methods. This is necessary to correlate the loads and stresses with the maneuver or operating conditions at which they occurred.
(3) The instrumentation system should be adequately calibrated and checked periodically throughout the flight strain measurement program to ensure consistent and accurate results.
(C) Parts to be Strain-Gauged. Fatigue critical portions of the rotor systems, control systems, landing gear, fuselage, and supporting structure for rotors, transmissions, and engine are to be strain-gauged. For rotorcraft of unusual or unique design, special consideration might be necessary to ensure that all the essential parts are evaluated.
(D) Flight Regimes and Conditions to be investigated.
(1) Typical flight and ground conditions to be investigated in the flight strain measurement program are given in paragraphs c. and d. of AC 27-1B, MG 11.
(2) The determination of flight conditions to be investigated in the flight strain measurement program should be based on the anticipated use of the rotorcraft and, if available, on past service records for similar designs. In any event, the flight conditions considered appropriate for the design and application should be representative of the actual operation in accordance with the rotorcraft flight manual. In the case of multiengine rotorcraft, the flight conditions concerning partial engine- out operation should be considered in addition to complete power-off operation. The flight conditions to be investigated should be submitted in connection with the flight evaluation program.
(3) The severity of the maneuvers investigated during the flight strain survey should be at least as severe as the maneuvers likely to occur in service.
(4) All flight conditions considered appropriate for the particular design are to be investigated over the complete rotor speed, airspeed, center of gravity, altitude, and weight ranges to determine the most critical stress levels associated with each flight condition. The temperature effects on loads as affected by elastomeric components are to be investigated. To account for data scatter and to determine the stress levels present, a sufficient amount of data points should be obtained at each flight condition. Consideration can be given to the use of scatter factors in determining the sufficiency of data points. In some instances, the critical weight, center of gravity, and altitude ranges for the various maneuvers can be based on past experience with similar design. This procedure is acceptable where adequate flight tests are performed to substantiate such selections. The combinations of flight parameters that produce the most critical stress levels should be used in the evaluation.
(vi) Frequency of loading.
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AC 29-2C, Chg 4 12/1/2011 (A) Types of Operation.
(1) The probable types of operation (transport, utility, etc.) for the rotorcraft should be established. The type of operation can have a major influence on the loading environment. In the past, rotorcrafts have been substantiated for the most critical general types of operation with some consideration of special, occasional types of operation. To assure that the most critical types of operation are considered, each major rotorcraft structural component should be substantiated for the most critical types of operation as established by the manufacturer. The types of operation shown below should be considered and, if applicable, used in the substantiation:
(i) Long flights to remote sites (low ground-air-ground cycles but high cruising speeds).
(ii) Typical, general types of operation.
(iii) Short flights as used in logging operations.
(2) One means is to substantiate for the most severe type of operation;
however, this method is not always economically feasible.
(3) A second means is to quantify the influence of mission type on fatigue damage by adding to or replacing hour limitations by flight cycle limitations (if properly defined and easily identifiable by the crew, for example: one landing, one load transportation). A special type of flight hour limitation replacement using factorization of flight hours for multiple types of operations may be feasible if continuing manufacturers’ technical support is provided and documented (i.e., the manufacturer either provides the factorization analyses or checks them on a continuing basis for each type of rotorcraft operation).
(4) Where one or more operations are not among the general uses intended for the rotorcraft, the rotorcraft flight manual should state in the limitations section that the intended use of the rotorcraft does not include certain missions or repeated maneuvers (e.g., logging with its high number of takeoffs and landings per hour). A note to this effect should also appear in the rotorcraft airworthiness limitations section of the maintenance manual prepared in accordance with §§ 29.573 and 29.1529.
(5) Should subsequent usage of the rotorcraft encompass a mission outside the original structural substantiation, the effects of this new mission environment on the frequency of loading and structural substantiation should be addressed and where practicable, in the interest of safety, a reassessment made. If this reassessment indicates the necessity for revised retirement times, those new times may be limited to specific rotorcraft model involved in the added mission provided:
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(i) changes are adopted through the airworthiness directives process and proper part re-identification is established; or
(ii) a Rotorcraft Flight Manual (RFM) supplement outlining the limitations is approved; or
(iii) an airworthiness limitations section (ALS) supplement is approved; or
(iv) an appropriate combination of part re-identification, RFM supplement, or airworthiness limitation section supplement is approved.
(B) Loading Spectrum. The spectrum allocating percentage of time or frequencies of occurrence to flight conditions or maneuvers is to be based on the expected usage of the rotorcraft. This spectrum is to be established so that it is unlikely that actual usage will subject the structure to damage beyond that associated with the spectrum. Considerations to be included in developing this spectrum should include prior knowledge based on flight history recorder data, design limitations established in compliance with § 29.309, and recommended operating conditions and limitations specified in the rotorcraft flight manual or instructions for continued airworthiness (ICA).
The distribution of times at various forward flight speeds should reflect not only the relation of these speeds to VNE but also the recommended operating conditions in the rotorcraft flight manual or ICA that govern Vc or cruise speed. It is desirable to conduct the flight strain-gauge program by simulating the usage as determined previously, with continuous recording of stresses and loads, thus obtaining directly the stress or load spectra for structural elements.
(7) The seventh major area is the dynamic loading and response requirements of §§ 29.241, 29.251, and 29.629 for vibration and resonance frequency determination and separation for aeroelastic stability and stability margin determination for dynamically critical flight structure. Critical parts, locations, excitation modes, and separations should be identified and substantiated. This substantiation should consist of analysis supported by tests, including tests that account for repeated loading effects and environment exposure effects on critical properties, such as stiffness, mass, and damping. This must be accomplished to assure that the initial stiffness, residual stiffness, proper critical frequency design, and structural damping are provided as necessary to prevent vibration, resonance, and flutter problems.
(i) All vibration and resonance critical composite structures must be identified and properly evaluated.
(ii) All flutter-critical composite structures must be identified and properly evaluated. This structure must be shown by analysis to be flutter free to 1.1 VNE (or any other critical operating limit, such as VD, for a VSTOL aircraft) with the extent of damage for which residual strength and stiffness are demonstrated.
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AC 29-2C, Chg 4 12/1/2011
(iii) Where appropriate, crash impact dynamics considerations should be taken into account to ensure proper crash resistance and a proper level of occupant safety for an otherwise survivable impact.
(8) The eighth area is the special repair and continued airworthiness requirements of §§ 29.611, 29.1529, and 14 CFR part 29 Appendix A, for composite structures. When repair and continued airworthiness procedures are provided in service documents (including approved sections of the maintenance manual or instructions for continued airworthiness), the resulting repairs and maintenance provisions should be shown to provide structure, which continually meets the guidance of paragraphs (1) through (7) of this AC. All certification- based repair and continued airworthiness standards, limits, and inspections must be clearly stated, and their provisions and limitations clearly documented to ensure continued airworthiness. No composite structural repair should be attempted that is beyond the scope of the applicable approved Structural Repair Manual (SRM) without an engineering design approval by a qualified FAA/AUTHORITY designated representative.
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CHAPTER 2. PART 29
AIRWORTHINESS STANDARDS
TRANSPORT CATEGORY ROTORCRAFT
SUBPART D - DESIGN AND CONSTRUCTION
DESIGN AND CONSTRUCTION - GENERAL AC 29.601. § 29.601 DESIGN.
a. Explanation.
(1) This rule requires that no design features or details be used that experience has shown to be hazardous or unreliable.
(2) Further, the rule requires that the suitability of each questionable design detail and part must be established by tests.
b. Procedures.
(1) This rule is met partially by a review of service history of earlier model rotorcraft, or for a new model, review of service experience of models with similar design features. Specifically, this rule covers “features or details” such as the following:
(i) Seat track-to-seat interface fittings should have adequate locking devices to prevent both premature structural failure and premature unlatching.
(ii) Seat belt and harness should be of a type and construction that service experience has shown to be easy to don, unlatch, and remove. They should also be of a type that is reliable, does not interfere with egress, and does not sustain unnecessary wear and tear under normal operations.
(iii) Metallic parts less than a certain thickness gauge and composite materials less than a certain number of plies should not be used. The minimum thickness and number of plies should be based to a large degree on service experience (normal wear and tear) with similar designs.
(2) The effects of service wear on the loading of critical components should be considered. Flight testing, ground testing, and analyses may be used in these considerations.
(3) Tests are required for details and parts which the applicant chooses to use after questions have arisen concerning their suitability.
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AC 29-2C, Chg 2 4/25/06 AC 29.602 § 29.602 CRITICAL PARTS .
a. Explanation.
(1) Critical parts requirements apply to structural components, rotor drive systems, rotors, and mechanical control systems.
(2) The objective of identifying critical parts is to ensure that critical parts are controlled during design, manufacture, and throughout their service life so that the risk of failure in service is minimized by ensuring that the critical parts maintain the critical characteristics on which certification is based.
(3) Definitions with respect to § 29.602:
(i) The use of the word “could” in paragraph 29.602(a) of the rule means that this failure assessment should consider the effect of flight regime (i.e., forward flight, hover, etc.). The operational environment need not be considered.
(ii) With respect to this rule, the term “catastrophic” means the inability to conduct an autorotation to a safe landing, without exceptional piloting skills, assuming a suitable landing surface is available.
(iii) The use of the word “and” in paragraph 29.602(a) of the rule means the part must have both a catastrophic failure mode together with one or more critical characteristics.
(iv) With respect to this rule, the term “part” means one piece, or two or more pieces permanently joined together.
(v) With respect to this rule, the term “critical characteristic” means any dimension, tolerance, finish, material, or any manufacturing or inspection process, or other feature which cannot tolerate variation from type design requirements and, if nonconforming, would cause failure of the critical part.
(4) Many rotorcraft manufacturers already have procedures in place within their companies for handling “critical parts.” These plans may be required by their dealings with other customers, frequently military (e.g., US DoD, UK MoD, Italian MoD).
Although these plans may have slightly different definitions of “critical parts” which have sometimes been called “Flight Safety Parts,” “Critical Parts,” “Vital Parts,” or “Identifiable Parts,” they have in the past been accepted as meeting the intent of this requirement and providing the expected level of safety. It is acceptable for these plans to use alternative names and terminology provided they meet the intent of this requirement.
b. Procedures. The rotorcraft manufacturer should establish a Critical Parts Plan that identifies and controls the critical characteristics. The policies and procedures which constitute that plan should be such as to ensure that-- Page D - 2
(1) All critical parts of the rotorcraft are identified by means of an appropriate failure assessment and a Critical Parts List is established.
(2) Documentation draws the attention of the personnel involved in the design, manufacture, maintenance, inspection, and overhaul of a critical part to the special nature of the part and details the relevant special instructions. For example all drawings, work sheets, inspection documents, etc., could be prominently annotated with the words "Critical Part" or equivalent and the Instructions for Continued Airworthiness and Overhaul Manuals (if applicable) should clearly identify critical parts and include the needed maintenance and overhaul instructions. The documentation should:
(i) Contain comprehensive instructions for the maintenance, inspection and overhaul of critical parts and emphasize the importance of these special procedures;
(ii) Indicate to operators and overhaulers that unauthorized repairs or modifications to critical parts may have hazardous consequences;
(iii) Emphasize the need for careful handling and protection against damage or corrosion during maintenance, overhaul, storage, and transportation and accurate recording and control of service life (if applicable);
(iv) Require notification of the manufacturer of any unusual wear or deterioration of critical parts and the return of affected parts for investigation when appropriate;
(3) Procedures should be established for identifying and controlling critical characteristics.
(4) To the extent needed for control of critical characteristics, procedures and processes for manufacturing critical parts (including test articles) are defined (for example material source, forging procedures, machining operations and sequence, inspection techniques, and acceptance and rejection criteria). Procedures for changing these manufacturing procedures should also be established.
(5) Any changes to the manufacturing procedures, to the design of a critical part, to the approved operating environment, or to the design loading spectrum are evaluated to establish the effects, if any, on the fatigue evaluation of the part.
(6) Materials review procedures for critical parts (i.e., procedures for determining the disposition of parts having manufacturing errors or material flaws) are in accordance with paragraphs (4) and (5) above.
(7) Critical parts are identified as required, and relevant records relating to the identification are maintained such that it is possible to establish the manufacturing history of the individual parts or batches of parts.
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AC 29-2C, Chg 4 5/1/2014
(8) The critical characteristics of critical parts produced in whole or in part by suppliers are maintained.
AC 29.603. § 29.603 (Amendment 29-17) MATERIALS.
a. Explanation. The rule requires that the suitability and durability of materials, the failure of which could adversely affect safety, must be determined by three-fold considerations:
(1) Considerations based on experience or tests.
(2) Meeting approved specifications.
(3) Taking into account environmental conditions such as temperature and humidity.
b. Procedures.
(1) Experience may be used to show a material’s resistance to wear and deterioration from environmental effects (environmental effects include both natural environmental effects such as exposure to sunlight, water, salt spray, etc., and installation environmental effects such as exposure to fuel, hydraulic fluids, deicing fluids, etc.). Installation environmental e ffects should consider both direct exposure contact and expected migration of potentially deleterious fluids and compounds.
Testing for environmental effects may use either coupon testing, full-scale testing, or a combination. A combination of te sting and experience may also be used.
(i) MIL-HDBK’s-5, -17, and -23 include consideration of some environmental effects and contain reference to additional methods of testing for environmental effects.
(ii) The use of AC 20-107A, Composite Aircraft Structure, is recommended for environmental and damage tolerance considerations of advanced composite materials. (Also see sections 29.573 and AC 29 MG 8 of this AC.)
(iii) The effects of excessive wear and delamination of elastomeric and self-lubricated bearings used in critical load carrying applications in relation to redistribution of loading should be considered.
(2) Where possible, materials that meet widely accepted specifications such as AISI, SAE, MIL, or AMS and alloys which have favorable experience or tests should be used. Where company-developed materials are used, approved specifications are required to ensure the developed properties are duplicated in each lot of material.
Documented specification usage is necessary to maintain quality assurance of materials.
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(3) Section 29.613 concerns strength properties and design values. (See paragraph AC 29.613.)
AC 29.605. § 29.605 (Amendment 29-17) FABRICATION METHODS.
a. Explanation. The basic requirement of this rule is that the methods of fabrication must produce sound structure and produce it consistently.
(1) A process specification is required for fabrication processes requiring close control.
(2) A test program is explicitly required for each new aircraft fabrication method.
b. Procedures.
(1) The approved specifications required by this rule may either be established government/industry specifications such as MIL, AISI, ASTM, or SAE, or the specifications may be company-developed proprietary specifications. Sufficient data should be provided to the FAA/AUTHORITY aircraft engineering offices to show that the desired features are provided by the process specification. In addition, sufficient process controls, inspections, and tests should be coordinated with FAA/AUTHORITY manufacturing inspection personnel to ensure that continued quality of the process is provided.
(2) In addition to the examples given by the rule; i.e., gluing, spot welding, and heat treating process, specifications should also be prepared for types of welding other than spot welding, for platings of metals, for protective finishes (other than decorative), for sealing, and for unique fabrication methods such as those used for composite materials.
(3) The required test programs should consider static strength effects, fatigue strength effects, and environmental effects as appropriate to the processes.
(4) During the fabrication of advanced composite materials, the effects of fabrication anomalies (i.e., disbonds, voids, porosity) should be considered. Special nondestruct testing inspection techniques and procedures should be developed to cover fabrication with allowable anomalies and permitted repair procedures. (See also paragraph AC 29 MG 8.)
AC 29.607. § 29.607 (Amendment 29-5) FASTENERS.
a. Explanation. Section 29.607 of Amendment 29-5 requires dual locking removable fasteners in critical locations. A nonfriction locking device is specifically required in any bolt subject to rotation, as stated in the rules.
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b. Procedures. Advisory Circular 20-71, Dual Locking Devices or Fasteners, December 8, 1970, contains information, procedures, and means of complying with § 29.607 of Amendment 29-5.
AC 29.609. § 29.609 PROTECTION OF STRUCTURE.
a. Explanation. The structure should be suitably protected as specified in the rule to maintain its design strength. Ventilation and drainage provisions must be provided as specified in the rule. Overboard drains should be furnished for corrosive or waste liquids. Drains for flammable fluids are specified in other rules such as §§ 29.999 and
29.1187.
b. Procedures.
(1) The structure may be preserved, painted, or treated with chemical films to protect it from strength deterioration. An approved process specification should be used for these types of treatments.
(2) Parts may be plated or chemically treated, such as anodized, for protection.
An evaluation and substantiation may be required to assure the structure or parts are not adversely affected during, or as a result of, the plating or treatment process.
(§ 29.605 concerns approval of process specifications and fabrication methods.)
(3) Plating or material surface hardness or composition changes may require fatigue substantiation to assure the fatigue strength is not altered or is otherwise properly assessed. An approved process specification should be used for these types of treatments.
(4) To prevent water accumulation, drain holes should be placed at possible dams such as bulkheads, and at low points in the fuselage and in the stabilizing surfaces.
(5) Control tubes and tubes used as primary mount structures (i.e., transmission support structure and engine mount structure) should be designed to prevent entry and collection of corrosive fluids or vapor, including water.
(i) A closed insert in each tube end may be used.
(ii) A s ealant applied around the tube ends and around each rivet head may be used.
(6) Overboar d drains should discharge clear of the entire rotorcraft. Dyed water discharged in flight, may be used to assure fluids are properly drained.
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(7) Welded tubes should be flushed and sealed after welding in accordance with an approved process specification.
(8) Refer to AC 43-4, “Corrosion Control for Aircraft,“ for further procedures.
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AC 29-2C, Chg 4 5/1/2014 AC 29.610. § 29.610 (Amendment 29-53) LIGHTNING AND STATIC ELECTRICITY PROTECTION.
a. Background. During the initial devel opment and promulgation of airworthiness standards, rotorcraft operated primarily in a VFR and non-icing environment. A prudent pilot avoided thunderstorms where the possibility of encountering severe weather and a lightning strike was much greater. Now, many rotorcraft are authorized to fly under IFR and into known icing conditions. Because many rotorcraft now use the same advanced technologies in structures and systems as fixed-wing aircraft, a specific rule on lightning protection of rotorcraft was adopted in Amendment 29-24. Am endment 29-40 revised the title of § 29.610 to include protection from static electricity, and a new § 29.610(d) was added specifying requirements for electrical bonding and protection against lightning and static electricity. Amendment 29-53 removed the system lightning protection requirements for electrical and electronic equipment in § 29.610(d)(4) because those requirements were implemented in the new § 29.1316.
b. Explanation.
(1) The regulation requires protection of rotorcraft from the catastrophic effects of lightning. This means that lightning must not prevent the continued safe flight and landing of the rotorcraft.
(2) Rotorcraft structural components, propulsion system, gearboxes, and mechanical and hydraulic control systems should be designed to ensure lightning will not prevent continued safe flight and landing of the rotorcraft (i.e., damage due to lightning currents that flow through any of the components must not result in catastrophic failure).
c. Procedures.
(1) Certification Plan. A formal written certification plan is an acceptable means to ensure demonstration of regulatory compliance. This plan is also useful to identify and define an acceptable resolution to the critical issues early in the certification process. These are the usual steps to follow when utilizing a certification plan:
(i) Prepare a certification plan that describes the analytical procedures and qualification tests to be utilized to demonstrate lightning and static electricity protection effectiveness. Test proposals should describe the rotorcraft and system to be utilized, test drawing(s) as required, the method of installation that simulates the production installation, the lightning zone(s) applicable, the lightning simulation method(s), test voltage or current waveforms to be used, diagnostic methods, and the appropriate schedules and location(s) of proposed test(s).
(ii) Obtain FAA/AUTHORITY concurrence that the certification plan is adequate.
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