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(1) Considerations based on experience or tests.
(2) By meeting approved specifications.
(3) By taking into account environmental conditions such as temperature and humidity.
b. Procedures.
(1) 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.
(2) Environmental conditions may be taken into account by service experience, coupon testing, full-scale testing, or a combination of testing and experience, MIL-HDBK's -5, -17, and -23 include some environmental effects and contain reference to additional methods of testing for environmental effects.
(3) Section 27.613 concerns strength properties and design values. (See Paragraph 248 of this document.)
243. § 27.605 (Amendment 27-16) 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, ASIM, 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.
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(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.
244. § 27.607 (Amendment 27-4) FASTENERS.
a. Explanation. Section 27.607 of Amendment 27-4 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.
b. Procedures. Advisory Circular 20-71 contains information, procedures, and means of complying with § 27.607 of Amendment 27-4.
245. § 27.609 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 §§ 27.999 and
27.1193.
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 ensure the structure or parts are not adversely affected during, or as a result of, the plating or treatment process.
(§ 27.605 concerns approval of process specifications and fabrication methods.)
(3) Plating or material surface hardness or composition changes may require fatigue substantiation to ensure 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.
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(5) Control tubes and tubes used as primary mount structures (l.e., transmission support structure and engine mount structure) should be designed to prevent entry and collection of corrosive fluids or vapor, including water.
(1) A closed insert in each tube end may be used.
(il) A sealant applied around the tube ends and around each rivet head may be used.
(6) Overboard drains should discharge clear of the entire rotorcraft. Dyed water discharged in flight may be used to ensure fluids are properly drained.
(7) Drains or vents which handle corrosive fumes (such as battery case vent line) may incorporate a container with an agent to neutralize the fumes prior to venting overboard.
(8) Welded tubes should be flushed and sealed after welding in accordance with an approved process specification.
(9) Refer to AC 43-4, "Corrosion Control for Aircraft," for further procedures.
246. § 27,610 (Amendment 27-21) LIGHTNING PROTECTION.
a. Background. During the initial development and promulgation of the standards concerning the airworthiness of rotorcraft, it was not necessary to specify design features that would protect the rotorcraft from the meteorological phenomenon of lightning. This was due, in part, to the fact that rotorcraft were primarily operated in a VFR and nonicing environment. Also, a prudent pilot avoided thunderstorms where the possibility of encountering severe weather and a lightning strike was much greater. The construction, design, and operating environment of civil rotorcraft have changed markedly within the past two decades. Many rotorcraft are now authorized to fly IFR in all types of weather environment. One transport design has been approved for flight into known icing conditions. Additionally, many rotorcraft now use the same advanced technologies in structures and systems as do airplanes. Because of these facts, a specific rule on lightning protection of rotorcraft was adopted in Amendment 27-21. For further information, see the preamble of Amendment 27-21 (49 FR 44433; 11/6/84), Proposal 2-14. Section 27.610 is similar to § 25.581 which applies to the protection of structures of transport airplanes. However, the standard provides for specific protection of the aircraft structures as well as the systems of the rotorcraft. Section 27.610 is the standard referenced in the requirement for lightning protection of systems in § 27.1309(d) (see Paragraph 621 of this AC). In addition, the protection of fuel systems from the effects of lightning is found and referenced in Report DOT/FAA/CT-83/3, User Manual for AC 20-53A, Protection of Airplane Fuel Systems Against Fuel Vapor Ignition Due to Lightning, dated April 12, 1985.
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b. Explanation.
(1) The regulation requires that the rotorcraft must be protected against the catastrophic effects of lightning. This means that a lightning strike encounter should not prevent the continued safe operation of the rotorcraft.
(2) Paragraph 621 of this AC addresses the protection required for systems.
The protection of the rotorcraft structures may be accomplished in a similar fashion.
(3) The structural components of the rotorcraft should be designed in such a manner that the lightning current may be safely diverted or conducted through the rotorcraft without damaging any critical structure or without causing damage to noncritical structure, the failure of which would preclude the continued safe flight and landing of the rotorcraft. A radome or fin cap which explodes due to a lightning strike and results in catastrophic damage to main or tail rotors is a scenario of lightning damage to a noncritical structure which has catastrophic results.
(4) This type of strike effect on the rotorcraft is generally referred to as direct effects. Direct effects are damage which includes the burning, eroding, blasting, or structural deformation produced by the high currents of the lightning flash passing through the rotorcraft structure.
c. Procedures.
(1) Certification Plan. Although not a regulatory requirement, it is recommended that a formal written certification plan be used to assure 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 test(s).
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, 304 Par 246
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.
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.
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.
(i¡) 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.
(ili) 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) Fuel Systems. Refer to Report DOT/FAA/CT-83/3 referenced in Paragraph 246a. For additional information on the lightning protection requirements for Par 246 305
fuel systems for rotorcraft with a certification basis which includes Amendment 27-23 refer to Paragraph 449 of this AC.
247. § 27.611 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 (§ 27.1541(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:
(i) Reviewing type design drawings.
(ii) Conformity inspections accomplished during certification testing.
(ili) Be evaluated during the control system proof and operation tests (§§ 27.681 and 27.683).
(iv) During type inspection tests and functioning and reliability testing.
(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.
248. § 27.613 (Amendment 27-16) 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.
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(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 § 27.613(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 § 27.613(a) and (d).
(3) Section 27.613(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 27.613(c) is an objective rule concerning minimizing fatigue failures and § 27.571 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 are also contained in this chapter.
(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-5.
(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.
Selection of materials with stress levels to 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.
(iii) Use of composite (multielement) duplicate structures so that a fatigue crack or failure occurring in one element of the composite (multielement) member will Par 248 307
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.
(v) 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 27 as a unique part of the aircraft type design.
248A. § 27.613 (Amendment 27-26) MATERIAL STRENGTH PROPERTIES AND DESIGN VALUES.
a. Explanation. Amendment 27-26 added explicit probability standards criteria to § 27.613(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 248 of this AC 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 § 27.613(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 308 Par 248
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 a minimum allowable strength, on the drawing.
249. § 27,619 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 § 27.303 is multiplied by a special factor as specified in the rule.
(2) Specific factors are prescribed for castings and fittings in §§ 27.621 and
27.625, respectively. Factors may be prescribed for bearings with free clearance as stated in § 27.623. In addition, any other factor may be prescribed "to ensure that the probability of the part being understrength because of the uncertainties specified in § 27.619(a) is extremely remote."
b. Procedures.
(1) One example of fitting factor use follows:
1,000-pound limit design load × 1.15 fitting factor x 1.5 ultimate load factor equals 1,725-pound ultimate design load.
(2) Other specific factors may be similarly applied. Refer to §§ 27.623 and
27.625.
(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 §$ 27.603 and 27.613. Regardless of the rule invoked, the variability of the material and/or assembly properties should be accounted for.
250. § 27.621 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 Par 248A 309
АС 27-1A 7/30/97 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 § 27.603, § 27.605, or § 27.613, 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 § 27.623 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.
(5) Structural static and fatigue substantiation, by test or analysis, is 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 § 27.621(c) and (d) are shown in the following chart:
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INSPECTION REQUIREMENTS
CRITICAL CASTINGS NONCRITICAL CASTINGS <(2)> <(3)>
CASTING
FACTOR FAA REQUIRE- OTHER FAA REQUIRE- OTHER RANGE MENT 27.621(c) CLASSIFICATION MENT 27.621(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, ref. § 27.613.
<(2)> Critical castings are those castings whose failure would preclude continued safe flight and landing or result in injury to any occupant, ref.
§ 27.621(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 magnetic particle or penetrant inspection or approved equivalent methods.
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<(6)> Each casting shall receive 100 percent visual and reduced radiographic and magnetic particle or penetrant inspection or approved equivalent methods.
<(7)> Each casting shall receive 100 percent inspection by visual, radiographic, and magnetic particle or penetrant inspections or approved equivalent methods.
<(8)> Three sample castings shall be static tested and shown to meet:
No failure at 1.25 x 1.5 x limit load, and no yielding at 1.15 x limit load.
<(9)> Castings shall be procured to a specification that guarantees the mechanical properties of the material in the casting and provides demonstration of these properties by test of coupons cut from the castings on a sampling basis.
This chart may be included in the casting test proposal report. It is recommended that the applicant include in the test proposal report additional information such as shown in Paragraph 250b(3).
(3) The casting test report may include the following sections or items in a Part I of the report. The report may also have a Part II that contains the test results as shown in the following example report. The following sections are a recommended format content of the report. Appropriate changes should be made as desired to accommodate the applicant's system.
EXAMPLE OF REPORT
INTRODUCTION This report presents the proposal for the static test of the castings used on the Model XYZ. The castings will be tested in compliance with Federal Aviation Regulations § 27.621. The purpose of this test is to substantiate the structural strength of the castings used on the Model XYZ. Part Il of this report, which will be published after static tests have been completed, will present test results.
All test specimens will be selected as radiographic standards of acceptance for the particular castings (see Test Specimen). Additional information on selecting the specific castings may be included in the test specimen section of this report.
Load sheets giving direction and magnitude of loads for each of the castings are presented in numerical order by part number at the end of this report. The test loads 312 Par 250