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:
310 Par 250
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.
Par 250 311
<(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
7130/97 AC 27-1A and design criteria for the castings are discussed in detail in the test loads section of this report.
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 Table I. 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 Table I 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 250b(2) for casting factors.)
Par 250 313
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 castings 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 tests) 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 two 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.
314 Par 250
7/30/97 АС 27-1A The tests will be conducted using calibrated load measuring devices such as hydraulic cylinders and pressure gages, load cells, strain gage bridges, or dead weights.
Deflections of the casting may be measured using graduated dial indicators or scales in all tests. The deflection indicators will be based or mounted on the casting and will measure casting deflection only when possible; otherwise, the indicators will be based on the fixture and measure deflection of the casting relative to the fixture. Deflection readings will be made at 20 percent increments of limit load through 100 percent of limit load and at 115 percent of limit load. These increments may be changed if necessary.
Permanent deformation readings will be made after relieving 115 percent and 150 percent of limit load.
See Figure 250-1 as an example of a load sheet.
Par 250 315
P +5,000 # TENS.
ULTIMATE LOADS -1,000 # COMP Tension Comp.
Tension Reaction Compression Reaction FIGURE 250-1
EXAMPLE OF CASTING LOAD SHEET
RETRACT ACTUATOR SUPPORT - LANDING GEAR 316 Par 250
Include spherical bearing with clamped-up bolt and a link in the test setup to confirm the stability. Loads are based on a jam condition with actuator operating at 1,700 PSI pressure maximum.
A 1.25 casting factor is included in these loads.
These loads were derived from data in approved structural loads and analysis report.
END OF SAMPLE REPORT
(4) The format of the previous guidance material may be changed to accommodate the applicant's method of data presentation.
(5) Nonstructural castings may be tested and included in the test report.
(6) Cast fluid containers, including hydraulic fluid containers, may be tested as prescribed in other rules of FAR Part 27 and a test proposal and test results report may be included in the casting test report, or an appropriate report may be referenced for convenience. We recommend use of one report to contain test data or reference to test data for all castings used on the rotorcraft.
Par 250 317
TABLEI EXAMPLE
CASTINGS TO BE STATIC TESTED FOR MODEL XYZ REF. LOAD
MACHINE OR SHEET CASTING NO. ASSY. NO. NAME AND LOCATION MATERIAL FIG. NO.
Base Assembly, Pilot's Collective Column 318 Par 250
251. § 27.623 BEARING FACTORS.
a. Explanation.
(1) The rule requires use of a minimum bearing factor in free fit joints to account for effects of typical relative motion. A minimum value is not specified in the rule. The factor, appropriate for the application, is applied to the ultimate bearing strength of the softest material used as a bearing. A definition of free fit (clearance fit) is noted in Paragraph 251b(7) below.
(2) A bearing factor, appropriate for the application, shall be used unless a larger factor is used.
(3) For reference, specific bearing factors are contained in § 29.685(e) for transport rotorcraft control system joints subject to angular rotation. These factors are applied to the ultimate bearing strength of the softest material used as a bearing in the control system. Control systems ball, roller, or needle bearings are covered by § 29.685(f) for transport rotorcraft.
(4) MIL-HDBK-5D, Paragraph 8.3, refers to design standards for plan or journal bearings or bushings. These standards are found in Air Force Systems Command Design Handbook AFSC DH-2-1, Airframe, Chapters 3 and 6.
b. Procedures.
(1) Control system joint bearings are discussed under Paragraph 284, § 27.685, of this AC, but the bearing factors are noted here for convenience. For transport rotorcraft control systems, § 29.685(e) requires a 2.0 bearing factor for cable systems and a 3.33 bearing factor for push-pull systems other than ball and roller bearing systems. The manufacturer's static, non-Brinell rating of ball and roller bearings should not be exceeded. Use of this for normal category rotorcraft is recommended.
(2) A landing gear pivot, grease-lubricated, plain bearing is one example of a free fit subject to pounding or vibration. A bearing factor of 2.0 may be used or another factor may be proven for a grease-lubricated plain bearing or bushing to account for the anticipated higher loads caused by pounding or vibration. See Paragraph 251b(6) for recommendations on ball or roller bearings.
(3) A typical engine mount bolt installation with a plain bearing having a free or loose fit (not interference fit) is another example of a sleeve bearing application subject to a design bearing factor. As an EXAMPLE OR ILLUSTRATION, a bearing factor of
1.85 may be applied to the design loads on the softest material reacting the bearing loads. A different but appropriate factor will be acceptable. The design limit load may be calculated for the example of a 0.312-inch-diameter bolt in a 2-inch-long bearing.
Par 251 319