(2) Except as provided in § 25.335(d)(2), VC may not be less than VB+ 1.32 UREF (with UREF as specified in § 25.341(a)(5)(i)). However VC need not exceed the maximum speed in level flight at maximum continuous power for the corresponding altitude.
(3) At altitudes where VD is limited by Mach number, VC may be limited to a selected Mach number.
(b) Design dive speed, VD. V D must be selected so that VC/MC is not greater than
0.8 VD/MD, or so that the minimum speed margin between VC/MC and VD/MD is the greater of the following values:
(1) From an initial condition of stabilized flight at VC/MC, the airplane is upset, flown for 20 seconds along a flight path 7.5º below the initial path, and then pulled up at a load factor of 1.5g (0.5g acceleration increment). The speed increase occurring in this maneuver may be calculated if reliable or conservative aerodynamic data is used. Power as specified in § 25.175(b)(1)(iv) is assumed until the pull-up is initiated, at which time power reduction and the use of pilot controlled drag devices may be assumed;
(2) The minimum speed margin must be enough to provide for atmospheric variations (such as horizontal gusts, and penetration of jet streams and cold fronts) and for instrument errors and airframe production variations. These factors may be considered on a probability basis. The margin at altitude where MC is limited by compressibility effects must not be less than 0.07M unless a lower margin is determined using a rational analysis that includes the effects of any automatic systems. In any case, the margin may not be reduced to less than
0.05M.
(c) Design maneuvering speed VA. For V A, the following apply:
(1) VA may not be less than VS1 �n where -
(i) n is the limit positive maneuvering load factor at VC; and
(ii) VS1 is the stalling speed with flaps retracted.
(2) VA and VS must be evaluated at the design weight and altitude under consideration.
(3) VA need not be more than VC or the speed at which the positive CN MAX curve intersects the positive maneuver load factor line, whichever is less.
(d) Design speed for maximum gust intensity, VB.
(1) VB may not be less than Ø Œº1 where__ VS1 = the 1-g stalling speed based on C NAMAX with the flaps retracted at the particular weight under consideration;
K U V ag ref C 1 / 2 VS1 ø + 498w œß 106
Vc = design cruise speed (knots equivalent airspeed);
Uref = the reference gust velocity (feet per second equivalent airspeed) from § 25.341(a)(5)(i);
w = average wing loading (pounds per square foot) at the particular weight under consideration.
.88mK = g . + m5 3 2 w m = r cag r = density of air (slugs/ft 3);
c = mean geometric chord of the wing (feet);
g = acceleration due to gravity (ft/sec 2);
a = slope of the airplane normal force coefficient curve, C NA per radian;
(2) At altitudes where VC is limited by Mach number__
(i) VB may be chosen to provide an optimum margin between low and high speed buffet boundaries; and,
(ii) VB need not be greater than VC.
(e) Design flap speeds, VF. For V F, the following apply:
(1) The design flap speed for each flap position (established in accordance with § 25.697(a)) must be sufficiently greater than the operating speed recommended for the corresponding stage of flight (including balked landings) to allow for probable variations in control of airspeed and for transition from one flap position to another.
(2) If an automatic flap positioning or load limiting device is used, the speeds and corresponding flap positions programmed or allowed by the device may be used.
(3) VF may not be less than
(i) 1.6 VS1 with the flaps in takeoff position at maximum takeoff weight;
(ii) 1.8 V S1 with the flaps in approach position at maximum landing weight; and
(iii) 1.8 VSO with the flaps in landing position at maximum landing weight.
(f) Design drag device speeds, VDD. The selected design speed for each drag device must be sufficiently greater than the speed recommended for the operation of the device to allow for probable variations in speed control. For drag devices intended for use in high speed descents, VDD may not be less than VD. When an automatic drag device positioning or load limiting means is used, the speeds and corresponding drag device positions programmed or allowed by the automatic means must be used for design.
[Amdt. 25-23, 35 FR 5672, April 8, 1970; Amdt. 25-86, 61 FR 5220, February 9, 1996; Amdt. 25-91, 62 FR 40702, July 29, 1997]
b. Intent of Rule. This rule establishes the airspeeds used for structural design of the airframe.
Structural design speeds are based on the structural design stall speed for the airplane as indicated in
25.333(b). These structural design airspeeds are selected to provide a margin above the likely operating speeds, including overspeed due to inadvertent upsets. Approved operating airspeeds must not exceed the approved structural design airspeeds.
c. Background. This rule was carried forward from §§ 4b.210 and 4b.711 of the Civil Air Regulations (CAR). Prior to Amendment 25-23, the design dive speed (VD/MD) was defined in the operating limitations section of the airworthiness standards as a margin above VMO/MMO. V D/MD was selected to provide a speed margin above VMO/MMO sufficient to make it highly improbable that VD /MD would be inadvertently exceeded in operation. Since VD/MD is a structural design speed, Amendment 25-23 brought the definition of VD/MD forward to the structures section of the airworthiness standards and redefined it as the speed margin above VC/MC. The speed margins were not changed.
Amendment 25-23 also addressed the design drag device speeds (VDD). Amendment 25-86 redefined the design speed for maximum gust intensity to be consistent with the tuned gust methodology adopted in that amendment. Amendment 25-91 provided a redefinition of the minimum speed margin for VC based on the gust intensities that had been revised by Amendment 25-86. Amendment 25-91 also clarified the meaning of § 25.335(b)(2) to require a rational analysis. Advisory Circular (AC) 25.335 1, or latest revision, was provided for that purpose. In lieu of a rational analysis, a minimum margin of Mach 0.07 can be used.
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 25-86, 61 FR 5220, February 9, 1996; and Amendment 25-91, 62 FR 40702, July 29, 1997. Advisory Circular 25.335-1, or latest revision, provides guidance information for paragraph (b)(2) for showing compliance with airspeeds used for structural design of the airframe.
e. References. Advisory Circular 25.335-1, or latest revision, Design Dive Speed.
78 - 86. [RESERVED]
87. SECTION 25.337 - LIMIT MANEUVERING LOAD FACTORS.
a. Rule Text.
(a) Except where limited by maximum (static) lift coefficients, the airplane is assumed to be subjected to symmetrical maneuvers resulting in the limit maneuvering load factors prescribed in this section. Pitching velocities appropriate to the corresponding pull-up and steady turn maneuvers must be taken into account.
(b) The positive limit maneuvering load factor "n" for any speed up to Vn may not be less than 2.1 + 24,000/(W+10,000), except that "n" may not be less than
2.5 and need not be greater than 3.8-- where "W" is the maximum design takeoff weight.
(c) The negative limit maneuvering load factor
(1) May not be less than -1.0 at speeds up to VC; and
(2) Must vary linearly with speed from the value at VC to zero at VD.
(d) Maneuvering load factors lower than those specified in this section may be used if the airplane has design features that make it impossible to exceed these values in flight.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970]
b. Intent of Rule. The purpose for establishing minimum maneuvering load factors is to ensure that the airplane is able to withstand any likely maneuver experienced in service without structural damage to the airplane.
c. Background. This rule was carried forward from § 4b.211 of the Civil Air Regulations (CAR).
Prior to Amendment 25-23 to this rule, the positive limit maneuvering load factor, n, was selected by the applicant, except that it could not be less than 2.5. Amendment 25-23 redefined the positive limit maneuvering load factor as a function of design maximum takeoff weight. It was allowed to range between 2.5 and 3.8. This resulted in a positive limit load factor greater than 2.5 for airplanes with less than 50,000 lb. design maximum takeoff weight. The negative limit maneuvering load factor remained unchanged.
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.
88 -96. [RESERVED]
97. SECTION 25.341 - GUST AND TURBULENCE LOADS.
a. Rule Text.
(a) Discrete Gust Design Criteria. The airplane is assumed to be subjected to symmetrical vertical and lateral gusts in level flight. Limit gust loads must be determined in accordance with the following provisions:
(1) Loads on each part of the structure must be determined by dynamic analysis.
The analysis must take into account unsteady aerodynamic characteristics and all significant structural degrees of freedom including rigid body motions.
(2) The shape of the gust must be:
ø
Ł ( ł )1 for 0 < s < 2H where__ s = distance penetrated into the gust (feet);
U = the design gust velocity in equivalent airspeed specified in paragraph ds
(a)(4) of this section; and H = the gust gradient which is the distance (feet) parallel to the airplane's flight path for the gust t o reach its peak velocity.
(3) A sufficient number of gust gradient distances in the range 30 feet to 350 feet must be investigated to find the critical response for each load quantity.
(4) The design gust velocity must be:
1 / 6 ) ( Ø Œº psUU = ds - Cos œß 2 H
350 U U = ds F H ref g where__ U = the reference gust velocity in equivalent airspeed defined in paragraphref
(a)(5) of this section.
Fg = the flight profile alleviation factor defined in paragraph (a)(6) of this section.
(5) The following reference gust velocities apply:
(i) At the airplane design speed VC: Positive and negative gusts with reference gust velocities of 56.0 ft/sec EAS must be considered at sea level. The reference gust velocity may be reduced linearly from 56.0 ft/sec EAS at sea level to 44.0 ft/sec EAS at 15000 feet. The reference gust velocity may be further reduced linearly from 44.0 ft/sec EAS at 15000 feet to 26.0 ft/sec EAS at 50000 feet.
(ii) At the airplane design speed VD: The reference gust velocity must be 0.5 times the value obtained under § 25.341(a)(5)(i).
(6) The flight profile alleviation factor, Fg, must be increased linearly from the sea level value to a value of 1.0 at the maximum operating altitude defined in § 25.1527. At sea level, the flight profile alleviation factor is determined by the following equation:
Fg = 0.5(Fgz + Fgm ) where__ ZmoFgz = 1 - ;
250000
(pR ) ;F = R Tan 1 gm 2 4Ł ł Maximum Landing WeightR1 = ;Maximum Take -off Weight Maximum Zero Fuel WeightR = ;2 Maximum Take - off Weight Z = Maximum operating altitude defined in § 25.1527. mo
(7) When a stability augmentation system is included in the analysis, the effect of any significant system nonlinearities should be accounted for when deriving limit loads from limit gust conditions.
(b) Continuous Gust Design Criteria. The dynamic response of the airplane to vertical and lateral continuous turbulence must be taken into account. The continuous gust design criteria of Appendix G of this part must be used to establish the dynamic response unless more rational criteria are shown.
[Amdt. 25-72, 55 FR 37607, Sep. 12, 1990; Amdt. 25-86, 61 FR 5220, Feb. 9, 1996, correction published in 61 FR 9533, on Mar. 3, 1996]
b. Intent of Rule. The purpose of this rule is to define dynamic discrete gust load conditions and continuous turbulence conditions that the airplane is likely to experience during its lifetime. The airplane structure must be designed to withstand these loads.
c. Background. Amendment 25-86 reorganized the gust requirement of the Federal Aviation Regulations (FAR) to bring all gust requirements under section 25.341. The previous static discrete gust criterion was replaced by a dynamic “tuned” discrete gust criterion. In addition, the design gust velocities were revised based on more recent measurements of gusts. The continuous turbulence requirements were brought from § 25.305(d) to § 25.341(b) without change.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-72, 55 FR 37607, September 12, 1990; and Amendment 25-86, 61 FR 5220, February 9, 1996, correction published in 61 FR 9533, on March 3, 1996.
e. References. None.
98 -108. [RESERVED]
109. SECTION 25.343 - DESIGN FUEL AND OIL LOADS.
a. Rule Text.
(a) The disposable load combinations must include each fuel and oil load in the range from zero fuel and oil to the selected maximum fuel and oil load. A structural reserve fuel condition, not exceeding 45 minutes of fuel under the operating conditions in § 25.1001(e) and (f), as applicable, may be selected.
(b) If a structural reserve fuel condition is selected, it must be used as the minimum fuel weight condition for showing compliance with the flight load requirements as prescribed in this subpart. In addition -
(1) The structure must be designed for a condition of zero fuel and oil in the wing at limit loads corresponding to -
(i) A maneuvering load factor of +2.25; and
(ii) The gust conditions of § 25.341(a) but assuming 85% of the design velocities prescribed in § 25.341(a)(4); and
(2) Fatigue evaluation of the structure must account for any increase in operating stresses resulting from the design condition of paragraph (b)(1) of this section; and
(3) The flutter, deformation, and vibration requirements must also be met with zero fuel.
[Amdt. 25-18, 33 FR 12226, Aug. 30,1968; Amdt. 25-72, 55 FR 37607, Sep. 12, 1990; Amdt. 25-86, 61 FR 5221, Feb. 9, 1996]
b. Intent of Rule. The purpose of this rule is to ensure that the inertia effects of fuel and oil are accounted for in calculating loads on the airframe. It is also intended to require that the airplane meet the flutter, deformation, and vibration requirements for the full range of fuel and oil loading conditions.
c. Background. This rule was carried forward from § 4b.210(c) of the Civil Air Regulations (CAR).
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-18, 33 FR 12226, August 30,1968; Amendment 25-72, 55 FR 37607, September 12, 1990; and Amendment 25-86, 61 FR 5221, February 9, 1996.
e. References. None.
110 - 118. [RESERVED]
119. SECTION 25.345 - HIGH LIFT DEVICES.
a. Rule Text.
(a) If wing flaps are to be used during takeoff, approach, or landing, at the design flap speeds established for these stages of flight under § 25.335(e) and with the wing flaps in the corresponding positions, the airplane is assumed to be subjected to symmetrical maneuvers and gusts. The resulting limit loads must correspond to the conditions determined as follows:
(1) Maneuvering to a positive limit load factor of 2.0; and
(2) Positive and negative gusts of 25 ft/sec EAS acting normal to the flight path in level flight. Gust loads resulting on each part of the structure must be determined by rational analysis. The analysis must take into account the unsteady aerodynamic characteristics and rigid body motions of the aircraft. The shape of the gust must be as described in § 25.341(a)(2) except that - Uds = 25 ft/sec EAS;
H = 12.5 c; and c = mean geometric chord of the wing (feet).
(b) The airplane must be designed for the conditions prescribed in paragraph (a) of this section, except that the airplane load factor need not exceed 1.0, taking into account, as separate conditions, the effects of
(1) Propeller slipstream corresponding to maximum continuous power at the design flap speeds VF, and with takeoff power at not less than 1.4 times the stalling speed for the particular flap position and associated maximum weight;
and
(2) A head on gust of 25 feet per second velocity (EAS).
(c) If flaps or other high lift devices are to be used in en route conditions, and with flaps in the appropriate position at speeds up to the flap design speed chosen for these conditions, the airplane is assumed to be subjected to symmetrical maneuvers and gusts within the range determined by -
(1) Maneuvering to a positive limit load factor as prescribed in
§ 25.337(b); and
(2) The discrete vertical gust criteria in § 25.341(a).
(d) The airplane must be designed for a maneuvering load factor of 1.5 g at the maximum take -off weight with the wing-flaps and similar high lift devices in the landing configurations.
[Amdt. 25-46, 43 FR 50595, Oct. 30, 1978; Amdt. 25-72, 55 FR 37607, Sep. 17, 1990; Amdt. 25-86, 61 FR 5221, Feb. 9, 1996; Amdt. 25-91, 62 FR 40702, Jul. 29, 1997]
b. Intent of Rule. The purpose of this rule is to ensure adequate strength in the airframe to carry loads that are likely to develop while operating with flaps or similar high lift devices extended. The referenced stalling speeds are 1g structural design stalling speeds.
c. Background. This rule was carried forward from § 4b.212 of the Civil Air Regulations (CAR).
Amendment 25-46 added subparagraph (d) that prescribed a load factor of 1.5 when landing at maximum takeoff weight with the flaps in the landing configuration. Amendment 25-72 amended subparagraph (c)(1) by changing the maneuver load factor of 2.5 to the load factor prescribed in
25.337(b).
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; Amendment 25-72, 55 FR 37607, September 17, 1990; Amendment 25-86, 61 FR 5221, February 9, 1996; and Amendment 25-91, 62 FR 40702, July 29, 1997. Advisory Circular (AC) 25-14, or latest revision, provides guidance information for showing compliance with structural and functional safety standards for high lift and drag devices and their operating systems.
e. References. Advisory Circular 25-14, or latest revision, High Lift and Drag Devices.
120 - 128. [RESERVED]
129. SECTION 25.349 - ROLLING CONDITIONS.
a. Rule Text.
The airplane must be designed for loads resulting from the rolling conditions specified in paragraphs (a) and (b) of this section. Unbalanced aerodynamic moments about the center of gravity must be reacted in a rational or conservative manner, considering the principal masses furnishing the reacting inertia forces.
(a) Maneuvering. The following conditions, speeds, and aileron deflections (except as the deflections may be limited by pilot effort) must be considered in combination with an airplane load factor of zero and of two-thirds of the positive maneuvering factor used in design. In determining the required aileron 115
deflections, the torsional flexibility of the wing must be considered in accordance with § 25.301(b):
(1) Conditions corresponding to steady rolling velocities must be investigated. In addition, conditions corresponding to maximum angular acceleration must be investigated for airplanes with engines or other weight concentrations outboard of the fuselage. For angular acceleration conditions, zero rolling velocity may be assumed in the absence of a rational time history investigation of the maneuver.
(2) At VA, a sudden deflection of the aileron to the stop is assumed.
(3) At VC, the aileron deflection must be that required to produce a rate of roll not less than that obtained in paragraph (a)(2) of this section.
(4) At VD, the aileron deflection must be that required to produce a rate of roll not less than one-third of that in paragraph (a)(2) of this section.
(b) Unsymmetrical gusts. The airplane is assumed to be subjected to unsymmetrical vertical gusts in level flight. The resulting limit loads must be determined from either the wing maximum airload derived directly from § 25.341(a), or the wing maximum airload derived indirectly from the vertical load factor calculated from § 25.341(a). It must be assumed that 100 percent of the wing air load acts on one side of the airplane and 80 percent of the wing air load acts on the other side.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970; Amdt. 25-86, 61 FR 5222, Feb. 9, 1996]
b. Intent of Rule. The purpose of this rule is to ensure that the airframe is designed to withstand any likely airplane rolling maneuver condition or unsymmetrical gust condition expected in service.
c. Background. The loading condition rule was carried forward from § 4b.214 of the Civil Air Regulations (CAR). Amendment 25-86 completely revised the unsymetrical gust requirement since it previously depended on the static discrete gust criteria and also on the gust envelope that were eliminated by Amendment 25-86. the new paragraph prescribes a simplified method for the unsymmetical gusts based on the dynamic discrete gust criteria of the revised § 25.341(a).
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 5222, February 9, 1996.
e. References. None.
130 - 137. [RESERVED] 116
138. SECTION 25.351 - YAW MANEUVER CONDITIONS.
a. Rule Text.
The airplane must be designed for loads resulting from the yaw maneuver conditions specified in paragraphs (a) through (d) of this section at speeds from VMC to VD. Unbalanced aerodynamic moments about the center of gravity must be reacted in a rational or conservative manner considering the airplane inertia forces. In computing the tail loads the yawing velocity may be assumed to be zero.
(a) With the airplane in unaccelerated flight at zero yaw, it is assumed that the cockpit rudder control is suddenly displaced to achieve the resulting rudder deflection, as limited by:
(1) the control system on control surface stops; or
(2) a limit pilot force of 300 pounds from VMC to VA and 200 pounds from VC/MC to VD/MD, with a linear variation between VA and VC/MC.
(b) With the cockpit rudder control deflected so as always to maintain the
maximum rudder deflection available within the limitations specified in
paragraph (a) of this section, it is assumed that the airplane yaws to the
overswing sideslip angle.
(c) With the airplane yawed to the static equilibrium sideslip angle, it is assumed that the cockpit rudder control is held so as to achieve the maximum rudder deflection available within the limitations specified in paragraph (a) of this section.
(d) With the airplane yawed to the static equilibrium sideslip angle of paragraph
(c) of this section, it is assumed that the cockpit rudder control is suddenly
returned to neutral.
[Amdt. 25-23, 35 FR 5672, Apr. 8, 1970; Amdt. 25-46, 43 FR 50595, Oct. 30, 1978; Amdt. 25-72, 55 FR 29775, Jul. 20, 1990; 55 FR 37607, Sept. 12, 1990; 55 FR 41415, Oct. 11, 1990; Amdt. 25-86, 61 FR 5222, Feb. 9, 1996; Amdt. 25-91, 62 FR 40702, Jul. 29, 1997]
b. Intent of Rule. The purpose of this rule is to ensure that the airframe is capable of withstanding any yawing maneuvers that are likely to occur in service.
c. Background. This rule was carried forward from part 4b of the Civil Air Regulations (CAR).
Amendment 25-23 added the requirement to deflect the rudder by a 300 pound rudder pedal force, from VMC to VA. Amendment 25-46 expanded this condition to include full deflection up to VD. Gust requirements were removed in Amendment 25-86. In Amendment 25-91 the rule was revised to reflect the policy letter of December 8, 1992, by stating that the yawing response of the airplane was to take place with a constant pedal force, not necessarily a constant rudder deflection.