(1) A lower drag reaction may be used if an effective drag force of 0.8 times the vertical reaction cannot be reached under any likely loading condition; and
(2) The forward acting load at the center of gravity need not exceed the
maximum drag reaction on one main gear, determined in accordance with
§ 25.493(b).
(e) With the airplane at design ramp weight, and the nose gear in any steerable position, the combined application of full normal steering torque and vertical force equal to 1.33 times the maximum static reaction on the nose gear must be considered in designing the nose gear, its attaching structure, and the forward fuselage structure.
[Amdt. 25-23, 35 FR 5673, Apr. 8, 1970; Amdt. 25-46, 43 FR 50595, Oct. 30, 1978; Amdt. 25-91, 62 FR 40702, Jul. 29, 1997]
b. Intent of Rule. The purpose of this rule is to ensure that the airplane, the nose gear, and its supporting structure, and the forward fuselage, are able to withstand any load likely to be encountered in service.
c. Background. The basic rule was carried forward from § 4b.235(e) of the Civil Air Regulations (CAR) with some editorial changes.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5673, April 8, 1970; Amendment 25-46, 43 FR 50595, October 30, 1978; and Amendment 25-91, 62 FR 40702, July 29, 1997.
e. References. None.
479 - 482. [RESERVED]
483. SECTION 25.503 - PIVOTING.
a. Rule Text.
(a) The airplane is assumed to pivot about one side of the main gear with the brakes on that side locked. The limit vertical load factor must be 1.0 and the coefficient of friction 0.8.
(b) The airplane is assumed to be in static equilibrium, with the loads being applied at the ground contact points, in accordance with figure 8 of Appendix A.
b. Intent of Rule. The purpose of this rule is to ensure that the airplane, landing gear and its supporting structure are able to withstand any load distributions likely to occur in service.
c. Background. This rule was carried forward from § 4b.235(d) 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.
484 - 488. [RESERVED]
489. SECTION 25.507 - REVERSED BRAKING.
a. Rule Text.
(a) The airplane must be in a three point static ground attitude. Horizontal reactions parallel to the ground and directed forward must be applied at the ground contact point of each wheel with brakes. The limit loads must be equal to
0.55 times the vertical load at each wheel or to the load developed by 1.2 times the nominal maximum static brake torque, whichever is less.
(b) For airplanes with nose wheels, the pitching moment must be balanced by rotational inertia.
(c) For airplanes with tail wheels, the resultant of the ground reactions must pass through the center of gravity of the airplane.
b. Intent of Rule. The purpose of this rule is to ensure that the airplane, landing gear and its supporting structure are able to withstand any load distributions likely to occur in service.
c. Background. This rule was carried forward from § 4b.235(g) 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.
490 - 495. [RESERVED]
496. SECTION 25.509 - TOWING LOADS.
a. Rule Text.
(a) The towing loads specified in paragraph (d) of this section must be considered separately. These loads must be applied at the towing fittings and must act parallel to the ground. In addition -
(1) A vertical load factor equal to 1.0 must be considered acting at the center of gravity;
(2) The shock struts and tires must be in their static positions; and
(3) With WT as the design ramp weight, the towing load, FTOW, is
(i) 0.3 WT for WT less than 30,000 pounds;
(ii) (6WT+450,000)/7 for WT between 30,000 and 100,000 pounds; and NOTE: The denumerator of 7 is incorrect in the CFR, it was published correctly in the Federal Register as 70.
(iii) 0.15 WT for WT over 100,000 pounds.
(b) For towing points not on the landing gear but near the plane of symmetry of the airplane, the drag and side tow load components specified for the auxiliary gear apply. For towing points located outboard of the main gear, the drag and side tow load components specified for the main gear apply. Where the specified angle of swivel cannot be reached, the maximum obtainable angle must be used.
(c) The towing loads specified in paragraph (d) of this section must be reacted as follows:
(1) The side component of the towing load at the main gear must be reacted by a side force at the static ground line of the wheel to which the load is applied.
(2) The towing loads at the auxiliary gear and the drag components of the towing loads at the main gear must be reacted as follows:
(i) A reaction with a maximum value equal to the vertical reaction must be applied at the axle of the wheel to which the load is applied. Enough airplane inertia to achieve equilibrium must be applied.
(ii) The loads must be reacted by airplane inertia.
(d) The prescribed towing loads are as follows:
Tow point Position Magnitude Load No. Direction Main 0.75 Ftow per main 1 Forward, parallel to drag axis.
gear gear unit 2 Forward, at 30 o to drag axis.
3 Aft, parallel to drag axis.
4 Aft, at 30 o to drag axis.
Auxiliary Swiveled forward 1.0 FTOW 5 Forward.
gear 6 Aft.
Swiveled aft 1.0 F TOW 7 Forward.
8 Aft.
Swiveled 45o from forward 0.5 F TOW 9 Forward, in plane of wheel.
10 Aft, in plane of wheel Swivel 45o from aft 0.5 Ft OW 11 Forward, in plane of wheel.
12 Aft, in plane of wheel.
[Amdt. 25-23, 35 FR 5673, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to ensure that the airplane, towing fittings and supporting structure are able to withstand any likely towing loads developed in service.
c. Background. This rule was carried forward from § 4b.235(h) of the Civil Air Regulations (CAR). Amendment 25-23 redefined WT as the design ramp weight.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5673, April 8, 1970.
NOTE: In paragraph a.(ii), the denominator of 7 is an error and should be 70.
e. References. None.
497 - 508. [RESERVED]
509. SECTION 25.511 - GROUND LOAD: UNSYMMETRICAL LOADS ON MULTIPLE-WHEEL UNITS.
a. Rule Text.
(a) General. Multiple-wheel landing gear units are assumed to be subjected to the limit ground loads prescribed in this subpart under paragraphs (b) through (f) of this section. In addition -
(1) A tandem strut gear arrangement is a multiple-wheel unit; and
(2) In determining the total load on a gear unit with respect to the provisions of paragraphs (b) through (f) of this section, the transverse shift in the load centroid, due to unsymmetrical load distribution on the wheels, may be neglected.
(b) Distribution of limit loads to wheels; tires inflated. The distribution of the limit loads among the wheels of the landing gear must be established for each landing, taxiing, and ground handling condition, taking into account the effects of the following factors:
(1) The number of wheels and their physical arrangements. For truck type landing gear units, the effects of any seesaw motion of the truck during the landing impact must be considered in determining the maximum design loads for the fore and aft wheel pairs.
(2) Any differentials in tire diameters resulting from a combination of manufacturing tolerances, tire growth, and tire wear. A maximum tire-diameter differential equal to 2/3 of the most unfavorable combination of diameter 379
variations that is obtained when taking into account manufacturing tolerances, tire growth, and tire wear, may be assumed.
(3) Any unequal tire inflation pressure, assuming the maximum variation to be ±5 percent of the nominal tire inflation pressure.
(4) A runway crown of zero and a runway crown having a convex upward shape that may be approximated by a slope of 1½ percent with the horizontal. Runway crown effects must be considered with the nose gear unit on either slope of the crown.
(5) The airplane attitude.
(6) Any structural deflections.
(c) Deflated tires. The effects of deflated tires on the structure must be considered with respect to the loading conditions specified in paragraphs (d) through (f) of this section, taking into account the physical arrangement of the gear components. In addition
(1) The deflation of any one tire for each multiple wheel landing gear unit, and the deflation of any two critical tires for each landing gear unit using four or more wheels per unit, must be considered; and
(2) The ground reactions must be applied to the wheels with inflated tires except that, for multiple-wheel gear units with more than one shock strut, a rational distribution of the ground reactions between the deflated and inflated tires, accounting for the differences in shock strut extensions resulting from a deflated tire, may be used.
(d) Landing conditions. For one and for two deflated tires, the applied load to each gear unit is assumed to be 60 percent and 50 percent, respectively, of the limit load applied to each gear for each of the prescribed landing conditions.
However, for the drift landing condition of § 25.485, 100 percent of the vertical load must be applied.
(e) Taxiing and ground handling conditions. For one and for two deflated tires -
(1) The applied side or drag load factor, or both factors, at the center of gravity must be the most critical value up to 50 percent and 40 percent respectively, of the limit side or drag load factors, or both factors, corresponding to the most severe condition resulting from consideration of the prescribed taxiing and ground handling conditions;
(2) For the braked roll conditions of § 25.493(a) and (b)(2), the drag loads on each inflated tire may not be less than those at each tire for the symmetrical load distribution with no deflated tires;
(3) The vertical load factor at the center of gravity must be 60 percent and 50 percent, respectively, of the factor with no deflated tires, except that it may not be less than 1g; and
(4) Pivoting need not be considered.
(f) Towing conditions. For one and for two deflated tires, the towing load, F TOW, must be 60 percent and 50 percent, respectively, of the load prescribed.
b. Intent of Rule. The purpose of this rule is to ensure that the airplane, landing gear and its supporting structure are able to withstand any load distributions likely to occur in service.
c. Background. This rule was carried forward from § 4b.236 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.
510 - 519. [RESERVED]
520. SECTION 25.519 - JACKING AND TIE-DOWN PROVISIONS.
a. Rule Text.
(a) General. The airplane must be designed to withstand the limit load conditions resulting from the static ground load conditions of paragraph (b) of this section and, if applicable, paragraph (c) of this section at the most critical combinations of airplane weight and center of gravity. The maximum allowable [limit] load at each jack pad must be specified.
(b) Jacking. The airplane must have provisions for jacking and must withstand the following limit loads when the airplane is supported on jacks -
(1) For jacking by the landing gear at the maximum ramp weight of the airplane, the airplane structure must be designed for a vertical load of 1.33 times the vertical static reaction at each jacking point acting singly and in combination with a horizontal load of 0.33 times the vertical static reaction applied in any direction.
(2) For jacking by other airplane structure at maximum approved jacking weight:
(i) The airplane structure must be designed for a vertical load of 1.33 times the vertical reaction at each jacking point acting singly and in combination with a horizontal load of 0.33 times the vertical static reaction applied in any direction.
(ii) The jacking pads and local structure must be designed for a vertical load of
2.0 times the vertical static reaction at each jacking point, acting singly and in combination with a horizontal load of 0.33 times the vertical static reaction applied in any direction.
(c) Tie-down. If tie-down points are provided, the main tie-down points and local structure must withstand the limit loads resulting from a 65-knot horizontal wind from any direction.
[Amdt. 25-81, 59 FR 22102, Apr. 28, 1994]
b. Intent of Rule. The purpose of this rule is to provide minimum design requirements for jacking and tie-down.
c. Background. This rule was incorporated into part 25 by Amendment 25-81.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-81, 59 FR 22102, April 28, 1994.
e. References. None.
521 - 528. [RESERVED] 383
Section 7. WATER LOADS.
529. SECTION 25.521 - GENERAL.
a. Rule Text.
(a) Seaplanes must be designed for the water loads developed during takeoff and landing, with the seaplane in any attitude likely to occur in normal operation, and at the appropriate forward and sinking velocities under the most severe sea conditions likely to be encountered.
(b) Unless a more rational analysis of the water loads is made, or the standards in ANC-3 are used, §§ 25.523 through 25.537 apply.
(c) The requirements of this section and §§ 25.523 through 25.537 apply also to amphibians.
b. Intent of Rule. This rule provides the general requirements for developing water loads on seaplanes.
c. Background. This rule was carried forward from § 4b.250 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.
530 - 540. [RESERVED]
541. SECTION 25.523 - DESIGN WEIGHTS AND CENTER OF GRAVITY POSITIONS.
a. Rule Text.
(a) Design weights. The water load requirements must be met at each operating weight up to the design landing weight except that, for the takeoff condition prescribed in § 25.531, the design water takeoff weight ( the maximum weight for water taxi and takeoff run) must be used.
(b) Center of gravity positions. The critical centers of gravity within the limits for which certification is requested must be considered to reach maximum design loads for each part of the seaplane structure.
[Amdt. 25-23, 35 FR 5673, Apr. 8, 1970]
b. Intent of Rule. The purpose of this rule is to account for all operating gross weights and c.g.
locations.
c. Background. This rule was carried forward from § 4b.251 of the Civil Air Regulations (CAR).
Amendment 25-23 added editorial changes.
d. Acceptable Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of Amendment 25-23, 35 FR 5673, April 8, 1970.
e. References. None.
542 - 548. [RESERVED]
549. SECTION 25.525 - APPLICATION OF LOADS.
a. Rule Text.
(a) Unless otherwise prescribed, the seaplane as a whole is assumed to be subjected to the loads corresponding to the load factors specified in § 25.527.
(b) In applying the loads resulting from the load factors prescribed in § 25.527, the loads may be distributed over the hull or main float bottom (in order to avoid excessive local shear loads and bending moments at the location of water load application) using pressures not less than those prescribed in § 25.533(b).
(c) For twin float seaplanes, each float must be treated as an equivalent hull on a fictitious seaplane with a weight equal to one-half the weight of the twin float seaplane.
(d) Except in the takeoff condition of § 25.531, the aerodynamic lift on the seaplane during the impact is assumed to be 2/3 of the weight of the seaplane.
b. Intent of Rule. The purpose of this rule is to provide criteria to be used in developing the waterloads on the seaplane.
c. Background. This rule was carried forward from § 4b.252 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.
550 - 559. [RESERVED] 456
560. SECTION 25.527 - HULL AND MAIN FLOAT LOAD FACTORS.
a. Rule Text.
(a) Water reaction load factors nw must be computed in the following manner:
(1) For the step landing case C V 2 nw = 1 so 2 3 / 1 3 / (Tan b)W
(2) For the bow and stern landing cases CV 2 K1 so 1n = x ;w 2 3 / 1 3 2 3 // 2(Tan b)W (1 + r )x
(b) The following values are used:
(1) nw = water reaction load factor (that is, the water reaction divided by seaplane weight).
(2) C1 = empirical seaplane operations factor equal to 0.012 (except that this factor may not be less than that necessary to obtain the minimum value of step load factor of 2.33).
(3) Vso = seaplane stalling speed in knots with flaps extended in the appropriate landing position and with no slipstream effects.
(4) b = angle of dead rise at the longitudinal station at which the load factor is being determined, in accordance with Figure 1 of Appendix B.
(5) W = seaplane design landing weight in pounds.
(6) K1 = empirical hull station weighing factor, in accordance with Figure 2 of Appendix B.
(7) rx = ratio of distance, measured parallel to hull reference axis, from the center of gravity of the seaplane to the hull longitudinal station at which the load factor is being computed to the radius of gyration in pitch of the seaplane, the hull reference axis being a straight line, in the plane of symmetry, tangential to the keel at the main step.
(c) For a twin float seaplane, because of the effect of flexibility of the attachment of the floats to the seaplane, the factor K 1 may be reduced at the bow and stern to 0.8 of the value shown in Figure 2 of Appendix B. This reduction applies only to the design of the carry-through and seaplane structure.
[Amdt. 25-23, 35 FR 5673, Apr. 8, 1970] 457