component tests should be consistent with the fatigue and damage tolerance behavior characterized for the materials, processes and other design details of the structure in building block tests.
(6) An inspection program should be developed consisting of frequency, extent, and methods of inspection for inclusion in the maintenance plan. Inspection intervals should be Page 17
established such that the damage will be reliably detected between the time it initially becomes detectable and the time at which the extent of damage reaches the limits for required residual strength capability. The potential for missed inspections should be considered.
(a) For the case of no-growth design concept, inspection intervals should be established as part of the maintenance program. In selecting such intervals, the residual strength level associated with the assumed damages should be considered. This point was illustrated in figures 4 and 5. Note that an acceptable inspecti on interval for the larger damages shown for the “no-growth” and “arrested growth” options in figures 4 and 5 was conceptually shown as related to an acceptable slow growth basis in terms of the residual strength and time below ultimate load before damage was detected and repaired. Data on the probability of occurrence for different damage sizes also helps define an inspection interval.
(b) A thorough composite damage threat assessment and the separation of different damage sizes into categories, each with associated detection methods, supports programs using a rigorous damage tolerance assessment to avoid conservative design criteria with very large damage assumptions. In such cases, Category 2 damage types will require the structural substantiation of well-specified and reliable inspection methods applied by trained inspectors at scheduled maintenance intervals (by default, Category 1 damage is at the threshold of this evaluation). Those damages classified as Categor y 3 may take advantage of shorter service time intervals provided sufficient structural substantiation exists with demonstrated proof that there will be early detection by untrained ramp maintenance or operations personnel. By definition, Category 4 damage will require residual strength substantiation to levels that complete a flight with limited maneuvers based on the associated regulatory loads. Due to the nature of service events leading to Category 4 damage, suitable inspections will need to be defined to evaluate the full extent of damage, prior to subsequent aircraft repair and return to service. By definition, Category 5 damages do not have associated damage tolerance design criteria or related structural substantiation tasks. Category 5 damage will require suitable inspections based on engineering assessment of the anomalous service event, and appropriate structural repair and/or part replacement, prior to the aircraft re-entering service.
(7) The structure should be able to withstand static loads (considered as ultimate loads) which are reasonably expected during a completion of the flight on which damage resulting from obvious discrete sources occur (i.e., uncontained engine failures, etc.). The extent of damage should be based on a rational assessment of service mission and potential damage relating to each discrete source. Structural substantiation will be needed for the most critical Category 4 damage as related to the associated load cases. Those Category 4 damages that will not require specific residual strength assessments for the associated get home loads because they have high margins (e.g., severe in-flight hail) will likely still require suitable inspections because their detectability may not be consistent with the substantiations validated for Category 2 damage types.
(8) The effects of temperature, humidity, and other environmental or time-related aging factors which may result in material property degradation should be addressed in the damage tolerance evaluation. Unless tested in the environment, appropriate enviro nmental factors should be derived and applied in the evaluation.
b. Fatigue Evaluation. Fatigue substantiation should be accomplished by component fatigue tests or by analysis supported by test evidence, accounting for the effects of the appropriate environment. The test articles should be fabricated and assembled in accordance with production specifications and processes so that the test articles are representative of production structures. Sufficient component, s ubcomponent, element or coupon tests should be performed to establish the fatigue scatter and the environmental effects. Component, subcomponent, and/or element tests may be used to evaluate the fatigue response of structure with impact damage levels typical of those that may occur during fabrication, assembly, and in service, consistent with the inspection procedures employed. Other allowed manufacturing and service defects, which would exist for the life of the structure, should also be included in fatigue testing. It should be demonstrat ed during the fatigue tests that the stiffness properties have not changed beyond acceptable levels. Replacement lives should be established based on the test results. By definition, Category 1 damage is s ubjected to fatigue evaluation and expected to retain ultimate load capability for the life of the aircraft structure.
c. Combined Damage Tolerance and Fatigue Evaluation. Generally, it is appropriate for a given structure to establish both an inspection program and demonstrate a service life to cover all detectable and non-detectable damage, respectively, which is anticipated for the intended aircraft usage. Extensions in service life should include evidence from component repeated load testing, fleet leader programs (including NDI and destructive tear-down inspections), and appropriate statistical assessments of accidental damage and environmental service data considerations.
9. Proof of Structure – Flutter and Other Aeroelastic Instabilities. The aeroelastic evaluations, which includes flutter, control reversal, divergence, and any undue loss of stability and control as a result of structural loading and resulting deformation, are required. Flutter and other aeroelastic instabilities must be avoided through design, quality control, maintenance, and systems interaction.
a. The evaluation of composite structure needs to account for the effects of repeated loading, environmental exposure, and service damage scenarios (e.g., large Category 2, 3 or 4 damage) on critical properties such as stiffness, mass, and damping. Some control surfaces exposed to large damage retain adequate residual strength margins, but the potential loss of stiffness or mass increase (e.g., sandwich panel disbond and/or water ingression) may adversely affect flutter and other aeroelastic characteristics. This is particularly important for control surfaces that are prone to accidental damage and environmental degradation. Other factors such as the weight or stiffness changes due to repair, manufacturing flaws, and multiple layers of paint need to be evaluated. There may also be issues associated with the proximity of high temperature heat sources near structural components (e.g., empennage structure in the path of jet engine exhaust streams or engine bleed air pneumatic system ducting). These effects may be determined by analysis supported by test evidence, or by tests at the coupon, element or subcomponent level.
10. Continued Airworthiness. The maintenance and repair of composite aircraft structure should meet all general, design and fabrication, static strength, fatigue/damage tolerance, flutter, Page 19
and other considerations covered by this AC as appropriate for the particular type of structure and its application.
a. Design for Maintenance. Composite aircraft structure should be designed for inspection and repair access in a field maintenance environment. The inspection and repair methods applied for structural details should recognize the special documentation and training needed for critical damage types that are difficult to detect, characterize, and repair. The inspection intervals and life limits for any structural details and levels of damage that preclude repair must be clearly documented in the appropriate continued airworthiness documents.
b. Maintenance Practices . Maintenance manuals should be developed by the appropriate organizations to include the necessary inspection, maintenance, and repair procedures for composite structures, including jacking, disassembly, handling, part drying methods, and repainting instructions (including restrictions for paint colors that increase structural temperatures). Special equipment, repair materials, ancillary materials, tooling, processing procedures, and other information needed for inspection or repair of a given part should be identified since standard field practices, which have been substantiated for different aircraft types and models, are not common.
(1) Damage Detection.
(a) Procedures used for damage detection must be shown to be reliable and capable of detecting degradation in structural integrity below ultimate load capability. These procedures must be documented in the appropriate sections of the instructions for continued airworthiness.
This should be substantiated in static strength, environmental resistance, fatigue, and damage tolerance efforts as outlined in paragraphs 6, 7, and 8. Substantiated detection procedures will be needed for all damage types identified by the threat assessment, including a wide range of foreign object impact threats, manufacturing defects, and degradation caused by overheating.
Degradation in surface layers (e.g., paints and coatings) that provide structural protection against ultraviolet exposure must be detected. Any degradation to the lightning strike protection system that affects structural integrity, fuel tank safety, and electrical systems must also be detected.
(b) Visual inspection is the predominant damage detection method used in the field and should be performed under prescribed lighting conditions. Visual inspection procedures should account for access, time relaxation in impact damage dent depth, and the color, finish, and cleanliness of part surfaces.
(2) Inspection. Visual indications of damage, which are often used for composite damage detection, provide limited details on the hidden parts of damage that require further investigation. As a result, additional inspection procedures used for complete composite damage characterization will generally be different than those used for initial damage detection and need to be well documented. Nondestructive inspection performed prior to repair and destructive processing steps performed during repair must be shown to locate and determine the full extent of the damage. In-process controls of repair quality and post-repair inspection methods must be shown to be reliable and capable of providing engineers with the data to determine degradation in structural integrity below ultimate load capability caused by the process itself. Certain Page 20
processing defects cannot be reliably detected at completion of the repair (e.g., weak bonds). In such cases, the damage threat assessment, repair design features, and limits should ensure sufficient damage tolerance.
(3) Repair. All bolted and bonded repair design and processing procedures applied for a given structure shall be substantiated to meet the appropriate requirements. Of particular safety concern are the issues associated with bond material compatibilities, bond surface preparation (including drying, cleaning, and chemical activation), cure thermal management, composite machining, special composite fasteners, and installation techniques, and the associated in-process control procedures. The surface layers (e.g., paints and coatings) that provide structural protection against ultraviolet exposure, structural temperatures, and the lightning strike protection system must also be properly repaired.
(4) Documentation and Reporting. Documentation on all repairs must be added to the maintenance records for the specific part number. This information supports future maintenance damage disposition and repair activities performed on the same part. It is recommended that service difficulties, damage, and degradation occurring to composite parts in service should be reported back to the original equipment manufacturer (OEM) to aid in continuous updates of damage threat assessments to support future design detail and process improvements. Such information will also support future design criteria, analysis, and test database development.
c. Substantiation of Repair.
(1) When repair procedures are provided in FAA-approved documents or the maintenance manual, it should be demonstrated by analysis and/or test that the method and techniques of repair will restore the structure to an airworthy condition. Repairable damage limits (RDL), which outline the details for damage to structural components that may be repaired based on existing data, must be clearly defined and documented. Allowable damage limits (ADL), which do not require repair, must also be clearly defined and documented. Both RDL and ADL must be based on sufficient analysis and test data to meet the appropriate structural substantiation requirements and other considerations outlined in this AC. Additional substantiation data will generally be needed for damage types and sizes not previously considered in design development. Some damage types may require special instructions for field repair and the associated quality control. Bonded repair is subjected to the same structural bonding considerations as the base design (refer to paragraph 6.c).
(2) Operators and maintenance repair organizations (MRO) wishing to complete major repairs or alterations outside the scope of approved repair documentation should be aware of the extensive analysis, design, process, and test substantiation required to ensure the airworthiness of a certificated structure. Docume nted records and the certification approval of this substantiation should be retained to support any subsequent maintenance activities.
d. Damage Detection, Inspection and Repair Competency.
(1) All technicians, inspectors, and engineers involved in damage disposition and repair should have the necessary skills to perform their supporting maintenance tasks on a specific Page 21
composite structural part. The continuous demonstration of acquired skills goes beyond initial training (e.g., similar to a welder qualification). The repair design, inspection methods, and repair procedures used will require approved structural substantiation data for the particular composite part. Society of Automotive Engineers International (SAE) Aerospace Information Report (AIR) 5719 outlines training for an awareness of the safety issues for composite maintenance and repair. Additional training for specific skill-building will be needed to execute particular engineering, inspection, and repair tasks.
(2) Pilots, ramp maintenance, and other operations personnel that service aircraft should be trained to immediately report anomalous ramp incidents and flight events that may potentially cause serious damage to composite aircraft structures. In particular, immediate reporting is needed for those service events that are outside the scope of the damage tolerance substantiation and standard maintenance practices for a given structure. The immediate detection of Category 4 and 5 damages are dependent on the proper reaction of personnel that operate and service the aircraft. Please refer to regulations in parts 21, 121, and 135 for reporting requirements.
11. Additional Considerations.
a. Crashworthiness.
(1) The crashworthiness of the aircraft is dominated by the impact response characteristics of the fuselage. Regulations, in general, evolve based on either experience gained through incidents and accidents of existing aircraft or in anticipation of safety issues raised by new designs. In the case of crashworthiness, regulations have evolved as experience has been gained during actual aircraft operations. For example, emergency load factors and passenger seat loads have been established to reflect dynamic conditions observed from fleet experience and from controlled FAA and industry research. Fleet experience has not demonstrated a need to have an aircraft level crashworthiness standard. As a result, the regulations reflect the capabilities of traditional aluminum aircraft structure under survivable crash conditions. This approach was satisfactory as aircraft have continued to be designed using traditional construction methods. With the advent of composite fuselage structure and/or the use of novel design, this historical approach may no longer be sufficient to substantiate the same level of protection for the passengers as provided by similar metallic designs.
(2) Airframe design should assure that occupants have every reasonable chance of escaping serious injury under realistic and survivable crash impact conditions. A composite design should account for unique behavior and structural characteristics, including major repairs or alterations, as compared with conventional metal airframe designs. Structural evaluation may be done by test or analysis supported by test evidence. Service experience may also support substantiation.
(3) The crash dynamics of an aircraft and the associated energy absorption are difficult to model and fully define representative tests with respect to structural requirements. Each aircraft product type (i.e., transport, small airplane, rotorcraft) has unique regulations governing the crashworthiness of particular aircraft structures. The regulations and guidance associated Page 22
with each product type should be used accordingly. The regulations for transport airplane and rotorcraft address some issues that go beyond those required of small airplanes.
(4) Special conditions are anticipated for transport category airplanes with composite fuselage structure to address crashworthiness survivability. The impact response of a composite transport fuselage structure must be evaluated to ensure the survivability is not significantly different from that of a similar-sized aircraft fabricated from metallic materials. Impact loads and resultant structural deformation of the supporting airframe and floor structures must be evaluated. Four main criteria areas should be considered in making such an evaluation.
(a) Occupants must be protected during the impact event from release of items of mass (e.g., overhead bins).
(b) The emergency egress paths must remain following a survivable crash.
(c) The acceleration and loads experienced by occupants during a survivable crash must not exceed critical thresholds.
(d) A survivable volume of occupant space must be retained following the impact event.
(5) The criticality of each of these four criteria will depend on the particular crash conditions. For example, the loads and accelerations experienced by passengers may be higher at lower impact velocities where structural failures have not started to occur. As a result, validated analyses may be needed to practically cover all the crashworthiness criteria for transport fuselage.
(6) Existing transport airplane requirements also require that fuel tank structural integrity be addressed during a survivable crash impact event as related to fire safety (also refer to paragraph 11.b). As related to crashworthiness, composite fuel tank structure must not fail or deform to the extent that fire becomes a greater hazard than with metal structure.
(7) Physics and mechanics of the crashworthiness for composite structures involve several issues. The local strength, energy absorbing characteristics, and multiple, competing failure modes need to be addressed for composite structure subjected to a survivable crash. This is not simply achieved for airframe structures made from anisotropic, quasi-brittle, composite materials. As a result, the accelerations and load histories experienced by passengers and equipment on a composite aircraft may differ significantly from that seen on a similar metallic aircraft unless specific considerations are designed into the composite structure. In addition, care should be taken when altering composite structure to achieve specific mechanical behaviors.
(For example, where the change in behavior of a metallic structure with a change in material thickness may be easily predicted, an addition or deletion of plies to a composite laminate may also require data for the effects of laminate stacking sequence on the failure mode and energy absorption characteristics of a composite element.)