(ii) The castings must be inspected as provided in paragraph (d)(1) of this section for casting factors of “1.25 through 1.50" and tested under paragraph (c)(2) of this section.
b. Intent of Rule. The purpose of this rule is to account for variability in strength due to uncertainties in manufacturing processes and inspection methods. These uncertainties are usually the result of porosity or inclusions in the castings.
c. Background. This rule was carried forward § 4b.307 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. References. None.
720 - 728. [RESERVED]
729. SECTION 25.623 - BEARING FACTORS.
a. Rule Text.
(a) Except as provided in paragraph (b) of this section, each part that has clearance (free fit), and that is subject to pounding or vibration, must have a bearing factor large enough to provide for the effects of normal relative motion.
(b) No bearing factor need be used for a part for which any larger special factor is prescribed.
b. Intent of Rule. The purpose for this rule is to provide for normal relative motion in parts subject to pounding or vibration.
c. Background. The basis for this rule was carried forward from § 4b.307 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. References. None.
730 - 738. [RESERVED] 741
739. SECTION 25.625 - FITTING FACTORS.
a. Rule Text.
For each fitting (a part or terminal used to join one structural member to
another), the following apply:
(a) For each fitting whose strength is not proven by limit and ultimate load tests in which actual stress conditions are simulated in the fitting and surrounding structures, a fitting factor of at least 1.15 must be applied to each part of -
(1) The fitting;
(2) The means of attachment; and
(3) The bearing on the joined members.
(b) No fitting factor need be used -
(1) For joints made under approved practices and based on comprehensive test data (such as continuous joints in metal plating, welded joints, and scarf joints in wood); or
(2) With respect to any bearing surface for which a larger special factor is used.
(c) For each integral fitting, the part must be treated as a fitting up to the point at which the section properties become typical of the member.
(d) For each seat, berth, safety belt, and harness, the fitting factor specified in § 25.785(f)(3) applies.
[Amdt. 25-23, 35 FR 5674, Apr. 8, 1970; Amdt. 25-72, 55 FR 29776, Jul. 20, 1990]
b. Intent of Rule. The purpose of this rule is to ensure that fittings will not be under strength because of the inability to predict the stress conditions in the part.
c. Background. This rule was carried forward from § 4b.307 of the Civil Air Regulations (CAR).
Amendment 25-23 added the reference to § 25.785 for seats, berths, safety belts, and harnesses.
Amendment 25-72 made minor editorial changes.
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 Amendment 25-72, 55 FR 29776, July 20, 1990.
e. References. None.
740 - 745. [RESERVED]
746. SECTION 25.629 - AEROELASTIC STABILITY REQUIREMENTS.
a. Rule Text.
(a) General. The aeroelastic stability evaluations required under this section include flutter, divergence, control reversal and any undue loss of stability and control as a result of structural deformation. The aeroelastic evaluation must include whirl modes associated with any propeller or rotating device that contributes significant dynamic forces. Compliance with this section must be shown by analyses, wind tunnel tests, ground vibration tests, flight tests, or other means found necessary by the Administrator.
(b) Aeroelastic stability envelopes. The airplane must be designed to be free from aeroelastic instability for all configurations and design conditions within the aeroelastic stability envelopes as follows:
(1) For normal conditions without failures, malfunctions, or adverse conditions, all combinations of altitudes and speeds encompassed by the VD/MD versus altitude envelope enlarged at all points by an increase of 15 percent in equivalent airspeed at both constant Mach number and constant altitude. In addition, a proper margin of stability must exist at all speeds up to VD/MD, and there must be no large and rapid reduction in stability as VD/MD is approached. The enlarged envelope may be limited to Mach 1.0 when MD is less than 1.0 at all design altitudes, and
(2) For the conditions described in § 25.659(d) below, for all approved altitudes, any airspeed up to the greater airspeed defined by;
(i) The VD/MD envelope determined by § 25.335(b); or,
(ii) An altitude-airspeed envelope defined by a 15 percent increase in equivalent airspeed above VC at constant altitude, from sea level to the altitude of the intersection of 1.15 VC with the extension of the constant cruise Mach number line, MC, then a linear variation in equivalent airspeed to MC + .05 at the altitude of the lowest VC/MC intersection; then, at higher altitudes, up to the maximum flight altitude, the boundary defined by a .05 Mach increase in MC at constant altitude.
(c) Balance weights. If concentrated balance weights are used, their effectiveness and strength, including supporting structure, must be substantiated.
(d) Failures, malfunctions, and adverse conditions. The failures, malfunctions, and adverse conditions which must be considered in showing compliance with this section are:
(1) Any critical fuel loading conditions, not shown to be extremely improbable, which may result from mismanagement of fuel.
(2) Any single failure in any flutter damper system.
(3) For airplanes not approved for operation in icing conditions, the maximum likely ice accumulation expected as a result of an inadvertent encounter.
(4) Failure of any single element of the structure supporting any engine, independently mounted propeller shaft, large auxiliary power unit, or large externally mounted aerodynamic body (such as an external fuel tank).
(5) For airplanes with engines that have propellers or large rotating devices capable of significant dynamic forces, any single failure of the engine structure that would reduce the rigidity of the rotational axis.
(6) The absence of aerodynamic or gyroscopic forces resulting from the most adverse combination of feathered propellers or other rotating devices capable of significant dynamic forces. In addition, the effect of a single feathered propeller or rotating device must be coupled with the failures of paragraphs (d)(4) and
(d)(5) of this section.
(7) Any single propeller or rotating device capable of significant dynamic forces rotating at the highest likely overspeed.
(8) Any damage or failure condition, required or selected for investigation by § 25.571. The single structural failures described in paragraphs (d)(4) and (d)(5) of this section need not be considered in showing compliance with this section if:
(I) the structural element could not fail due to discrete source damage resulting from the conditions described in § 25.571(e), and;
(ii) A damage tolerance investigation in accordance with § 25.571(b) shows that the maximum extent of damage assumed for the purpose of residual strength evaluation does not involve complete failure of the structural element.
(9) Any damage, failure, or malfunction considered under §§ 25.631, 25.671,
25.672, and 25.1309.
(10) Any other combination of failures, malfunctions, or adverse conditions not shown to be extremely improbable.
(e) Flight flutter testing. Full scale flight flutter tests at speeds up to V DF/MDF must be conducted for new type designs and for modifications to a type design unless the modifications have been shown to have an insignificant effect on the aeroelastic stability. These tests must demonstrate that the airplane has a proper margin of damping at all speeds up to VDF/MDF, and that there is no large and rapid reduction in damping as VDF/MDF is approached. If a failure, malfunction, or adverse condition is simulated during flight test in showing compliance with paragraph (d) of this section, the maximum speed investigated need not exceed VFC/MFC if it is shown, by correlation of the flight test data with other test data or analyses, that the airplane is free from any aeroelastic instability at all speeds within the altitude-airspeed envelope described in paragraph (b)(2) of this section.
[Amdt. 25-23, 35 FR 5674, Apr. 8,1970; Amdt. 25-46, 43 FR 50595, Oct.
30,1978; Amdt. 25-72, 55 FR 29776, Jul. 20, 1990; Amdt. 25-77, 57 FR 28949, Jun. 29,1992] 744
b. Intent of Rule. The purpose of this rule is to ensure that the airplane will be free from any aeroelastic instabilities including flutter, divergence, control reversal, and any undue loss of stability and control as a result of structural deformation or failure. Amendment 25-77 recognized the improved substantiation capability of modern methods and revised the safety margins from 20 percent to 15 percent for flutter substantiation.
c. Background. This rule was carried forward from § 4b.307 of the Civil Air Regulations (CAR).
Amendment 25-23 made extensive changes to the rule.
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; Amendment 25-46, 43 FR 50595, October 30,1978; Amendment 25-72, 55 FR 29776, July 20, 1990; and Amendment 25-77, 57 FR 28949, June 29,1992. Advisory Circular (AC) 25.629-1, or latest revision, provides guidance information for demonstrating compliance with the design requirements for transport category airplanes to preclude flutter and other aeroelastic phenomena.
e. References. Advisory Circular 25.629-1, or latest revision, Flutter Substantiation of Transport Category Airplanes.
747 - 759. [RESERVED]
760. SECTION 25.631 - BIRD STRIKE DAMAGE.
a. Rule Text.
The empennage structure must be designed to assure capability of continued safe flight and landing of the airplane after impact with an 8-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, selected under § 25.335(a). Compliance with this section by provision of redundant structure and protected location of control system elements or protective devices such as splitter plates or energy absorbing material is acceptable. Where compliance is shown by analysis, tests, or both, use of data on airplanes having similar structural design is acceptable.
[Amdt. 25-23, 35 FR 5674, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that the empennage structure is able to withstand impacts with birds likely to be encountered in service.
c. Background. This rule was introduced by Amendment 25-23.
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.
761 - 769. [RESERVED] 746
Section 2. CONTROL SURFACES
770. SECTION 25.651 - PROOF OF STRENGTH.
a. Rule Text.
(a) Limit load tests of control surfaces are required. These tests must include the horn or fitting to which the control system is attached.
(b) Compliance with the special factors requirements of §§ 25.619 through
25.625 and 25.657 for control surface hinges must be shown by analysis or individual load tests.
b. Intent of Rule. The purpose of this rule is to ensure that the control surfaces and their attachments are able to withstand the design loads.
c. Background. This rule was carried forward from § 4b.311 of the Civil Air Regulations (CAR) with editorial changes.
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. References. None.
771 - 778. [RESERVED]
779. SECTION 25.655 - INSTALLATION.
a. Rule Text.
(a) Movable tail surfaces must be installed so that there is no interference between any surfaces when one is held in its extreme position and the others are operated through their full angular movement.
(b) If an adjustable stabilizer is used, it must have stops that will limit its range of travel to the maximum for which the airplane is shown to meet the trim requirements of § 25.161.
b. Intent of Rule. The purpose of this rule is to ensure that control surfaces will not interfere with each other in any possible condition of deflection. It also requires that the stabilizer trim is physically 845
limited by stops so that the airplane can meet minimum control requirements in the most adverse trim positions. This is intended to limit the effects of trim system failures.
c. Background. This rule was carried forward basically unchanged from § 4b.312 of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. Compliance with this requirement may be shown by test(s) and/or analysis.
e. References. None.
780 - 787. [RESERVED]
788. SECTION 25.657 - HINGES.
a. Rule Text.
(a) For control surface hinges, including ball, roller, and self-lubricated bearing hinges, the approved rating of the bearing may not be exceeded. For nonstandard bearing hinge configurations, the rating must be established on the basis of experience or tests and, in the absence of a rational investigation, a factor of safety of not less than 6.67 must be used with respect to the ultimate bearing strength of the softest material used as a bearing.
(b) Hinges must have enough strength and rigidity for loads parallel to the hinge line.
[Amdt. 25-23, 35 FR 5674, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that control surface hinges do not fail due to any flight or ground loading conditions.
c. Background. This rule was carried forward from § 4b.313 of the Civil Air Regulations (CAR).
Amendment 25-23 added self-lubricated bearings to the list of bearings that may not exceed their rated capacity.
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.
789 -793. [RESERVED] 846
Section 3. CONTROL SYSTEMS
794. SECTION 25.671 - GENERAL.
a. Rule Text.
(a) Each control and control system must operate with the ease, smoothness, and positiveness appropriate to its function.
(b) Each element of each flight control system must be designed, or distinctively and permanently marked, to minimize the probability of incorrect assembly that could result in the malfunctioning of the system.
(c) The airplane must be shown by analysis, tests, or both, to be capable of continued safe flight and landing after any of the following failures or jamming in the flight control system and surfaces (including trim, lift, drag, and feel systems) within the normal flight envelope, without requiring exceptional piloting skill or strength. Probable malfunctions must have only minor effects on control system operation and must be capable of being readily counteracted by the pilot.
(1) Any single failure, excluding jamming (for example, disconnection or failure of mechanical elements, or structural failure of hydraulic components, such as actuators, control spool housing, and valves).
(2) Any combination of failures not shown to be extremely improbable, excluding jamming (for example, dual electrical or hydraulic system failures, or any single failure in combination with any probable hydraulic or electrical failure).
(3) Any jam in a control position normally encountered during takeoff, climb, cruise, normal turns, descent, and landing unless the jam is shown to be extremely improbable, or can be alleviated. A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable.
(d) The airplane must be designed so that it is controllable if all engines fail.
Compliance with this requirement may be shown by analysis where that method has been shown to be reliable.
[Amdt. 25-23, 35 FR 5674, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that failures in the flight control systems will not result in catastrophic loss of the airplane.
c. Background. This rule was carried forward from § 4b.320 of the Civil Air Regulations (CAR).
Amendment 25-23 made major changes to this rule by addressing combinations of failure conditions.
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) 25-14, or latest revision, provides guidance information for showing compliance with the requirements for structural and functional safety standards for high lift and drag devices and their operating systems.
Advisory Circular 25.672-1, or latest revision, provides guidance information for showing an equivalent means of compliance by setting forth procedures applying to load alleviation systems (SAS), and flutter suppression systems (FSS).
e. References. Advisory Circular 25-14, or latest revision, High Lift and Drag Devices; and AC
25.672-1, or latest revision, Active Flight Controls.
795 - 800. [RESERVED]
801. SECTION 25.672 - STABILITY AUGMENTATION AND AUTOMATIC AND POWER-OPERATED SYSTEMS.
a. Rule Text.
If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with § 25.671 and the following:
(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.
(b) The design of the stability augmentation system or of any other automatic or power-operated system must permit initial counteraction of failures of the type specified in § 25.671(c) without requiring exceptional pilot skill or strength, by either the deactivation of the system, or a failed portion thereof, or by overriding the failure by movement of the flight controls in the normal sense.
(c) It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated system
(1) The airplane is safely controllable when the failure or malfunction occurs at any speed or altitude within the approved operating limitations that is critical for the type of failure being considered;
(2) The controllability and maneuverability requirements of this part are met within a practical operational flight envelope (for example, speed, altitude, normal acceleration, and airplane configurations) which is described in the Airplane Flight Manual; and 922
(3) The trim, stability, and stall characteristics are not impaired below a level needed to permit continued safe flight and landing.
[Amdt. 25-23, 35 FR 5675, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that failures in automatic and power operated systems will not be a hazard to the airplane.
c. Background. This fundamental rule was carried forward from § 4b.320 of the Civil Air Regulations (CAR). The rule was substantially revised by Amendment 25-23 to raise the level of safety.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5675, April 8, 1970. Advisory Circular (AC) 25.672-1, or latest revision, provides guidance information for showing an equivalent means of compliance by setting forth procedures applying to load alleviation systems (SAS), and flutter suppression systems (FSS).
Advisory Circular 25.1309-1A, or latest revision, describes various acceptable means of compliance to provide guidance for the engineering and operational judgment that form the basis for compliance findings.
e. References. Advisory Circular 25.672-1, or latest revision, Active Flight Controls;
AC 25.1309-1A, or latest revision, System Design Analysis; AC 25.1329-1A, or latest revision, Automatic Pilot Systems Approval; and Special Conditions (SC) issued against fly-by-wire airplanes.
Reference SC 25-ANM-23 for the A320, and reference SC 25-ANM-78 for the Boeing Model 777.
802 - 810. [RESERVED]
811. SECTION 25.673 - Two-Control Airplanes. [Removed] NOTE: This rule was removed because it is obsolete in regard to transport category airplanes.
[Amdt. 25-72, 55 FR 29776, Jul. 20, 1990]
812. SECTION 25.675 - STOPS.
a. Rule Text.
(a) Each control system must have stops that positively limit the range of motion of each movable aerodynamic surface controlled by the system.