c. Background1279
d. Acceptable compliance methods1279
e. References1279
[1087-1095] [RESERVED]1279
1096. Section 25.867 Fire protection: other components1279
a. Rule text1279
b. Intent of rule1280
c. Background1280
d. Acceptable compliance methods1280
e. References1280
[1097-1105] [RESERVED]1280
Section 9. MISCELLANEOUS
1106. Section 25.871 Leveling means1355
a. Rule text1355
b. Intent of rule1355
c. Background1355
d. Acceptable compliance methods1355
e. References1355
[1107-1112] [RESERVED]1355
Section 10. FUEL SYSTEM
1113. Section 25.963 Fuel tanks: general1375
a. Rule text1375
b. Intent of rule1375
c. Background1375
d. Acceptable compliance methods1375
e. References1376
[1114-1122] [RESERVED]1376
xxviii
Section 11. OPERATING LIMITATIONS 1123. Section
25.1517 Rough air speed, VRA1451
a. Rule text1451
b. Intent of rule1451
c. Background1451
d. Acceptable compliance methods1451
e. References1451
[1124-1129] [RESERVED]1451
1130. Section
25.1529/
Appendix H Instructions for continued airworthiness1452
a. Rule text1452
b. Intent of rule1452
c. Background1452
d. Acceptable compliance methods1452
e. References1452
[1131-1140] [RESERVED]1452
APPENDICES
APPENDIX 1 Glossary of acronyms and abbreviations1501
APPENDIX 2 Index of incorporated guidance material and references1502
1. Advisory circulars1502
2. Special conditions1504
3. Miscellaneous documents1504
xxix
Chapter 1 - STRUCTURE Section 1. GENERAL
1. SECTION 25.301 - LOADS.
a. Rule Text.
(a) Strength requirements are specified in terms of limit loads (the maximum loads to be expected in service) and ultimate loads (limit loads multiplied by prescribed factors of safety). Unless otherwise provided, prescribed loads are limit loads.
(b) Unless otherwise provided, the specified air, ground, and water loads must be placed in equilibrium with inertia forces, considering each item of mass in the airplane. These loads must be distributed to conservatively approximate or closely represent actual conditions. Methods used to determine load intensities and distribution must be validated by flight load measurement unless the methods used for determining those loading conditions are shown to be reliable.
(c) If deflections under load would significantly change the distribution of
external or internal loads, this redistribution must be taken into account.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that the loads developed on the airplane are accurately represented and are applied in a realistic manner. This rule also ensures that a margin of safety is provided above the expected loads by applying a factor of safety to the limit loads.
c. Background. This rule was carried forward from § 4b.200 of the Civil Air Regulations (CAR) with minor editorial changes. Amendment 25-23 requires that the methods used to determine load intensities and distributions must be validated by flight load measurements, unless the methods used for determining those loading conditions are shown to be reliable.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5672, April 8, 1970.
e. References. None.
2 - 10. [RESERVED] 1
11. SECTION 25.303 - FACTOR OF SAFETY.
a. Rule Text.
Unless otherwise specified, a factor of safety of 1.5 must be applied to the prescribed limit loads which are considered external loads on the structure. When a loading condition is prescribed in terms of ultimate loads, a factor of safety need not be applied unless otherwise specified.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure airplane structural integrity by providing a margin of safety above the loads expected in service. This is achieved by application of a factor of safety to the limit loads.
c. Background. This rule was carried forward from § 4b.200 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5672, April 8, 1970.
e. References. None.
12 - 22. [RESERVED]
23. SECTION 25.305 - STRENGTH AND DEFORMATION.
a. Rule Text.
(a) The structure must be able to support limit loads without detrimental permanent deformation. At any load up to limit loads, the deformation may not interfere with safe operation.
(b) The structure must be able to support ultimate loads without failure for at least 3 seconds. However, when proof of strength is shown by dynamic tests simulating actual load conditions, the 3-second limit does not apply. Static tests conducted to ultimate load must include the ultimate deflections and ultimate deformation induced by the loading. When analytical methods are used to show compliance with the ultimate load strength requirement, it must be shown that -
(1) The effects of deformation are not significant;
(2) The deformations involved are fully accounted for in the analysis; or
(3) The methods and assumptions used are sufficient to cover the effects of these deformations.
(c) Where structural flexibility is such that any rate of load application likely to occur in the operating conditions might produce transient stresses appreciably higher than those corresponding to static loads, the effects of this rate of application must be considered.
(d) [Reserved]
(e) The airplane must be designed to withstand any vibration and buffeting that might occur in any likely operating condition up to VD/MD, including stall and probable inadvertent excursions beyond the boundaries of the buffet onset envelope. This must be shown by analysis, flight tests, or other tests found necessary by the Administrator.
(f) Unless shown to be extremely improbable, the airplane must be designed to withstand any forced structural vibration resulting from any failure, malfunction or adverse condition in the flight control system. These must be considered limit loads and must be investigated at airspeeds up to VC/MC.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970; Amdt. 25-54, 45 FR 60172, Sep. 11, 1980; Amdt. 25-77, 57 FR 28949, Jun. 29, 1992; Amdt. 25-86, 61 FR 5220, Feb.
9, 1996]
b. Intent of Rule. This rule is intended to ensure that the structure is able to withstand loading conditions, including dynamic loading conditions, likely to be encountered in service. This includes loads due to continuous turbulence, buffeting, and vibration. The rule addresses the effects of deformation up to limit loads and up to ultimate loads.
c. Background. This rule was carried forward from § 4b.201 of the Civil Air Regulations (CAR) with some editorial changes. Amendment 25-23 added the requirement to investigate the dynamic response of the airplane to continuous turbulence. Amendment 25-54 added Appendix G (Continuous Gust Design Criteria). Vibration and buffeting were previously covered in § 25.251. Amendment 25 77 transferred the structural requirements of § 25.251 to §§ 25.305(e) and (f). Amendment 25-86 transferred the continuous turbulence requirement of § 25.305(d) to § 25.341(b).
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5672, April 8, 1970; Amendment 25-54, 45 FR 60172, September 11, 1980; Amendment 25-77, 57 FR 28949, June 29, 1992; and Amendment 25-86, 61 FR 5220, February 9, 1996.
e. References. None.
24 - 34. [RESERVED] 3
35. SECTION 25.307 - PROOF OF STRUCTURE.
a. Rule Text.
(a) Compliance with the strength and deformation requirements of this subpart must be shown for each critical loading condition. Structural analysis may be used only if the structure conforms to that for which experience has shown this method to be reliable. The Administrator may require ultimate load tests in cases where limit load tests may be inadequate.
(b)-(c) Reserved
(d) When static or dynamic tests are used to show compliance with the requirements of § 25.305(b) for flight structures, appropriate material correction factors must be applied to the test results, unless the structure, or part thereof, being tested has features such that a number of elements contribute to the total strength of the structure and the failure of one element results in the redistribution of the load through alternate load paths.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970; Amdt. 25-54, 45 FR 60172, Sep. 11, 1980; Amdt. 25-72, 55 FR 29775, Jul. 20, 1990]
b. Intent of Rule. The purpose of this rule is to ensure that flight structures will meet the required strength and deformation levels by analysis and tests. It also requires that appropriate material correction factors be applied to certain test results.
c. Background. Section 25.307 of the FAR was recodified from § 4b.202 of Civil Air Regulations (CAR) in 1964. During recodification, the text was revised for the purpose of technical clarification, but the intent of the requirement remained the same. Prior to adoption of part 4b of the CAR, a similar requirement existed in CAR 04, where § 04.3 required proof of compliance with the strength and deformation requirements of § 04.2.
(1) Amendment 25-23 revised § 25.307 by adding paragraph (d), the requirement for material correction factors which must be used for single load path flight structure substantiated by test only. While there was no corresponding requirement in part 4b of the CAR, § 04.3021 of CAR 04 required the use of material correction factors for all primary structure substantiated by test only.
Section 25.307 was further revised by Amendment 25-54 that added the last sentence in § 25.307(a).
Amendment 25-72 made further minor revisions, rescinding two redundant sections.
(2) The basic intent of the requirement has remained unchanged since the early 1940's. The regulation required proof of structural compliance with the strength and deformation requirement of Part 25 for each critical loading condition. Structural analysis was allowed only if the structure conformed to that which experience had shown the methods to be reliable.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5672, April 8, 1970; Amendment 25-54, 45 FR 60172, September 11, 1980; and Amendment 25-72, 55 FR 29775, July 20, 1990.
e. References. None.
36 - 44. [RESERVED] 5
Section 2. FLIGHT LOADS
45. SECTION 25.321 - GENERAL.
a. Rule Text.
(a) Flight load factors represent the ratio of the aerodynamic force component (acting normal to the assumed longitudinal axis of the airplane) to the weight of the airplane. A positive load factor is one in which the aerodynamic force acts upward with respect to the airplane.
(b) Considering compressibility effects at each speed, compliance with the flight loads requirements of this subpart must be shown
(1) At each critical altitude within the range of altitudes selected by the
applicant;
(2) At each weight from the design minimum weight to the design maximum weight appropriate to each particular flight load condition; and
(3) For each required altitude and weight, for any practicable distribution of disposable load within the operating limitations recorded in the Airplane Flight Manual.
(c) Enough points on and within the boundaries of the design envelope must be investigated to ensure that the maximum load for each part of the airplane structure is obtained.
(d) The significant forces acting on the airplane must be placed in equilibrium in a rational or conservative manner. The linear inertia forces must be considered in equilibrium with the thrust and all aerodynamic loads, while the angular (pitching) inertia forces must be considered in equilibrium with thrust and all aerodynamic moments, including moments due to loads on components such as tail surfaces and nacelles. Critical thrust values in the range from zero to maximum continuous thrust must be considered.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970; Amdt. 25-86, 61 FR 5220, Feb. 9, 1996]
b. Intent of Rule. The purpose for this rule is to ensure that all flight load conditions required by this subpart accurately account for all critical conditions.
c. Background. This rule was carried forward from § 4b.210 of the Civil Air Regulations (CAR).
Amendment 25-23 made minor editorial changes to the rule. Amendment 25-86 added paragraphs (c) and (d) taken from § 25.331 in order to re-organize the presentation accounting for the deletion of the gust envelope by Amendment 25-86.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5672, April 8, 1970; and Amendment 25-86, 61 FR 5220, February 9, 1996.
e. References. None.
46 - 54. [RESERVED] 52
Section 3. FLIGHT MANEUVER AND GUST CONDITIONS
55. SECTION 25.331 - SYMMETRIC MANEUVERING CONDITIONS.
a. Rule Text.
(a) Procedure. For the analysis of the maneuvering flight conditions specified in paragraphs (b) and (c) of this section, the following provisions apply:
(1) Where sudden displacement of a control is specified, the assumed rate of control surface displacement may not be less than the rate that could be applied by the pilot through the control system.
(2) In determining elevator angles and chordwise load distribution in the maneuvering conditions of paragraphs (b) and (c) of this section, the effect of corresponding pitching velocities must be taken into account. The in-trim and out-of-trim flight conditions specified in § 25.255 must be considered.
(b) Maneuvering balanced conditions. Assuming the airplane to be in equilibrium with zero pitching acceleration, the maneuvering conditions A through I on the maneuvering envelope in § 25.333(b) must be investigated.
(c) Pitch maneuver conditions. The conditions specified in paragraphs (c)(1) and
(c)(2) of this section must be investigated. The movement of the pitch control surfaces may be adjusted to take into account limitations imposed by the maximum pilot effort specified by § 25.397(b), control system stops and any indirect effect imposed by limitations in the output side of the control system (for example, stalling torque or maximum rate obtainable by a power control system.)
(1) Maximum pitch control displacement at VA. The airplane is assumed to be flying in steady level flight (point A1, § 25.333(b)) and the cockpit pitch control is suddenly moved to obtain extreme nose up pitching acceleration. In defining the tail load, the response of the airplane must be taken into account. Airplane loads that occur subsequent to the time when normal acceleration at the c.g. exceeds the positive limit maneuvering load factor (at point A2 in § 25.333(b)), or the resulting tailplane normal load reaches its maximum, whichever occurs first, need not be considered.
(2) Specified control displacement. A checked maneuver, based on a rational pitching control motion vs. time profile, must be established in which the design limit load factor specified in § 25.337 will not be exceeded. Unless lesser values cannot be exceeded, the airplane response must result in pitching accelerations not less than the following:
(i) A positive pitching acceleration (nose up) is assumed to be reached concurrently with the airplane load factor of 1.0 (points A1 to D1, § 25.333(b)).
The positive acceleration must be equal to at least 103
39n (n - 1.5),(Radians /sec2 ) V
Where n is the positive load factor at the speed under consideration, and V is the
airplane equivalent speed in knots.
(ii) A negative pitching acceleration (nose down) is assumed to be reached
concurrently with the positive maneuvering load factor (points A2 to D2,
§ 25.333(b)). This negative pitching acceleration must be equal to at least
- 26n 2 )(n - 1.5),(Radians / sec.
V
Where n is the positive load factor at the speed under consideration; and V is the
airplane equivalent speed in knots.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970; Amdt. 25-46, 43 FR 50594, Oct. 30, 1978; 43 FR 52495, Nov. 13, 1978; 43 FR 54082, Nov. 20, 1978; Amdt. 25-72, 55 FR 29775, Jul. 20, 1990; 55 FR 37607, Amdt. 25-86, 61 FR 5220, Feb. 9, 1996;
Amdt. 25-91, 62 FR 40704, Jul. 29, 1997]
b. Intent of Rule. The purpose of this rule is to ensure that the airplane structure is capable of withstanding maneuvering pitching conditions by requiring investigation of specific maneuvers. It requires the applicant to investigate all significant forces acting on the airplane during pitching maneuvers, including both inertia and aerodynamic forces.
c. Background. This rule was carried forward from § 4b.213 of the Civil Air Regulations (CAR) with minor changes. Amendment 25-23 made changes for clarification. Amendment 25-86 removed all reference to gusts and moved the general requirements to § 25.321. Amendment 25-91 revised the “unchecked” pitch maneuver so that the time history analysis no longer had to be carried out past the point where the maximum tail load was reached.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5672, April 8, 1970; Amendment 25-46, 43 FR 50594, October 30, 1978; 43 FR 52495, November 13, 1978; 43 FR 54082, November 20, 1978;
Amendment 25-72, 55 FR 29775, July 20, 1990; 55 FR 37607, Amendment 25-86, 61 FR 5220, February 9, 1996; and Amendment 25-91, 62 FR 40704, July 29, 1997.
e. References. None.
56 - 64. [RESERVED]
65. SECTION 25.333 - FLIGHT MANEUVERING ENVELOPE.
a. Rule Text.
(a) General. The strength requirements must be met at each combination of airspeed and load factor on and within the boundaries of the representative maneuvering and gust envelopes (V-n diagram) of paragraph (b) of this section.
This envelope must also be used in determining the airplane structural operating limitations as specified in § 25.1501.
(b) Maneuvering envelope.
NOTE: See Title 14, Code of Federal Regulations (14 CFR) for figure following paragraph (b).
[Amdt. 25-86, 61 FR 5220, Feb. 9, 1996]
b. Intent of Rule. The purpose of this rule is to provide a structural design envelope that will allow a realistic operating envelope for the airplane.
c. Background. This rule was carried forward from § 4b.211 of the Civil Air Regulations (CAR).
Amendment 25-86 revised this section by removing the gust envelope.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-86, 61 FR 5220, February 9, 1996.
e. References. None.
66 - 76. [RESERVED]
77. SECTION 25.335 - DESIGN AIRSPEEDS.
a. Rule Text.
The selected design airspeeds are equivalent airspeeds (EAS). Estimated values of VSO and VS1 must be conservative.
(a) Design cruising speed, VC. For V C, the following apply:
(1) The minimum value of VC must be sufficiently greater than VB to provide for inadvertent speed increases likely to occur as a result of severe atmospheric turbulence.