c. Procedures.
(1) Certification Plan. Although not a regulatory requirement, it is recommended that a formal written certification plan be used to ensure regulatory compliance. The use of this plan is beneficial to both the applicant and the FAA/AUTHORITY because it identifies and defines an acceptable resolution to the critical issues early in the certification process. These are the usual steps to be followed when utilizing a certification plan:
(i) Prepare a certification plan which describes the analytical procedures and/or the qualification tests to be utilized to demonstrate 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 tests).
NOTE: The recommended reference for quantification of the lightning environment, the determination of the aircraft lightning strike zones, and the determination of appropriate test methods is SAE AE4L Committee Report, dated June 20, 1978, Lightning Test Waveforms and Techniques for Aerospace Vehicles and Hardware. Additionally, information may also be found in the NASA publication No. RP-1008, Lightning Protection of Aircraft.
(ii) Obtain FAA/AUTHORITY concurrence that the certification plan is adequate.
(iii) Obtain FAA/AUTHORITY detail part conformity of the test articles and installation conformity of applicable portions of the test setup. Obtain FAA/AUTHORITY approval of the test proposal. A comprehensive test proposal may be used.
(iv) Schedule FAA/AUTHORITY witnessing of the test or tests proposed.
(V) Submit a test report describing all results and obtain FAA/AUTHORITY approval of each report prepared.
(2) Test Conditions. Refer to SAE AE4L Committee Report, dated June 20, 1978, and the NASA publication noted in Paragraph c(1)(i) to determine the appropriate test parameters.
(3) Aircraft Design Features and Criteria. MIL-B-5087B, Amendment 2, or later amendment contains valuable information to assist the designer. Figure 6 in the specification contains fault current versus bond resistance information. Refer to the NASA publication noted above also.
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Aluminum wire screen or mesh applied to the control or stabilizing surface and electrically bonded at each joint or juncture has been successful in conducting the current without serious damage.
(її) Metal skin surfaces combined with surface wire screen or mesh have been successful. Also, successful use of surface treatment has been reported. For composites, treatments such as the following have been used: flame spray coatings, aluminized glass, metal foil, metallized fabrics, and conductive paint.
(iii) Ball or roller bearings may be used to conduct the current at rotating joints. However, increased friction or possible seizure of the bearing may occur. The potential for this should be evaluated. Inspection and replacement criteria for possible damage should be addressed in the manual for continued airworthiness. Bearings are especially susceptible to pitting and internal arcing.
(iv) Report DOT/FAA/CT-86/8, April 1987, Determination of Electrical Properties of Grounding, Bonding, and Fastening Techniques for Composite Materials, may assist the applicant.
(4) Euel Systems. Refer to Report DOT/FAA/CT-83/3 referenced in Paragraph 245a. For additional information on the lightning protection requirements for fuel systems for rotorcraft with a certification basis which includes Amendment 29-26 refer to Paragraph 449 of this AC.
d. Aircraft Design Criteria for Lightning and Static Electricity Protection.
(1) Lightning Protection.
General. The rotorcraft structure should be provided with means to conduct lightning so that the rotorcraft and its occupants will not be endangered.
(ii) Rotors and Control Systems.
(A) It should be established that an adequate primary bonding path exists between the rotors and the airframe, such that a lightning strike on a rotor will not result in damage to or seizure of gearbox or swashplate bearings, control jacks, etc.
(B) Each hinge and bearing of rotor blades and control surfaces should either-
(1) Be of a type that is capable of withstanding a lightning discharge without damage or seizure leading to loss of function, or
(2) Be provided with at least one primary bonding conductor.
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Where bonding conductors are provided, they should be as flexible and short as possible and should be installed so that there is no danger of the conductor jamming the hinge or bearing, particularly if partially disrupted by a lightning strike.
(IiI) External Non-metallic Parts.
(A) Where non-metallic parts are fitted externally to the rotorcraft
(e.g., rotors, radomes, composite skin panels) and are subjected to lightning, they should be protected against the following risks:
(1) The disruption of the materials because of rapid expansion of gases within them (e.g., water vapor);
(2) The rapid build-up of pressure in voids or in the enclosure provided by the parts resulting in mechanical disruption of the parts themselves or of the structure enclosed by them;
(3) Fire caused by the ignition of the materials themselves or of the materials contained within the enclosures.
(B) Materials used for external non-metallic parts should have low water-absorption characteristics, should not occlude gases, and should be of high dielectric strength in order to encourage surface flashover rather than puncture.
(C) Rotors and other external parts of nonmetallic construction should be provided with effective lightning diverters and/or primary conductors, which are capable of safely carrying lightning discharges, unless it can be shown that damage due to lightning discharge will not endanger the rotorcraft or its occupants.
(D) In some cases (e.g. radomes and rotors), confirmatory tests may be required to check the adequacy of the lightning protection provided.
(2) Characteristics of Lightning Discharges. The data contained in FAA AC 20-53 should be used for the purpose of assessing the adequacy of lightning discharge protection of rotorcraft.
(3) Protection Against the Accumulation of Static Charges.
(i) General. All items, which by the accumulation and discharge of static charges may cause a danger of electrical shock, ignition of flammable vapors or interference with essential equipment (e.g. radio communications and navigational aids) should be adequately bonded to the main aircraft grounding system.
Intermittent Contact. The design should ensure that random intermittent contact between metallic and/or metallized parts (such as could cause Par 245 367
unwanted radio interference or degradation of the components due to sparking) will not occur.
(ili) High Pressure Refueling and Fuel Transfer. Where provision is made for high pressure refueling and/or high rates of fuel transfer, it should be established, by test, or by consultation with the appropriate fuel manufacturers, that dangerously high voltages will not be induced within the fuel system. If compliance with this requirement involves any restriction on the types of fuel to be used or on the use of additives, an appropriate operating limitation should be established under FAR 29.1501(a). The critical refueling rates are related to the rotorcraft refueling installations, and the designer should seek the advice of fuel suppliers on this problem.
(A) With standard refueling equipment and standard aircraft turbine tuels, voltages high enough to cause sparking may be induced between the surface of the fuel and metal parts of the tank at refueling rates above approximately 250 gal/min.
These induced voltages may be increased by the presence of additives and contaminants (e.g., anti-corrosion inhibitors, lubricating oil, free water) and by splashing or spraying of the fuel in the tank.
(B) The static charge can be reduced as follows:
(1) By means taken in the refueling equipment such as increasing the diameter of refueling lines and designing filters to give the minimum of electrostatic charging, or
(2) By changing the electrical properties of the fuel by the use of anti-static additives and thus reducing the accumulation of static charge in the tank to a negligible amount.
(4) Primary and Secondary Bonding Paths.
(i> Primary bonding paths are those paths that are required to carry lightning discharge currents. These paths should be of as low an electrical impedance as is practicable. Secondary bonding paths are those paths provided for other forms of bonding.
(ii) Where additional conductors are required to provide or supplement the inherent primary bonding paths provided by the structure or equipment, the cross-sectional area of such primary conductors made from copper should be not less than 3mm? except that, where a single conductor is likely to carry the whole discharge Trom an isolated section, the cross-sectional area should be not less than 6mm?
Aluminum primary conductors should have a cross-sectional area giving an equivalent surge carrying capacity.
(Ili) Primary bonding paths should be used for-- 368 Par 245
(A) Connecting together the main grounding points of separable major components which may carry lightning discharges, (B) Connecting engines to the main aircraft ground, (C) Connecting to the main aircraft ground all metal parts presenting a surface on or outside of the external surface of the rotorcraft, and (D) Conductors and lightning diverters on external non-metallic parts.
(iv) Where additional conductors are required to provide or supplement the inherent secondary bonding paths provided by the structure or equipment, the cross-sectional area of such secondary conductors made from copper should be not less than 1mm2.
(5) Resistance and Continuity Measurements.
Measurements should be made to determine the efficacy of the bonding and connection between at least the following:
(1) Primary Bonding Paths (A) The extremities of the fixed portions of the rotorcraft and such fixed external panels and components where the method of construction and/or assembly leads to doubt as to the repeatability of the bond, e.g., removable panels.
(B) The engines and the main aircraft ground.
(C) External movable metal surfaces or components and the main aircraft ground.
(D) The bonding conductors of external non-metallic parts and the main aircraft ground.
(E) Internal components for which a primary bond is specified and the main aircraft ground.
(ii) Secondary Bonding Paths.
(A) Metallic parts, normally in contact with flammable fluids, and the main aircraft ground.
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(B) Isolated conducting parts subject to appreciable electrostatic charging and the main aircraft ground.
(C) Electrical panels and other equipment accessible to the occupants of the rotorcraft and the main aircraft ground.
(D) Grounding connections that normally carry the main electrical supply and the main electrical return. The test on these connections should be such as to ensure that the connections can carry, without risk of fire or damage to the bond, or excessive volt drop, such continuous normal currents and intermittent fault currents as are applicable.
(E) Electrical and electronic equipment and the main earth, where applicable, and as specified by the rotorcraft manufacturer.
(F) Static dischargers and the main rotorcraft structure.
246. § 29.611 INSPECTION PROVISIONS.
a. Explanation. The rotorcraft must have access panels, or openings, that will allow for proper maintenance and/or adjustment of the rotorcraft systems.
(1) The rule states: There must be means to allow close examination of each part that requires recurring inspection, adjustment for proper alignment and functioning, or lubrication.
(2) "Structural" or load-carrying access panels may be used to comply with the rule. Structural panels should have stencils or permanent labels (§ 29.1541(a)(2)) stating the panels must be installed prior to ground or flight operation.
(3) Holes or "nonstructural" access panels should be used whenever possible.
b. Procedures.
(1) The determination of compliance can be accomplished in conjunction with the following activities:
(1) Reviewing type design drawings.
(ii) Conformity inspections accomplished during certification testing.
(iii) Be evaluated during the control system proof and operation tests (§§ 29.681 and 29.683).
(iv) During type inspection tests and functioning and reliability testing.
370 Par 245
(2) Equipment requiring frequent inspections (at less than 25-hour intervals), lubrication, or adjustments should be accessible through "nonstructural" doors. Areas or items requiring daily attention should be accessible through "nonstructural" doors since properly rated maintenance personnel are required to "open and close," or reinstall structural panels and special design features, such as multiple pins and latches, are generally necessary for structural doors.
(3) If the rotorcraft is subject to an FAA Maintenance Review Board Approval Program, further review of the rotorcraft inspection provisions will be obtained.
247. § 29.613 (Amendment 29-17) MATERIAL STRENGTH PROPERTIES AND DESIGN VALUES.
a. Explanation. The rule requires the use of materials that have a known minimum strength value. The structure must not be understrength and must be designed to minimize fatigue failure.
(1) Material design values in certain specified documents may be used. The FAA/AUTHORITY may approve other material design values thus allowing the applicant greater flexibility in selection of materials by proving their strength properties and design values as stated in § 29.613(d).
(2) Other materials that may be new or are not included in the specified documents may be tested and design values established as provided by § 29.613(a) and (d).
(3) Section 29.613(d) requires the selection of materials that will retain design values and properties in the type of service environment and for the length of service time intended for the structure.
(4) Section 29.613(c) is an objective rule concerning minimizing fatigue failures. Paragraph 230, § 29.571, of this advisory circular, concerns quantitative fatigue substantiation requirements.
b. Procedures.
(1) The properties and design values in the documents noted in the rule may be used.
(2) MIL-HDBK-5, Metallic Materials and Elements for Flight Vehicle Structure, Chapter 9, contains procedures for establishing design values of additional materials.
Uniform means of presenting the data is also contained in this chapter.
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(3) Design values and properties must include effects of the service environment and service time. An example is exposure at elevated temperatures on the ultimate tensile strength of 7079-T6 aluminum alloys as found in Figure 3.7.4.1.1(c) of MIL-HDBK-5C.
(4) The probability of disastrous fatigue failures must be minimized. This may be accomplished by using design features usually identified as fail-safe features, such as the following, which were obtained from Advisory Circular 20-95. See Paragraph 230 of this document for the fatigue requirement information.
(i) Selection of materials and stress levels that provide a controlled slow rate of crack propagation combined with high residual strength after initiation of cracks (lightly loaded structures).
(її) Use of multipath construction and the provision of crack stoppers to limit the growth of cracks.
(ili) Use of composite (multielement) duplicate structures so that a fatigue crack or failure occurring in one element of the composite (multielement) member will be confined to that element and the remaining structure will still possess adequate load-carrying ability.
(iv) Use of backup structure wherein one member carries all the load, with a second member available and capable of assuming the extra load if the primary member fails.
Design to permit detection of cracks including the use of crack detection systems, in all critical structural elements before the cracks can become dangerous or result in appreciable strength loss, and to permit replacement or repair.
(5) Acceptable standards for pressurized containers or cylinders, such as cylinders of nitrogen, used to inflate emergency floats may be found in 49 CFR 178 Subpart C, §§ 178.36 through 178.68. Specifically, § 178.44 concerns standards for steel cylinders used in aircraft that are subjected to at least 900 PSI service pressure.
This standard includes strength, test, material property, inspection, quality, design features, identification and inspection report requirements. As an example, § 178.44-14, entitled "Hydrostatic Test," requires that each cylinder must be (proof) tested to at least 5/3 times the service pressure. Section 178.44-16, entitled "Burst Test," also states that one cylinder taken at random out of each lot of cylinders shall be hydrostatically tested to destruction.
(6) Other design criteria may be developed and approved under the provisions of FAR Part 29 as a unique part of the aircraft type design.
372 Par 247
247A § 29,613 (Amendment 29-30) MATERIAL STRENGTH PROPERTIES AND DESIGN VALUES.
a. Explanation. Amendment 29-30 added explicit probability standards criteria to § 29.613(b). This amendment also provided for testing or proving the strength of selected individual items rather than conducting coupon tests to develop generic material strength properties that would be used for design purposes.
b. Procedures. The basic procedures of Paragraph 247 still apply, except:
(1) Probability criteria common with MIL-HDBK-5D are explicitly allowed to determine strengths for metallic materials whose data are not available in MIL-HDBK-5D. These specific probability criteria should be used in conjunction with MIL-HDBK-17B whenever determining material strength properties for non-metallics.
(Also, reference Paragraph 788 of this AC.)
(2) New § 29.613(e) provides for the premium selection of materials. The premium selection of materials method uses a specimen from each individual item (part) to determine its properties before its use is allowed. This is a highly specialized and possibly costly method which applies only to parts that have areas available from which specimens can be obtained without destroying the part. The rotorcraft type design data of those parts made from premium selection should have the necessary information, such as minimum allowable strength, on the part drawing.
248. § 29.619 SPECIAL FACTORS.
a. Explanation.
(1) This is a general rule to complement other rules. Special factors are employed for reasons cited in the rule to ensure an airworthy aircraft structure. The
1.5 ultimate load factor in § 29.303 is multiplied by a special factor as specified in the rule.
(2) Specific factors are prescribed for castings and fittings in 99 29.621 and
29.625 respectively. Factors may be prescribed for bearings with free clearance as stated in § 29.623. In addition, any other factor may be prescribed "to ensure that the probability of the part being understrength because of the uncertainties specified in § 29.619(a) "is extremely remote."
b. Procedures.
(1) One example of fitting factor use follows:
1,000 pounds limit design load x 1.15 fitting factor x 1.5 ultimate load factor equals 1,725 pounds ultimate design load.
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(2) Other specific factors may be similarly applied. Refer to §§ 29.623, 29.625,
29.685, and 29.785.
(3) Other factors may be imposed as cited in the rule. Advisory Circular 20-107, Paragraphs 5 and 6, are examples of requiring tests of component and subcomponent structure to account for variability of strength and stiffness of composite structures. Factors appropriate for the particular design are obtained and used in substantiation of the composite structure.
(4) The rule complements §$ 29.603 and 29.613. Regardless of the rule invoked, the variability of the material and/or assembly properties must be accounted
(5) Ground resonance can occur due to flexibility in the rotor pylon restraint system as well as with landing gear flexibilities. This evaluation should include variations in stiffness and damping of the rotor pylon restraints that may occur in service (reference "Ground Vibrations of Helicopters," M.L. Deutsch, JAS, Vol. 13, No. 5, May 1946).
249. § 29.621 CASTING FACTORS.
a. Explanation. Casting design, test, and inspection criteria are included in this rule for critical and noncritical structural castings. Hydraulic or other fluid containers are not subjected to "structural loads" but are subject to pressure testing as a part of hydraulic or other flight systems. Critical and noncritical castings are defined in the rule.
(1) Factors, tests, and inspections are specified for structural castings.
Additional factors, tests, and inspections may be applied, as prescribed by §§ 29.603,
29.605, or 29.613, for foundry quality control.
(2) For castings that have surfaces subject to bearing structural design loads, the casting factor need not exceed 1.25 with respect to bearing stresses and need not be used with respect to the bearing surfaces if the bearing factor of § 29.623 exceeds the applicable casting factor.
(3) Critical castings must have a casting factor not less than 1.25 and must receive 100 percent inspection as specified including radiographic inspection. Static test requirements are also specified in addition to the inspection requirements.
(4) Noncritical structural castings may have a casting factor as small as 1.0 with attendant increased inspection and quality control requirements. Use of larger casting factors reduces the inspection and quality control requirements.
374 Par 248
(5) Structural static and fatigue substantiation, by test or analysis, are still required in addition to any casting static tests required by this rule.
b. Procedures.
(1) The rotorcraft castings should be classified as critical, or noncritical, or nonstructural, or fluid container as soon as possible in the certification program. The applicant should then be prepared to propose the tests required for certification.
(2) The casting factors and associated inspection requirements dictated by § 29.621(c) and (d) are shown below:
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INSPECTION REQUIREMENTS
CRITICAL CASTINGS NONCRITICAL CASTINGS <(2)> <(3)>
CASTING
FACTOR FAA REQUIRE- OTHER FAA REQUIRE- OTHER RANGE MENT 29.621(c) CLASSIFICATION MENT 29.621(d) CLASSIFICATION <(1)>
2.01 OR <(7)> <(4)>
GREATER
1.50 TO <(7)> <(5)>
2.00
1.250 TO <(7)> <(8>> <(6)>
1.499
1.00 TO NOT NOT <(7)> <(8)>
1.249 ALLOWED ALLOWED <(9)> <(1)> Ultimate load = Casting factor x 1.5 x limit load. CAUTION: For casting factor range of 1.25 to 1.5 see yield test requirements of NOTE 8.
The mechanical properties to be used for analysis shall be based on the tabulated values of MIL-HDBK-5 or other approved sources, reference § 29.613.
<(2)> Critical castings are those castings whose failure would preclude continued safe flight and landing or result in injury to any occupant, reference § 29.621(c).
<(3)> Noncritical castings are castings other than those defined by NOTE 2.
<(4)> Each casting shall receive 100 percent visual inspection.
<(5)> Each casting shall receive 100 percent visual and reduced 376 Par 249