§ 3.244-1 to Landing cases and attitudes (FAA policies which apply § 3.244). The supporting structure as well as the landing gear itself should be capable of withstanding the loads occurring at the critical extension of the shock struts in accordance with Note (2) of Figure 3-12(a). [Supp. 10, 16 F. R. 3288, Apr. 14, 1951] § 3.245 Level landing. (a) Tail wheel type. Normal level flight attitude. (b) Nose wheel type. Two cases shall be considered: (1) Nose and main wheels contacting the ground simultaneously, (2) Main wheels contacting the ground, nose wheel just clear of the ground. (The angular attitude may be assumed the same as in subparagraph (1) of this paragraph for purposes of analysis.) (c) Drag components. In this condition, drag components simulating the forces required to accelerate the tires and wheels up to the landing speed shall be properly combined with the corresponding instantaneous vertical ground reactions. The wheel spin-up drag loads may be based on vertical ground reactions, assuming wing lift and a tiresliding coefficient of friction of 0.8, but in any case the drag loads shall not be less than 25 percent of the maximum vertical ground reactions neglecting wing lift. § 3.245-1 Wheel spin-up loads (FAA policies which apply to § 3.245). (a) Section 3.245 requires that spin-up loads be taken into account in structural designs. Section 3.244 permits the use of arbitrary drag loads for this purpose. (b) If it is desired to use a method more rational than the arbitrary drag components referred to in § 3.244 in determining the wheel spin-up loads for landing conditions, the Administrator will accept the following method from NACA T. N. 863 for this purpose (however, the minimum drag component of 0.25 times the vertical component will still apply): FHmax=maximum rearward horizontal force acting on the wheelpounds. Te effective rolling radius of wheel VH-linear velocity of airplane par- of Vc peripheral speed of tire if pre- n= effective coefficient of friction; Fvmax=maximum vertical force on whee (pounds)=n;We, where We and n are defined in §§ 3.353 and 3.354. tz=time interval between ground contact and attainment of maximum vertical force оп wheel (seconds). If the value of FHmax from the above equa tion exceeds 0.8Fvmax, the latter value should be used fo FHmax. NOTE: This equation assumes a linear vari ation of load factor with time until the peal load is reached and under this assumption determines the drag force at the time tha the wheel peripheral velocity at radius equals the airplane velocity. Most shock absorbers do not exactly follow a linear vari ation of load factor with time. Hence, ra tional or conservative allowances should b made to compensate for these variations On most landing gears the time for whee spin-up will be less than the time require to develop maximum vertical load factor fo the specified rate of descent and forwar velocity. However, for exceptionally larg wheels, a wheel peripheral velocity equal t the ground speed may not have been at tained at time of maximum vertical gea load. This case is covered by the statemen above that the drag spin-up load need no exceed 0.8 of the maximum vertical load. (c) Dynamic spring-back of the land ing gear and adjacent structure at th (d) The arbitrary drag loads referred to in § 3.244 (Fig. 3-12) are usually sufficient to provide for wheel spin-up except for airplanes having large diameter wheels or high stalling speeds. For the latter, it is recommended that a more rational investigation, such as that described above, be made. [Supp. 1, 12 F. R. 3436, May 28, 1947, as amended by Amdt. 1, 14 F. R. 36, Jan. 5, 1949] § 3.245-2 Level landing inclined reaction resultant (FAA policies which apply to § 3.245). In Figure 3-12 (b) the level landing inclined reaction resultant for both tail wheel and nose wheel type landing gears is assumed to pass through the wheel axles. [Supp. 10, 16 F. R. 3288, Apr. 14, 1951] § 3.246 Tail down. (a) Tail wheel type. Main and tail wheels contacting ground simultaneously. (b) Nose wheel type. Stalling attitude or the maximum angle permitting clearance of the ground by all parts of the airplane, whichever is the lesser. (c) Vertical ground reactions. In this condition, it shall be assumed that the ground reactions are vertical, the wheels E having been brought up to speed before the maximum vertical load is attained. §3.247 One-wheel landing. One side of the main gear shall contact the ground with the airplane in the level attitude. The ground reactions shall be the same as those obtained on the one side in the level attitude. (See § 3.245.) GROUND ROLL CONDITIONS The limit ground reaction obtained in the tail down landing condition shall be assumed to act up and aft through the axle at 45 degrees. The shock absorber and tire may be assumed deflected to their static positions. § 3.252 Side load. A limit vertical ground reaction equal to the static load on the tail wheel, in combination with a side component of equal magnitude. When a swivel is provided, the tail wheel shall be assumed swiveled 90 degrees to the airplane longitudinal axis, the resultant ground load passing through the axle. When a lock steering device or shimmy damper is provided, the tail wheel shall also be assumed in the trailing position with the side load acting at the ground contact point. The shock absorber and tire shall be assumed deflected to their static positions. NOSE WHEELS § 3.253 Supplementary conditions for nose wheels. The conditions set forth in §§ 3.2543.256 apply to nose wheels and affected supporting structure. The shock absorbers and tires shall be assumed deflected to their static positions. § 3.257 Supplementary conditions for skiplanes. The airplane shall be assumed resting on the ground with one main ski frozen in the snow and the other main ski and the tail ski free to slide. A limit side force equal to P/3 shall be applied at the most convenient point near the tail assembly, where P is the static ground reaction on the tail ski. For this condition the factor of safety shall be assumed equal to 1.0. § 3.257-1 Type certification of skis (FAA policies which apply to § 3.257). Type certification of skis is not contingent upon compliance with § 3.257 which applies to skiplanes only. [Supp. 10, 16 F. R. 3288, Apr. 14, 1951] § 3.257-2 Supplementary conditions for skiplanes (FAA policies which apply to § 3.257). (a) The following material outlines acceptable supplementary structural conditions for skiplanes with a tricycle type gear, in order to show "the same level of safety" under § 3.10. (1) To provide adequate strength for normal landing, taxiing and ground handling conditions for skiplanes equipped with a tricycle gear, a limit torque equal to 0.667W foot pounds should be separately applied about the vertical axis through the centerline of each main pedestal bearing of each main gear, W being the maximum design weight of the airplane in pounds. (2) For the nose gear, a limit torque equal to 1.333 WK foot pounds should be separately applied about the vertical axis through the centerline of the nose gear pedestal bearing, where K is the ratio of the nose gear ground reaction (total of both sides), as determined from § 3.245 (b) (1), proper account being taken of the increase of load on the nose gear due to pitching of the airplane. (3) In the case of a steerable nose gear, the limit torque on the nose gear need not exceed the pilot effort. (b) An ultimate factor of safety of 1.5 should be applied to the limit torques specified in paragraph (a) (1), (2) and (3) of this section. [Supp. 10, 16 F. R. 3288, Apr. 14, 1951] § 3.257-3 Factor of safety of 1.0 (FAA policies which apply to § 3.257). (a) The load P/3 in §3.257 is considered an ultimate loading. Therefore the factor of 1.0 is considered an ultimate factor. [Supp. 10, 16 F. R. 3288, Apr. 14, 1951] WATER LOADS § 3.265 Water load conditions. The structure of boat and float type seaplanes shall be designed for water loads developed during take-off and landing with the seaplane in any attitude likely to occur in normal operation at appropriate forward and sinking velocities under the most severe sea conditions likely to be encountered. Unless a more rational analysis of the water loads is performed, the requirements of §§ 4b.251 through 4b.258 of this subchapter shall apply. § 3.265-1 Float loads (FAA policies which apply to § 3.265). (a) Floats which are presently certificated on the basis of Part 4a of this subchapter in effect prior to November 9 1945, are considered satisfactory structurally for installation on airplanes which are designed in accordance with this part. (b) New float designs which are submitted for approval should be investigated for the structural design requirements of this part. [Supp. 10, 16 F.R. 3288, Apr. 14, 1951, as amended by Supp. 14, 17 F.R. 9066, Oct. 11 1952] § 3.265-2 Water loads; alternate standards (FAA policies which apply to §§ 3.10 and 3.265). ANC-3 provides a level of safety equivalent to, and may be applied in lieu of § 3.265. [Supp. 14, 17 F. R. 9066, Oct. 11, 1952] FATIGUE EVALUATION § 3.270 Pressurized cabins. The strength, detail design, and fabrication of the pressure cabin structure shall be evaluated in accordance with the provisions of either paragraph (a) or - paragraph (b) of this section. (a) Fatigue strength. The structure shall be shown by analysis and/or tests to be capable of withstanding the repeated loads of variable magnitude expected in service. (b) Fail safe strength. It shall be shown by analysis and/or tests that catastrophic failure is not probable after fatigue failure or obvious partial failure of a principal structural element. After : such failure the remaining structure shall be capable of withstanding a static ultimate load factor of 75 percent of the limit load factor at Vc, taking into account the combined effect of normal operating pressures, the expected external aerodynamic pressures, and flight loads. These loads shall be multiplied by a factor of 1.15 unless the dynamic effects of failure under static load are otherwise taken into consideration. § 3.292 Materials and workmanship. The suitability and durability of all materials used in the airplane structure shall be established on the basis of experience or tests. All materials used in the airplane structure shall conform to approved specifications which will insure their having the strength and other properties assumed in the design data. All workmanship shall be of a high standard. § 3.293 Fabrication methods. The methods of fabrication employed in constructing the airplane structure shall be such as to produce consistently 4sound structure. When a fabrication process such as gluing, spot welding, or heat-treating requires close control to attain this objective, the process shall be performed in accordance with an approved process specification. § 3.294 Standard fastenings. All bolts, pins, screws, and rivets used in the structure shall be of an approved type. The use of an approved locking device or method is required for all such bolts, pins, and screws. Self-locking nuts shall not be used on bolts subject to rotation during the operation of the airplane. § 3.295 Protection. All members of the structure shall be suitably protected against deterioration or loss of strength in service due to weathering, corrosion, abrasion, or other causes. In seaplanes, special precaution shall be taken against corrosion from salt water, particularly where parts made from different metals are in close proximity. Adequate provisions for ventilation and drainage of all parts of the structure shall be made. §3.296 Inspection provisions. Adequate means shall be provided to permit the close examination of such parts of the airplane as require periodic inspection, adjustments for proper alignment and functioning, and lubrication of moving parts. will be equalled or exceeded by the properties possessed by approximately 90 percent of the material. All other allowable design property columns relate to the minimum guaranteed properties and are based on values given in the various material specifications. The Administrator will permit uses of these design properties as outlined in subparagraphs (1) and (2) of this section, based on the objectives of § 3.301. (1) In the case of structures where the applied loads are eventually distributed through single members within an assembly, the failure of which would result in the loss of the structural integrity of the component involved, the guaranteed minimum design mechanical properties listed in ANC-5 shall be used. NOTE: Typical examples of such items are: 1. Wing lift struts. 2. Spars in two-spar wings. 3. Sparcaps in regions such as wing cutouts and wing center sections where loads are transmitted through caps only. 4. Primary attachment fittings dependent on single bolts for load transfer. wherein (2) Redundant structures partial failure of individual elements would result in the applied load being safely distributed to other load carrying members, may be designed on the basis of the "90 percent probability" allowable. NOTE: Typical examples of such items are: 1. Sheet-stiffener combinations. 2. Multi-rivet or multiple bolt connections. (b) Certain manufacturers have indicated a desire to use design value greater than the guaranteed minimums even in applications where only guaranteed minimum values would be permitted under paragraph (a) of this section, and have advocated that such allowables be based on "premium selection" of the material. Such increased design allowables will be acceptable to the Administrator: Provided, That a specimen or specimens of each individual item are tested prior to its use, to determine that the actual strength properties of that particular item will equal or exceed the properties used in design. This, in effect, results in the airplane or materials manufacturer guaranteeing higher minimum properties than those given in the basic procurement specifications. (c) See § 3.174-1 (a). [Supp. 1, 12 F. R. 3435, May 28, 1947, as amended by Amdt. 1, 14 F. R. 36, Jan. 5, 1949; Supp. 10, 16 F. R. 3289, Apr. 14, 1951] § 3.301-2 Substitution of seam-welded for seamless steel tubing (FAA policies which apply to § 3.301). Seam welded tubing may be substituted for seamless steel tubing as follows: (a) SAE 4130 welded tubing as per Specification AN-T-3, may be substituted for SAE 4130 seamless tubing conforming to Specification AN-WW-T850a, and vice versa. (b) SAE 1025 welded tubing as per Specification AN-T-4, may be substituted for SAE 1025 seamless tubing conforming to Specification AN-WW-T-846, and vice versa. (c) SAE 8630 welded tubing conforming to Specification AN-T-33a may be substituted for SAE 8630 seamless tubing conforming to Specification AN-T15 and vice versa. [Supp. 10, 16 F. R. 3289, Apr. 14, 1951] § 3.302 Special factors. Where there may be uncertainty concerning the actual strength of particular parts of the structure or where the strength is likely to deteriorate in service prior to normal replacement, increased factors of safety shall be provided to insure that the reliability of such parts is not less than the rest of the structure! as specified in §§ 3.303-3.306. § 3.303 Variability factor. For parts whose strength is subject to appreciable variability due to uncertainties in manufacturing processes and inspection methods, the factor of safety shall be increased sufficiently to make the probability of any part being understrength from this cause extremely remote. Minimum variability factors (only the highest pertinent variability factor need be considered) are set forth in §§ 3.304-3.306. § 3.304 Castings. (a) Where visual inspection only is to be employed, the variability factor shall be 2.0. (b) The variability factor may be reduced to 1.25 for ultimate loads and, 1.15 for limit loads when at least three sample castings are tested to show compliance with these factors, and all sample and production castings are visually and. radiographically inspected in accordance with an approved inspection specification. |