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 Qualification for Casting Process. .............................................................5

7.3 Proof of Product. ...............................................................................................................8

7.4 Monitoring the Process. ..................................................................................................10

7.5 Modifications to the Casting Design, Material, and Process. .........................................10

Tables 

Page 

Table 1. Recommended Verification Testing ............................................................................... 11 
10/06/14 AC 25.621-1 

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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 outlines 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 certification engineers and their designees. 

2.2 The material in this AC is neither mandatory nor regulatory in nature and does not 

constitute a regulation. While these guidelines are not mandatory, they are derived from 

extensive FAA and industry experience in determining 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 change or create any additional regulatory 

requirements, nor does it authorize changes in, or permit deviations from, regulatory 

requirements. The applicant should be aware that § 25.621 is an additional requirement 

for structural substantiation of cast parts and components. It is used in combination with 

a number of other sections, and it does not replace or negate 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 applicability of this particular method of 

compliance when the acceptable method of compliance 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 the U.S. Superintendent of Documents, 

U.S. Government Printing Office, Washington, D.C. 20402-0001; or by calling 

telephone number (202) 512-1800; or by sending 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. 
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 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 continued safe flight and landing of the airplane 

or could result in serious injury to occupants is considered a critical casting. Examples 

of castings that may be critical are structural attachment fittings; parts of flight control 

systems; control surface hinges and balance weight attachments; seat, berth, and safety 

belt supports and attachments; fuel and oil tank supports and attachments; pressurized 

doors; and cabin pressure valves. 

4.3 Premium Casting Process.

A casting process that produces castings characterized by a high quality and reliability. 

4.4 Prolongation.

An integrally cast test 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 structural castings as either “critical” or “non-critical.” 

Depending on classification, the current standards specify the accomplishment of 

certain inspection and test requirements, and the 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 limit 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 minimum additional ultimate strength factor of 
10/06/14 AC 25.621-1 

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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 castings 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 inspection techniques, for example radiographic 

(X-ray), it is possible to detect such imperfections 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 appreciable variability because of uncertainties in the 

manufacturing processes or inspection methods. Since the mechanical properties of a 

casting depend on the casting design, the 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 same specification. Thus, casting factors have 

been necessary for castings produced by normal techniques and methodologies to 

ensure the structural integrity of castings in light of these uncertainties. 

5.2.3 Another approach is to reduce the uncertainties in the casting manufacturing process by 

use of a “premium casting process” (discussed 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 tight 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 special factor of safety prescribed 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. 
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6.2 Inspections. 

Guidance on non-destructive inspection 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 “equivalent inspection method” to the methods 

specified in the rule, the applicant should consider the types and sizes of the internal and 

external defects detectable by the methods; the 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 configuration, 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 selected 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 casting to be tested and any additional parts 

(production or test specific) required to ensure that loads and stresses experienced by 

the sample casting are representative of expected 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 desirable 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 significant areas of the 

cast part. 

6.4 Rework of Castings. 

If applicable, the applicant should address the effects on material properties of 

processes intended to correct a non-conforming condition in the cast part (e.g., weld 

rework). Evaluation of effects of rework processes on material properties should include 

analysis supported by test data. Material properties of reworked areas should have a 

population coefficient of variation consistent with the type of casting factor selected for 

the casting. The extent and limitations of such rework processes should be detailed in 

the manufacturing specifications as well as on the design drawings, either directly or by 

reference to a qualified rework process. 

7 PREMIUM CASTING PROCESS. 

This paragraph provides guidance for compliance 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 process 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. 
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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 testing methods, or from destructive 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 adequately 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. 
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7.2.1.2 The mechanical properties for the material produced by the process have 

population coefficients of variation equivalent to that of wrought products 

of similar composition (in other words, plate, extrusions, and bar). Using 

the population coefficient of variation from forged products does not 

apply. In most cases, the coefficients of variation for tensile ultimate 

strength less than or equal to 3.5 percent, and tensile yield strength less 

than or equal to 4.0 percent, is adequate to demonstrate this equivalency of 

mechanical properties. 

7.2.1.3 The casting process is capable of producing a casting with uniform 

properties throughout the casting or, if not uniform, with a distribution of 

material properties that can be predicted 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 test castings from a number of melts, using 

the appropriate production procedures of the foundry, should be manufactured. The test 

casting produced should undergo a standardized inspection or investigation using 

non-destructive inspection and cut-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 should 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 toughness, and crack propagation tests. If 

the production component complies with these requirements, it may be used to qualify 

the process. The number of melts sampled should 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 components to be heat-treated, this should be 

done in no fewer than 10 heat treatment batches 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 aerospace 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 internal areas and areas where defects 

are likely to occur using radiographic methods or equivalent inspection 
10/06/14 AC 25.621-1 

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methods. The specific radiographic 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 objectives 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 the 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-destructive inspection techniques and methods can be 

obtained from national and international standards. The standard listing 

below is not a comprehensive list, but is given as an initial reference 

guide: 

 American Society for Testing and Materials (ASTM) A802/A802M, 

Standard Practice for Steel Castings, 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 for 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. 
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 ISO 9916, Aluminum Alloy and Magnesium Alloy Castings—Liquid 

Penetrant Inspection. 

 ISO 10049, Aluminum Alloy Castings—Visual 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 the foundry and process combination is 

capable of producing a product free of cracks, 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 these 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 paragraph 7.2.3 of this AC. Principally, the tests 

are to establish the variability within the cast component, as well as determine the 

variability between components from the same 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. The data gathered also may be used during 

later phases to identify deviations from the limits established in the process qualification 

and product proving programs. 

7.2.6 All the fracture surfaces generated during the qualification program should be inspected 

at least visually for detrimental defects. Evidence of inclusions, oxide films, porosity, or 

shrinkage cavities would indicate 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 and microstructural characterization. 

7.2.8 When the process has been qualified, it should not be altered without completing 

comparability studies and necessary testing of differences. 

7.3 Proof of Product. 

7.3.1 Subsequent to the qualification of the process, 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 program should establish 

the following: 

7.3.1.1 The design values developed during the process qualification program are 

valid (for example, same statistical distribution) for the production casting. 
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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 representative of the critical area(s) of the casting. 

7.3.1.5 The prolongation(s) consistently reflects 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 inspection. All of the selected castings should be 

non-destructively inspected in accordance 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 metallographic inspection. 

7.3.2.2 The casting(s) selected for cut-up should 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 produced 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 inspection similar to that performed for the 

pre-production casting(s). The cut-up procedure used should be standardized, 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 property tests should be compared with 

the results obtained from the prolongations to further substantiate the 

correlation between prolongation(s) and the critical area(s) of the casting. 

7.3.3.3 In addition, if the distribution of mechanical properties derived from these 

tests is acceptable, when compared to the property values determined in 

the qualification program, the frequency 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 

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7.3.4 At no point in the production should the castings 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 impact on the quality of the product. The 

applicant should show that these variables are controlled with corrective action 

throughout production. 

7.4.2 During production, every casting should be non-destructively inspected 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 employed 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 should comply with limits developed 

in the process qualification and product proving programs. 

7.4.2.4 Additionally, as mentioned in paragraph 7.3 of this AC, a periodic casting 

cut-up inspection should be undertaken with the inspection schedule as 

agreed upon during the proof 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 alterations are made to the casting geometry, 

material, significant/critical process variables, 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 

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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 show 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 establish 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 partial static test samples. With larger castings, 

re-qualification could be undertaken by a static test of the casting’s critical region only. 
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