Page D - 11
(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.
(ii) Intermittent Contact . The design should ensure that random intermittent contact between metallic and/or metallized parts (such as could cause unwanted radio interference or degradation of the components due to sparking) will not occur.
(iii) 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 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 properti es 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 ar e 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
Page D - 12 than 3mm2 except that, where a single conductor is likely to carry the whole discharge from an isolated section, the cross-sectional area should be not less than 6mm2.
Aluminum primary conductors should have a cross-sectional area giving an equivalent surge carrying capacity.
(iii) Primary bonding paths should be used for--
(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 dive rters on external non-metallic parts.
(iv) Where additional conductors ar e 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:
(i) 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 surfac es 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 wh ich 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.
Page D - 13
(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.
AC 29.611. § 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 a ccess 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” acce ss 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 a ccomplished during certification testing.
(iii) Be evaluated during the c ontrol system proof and operation tests (§§ 29.681 and 29.683).
(iv) During type inspection tests and functioning and reliability testing.
Page D - 14
(2) Equipment requiring frequent inspec tions (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.
AC 29.613. § 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 certai n 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).
Page D - 15
(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 select ion 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 AC 29.571, § 29.571, 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 is also contained in this chapter.
(3) Design values and properties mu st 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. (See paragraph AC 29.571 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).
(ii) Use of multipath constructi on 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.
AC 29-2C, Chg 1 2/12/03 Page D - 16
(iv) Use of backup structure wher ein 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 pressuri zed 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.
AC 29.613A. § 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 AC 29.613 still apply, except:
(1) Probability criteria common with MI L-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 29 MG 8.)
(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.
Page D - 17 AC 29.619. § 29.619 SPECIAL FACTORS .
a. Explanation .
(1) This is a general rule to comple ment 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 §§ 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.
(2) Other specific factors may be sim ilarly 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 for.
(5) Ground resonance can occur due to fl exibility 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).
AC 29.621. § 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
Page D - 18 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 ar e 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.
(5) Structural static and fatigue subst antiation, 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 cla ssified 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 associat ed 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 RANGE <(1)>
FAA REQUIRE- MENT 29.621(c)
OTHER
CLASSIFICATION
FAA REQUIRE- MENT 29.621(d)
OTHER
CLASSIFICATION
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, reference § 29.613.
<(2)> Critical castings are those ca stings whose failure would preclude continued safe flight and landing or result in injury to any occupant, reference § 29.621(c).
<(3)> Noncritical castings are ca stings 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.
Page D - 20 <(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 AC 29.621b(3).
(3) The casting test report may incl ude 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, Part 29, § 29.621. The purpose of this test is to substantiate the structural strength of the castings used on the Model XYZ. Part II 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 and design criteria for the castings are discussed in detail in the test loads section of this report.
Page D - 21 The test loads will be applied and reacted using mating aircraft parts or special fixtures which simulate the mating parts. The methods and apparatus to be used for the static tests of the castings are discussed in the apparatus and method section of this report.
Testing will be conducted in…(location).
TEST SPECIMEN
The castings which will be tested are listed in numerical order in figure AC 29.621-2.
Those castings which, after structural analysis, show less than a 1.5 casting factor will be tested. All directions are given with reference to a forward facing position in the rotorcraft.
On the basis of a radiographic examination, the three castings which are of the poorest acceptable quality in the first production lot of castings will be selected as test specimens. The poorest of the three castings will be selected as the initial test casting and its radiograph or ASTM standard will be used as the standard for accepting future castings of the particular part unless later standards are approved. Three castings must be tested for each critical condition for each part.
Conformity Inspection
Each machined casting will be subjected to an FAA/AUTHORITY conformity inspection prior to testing to determine compliance with the type design drawings. A conformity report for each casting may be incorporated in Part II, Test Results, of this report.
The test specimen will be permanently marked or defaced after testing to preclude its use on a rotorcraft.
See figure AC 29.621-2 for an example of a convenient means of listing castings.
TEST LOAD
The test load(s) to be applied to each casting represents the critical loading condition(s) for that casting. The critical conditions on each of the castings were determined by the design criteria and substantiating data approved by the FAA/AUTHORITY.
The design criteria for all of the castings to be static tested may fall into one of two categories. The load factors and structural acceptability requirements for each category are discussed below. Casting factors that are included on the load sheets of each part do not apply in the discussion below. (See paragraph AC 29.621b(2) for casting factors.)
Page D - 22 Castings Designed to Limit Load Conditions
A structural analysis of each test casting showing the critical design limit load conditions is given in the data, (reference report number here). The load factors for the static test of the castings are as follows:
1.15 x design limit load = design yield load
1.50 x design limit load = design ultimate load
Castings Designed Only to Crash Landing Conditions
The castings in this category were designed using a crash landing load factor for the design ultimate load. The design yield load criteria of 1.15 x limit load need not apply to these castings. The test loads for these castings may be given in terms of design ultimate load on the individual casting load sheets shown in Part I of this report.
Test Procedures
Depending on the results of the initial static test of each casting, the following procedure will be used.
a. If in the initial test of critical ca stings the casting is found to have a casting factor of 1.5 (1.5 x design ultimate load), the casting will be considered acceptable and no further tests will be conducted.
b. If in the initial test(s) the critical casting is found to have a casting factor less than 1.5 but equal to or greater than 1.25, two additional castings will be tested for each critical load condition. Each must also show a minimum casting factor of 1.25.
c. If in the initial test, or in one of tw o additional tests, a casting shows a casting factor less than 1.25 times design ultimate or yields prior to reaching 1.15 times design limit load, the casting will be redesigned and retested. The yield criteria are also applicable to the first two procedures with the exception of critical castings designed to crash landing conditions.
TEST APPARATUS AND METHOD
The Model XYZ casting static tests will be conducted using fixtures designed to simulate the installation of the castings in the aircraft. Where practical, mating aircraft parts will be used to apply and react test loads. When practical, the static tests will be conducted with mating castings assembled when the critical loads for the mating castings are compatible; otherwise, fixtures simulating the mating parts may be designed and fabricated for the tests. Assembly hardware used to mount test castings will be the same as hardware used on the rotorcraft. All bolt torques and other assembly notes will conform to the type design assembly instructions.