§ 4b.234a Rebound landing condition.

The landing gear and its supporting structure shall be investigated for the loads occurring during rebound of the airplane from the landing surface. With the landing gear fully extended and not in contact with the ground, a load factor of 20.0 shall act on the unsprung weights of the landing gear. This load factor shall act in the direction of motion of the unsprung weights as they reach their limiting positions in extending with relation to the sprung portions of the landing gear.

[Amdt. 4b-3, 21 F. R. 991, Feb. 11, 1956]

§ 4b.235

Ground handling conditions.

The landing gear and airplane structure shall be investigated for the conditions of this section with the airplane at the design take-off weight, unless otherwise prescribed. No wing lift shall be considered. It shall be acceptable to assume the shock absorbers and tires to be deflected to their static position. In the conditions of paragraph (b)(1) and (2), it shall be acceptable to use a drag reaction lower than prescribed therein if it is substantiated that an effective drag force of 0.8 times the vertical reaction cannot be attained under any likely loading condition.

(a) Take-off run. The landing gear and the airplane structure shall be assumed to be subjected to loads not less than those encountered under conditions described in § 4b.172.

(b) Braked roll—(1) Tail-wheel type. The airplane shall be assumed to be in the level attitude with all load on the main wheels. The limit vertical load factor shall be 1.2 for the airplane at the design landing weight, and 1.0 for the airplane at the design take-off weight. A drag reaction equal to the vertical reaction multiplied by a coefficient of friction of 0.8 shall be combined with the vertical ground reaction and applied at the ground contact point. (See fig. 4b-12.)

(2) Nose-wheel type. The limit vertical load factor shall be 1.2 for the airplane at the design landing weight, and 1.0 for the airplane at the design take-off weight. A drag reaction equal to the vertical reaction multiplied by a coefficient of friction of 0.8 shall be combined with the vertical reaction and applied at the ground contact point of each wheel having brakes. The following

two airplane attitudes shall be considered: (See fig. 4b-12.)

(i) The airplane shall be assumed to be in the level attitude with all wheels contacting the ground and the loads distributed between the main and nose gear. Zero pitching acceleration shall be assumed.

(ii) The airplane shall be assumed to be in the level attitude with only the main gear contacting the ground and the pitching moment resisted by angular acceleration.

(c) Turning. The airplane in the static position shall be assumed to execute a steady turn by nose gear steering or by application of differential power such that the limit load factors applied at the center of gravity are 1.0 vertically and 0.5 laterally. (See fig. 4b-13.) The side ground reaction of each wheel shall be 0.5 of the vertical reaction.

(d) Pivoting. The airplane shall be assumed to pivot about one side of the main gear, the brakes on that side being locked. The limit vertical load factor shall be 1.0 and the coefficient of friction 0.8. The airplane shall be assumed to be in static equilibrium, the loads being applied at the ground contact points. (See fig. 4b-14.)

(e) Nose-wheel yawing. (1) A vertical load factor of 1.0 at the airplane center of gravity and a side component at the nose wheel ground contact equal to 0.8 of the vertical ground reaction at that point shall be assumed.

(2) The airplane shall be assumed to be in static equilibrium with the loads resulting from the application of the brakes on one side of the main gear. The vertical load factor at the center of gravity shall be 1.0. The forward acting load at the airplane center of gravity shall be 0.8 times the vertical load on one main gear. The side and vertical loads at the ground contact point on the nose gear shall be those required for static equilibrium. The side load factor at the airplane center of gravity shall be assumed to be zero. Where this condition results in a nose gear side load in excess of 0.8 times the vertical nose gear load, it shall be acceptable to limit the design nose gear side load to 0.8 times the vertical load with the unbalanced yawing moments assumed to be resisted by aircraft inertia forces.

(f) Tail-wheel yawing. (1) A vertical ground reaction equal to the static

load on the tail wheel in combination with a side component of equal magnitude shall be assumed.

(2) When a swivel is provided, the tail wheel shall be assumed to be swiveled 90° to the airplane longitudinal axis with the resultant load passing through the axle. When a lock, steering device, or shimmy damper is provided, the tail wheel shall also be assumed to be in the trailing position with the side load acting at the ground contact point.

(g) Reversed braking. The airplane shall be in a three point static ground

attitude. Horizontal reactions parallel to the ground and directed forward shall be applied at the ground contact point of each wheel equipped with brakes. The limit loads shall be equal either 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 the lesser. For nosewheel types, the pitching moment shall be balanced by rotational inertia. For tailwheel types, the resultant of the ground reactions shall pass through the center of gravity of the airplane.

(h) Towing loads. Towing loads shall be those specified in Figure 4b-26, considering each condition separately. These loads shall be applied at the towing fittings and shall act parallel to the ground. A vertical load factor equal to 1.0 shall be considered acting at the center of gravity. The shock struts and tires shall be in their static positions. The towing load, Frow, shall be defined as equal to 0.3Wr for Wr less than 6WT+450,000 for 70 Wr between 30,000 and 100,000 pounds and equal to 0.15 Wr for Wr over 100,000 pounds, where Wr is the design maximum take-off weight. For towing points not on the landing gear but located near the plane of symmetry of the airplane, the drag and side tow load components specified for the auxiliary gear shall apply. For tow points located outboard of the main gear, the drag and side tow load components specified for the main gear shall apply. In cases where the specified angle of swivel cannot be obtained, the maximum obtainable angle shall be used.

30,000 pounds, equal to

[15 F. R. 3543, June 8, 1950; 15 F. R. 4171, June 29, 1950, as amended by Amdt. 4b-3, 21 F.R. 992, Feb. 11, 1956; Amdt. 4b-6, 22 F.R. 5564, July 16, 1957; Amdt. 4b-11, 24 F.R. 7069, Sept. 1, 1959]

§ 4b.236 Unsymmetrical loads on multiple-wheel units.

(a) General. Multiple-wheel landing gear units shall be assumed to be subjected to the limit ground loads prescribed in this subpart in accordance with the provisions of paragraphs (b) and (c) of this section. A tandem strut gear arrangement shall be considered to be a multiple-wheel unit.

(b) Distribution of limit loads to wheels; all tires inflated. The distribution of the limit loads among the wheels of the landing gears shall be established for all landing, taxiing, and ground handling conditions, taking into account the effects of the factors enumerated in subparagraphs (1) through (6) of this paragraph.

(1) Number of wheels and their physical arrangement. In the case of truck type landing gear units, the effects of any see-saw motion of the truck during the landing impact shall be considered in determining the maximum design loads for the fore and aft wheel pairs.

(2) Differentials in tire diameters resulting from a combination of manufacturing tolerances, tire growth, and tire wear. It shall be acceptable to assume a maximum tire-diameter differential equal to % of the worst combination of diameter variations which is obtained when taking into account manufacturing tolerances, tire growth, and tire wear.

(3) 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 which may be approximated by a slope of 12 percent with the horizontal. Runway crown effects shall be considered with the nose gear unit on either slope of the crown.

(5) Airplane attitude.

(6) Structural deflections.

(c) Deflated tires. The effect of deflated tires on the structure shall be considered with respect to the loading conditions specified in subparagraphs (1), (2), and (3) of this paragraph taking into account the physical arrangement of the gear components. Consideration shall be given to the deflation of any one tire for all multiple wheel landing gear units and, in addition, to the deflation of any 2 critical tires for landing gear units employing 4 or more wheels per unit. The ground reactions shall be applied to the wheels with inflated tires, except that for multiple-wheel gear units incorporating more than one shock strut, it shall be permissible to use a rational distribution of the ground reactions between the deflated and inflated tires, taking into account the differences in shock strut extensions resulting from a deflated tire.

(1) Landing conditions. For one deflated tire and for two deflated tires, the applied load to each gear unit shall be assumed to be 60 percent and 50 percent, respectively, of the limit load applied to each gear for each of the prescribed landing conditions except that, for the drift landing condition of § 4b.234, 100 percent of the vertical load shall be applied.

(2) Taxiing and ground handling conditions. For one deflated tire and for two deflated tires, the applied side and/or drag load factor at the center of gravity shall be the most critical value up to 50