10/06/14 AC 25.621-1
Contents Paragraph Page

ii 1 Purpose. ...................................................................................................................... ................1 2 Applicability. ................................................................................................................ .............1 3 Related Regulations. .......................................................................................................... ........1 4 Definitions.................................................................................................................... ..............2 4.1 Casting. .................................................................................................................. ...........2 4.2 Critical Casting. ......................................................................................................... .......2 4.3 Premium Casting Process. ................................................................................................2 4.4 Prolongation. ............................................................................................................. ........2 4.5 Test Casting. ............................................................................................................. ........2 5 Background. ................................................................................................................... ............2 5.1 Regulatory Background. ...................................................................................................2 5.2 Application of Special Factors of Safety. .........................................................................3 6 General Guidance for Use of Casting Factors. ..........................................................................3 6.1 Factors of Safety. ........................................................................................................ ......3 6.2 Inspections. .............................................................................................................. .........4 6.3 Static Testing. ........................................................................................................... ........4 6.4 Rework of Castings. ....................................................................................................... ...4 7 Premium Casting Process. ...................................................................................................... ...4 7.1 General. .................................................................................................................. ...........5 7.2 One Means of Qualificati on for Casting Process. .............................................................5 7.3 Proof of Product. ......................................................................................................... ......8 7.4 Monitoring the Process. ..................................................................................................1 0 7.5 Modifications to the Casting De sign, Material, and Process. .........................................10

Tables Page Table 1. Recommended Ve rification Testing ............................................................................... 1110/06/14 AC 25.621-1

1 1 PURPOSE. This advisory circular (AC) describes an acceptable means for showing compliance with the requirements of § 25.621, Casting factors , of Title 14, Code of Federal Regulations (14 CFR) part 25. Section 25.621 outlin es the factors, tests, and inspections that must be applied to castings used in structural applications.

2 APPLICABILITY. 2.1 The guidance provided in this document is directed to airplane manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation Administration (FAA) transport airplane type certificat ion engineers and their designees. 2.2 The material in this AC is neither manda tory nor regulatory in nature and does not constitute a regulation. While these guideline s are not mandatory, they are derived from extensive FAA and industry experience in de termining compliance with the relevant regulations. These means are issued, in the interest of standardization, for guidance purposes and to outline a method that has been found acceptable in showing compliance with the standards set forth in the rule. If , however, we become aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC, and we may require additional substantiation or design changes as a basis for finding compliance. 2.3 The material in this AC does not chan ge or create any addi tional regulatory requirements, nor does it authorize changes in, or permit deviations from, regulatory requirements. The applicant should be awar e that § 25.621 is an a dditional requirement for structural substantiation of cast parts and components. It is used in combination with a number of other sectio ns, and it does not replace or ne gate compliance with any other paragraph of 14 CFR part 25. 2.4 Except in the explanations of what the regulations require, the term “must” is used in this AC only in the sense of ensuring app licability of this particular method of compliance when the acceptable method of co mpliance described in this AC is used.

3 RELATED REGULATIONS. The following 14 CFR regulations are referenced in this AC. The full text of these regulations can be downloaded at the U.S. Government Printing Office e-CFR . You can order a paper copy by sending a request to th e U.S. Superintendent of Documents, U.S. Government Printing Office, Wa shington, D.C. 20402-0001; or by calling telephone number (202) 512-1800; or by se nding a request by facsimile to (202) 512-2250.

 Section 25.303, Factor of safety

.  Section 25.305, Strength and deformation

.  Section 25.307, Proof of structure

.10/06/14 AC 25.621-1

2  Section 25.571, Damage-tolerance and fatigue evaluation of structure

.  Section 25.613, Material strength properties and material design values

.  Section 25.619, Special factors

. 4 DEFINITIONS. For the purposes of this AC, the following definitions apply: 4.1 Casting. A method of forming an object by pouring molten metal into a mold, allowing the material to solidify inside the mold, and removing it when solidification is complete. 4.2 Critical Casting. A casting whose failure could preclude continue d safe flight and la nding of the airplane or could result in serious injury to occupant s is considered a cri tical casting. Examples of castings that may be critical are structural attachment fittings; parts of flight control systems; control surface hinges and balance we ight attachments; seat, berth, and safety belt supports and attachments; fuel and oil tank supports and attach ments; pressurized doors; and cabin pressure valves. 4.3 Premium Casting Process. A casting process that produces castings char acterized by a high quali ty and reliability. 4.4 Prolongation. An integrally cast te st bar or test coupon. 4.5 Test Casting. A casting produced specifically for the purpose of qualifying the casting process.

5 BACKGROUND. 5.1 Regulatory Background. 5.1.1 The airworthiness standards of 14 CFR part 25 relative to transport category airplanes require classification of stru ctural castings as either “c ritical” or “non-critical.” Depending on classification, the current standards specify the accomplishment of certain inspection and test requirements, and th e application of special factors of safety for ultimate strength and deformation. 5.1.2 The original requirements specified in § 25.621, Casting factors , stated that critical castings in structural applications must have a minimum casting factor of 1.25, which was to be applied in addition to the lim it load factor of safety required by § 25.303, Factor of safety . Those requirements were in effect for many years, having been carried forward in 1964 from Part 4b of the Civil Air Regulations (CAR 4b.307). Prior to that, Civil Aeronautics Manual 04 required a mini mum additional ultimate strength factor of10/06/14 AC 25.621-1

3 2.0 for castings used in primary structure. Under those regulations, the Administrator had the authority to prevent the use of any casting that was not considered acceptable for a given application. 5.2 Application of Special Factors of Safety. 5.2.1 The application of factors of safety to cas tings is based on the fact that the casting process can be inconsistent. Casting is a method of forming an object by pouring molten metal into a mold, allowing the material to solidify inside the mold, and removing it when solidification is complete. Castings are subject to variability in mechanical properties due to this casting process, which can result in imperfections, such as voids, within the cast part. Using certain insp ection techniques, for example radiographic (X-ray), it is possible to detect such imperf ections above a minimum detectable size, but accurate detection depends on the dimensions of the part, the inspection equipment used, and the skill of the inspector. 5.2.2 Section 25.619, Special factors , includes a requirement to apply a special factor to the factor of safety prescribed in § 25.303 for each part of the airplane structure whose strength is subject to ap preciable variability becaus e of uncertainties in the manufacturing processes or inspection methods. Since the mechanical properties of a casting depend on the casting design, th e design values established under § 25.613, Material strength properties and material design values , for one casting might not be applicable to another casting made to the sa me specification. Thus, casting factors have been necessary for castings produced by normal techniques and methodologies to ensure the structural inte grity of castings in light of these uncertainties. 5.2.3 Another approach is to reduce the uncertain ties in the casting manufacturing process by use of a “premium casting process” (discusse d below), which enables the applicant to use a casting factor of 1.0. Section 25.621 does permit the use of a casting factor of 1.0 for critical castings, provided that— 5.2.3.1 The manufacturer has established tigh t controls for the casting process, inspection, and testing; and 5.2.3.2 The material strength properties of the casting have no more variability than equivalent wrought alloys.

6 GENERAL GUIDANCE FOR USE OF CASTING FACTORS. 6.1 Factors of Safety. Section 25.619 requires that the factor of safety specified in § 25.303 be multiplied by the highest pertinent specia l factor of safety prescr ibed in §§ 25.621 through 25.625. While castings are subject to the fatigue and damage tolerance requirements of § 25.571, the factors prescribed in §§ 25.621 through 25.625 need not be considered in demonstrating compliance with § 25.571.10/06/14 AC 25.621-1

4 6.2 Inspections. Guidance on non-destructive in spection techniques and methods can be obtained from national and international standards. See paragraph 7.2.4.3 of this AC. To determine whether an inspection method is an “equiva lent inspection method” to the methods specified in the rule, the applicant should cons ider the types and sizes of the internal and external defects detectable by the methods; th e capability of the methods to inspect the structurally significant areas and areas where defects are likely to occur; and the capability of the methods relative to the conf iguration, material specification, and types and sizes of defects expected for the casting being inspected. 6.3 Static Testing. The static test specimen(s) should be select ed on the basis of the foundry quality control inspections, in conjunction with those inspections prescribed in § 25.621(c) and (d). The test article may consist of the sample cas ting to be tested and any additional parts (production or test specific) required to ensu re that loads and stresses experienced by the sample casting are representative of e xpected in-service loading conditions. An attempt should be made to select sample casting(s) for testing from the first batch produced to the production standard. It is de sirable that the test specimen(s) contain detectable surface and/or internal defects in structurally significant areas that do not otherwise exceed the threshold for rejection of the cast part. Multiple test conditions may be required to provide complete coverage of all structurally si gnificant areas of the cast part. 6.4 Rework of Castings. If applicable, the applican t should address the effects on material properties of processes intended to correct a non-confor ming condition in the cast part (e.g., weld rework). Evaluation of effects of rework pr ocesses on material pr operties should include analysis supported by test data. Material properties of reworked areas should have a population coefficient of variation consistent w ith the type of casting factor selected for the casting. The extent and limita tions of such rework pro cesses should be detailed in the manufacturing specifications as well as on the design drawings, e ither directly or by reference to a qualified rework process.

7 PREMIUM CASTING PROCESS. This paragraph provides guidance for co mpliance with § 25.621 for using a casting factor greater than or equal to 1.0, but less than 1.25, for “critical” castings used in structural applications. A premium casting pr ocess is capable of producing castings with predictable properties, thus allowing a casting factor of 1.0 to be used for these components. Three major steps, required by § 25.621(c)(1)(i), are essential in characterizing a premium casting process:

 Qualification of the process.

 Proof of the product.

 Monitoring of the process.10/06/14 AC 25.621-1

5 7.1 General. 7.1.1 The objective of a premium casting process is to consistently produce castings with high quality and reliability. To this end, the casting process is one that is capable of consistently producing castings that include the following characteristics: 7.1.1.1 Good dimensional tolerance. 7.1.1.2 Minimal distortion. 7.1.1.3 Good surface finish. 7.1.1.4 No cracks. 7.1.1.5 No cold shuts. 7.1.1.6 No laps. 7.1.1.7 Minimal shrinkage cavities. 7.1.1.8 No harmful entrapped oxide films. 7.1.1.9 Minimal porosity. 7.1.1.10 A high level of metallurgical cleanness. 7.1.1.11 Good microstructural characteristics. 7.1.1.12 Minimal residual internal stress. 7.1.1.13 Consistent mechanical properties. 7.1.2 The majority of these characteristics can be detected, evaluated, and quantified by standard non-destructive te sting methods, or from dest ructive methods on prolongation or casting cut-up tests. However, a number of them cannot. Thus, to ensure an acceptable quality of product, the significant and critical process variables must be identified and ade quately controlled. 7.2 One Means of Qualification for Casting Process. 7.2.1 To demonstrate a premium casting process, the applicant should subject it to a qualification program that is specific to a foundry and material combination. The qualification program should establish the following: 7.2.1.1 The capability of the casting process to produce a consistent quality of product for the specific material grade selected for the intended production component.10/06/14 AC 25.621-1

6 7.2.1.2 The mechanical properties for the material produced by the process have population coefficients of variation e quivalent to that of wrought products of similar composition (in other words, plate, extrusions, and bar). Using the population coefficient of variat ion from forged products does not apply. In most cases, the coefficients of variation for tensile ultimate strength less than or equal to 3.5 pe rcent, and tensile yield strength less than or equal to 4.0 percent, is adequa te to demonstrate this equivalency of mechanical properties. 7.2.1.3 The casting process is capable of producing a casti ng with uniform properties throughout the casting or, if not uniform, with a distribution of material properties that can be pr edicted to an acceptable level of accuracy. 7.2.1.4 The (initial) material design data for the specified material are established. 7.2.1.5 The material and process specifications are clearly defined. 7.2.2 For each material specification, a series of te st castings from a number of melts, using the appropriate production pro cedures of the foundry, should be manufactured. The test casting produced should undergo a standardi zed inspection or investigation using non-destructive inspection and cu t-up testing to determine the consistency of the casting process. 7.2.3 The test casting should be representative of the intended cast product(s) with regard to section thicknesses and complexity, and it s hould expose any limitations of the casting process. In addition, the standard test casting should be large enough to provide mechanical test specimens from various areas for tensile and, if applicable, compression, shear, bearing, fatigue, fracture t oughness, and crack pr opagation tests. If the production component complies with these requirements, it may be used to qualify the process. The number of melts sampled s hould be statistically significant. Typically, at least 10 melts are sampled with no more than 10 castings produced from each melt. If the material specification requires the compone nts to be heat-treated, this should be done in no fewer than 10 heat treatment batc hes consisting of castings from more than one melt. Reduction of qualification tests may be considered if the casting process and the casting alloy is already well known for aer ospace applications and the relevant data are available. 7.2.4 Inspection Program for Test Casting. 7.2.4.1 Each test casting should receive a non-destructive inspection program, which should include the following as a minimum: 7.2.4.1.1 Inspection of 100 percent of its surface using visual and liquid penetrant or equivalent inspection methods. 7.2.4.1.2 Inspection of structurally significant in ternal areas and areas where defects are likely to occur using radiographi c methods or equi valent inspection10/06/14 AC 25.621-1

7 methods. The specific radi ographic reference standard to be employed is to be determined, and the margin by which the test castings exceed the minimum required standard should be recorded. 7.2.4.2 The program of inspection is intended to— 7.2.4.2.1 Confirm that the casting process is capable of producing a consistent quality of product. 7.2.4.2.2 Verify compliance with the stated obj ectives of a premium casting process with regard to surface finish, cracks, cold shuts, laps, shrinkage cavities, and porosity. (See paragraph 7.1.1 of this AC.) 7.2.4.2.3 Ensure that the areas from which th e mechanical property test samples were taken were typical of the casting as a whole with respect to porosity and cleanness. 7.2.4.3 Guidance on non-destructiv e inspection techniques and methods can be obtained from national and internationa l standards. The standard listing below is not a comprehensive list, bu t is given as an initial reference guide:

 American Society for Testing a nd Materials (ASTM) A802/A802M, Standard Practice for Steel Casti ngs, Surface Acceptance Standards, Visual Examination

.  ASTM A903/A903M, Standard Specification for Steel Castings, Surface Acceptance Standards, Magnetic Particle and Liquid Penetrant Inspection

.  ASTM E155, Standard Reference Radiographs for Inspection of Aluminum and Magnesium Castings

.  ASTM E192, Standard Reference Radiographs of Investment Steel Castings for Aerospace Applications

.  ASTM E433, Standard Reference Photographs for Liquid Penetrant Inspection

.  ASTM E1030, Standard Test Method for Radiographic Examination of Metallic Castings

.  ASTM E1320, Standard Reference Radiographs for Titanium Castings

.  International Organization fo r Standardization (ISO) 4986, Steel Castings—Magnetic Particle Inspection

.  ISO 4987, Steel Castings—Liquid Penetrant Inspection

.  ISO 4993, Steel and Iron Castings—Radiographic Inspection

.  ISO 9915, Aluminum Alloy Castings—Radiography Testing

.10/06/14 AC 25.621-1

8  ISO 9916, Aluminum Alloy and Magnesium Alloy Castings—Liquid Penetrant Inspection

.  ISO 10049, Aluminum Alloy Castings—V isual Method for Assessing the Porosity

.  ISO 11971, Steel and Iron Castings—Visual Examination of Surface Quality

. 7.2.4.4 The test castings must show that th e foundry and process combination is capable of producing a product free of cr acks, laps, and cold shuts. Ideally, the test castings should be free of detectable shrinkage cavities and porosity. Guidance for acceptance criteria regarding dimensional tolerance, distortion, and surface finish can be obtained from the standards cited in paragraph 7.2.4.3 of this AC . When these standards are used, it should be taken into account that th ese standards are for general quality castings. 7.2.5 All test castings should be cut up using a standardized methodology to produce the mechanical test specimens detailed in para graph 7.2.3 of this AC. Principally, the tests are to establish the variability within th e cast component, as well as determine the variability between components from the sa me melt and from melt to melt. Typically, the evaluation to compare the cast material to wrought material should use the ultimate tensile strength and tensile yield strength. Th e data gathered also may be used during later phases to identify deviati ons from the limits establishe d in the proces s qualification and product proving programs. 7.2.6 All the fracture surfaces gene rated during the qualification program should be inspected at least visually for detrimental defects. Evid ence of inclusions, oxide films, porosity, or shrinkage cavities would i ndicate inadequate control of the casting process. 7.2.7 As part of the cut-up investigation, it is usually necessary to take metallographic samples for cleanness determination a nd microstructural characterization. 7.2.8 When the process has been qualified, it should not be altered without completing comparability studies and necessa ry testing of differences. 7.3 Proof of Product. 7.3.1 Subsequent to the qualification of the pr ocess, the production castings should be subjected to a production-proving program. Such castings should have at least one prolongation; however, large and/or complex castings may require more than one. If a number of castings are produced from a single mold with a single runner system, they may be treated as one single casting. The production-proving progr am should establish the following: 7.3.1.1 The design values developed during th e process qualification program are valid (for example, same statistical distribution) fo r the production casting.10/06/14 AC 25.621-1

9 7.3.1.2 The production castings have the same or less than the level of internal defects as the test castings produced during qualification. 7.3.1.3 The cast components have a predictable distribution of tensile properties. 7.3.1.4 The prolongation(s) is repr esentative of the critical area(s) of the casting. 7.3.1.5 The prolongation(s) consistently refl ects the quality process and material properties of the casting. 7.3.2 At least two (or more) pre- production castings of each part number to be produced should be selected for testing and inspecti on. All of the selected castings should be non-destructively inspected in accord ance with the qualification program. 7.3.2.1 One of these castings should be used as a dimensional tolerance test article. The other selected casting( s) should be cut up for mechanical property testing and meta llographic inspection. 7.3.2.2 The casting(s) selected for cut-up shoul d be cut in a standardized manner to yield a number of tensile test specimens and metallographic samples. There should be sufficient cut-up tensile specimens to cover all critical (“critical” with respect to both the casting process and service loading) areas of the casting. 7.3.2.3 All prolongations should be machined to give tensile specimens and subsequently tested. 7.3.2.4 The production castings should be pr oduced under production procedures identical to those used for these pre-production castings. 7.3.3 On initial production, a number of castings should undergo a cut-up for mechanical property testing and metallographic inspect ion similar to that performed for the pre-production casting(s). The cut-up proce dure used should be st andardized, although it may differ from that used for the pre- production casting(s). Tensile specimens should be obtained from the most critical areas. 7.3.3.1 For the first 30 castings produced, at least 1 casting in 10 should undergo this testing program. 7.3.3.2 The results from the mechanical prop erty tests should be compared with the results obtained from the prolongations to further substantiate the correlation between prolongation(s) and th e critical area(s) of the casting. 7.3.3.3 In addition, if the distribution of mech anical properties derived from these tests is acceptable, when compared to the property values determined in the qualification program, the fre quency of testing may be reduced. However, if the comparison is found unacceptable, the test program may require extension.10/06/14 AC 25.621-1

10 7.3.4 At no point in the production should the casti ngs contain shrinkage cavities, cracks, cold shuts, laps, porosity, or entrapped oxide film, or have a poor surface finish exceeding the acceptance level defined in the technical specifications. 7.4 Monitoring the Process. 7.4.1 The applicant should employ quality techniques to establish the significant/critical foundry process variables that have an im pact on the quality of the product. The applicant should show that these variable s are controlled with corrective action throughout production. 7.4.2 During production, every casting should be non-destructively in spected using the techniques and the acceptance standards employed during the qualification program. 7.4.2.1 Rejections should be investigated and process corrections made as necessary. 7.4.2.2 Alternative techniques may be empl oyed if the equivalence in the acceptance levels can be demonstrated. 7.4.2.3 In addition, tensile tests should be taken from the prolongations on every component produced, and the results s hould comply with limits developed in the process qualification a nd product proving programs. 7.4.2.4 Additionally, as mentioned in paragr aph 7.3 of this AC, a periodic casting cut-up inspection should be undertaken with the inspection schedule as agreed upon during the pr oof of product program. 7.4.2.5 Deviations from the limits established in the process qualification and production-proving programs should be investigated and corrective action taken. 7.5 Modifications to the Casting Design, Material, and Process. Additional testing may be required when alte rations are made to the casting geometry, material, significant/critical process variable s, process, or production foundry to verify that the alterations have not significantly changed the casting’s properties. The recommended verification testing is detailed in table 1 of this AC.10/06/14 AC 25.621-1

11 Table 1. Recommended Verification Testing Modifications Verification Testing Case Geometry Material Process Foundry Qualification of Process Proof of Product Tests per § 25.621(c)(1) 1 Yes None None None Not necessary Yes Yes 2 2 None Yes None None Yes 1 Yes Yes 2 3 Yes Yes None None Yes Yes Yes 4 None None Yes None Yes 1 Yes Yes 2 5 None None None Yes Yes 1 Yes Yes 2

1 The program described in paragraph 7 of this AC to qualify a new material, process, and foundry combination may not be necessary if the following three conditions exist for the new combination: 1. Sufficient data from relevant castings to s how that the process is capable of producing a consistent quality of product, and that the quality is comparable to, or better than, the old combination. 2. Sufficient data from relevant castings to estab lish that the mechanical properties of the castings produced from the new combination have a similar or better statistical distribution than the old combination. 3. Clearly defined material and process specifications. 2 The casting may be re-qualified by testing partia l static test samples. With larger castings, re-qualification could be undertaken by a sta tic test of the casting’s critical region only.Advisory Circular Feedback If you find an error in this AC, have recommenda tions for improving it, or have suggestions for new items/subjects to be added, you may let us know by (1) emailing this form to 9-AWA-AVSAIR500-Coord@faa.gov or (2) faxing it to the attention of the Aircraft Certification Service Directives Management Officer at (202) 267-3983. Subject: (insert AC title/number here) Date: Click here to enter text. Please check all appropriate line items: ☐ An error (procedural or typographi cal) has been noted in paragraph Click here to enter text. on page Click here to enter text.

. ☐ Recommend paragraph Click here to enter text. on page Click here to enter text. be changed as follows: Click here to enter text. ☐ In a future change to this AC , please cover the following subject: (Briefly describe what you want added.) Click here to enter text. ☐ Other comments: Click here to enter text. ☐ I would like to discuss th e above. Please contact me. Submitted by: Date: