(b) Damage-tolerance evaluation. The evaluation must include a determination of the probable locations and modes of damage due to fatigue, corrosion, or accidental damage. Repeated load and static analyses supported by test evidence and (if available) service experience must also be incorporated in the evaluation.
Special consideration for widespread fatigue damage must be included where the design is such that this type of damage could occur. It must be demonstrated with sufficient full-scale fatigue test evidence that widespread fatigue damage will not occur within the design service goal of the airplane. The type certificate may be issued prior to completion of full-scale fatigue testing, provided the Administrator has approved a plan for completing the required tests, and the Airworthiness Limitations Section of the Instructions for Continued Airworthiness required by § 25.1529 of this part specifies that no airplane may be operated beyond a number of cycles equal to 1/2 the number of cycles accumulated on the fatigue test article, until such testing is completed.. The extent of damage for residual strength evaluation at any time within the operational life of the airplane must be consistent with the initial detectability and subsequent growth under repeated loads. The residual strength evaluation must show that the remaining structure is able to withstand loads (considered as static ultimate loads) corresponding to the following conditions:
(1) The limit symmetrical maneuvering conditions specified in § 25.337 at all speeds up to VC and in § 25.345.
(2) The limit gust conditions specified in § 25.341 at speeds up to VC, and in § 25.345.
(3) The limit rolling conditions specified in § 25.349 and the limit unsymmetrical conditions specified in §§ 25.367 and 25.427(a) through (c), at speeds up to VC.
(4) The limit yaw maneuvering conditions specified in § 25.351(a) at the specified speeds up to VC.
(5) For pressurized cabins, the following conditions:
(i) The normal operating differential pressure combined with the expected external aerodynamic pressures applied simultaneously with the flight loading conditions specified in paragraphs (b)(1) through (4) of this section, if they have a significant effect.
(ii) The maximum value of normal operating differential pressure (including the expected external aerodynamic pressures during 1g level flight) multiplied by a factor of 1.15, omitting other loads.
(6) For landing gear and directly-affected airframe structure, the limit ground loading conditions specified in §§ 25.473, 25.491, and 25.493.
If significant changes in structural stiffness or geometry, or both, follow from a structural failure, or partial failure, the effect on damage tolerance must be further investigated.
(c) Fatigue (safe-life) evaluation. Compliance with the damage-tolerance requirements of paragraph (b) of this section is not required if the applicant establishes that their application for particular structure is impractical. This 594
structure must be shown by analysis, supported by test evidence, to be able to withstand the repeated loads of variable magnitude expected during its service life without detectable cracks. Appropriate safe-life scatter factors must be applied.
(d) Sonic fatigue strength. It must be shown by analysis, supported by test evidence, or by the service history of airplanes of similar structural design and sonic excitation environment, that -
(1) Sonic fatigue cracks are not probable in any part of the flight structure subject to sonic excitation; or
(2) Catastrophic failure caused by sonic cracks is not probable assuming that the loads prescribed in paragraph (b) of this section are applied to all areas affected by those cracks.
(e) Damage-tolerance (discrete source) evaluation. The airplane must be capable of successfully completing a flight during which likely structural damage occurs as a result of -
(1) Impact with a 4-pound bird when the velocity of the airplane relative to the bird along the airplane's flight path is equal to VC at sea level or 0.85VC at 8,000 feet, whichever is more critical;
(2) Uncontained fan blade impact;
(3) Uncontained engine failure; or
(4) Uncontained high energy rotating machinery failure.
The damaged structure must be able to withstand the static loads (considered as ultimate loads) which are reasonably expected to occur on the flight. Dynamic effects on these static loads need not be considered. Corrective action to be taken by the pilot following the incident, such as limiting maneuvers, avoiding turbulence, and reducing speed, must be considered. If significant changes in structural stiffness or geometry, or both, follow from a structural failure or partial failure, the effect on damage tolerance must be further investigated.
[Amdt. 25-45, 43 FR 46242, Oct. 5, 1978; Amdt. 25-54, 45 FR 60173, Sep. 11, 1980; Amdt. 25-72, 55 FR 29776, Jul. 20, 1990; Amdt. 25-86, 61 FR 5222, Feb. 9, 1996; Amdt. 25-96, 63 FR 15708, Mar. 31, 1998]
b. Intent of Rule. The purpose of this rule is to ensure that fatigue, or corrosion, or accidental damage will be detected and repaired before it becomes catastrophic. This is achieved by adhering to damage tolerant design practices and by prescribing inspections that will detect any such damage before it becomes critical.
c. Background. This rule was carried forward from § 4b.270 of the Civil Air Regulations (CAR), Fatigue Evaluation. The fatigue evaluation provided an option of fatigue strength (safe life) or fail safe strength for flight critical structures. Amendment 25-45 to part 25 introduced the new damagetolerance rule and mandated an inspection program that is based on the damage tolerance evaluation 595
required by this section. It also required a damage-tolerance (discrete source) evaluation to cover accidental damage. Amendment 25-54 requires that inspections or other procedures must be included in the Airworthiness Limitations section of the Instructions for Continued Airworthiness required by § 25.1529. Amendment 25-72 removes the requirement to consider propeller blade impact and to define the likely operating speed to consider for the 4-pound bird impact. Editorial changes were made in Amendment 25-86 to conform to changes in the gust load requirements. Amendment 25-96 added requirements for fatigue tests and inspection thresholds. It also corrected an editorial error in paragraph
(e)(1) that reduced the birdstrike speed to 0.85 VC at 8,000 feet, and revised the residual strength cabin pressure loads.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-45, 43 FR 46242, October 5, 1978; Amendment 25-54, 45 FR 60173, September 11, 1980; Amendment 25-72, 55 FR 29776, July 20, 1990; and Amendment 25 86, 61 FR 5222, February 9, 1996. Advisory Circular (AC) 25.571-1B, or latest revision, provides an acceptable means of compliance with this regulation.
e. References. Advisory Circular 25.571-1B, or latest revision, Damage-Tolerance and Fatigue Evaluation of Structure; and AC 20-107A, or latest revision, Composite Aircraft Structures.
630 - 638. [RESERVED] 596
Section 10. LIGHTNING PROTECTION
639. SECTION 25.581 - LIGHTNING PROTECTION.
a. Rule Text.
(a) The airplane must be protected against catastrophic effects from lightning.
(b) For metallic components, compliance with paragraph (a) of this section may be shown by-
(1) Bonding the components properly to the airframe; or
(2) Designing the components so that a strike will not endanger the airplane.
(c) For nonmetallic components, compliance with paragraph (a) of this section may be shown by-
(1) Designing the components to minimize the effects of a strike; or
(2) Incorporating acceptable means of diverting the resulting electrical current so as not to endanger the airplane.
[Amdt. 25-23, 35 FR 5674, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that the airplane is able to withstand any likely lightning strike and continue safe flight and landing.
c. Background. This rule was adopted by Amendment 25-23 on April 8, 1970.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5674, April 8, 1970. Advisory Circular (AC) 20-136, or latest revision, provides guidance information concerning an acceptable means, but not the only means, of compliance for preventing hazardous effects, due to lightning, from occurring to electrical/electronic systems performing critical essential functions. Advisory Circular 20-53A, or latest revision, provides guidance information applicable to preventing ignition of fuel vapors due to lightning.
e. References. Advisory Circular 20-53A, or latest revision, Protection of Aircraft Fuel Systems Against Fuel Vapor Ignition due to Lightning, AC 20-107A, or latest revision, Composite Aircraft Structure; and AC 20-136, or latest revision, Protection of Aircraft Electrical/Electronic Systems Against the Indirect Effects of Lightning.
640 - 650. [RESERVED] 657
Chapter 2 - DESIGN AND CONSTRUCTION Section 1. GENERAL
651. SECTION 25.601 - GENERAL.
a. Rule Text.
The airplane may not have design features or details that experience has shown to be hazardous or unreliable. The suitability of each questionable design detail and part must be established by tests.
b. Intent of Rule. The purpose of this rule is to prohibit approval of any design that service experience has shown to be unsafe. This includes materials that experience has shown to be unsuitable for aircraft use.
c. Background. This rule was carried forward from § 4b.300 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by service experience review.
e. References. None.
652 - 658. [RESERVED]
659. SECTION 25.603 - MATERIALS.
a. Rule Text.
The suitability and durability of materials used for parts, the failure of which could adversely affect safety, must -
(a) Be established on the basis of experience or tests;
(b) Conform to approved specifications (such as industry or military
specifications, or Technical Standard Orders) that ensure their having the
strength and other properties assumed in the design data; and
(c) Take into account the effects of environmental conditions, such as
temperature and humidity, expected in service.
[Amdt. 25-38, 41 FR 55466, Dec. 20, 1976; Amdt.. 25-46, 43 FR 50595, Oct. 30, 1978]
b. Intent of Rule. The purpose of this rule is to ensure that materials used in the design of airplane parts are suitable for their intended purpose.
c. Background. This rule was carried forward from § 4b.301 of the Civil Air Regulations (CAR), along with the § 4b.301-1 Civil Aeronautics Manual (CAM) material. Amendment 25-38 made minor editorial changes, and Amendment 25-46 added the requirement to account for environmental effects.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-38, 41 FR 55466, December 20, 1976; and Amendment 25-46, 43 FR 50595, October 30, 1978.
e. References. None.
660 - 668. [RESERVED]
669. SECTION 25.605 - FABRICATION METHODS.
a. Rule Text.
(a) The methods of fabrication used must produce a consistently sound structure.
If a fabrication process (such as gluing, spot welding, or heat treating) requires close control to reach this objective, the process must be performed under an approved process specification.
(b) Each new aircraft fabrication method must be substantiated by a test
program.
[Amdt. 25-46, 43 FR 50595, Oct. 30, 1978]
b. Intent of Rule. The purpose of this rule is to ensure a consistent design that will meet the strength and other properties assumed in the design data.
c. Background. This rule was carried forward from § 4b.302 of the Civil Air Regulations (CAR).
Amendment 25-46 required that each new fabrication method be substantiated by a test program.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-46, 43 FR 50595, October 30, 1978.
e. References. None.
670 - 675. [RESERVED]
676. SECTION 25.607 - FASTENERS.
a. Rule Text.
(a) Each removable bolt, screw, nut, pin, or other removable fastener must incorporate two separate locking devices if -
(1) Its loss could preclude continued flight and landing within the design
limitations of the airplane using normal pilot skill and strength; or
(2) Its loss could result in reduction in pitch, yaw, or roll control capability or response below that required by Subpart B of this chapter.
(b) The fasteners specified in paragraph (a) of this section and their locking devices may not be adversely affected by the environmental conditions associated with the particular installation.
(c) No self-locking nut may be used on any bolt subject to rotation in operation, unless a non-friction locking device is used in addition to the self-locking device.
[Amdt. 25-23, 35 FR 5674, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that removable fasteners used in critical locations will not be lost due to failure of any single retention device.
c. Background. This rule was carried forward from § 4b.303 of the Civil Air Regulations (CAR) with considerable revision. Amendment 25-23 added the requirement for dual locking devices on fasteners in critical locations.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5674, April 8, 1970, and Advisory Circular (AC) 20-71, or latest revision.
e. References. Advisory Circular 20-71, or latest revision, Dual Locking Devices on Fasteners.
677 - 683. [RESERVED] 735
684. SECTION 25.609 - PROTECTION OF STRUCTURE.
a. Rule Text.
Each part of the structure must -
(a) Be suitably protected against deterioration or loss of strength in service due to any cause, including -
(1) Weathering;
(2) Corrosion; and
(3) Abrasion; and
(b) Have provisions for ventilation and drainage where necessary for protection.
b. Intent of Rule. The purpose of this rule is to ensure that the structure is adequately protected to provide resistance to deterioration in service.
c. Background. This rule was carried forward from § 4b.304 of the CAR with editorial changes.
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by review of drawings and by inspection of aircraft.
e. References. None.
685 - 692. [RESERVED]
693. SECTION 25.611 - ACCESSIBILITY PROVISIONS.
a. Rule Text.
Means must be provided to allow inspection (including inspection of principal structural elements and control systems), replacement of parts normally requiring replacement, adjustment, and lubrication as necessary for continued airworthiness. The inspection means for each item must be practicable for the inspection interval for the item. Nondestructive inspection aids may be used to inspect structural elements where it is impracticable to provide means for direct visual inspection if it is shown that the inspection is effective and the inspection procedures are specified in the maintenance manual required in § 25.1529.
[Amdt. 25-23, 35 FR 5674, Apr. 8, 1970] 736
b. Intent of Rule. The purpose of this rule is to make it possible to replace parts when needed and to inspect critical structures. Continued safe operation depends on the ability to evaluate critical systems and structures and to replace or repair as necessary.
c. Background. This rule was carried forward from § 4b.305 of the Civil Air Regulations (CAR) and was revised by Amendment 25-23.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble to Amendment 25-23, 35 FR 5674, April 8, 1970. Compliance may be shown by review of drawings or by inspection of aircraft.
e. References. None.
694 - 698. [RESERVED]
699. SECTION 25.613 - MATERIAL STRENGTH PROPERTIES AND DESIGN VALUES.
a. Rule Text.
(a) Material strength properties must be based on enough tests of material meeting approved specifications to establish design values on a statistical basis.
(b) Design values must be chosen to minimize the probability of structural failures due to material variability. Except as provided in paragraph (e) of this section, compliance with this paragraph must be shown by selecting design values which assure material strength with the following probability:
(1) Where applied loads are eventually distributed through a single member within an assembly, the failure of which would result in loss of structural integrity of the component, 99 percent probability with 95 percent confidence.
(2) For redundant structure, in which the failure of individual elements would result in applied loads being safely distributed to other load carrying members, 90 percent probability with 95 percent confidence.
(c) The effects of temperature on allowable stresses used for design in an essential component or structure must be considered where thermal effects are significant under normal operating conditions.
(d) The strength, detail design, and fabrication of the structure must minimize the probability of disastrous fatigue failure, particularly at points of stress concentration.
(e) Greater design values may be used if a “premium selection" of the material is made in which a specimen of each individual item is tested before use to 737
determine that the actual strength properties of that particular item will equal or exceed those used in design.
[Amdt. 25-46, 43 FR 50595, Oct. 30, 1978; Amdt. 25-72, 55 FR 29776, Jul. 20, 1990]
b. Intent of Rule. The purpose of this rule is to ensure that design values are based on enough tests of material meeting an approved specification to minimize the probability of structural failures due to material variability.
c. Background. This rule was carried forward from § 4b.306 of the Civil Air Regulations (CAR) with editorial changes. Amendment 25-46 permitted the Administrator to approve other design values.
Amendment 25-72 combined the requirements of § 25.613 with § 25.615, and removed the reference to the military handbooks (MIL-HDBK’s). This was done, in part, to harmonize with the Joint Aviation Requirements (JAR).
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-46, 43 FR 50595, October 30, 1978; and Amendment 25-72, 55 FR 29776, July 20, 1990.
e. References. Military handbook (MIL-HDBK)-5, Metallic Materials and Elements for Flight Vehicle Structure; MIL-HDBK-17, Plastics for Flight Vehicles Composite Construction for Flight Vehicles.
700 - 707. [RESERVED]
708. SECTION 25.615 [Removed].
NOTE: In Amendment 25-72, Sections 25.613 and 25.615 were revised and
consolidated into a single rule in Section 25.613.
[Amdt. 25-72, 55 FR 29776, Jul. 20, 1990]
709. SECTION 25.619 - SPECIAL FACTORS.
a. Rule Text.
The factor of safety prescribed in § 25.303 must be multiplied by the highest pertinent special factor of safety prescribed in §§ 25.621 through 25.625 for each part of the structure whose strength is - 738
(a) Uncertain;
(b) Likely to deteriorate in service before normal replacement; or
(c) Subject to appreciable variability because of uncertainties in manufacturing processes or inspection methods.
[Amdt. 25-23, 35 FR 5674, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to account for uncertainties in structural strength, likely deterioration in service, and appreciable variability in manufacturing. This is necessary to ensure that the probability of failure of the part being under strength from these causes is extremely remote.
c. Background. This rule was carried forward from § 4b.307 of the Civil Air Regulations (CAR).
Amendment 25-23 revised this rule for clarification of the applicability of the appropriate special factors.
The revised rule makes it clear that the safety factor prescribed in § 25.303 is to be multiplied by the highest pertinent special factor prescribed in §§ 25.621 through 25.625. These special factors are not to be compounded.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5674, April 8, 1970.
e. References. None.
710 - 718. [RESERVED]
719. SECTION 25.621 - CASTING FACTORS.
a. Rule Text.
(a) General. The factors, tests, and inspections specified in paragraphs (b) through (d) of this section must be applied in addition to those necessary to establish foundry quality control. The inspections must meet approved specifications. Paragraphs (c) and (d) of this section apply to any structural castings except castings that are pressure tested as parts of hydraulic or other fluid systems and do not support structural loads.
(b) Bearing stresses and surfaces. The casting factors specified in paragraphs (c) and (d) of this section -
(1) Need not exceed 1.25 with respect to bearing stresses regardless of the
method of inspection used; and
(2) Need not be used with respect to the bearing surfaces of a part whose bearing factor is larger than the applicable casting factor.
(c) Critical castings. For each casting whose failure would preclude continued safe flight and landing of the airplane or result in serious injury to occupants, the following apply:
(1) Each critical casting must - (I) Have a casting factor of not less than 1.25; and
(ii) Receive 100 percent inspection by visual, radiographic, and magnetic particle or penetrant inspection methods or approved equivalent nondestructive inspection methods.
(2) For each critical casting with a casting factor less than 1.50, three sample castings must be static tested and shown to meet -
(i) The strength requirements of § 25.305 at an ultimate load corresponding to a casting factor of 1.25; and
(ii) The deformation requirements of § 25.305 at a load of 1.15 times the limit load.
(3) Examples of these castings are structural attachment fittings, parts of flight control systems, control surface hinges and balance weight attachments, seat, berth, safety belt, and fuel and oil tank supports and attachments, and cabin pressure valves.
(d) Noncritical castings. For each casting other than those specified in paragraph
(c) of this section, the following apply:
(1) Except as provided in paragraphs (d) (2) and (3) of this section, the casting factors and corresponding inspections must meet the following table:
Casting factor Inspection
2.0 or more 100 percent visual.
Less than 2.0 but 100 percent visual, and magnetic particle or more than 1.5 penetrant or equivalent nondestructive inspection methods.
1.25 through 1.50 100 percent visual, magnetic particle or penetrant, and radiographic, or approved equivalent nondestructive inspection methods.
(2) The percentage of castings inspected by nonvisual methods may be reduced below that specified in paragraph (d)(1) of this section when an approved quality control procedure is established.
(3) For castings procured to a specification that guarantees the mechanical properties of the material in the casting and provides for demonstration of these properties by test of coupons cut from the castings on a sampling basis-
(i) A casting factor of 1.0 may be used; and 740