AC 27-1B, Chg 4 3/31/2014 AC 27.610. § 27.610 (Amendment 27-46) LIGHTNING AND STATIC ELECTRICITY PROTECTION.
Background. During the initial development 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 27-21. Amendment 27-37 revised the title of § 27.610 to include protection from static electricity, and a new § 27.610(d) was added specifying requirements for electrical bonding and protection against lightning and static electricity. Amendment 27-46 removed the system lightning protection requirements for electrical and electronic equipment in § 27.610(d)(4) because those requirements were implemented in the new § 27.1316.
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 operation 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 these components must not result in catastrophic failure).
c. Procedures.
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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(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) Lightning Environment and Zones. AC 20 -155 refers to SAE documents ARP5412 (or EUROCAE ED-84) and ARP5414 (or EUROCAE ED-91), which provide acceptable definitions for the rotorcraft lightning environment and for rotorcraft lightning attachment zones.
(3) Testing. Tests may be required to check the adequacy of the lightning and electrostatic charge protection. Refer to SAE ARP5416 (or EUROCAE ED-105) for acceptable test methods to show that the lightning protection is effective and to SAE ARP5672 for acceptable test methods to show rotorcraft electrostatic charge control.
(4) Aircraft Lightning Protection Design Features. The Aircraft Lightning Protection Handbook (DOT/FAA/CT-89/22) provides information on aircraft lightning protection design. The following are examples where lightning protection should be considered.
(i) Rotors and Control Systems.
(A) It should be established that an adequate bonding path exists between the rotors and the airframe, such that a lightning strike to 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 capable of withstanding lightning without damage or seizure leading to loss of function, or
(2) be provided with at least one bonding conductor, as flexible and short as possible and installed so that there is no danger of the conductor jamming the hinge or bearing following a lightning strike.
(ii) External Non-Metallic Parts.
(A) Where non-metallic parts are fitted externally to the rotorcraft (e.g., rotors, radomes, composite skin panels) and may be subjected to lightning, they should be protected. The protection should consider disruption of the materials because of:
(1) rapid expansion of gases within them (e.g., water vapor), Page D - 9
AC 27-1B, Chg 4 3/31/2014
(2) 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, and
(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 conductors that are capable of safely carrying lightning current, unless it can be shown that damage due to lightning will not endanger the rotorcraft or its occupants. Bonding straps and leads are not required for small gaps between metallic structure and diverters in non-conducting panels in order to comply with the lightning protection criteria. However, an electrical bonding path may be required to achieve static electricity protection.
(5) Protection Against the Effects of Static Electricity.
(i) General. Rotorcraft structure, rotor systems, and equipment should be electrically bonded together to minimize the accumulation and discharge of electrostatic charge, which could result in electrical shock, ignition of flammable vapors, or interference with essential equipment such as radio communications and navigational aids.
(ii) Intermittent Contact. The design should prevent random intermittent contact between metallic or metallized parts such that unwanted radio interference or degradation of the components due to sparking will not occur.
(iii) Rotors and other external parts of nonmetallic construction should be provided with effective electrical conductors that are capable of safely conducting electrostatic charge.
(iv) 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 § 27.1501(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.
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(A) With standard refueling equipment and standard aircraft turbine fuels, 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 antistatic additives and thus reducing the accumulation of static charge in the tank to a negligible amount.
(6) Fuel Systems. Requirements for lightning protection for fuel systems are in § 27.954. AC 20-53 provides guidance on compliance with § 27.954. Section 27.954 of this AC addresses the protection required for systems.
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AC 27-1B, Chg 4 3/31/2014 AC 27.611. § 27.611 INSPECTION PROVISIONS.
a. Explanation. The rotorcraft must have access panels or openings that wil l 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.
(iii) 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.
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AC 27.613. § 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.
(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:
(i) 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).
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AC 27-1B, Chg 4 3/31/2014
(ii) 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 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. Specifical ly, § 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.
AC 27.613A. § 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 AC 27.613 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 AC 27 MG 8).
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(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 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.
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AC 27-1B, Chg 4 3/31/2014 AC 27.619. § 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.
[Section AC 27.619 continued on next page.] .
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(1) One example of fitting factor use follows:
1,000-pound limit design load x 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.
AC 27.621. § 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 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.
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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 RANGE <(1)> FAA
REQUIRE- MENT
27.621(c) OTHER
CLASSIFICATI ON FAA
REQUIRE- MENT
27.621(d) OTHER
CLASSIFICATI ON
2.01 OR
GREATER <(7)> <(4)>
1.50 TO
2.00 <(7)> <(5)>
1.250 TO
1.499 <(7)> <(8)> <(6)>
1.00 TO
1.249 NOT
ALLOWED NOT
ALLOWED <(7)> <(8)> <(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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