NOTE: The method is valid for aspect ratios of 5 through 12 and for rectangular plantorms such as used on rotorcraft, other planforms may be acceptable as prescribed in the TM.
(ii) The chordwise distribution appropriate for the aerodynamic shape or a conservative distribution should be used.
(III) The flight load survey conducted under § 27.571 may be used to confirm design parameters and possible load distribution data. On controllable surfaces, the pitching moment (control loads) may be measured for fatigue substantiation of the control system. The control stabilizing surfaces may be subject to loads measurement and possible fatigue tests for fatigue substantiation also.
(3) Proof of the structure for the required loads is specified in §§ 27.301,
27.303, 27.305, and 27.307. Tests or analysis may be used as prescribed. If analysis is used, fitting factors and other appropriate factors prescribed by the rules of §§ 27.625, 27.621, and 27.623 will be required in the analysis.
AC 27.413A. § 27.413 (Amendment 27-27) STABILIZING AND CONTROL SURFACES.
a. Explanation. Amendment 27-27 removed this section since the requirements are adequately addressed in §§ 27.337, 27.339, and 27.341.
b. Procedures. The policy material pertaining to this section is retained as supplemental information especially as reference material for paragraph AC 27.341 (§ 27.341).
AC 27.427. § 27.427 (Amendment 27-27) UNSYMMETRICAL LOADS.
a. Explanation. Amendment 27-26 added the standard and Amendment 27-27 amended it. Minimum unsymmetrical design loads are specified for horizontal tail surfaces and also vertical tail surfaces whenever they support the horizontal tail surfaces.
(1) Loads are derived by rational analysis, or for earlier certification bases, the prescribed empirical loads of § 27.413 may be used. Section 27.413 was removed by Amendment 27-27 since the requirements are adequately addressed in §§ 27.337,
27.339, and 27.341.
(2) Rational loads, appropriate for the aerodynamic surfaces, should be distributed according to the standard.
(3) When vertical tail surfaces support the horizontal tail surfaces, the vertical tail surfaces and supporting surfaces are required to support the critical combination of vertical and horizontal surface loads.
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b. Procedures. Two basic loading conditions are required by § 27.427 for each of the two basic empennage configurations.
(1) Horizontal surfaces supported by the tail boom or fuselage. Structural substantiation should be provided for all six combinations shown in figure AC 27.427-1.
All of these empirical loading distributions should be used unless rational analysis shows one or more of each set of conditions to be non-critical or equal or more realistic distributions are substantiated. Rectangular spanwise air load distribution should be used unless more rational distribution is substantiated. If end plates are used, the air loads should be distributed accordingly.
(i) First unsymmetrical loading condition:
(A) 100 percent of the flight load is applied to one side of the plane of symmetry; and 0 percent of the flight load is applied to the other side of the plane of symmetry.
(B) For surfaces with end plates or other similar devices, the load distribution will be changed accordingly.
(ii) Second unsymmetrical loading condition:
50 percent of the flight load is applied to one side of the plane of symmetry acting up;
and 50 percent of the flight load is applied to the other side of the plane of symmetry acting down.
(2) Horizontal surfaces supported by a vertical surface. Structural substantiation should be provided for all six combinations shown in figure AC 27.427-2.
All of these empirical loading distributions should be used unless rational analysis shows one or more of each set of conditions to be non-critical or equal or more realistic distributions are substantiated. Rectangular spanwise air load distribution should be used unless more rational distribution is substantiated. If end plates are used, the air loads should be distributed accordingly.
First unsymmetrical loading condition:
100 percent of the flight load is applied to one side of the plane of symmetry; and O percent of the flight load is applied to the other side of the plane of symmetry.
(ii) Second unsymmetrical loading condition:
50 percent of the flight load is applied to one side of the plane of symmetry acting up;
and 50 percent of the flight load is applied to the other side of the plane of symmetry acting down.
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Figure AO 27 427-1 (View Looking Forward) Page C - 26
Figure AO 27 427-2 (View Looking Forwerd) Page C - 27
SUBPART C - STRENGTH REQUIREMENTS
GROUND LOADS AC 27.471 $ 27.471 GROUND LOADS--GENERAL.
a. Explanation. This regulation specifies that limit ground loads must be considered which are:
(1) External loads caused by landing (ground) conditions for skid and wheel landing gear equipped rotorcraft and by ground taxiing loads as specified in § 27.235 for wheel landing gear equipped rotorcraft.
(2) Loads considering the rotorcraft structure as a rigid body.
(3) Loads in equilibrium with linear and angular inertia loads.
(4) The critical center of gravity "must be selected so that the maximum design loads are obtained in each landing gear element."
b. Procedures.
(1) The standards to be considered are specified in §$ 27.473 through 27.505.
These associated standards cover landing gear arrangements, landing conditions, and ground loading conditions (for wheel landing gear rotorcraft).
(2) Drop tests may be used to verify landing load factors. (See paragraph AC 27.725.)
(3) The application of the design loads derived from the landing load factors will be as specified for each element affected by landing or ground loading conditions (for wheel landing gear rotorcraft).
(4) During the applicant's flight test program, the landing load factors for skid and wheel landing gear rotorcraft and taxiing load factors for wheel landing gear rotorcraft are monitored to assure the design load factors used are adequate. See paragraph AC 27.235 of this document for § 27.235 policy.
AC 27.473 § 27.473 (Amendment 27-2) GROUND LOADING CONDITIONS AND ASSUMPTIONS.
a. Explanation. The rotorcraft is to be designed for the maximum weight. A rotor lift of two-thirds of the design maximum weight may be used. The minimum limit landing load factor is determined by the drop tests of § 27.725.
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b. Procedures. Loads for the landing conditions are derived considering mass (equal to the maximum weight) and rotor lift (equal to two-thirds of the maximum weight) acting through the center of gravity throughout the landing impact. Unbalanced external loads resulting from asymmetric loading conditions are reacted as specified in the individual subparagraphs. The rotorcraft must be substantiated for ultimate landing loads by either test or analysis utilizing an ultimate load factor of 1.5 applied to the limit load factor of not less than that substantiated under § 27.725.
AC 27.475 $ 27.475 TIRES AND SHOCK ABSORBERS.
a. Explanation. This section specifies the tire and shock absorber position to be used in ground load derivations.
b. Procedures. Ground loads are to be derived with the tires in static (1g) position and the shock absorbers "in their most critical position." The determination of the "most critical positioni for the shock absorbers generally requires a load versus deflection test or analysis of the shock absorber system and a determination of the effect of both load and deflections on the shock absorber, attachment structure, and substructure designed by ground loads.
AC 27.477 § 27.477 LANDING GEAR ARRANGEMENT.
a. Explanation. This section specifies the individual standards to be used for ground load conditions for rotorcraft having two wheels aft and one or more wheels forward of the center of gravity.
b. Procedures. The ground loading conditions of §§ 27.235, 27.479 through
27.485, and 27.493 will be used for rotorcraft having two wheels aft and one or more wheels forward of the center of gravity. This includes forward wheels on separate axles.
AC 27.479 § 27.479 LEVEL LANDING CONDITIONS.
a. Explanation. This section provides explicit level landing load criteria for landing gear with two wheels aft and one or more wheels forward of the center of gravity.
(1) Level landings--
(i) Each wheel contacting the ground simultaneously; and
(ii) Aft wheels contacting the ground with forward wheels just clear of the ground.
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(2) Application of loads--
(i) Maximum design vertical loads applied alone; and
(ii) The maximum design vertical loads applied with a drag load of at least 25 percent of the vertical load (applied at the ground contact area).
(3) A 40 percent/60 percent load distribution between wheels for configurations having two forward wheels including quadricycle. This distribution between wheels on a common axis is to be applied for the conditions of vertical loads only and for vertical loads combined with drag loads of 25 percent of the vertical loads.
(4) Aircraft pitching moments are to be reacted by the forward landing gear for simultaneous wheel contact or by the angular inertia forces when the forward landing gear is clear of the ground as specified.
b. Procedures.
(1) The specified loading conditions will be used in load derivations.
(2) The critical center of gravity condition will be used for each gear and gear support structure.
The aft center of gravity condition with the forward gear clear will normally be critical for the aft gear and gear supports.
The forward center of gravity condition with each gear contacting the ground simultaneously will normally design forward gear elements critical for vertical loads.
(Ш) The forward center of gravity condition with the forward gear clear may result in high load factors, angular plus linear, that will greatly affect security of items of significant mass.
AC 27.481 § 27.481 TAIL-DOWN LANDING CONDITIONS.
a. Explanation. This section provides the criteria for tail-down landing conditions;
i.e., "the maximum nose-up attitude allowing ground clearance" with ground loads acting "perpendicular to the ground."
b. Procedures.
(1) The tail-down landing condition will be used to check (by analysis or test) for criticality of landing gear or support structure. This attitude generally creates the highest forward loads on the main landing gear in combination with vertical loads.
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(2) The tail-down landing condition may be the critical condition for both landing load factor and for energy absorption by the main gear. Section 27.725 requires that "each landing gear must be tested in the attitude simulating the landing condition that is most critical." Where questions exist as to the critical attitude, both level landing and tail-down landing attitudes should be used in drop tests required by § 27.725.
AC 27.483 § 27.483 ONE-WHEEL LANDING CONDITIONS.
a. Explanation. This section gives the condition to be used for one-wheel landing conditions. Only the vertical load condition of § 27.479(b)(1) is required.
b. Procedures. The one-wheel landing condition is generally critical for the landing gear-to-fuselage attachments and the landing gear elements between the attachments. Unbalanced external loads are reacted by rotorcraft inertia.
AC 27.485 $ 27.485 LATERAL DRIFT LANDING CONDITIONS.
a. Explanation.
(1) This section provides the loading conditions which impose side (and vertical) loads on the landing gear. A level landing attitude is specified. Two main conditions required are--
(i) Only the aft wheels in contact with the ground; and (II) All wheels contacting the ground simultaneously.
(2) Loads. The vertical loads to be applied with the side loads are specified as "one-half of the maximum ground reactions of § 27.479(b)(1)." These vertical loads are the level landing loads considering both contact and noncontact with the ground by the forward wheels.
(i) One side load condition is specified as "0.8 times the vertical reaction acting inward on one side and 0.6 times the vertical reaction acting outward on the other side"i when only the aft wheels contact the ground.
(ii) The other side load condition (for all wheels contacting the ground) specifies the 80 percent inward/60 percent outward distribution for the aft wheels and
0.8 times (80 percent) the vertical reaction for the forward wheels.
b. Procedures. The loading conditions, as specified, are applied to the landing gear and attaching structure. The loads are applied at the ground contact point, except for full swiveling gear which has the load applied at the center of the axle. In other words, full swiveling gear is considered to have swiveled to a static position under the side load before the design vertical and side loads are achieved. The rotorcraft as well as the landing gear itself will be substantiated for these side load conditions.
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AC 27.493 § 27.493 BRAKED ROLL CONDITIONS.
a. Explanation. This section provides two loading conditions for ground braking operations. Specific vertical loads in conjunction with drag loads (due to braking) are to be considered. The limit vertical load factor is 1.33 for condition of all wheels in contact with the ground and 1.0 for condition of aft wheels only in contact with the ground and nose wheel clear. The drag load on wheels with brakes is 0.8 times the vertical load or the drag load value based on limiting brake torque, whichever is less. The drag load value for limiting brake torque may be that determined in the performance testing to TSO C26 or equivalent, as required.
b. Procedures. The braking loads are calculated from the specified criteria with the shock absorbers in their static (normal) positions and with the drag loads applied at the ground contact point. Structural substantiation of the affected structure may be accomplished by test or analysis. If tests are used, the wheel and tire assembly is commonly replaced with a test fixture so the limit loads and static deflections specified can be more accurately controlled. The test specimen should be complete enough to ensure that the landing gear structure and the attach and backup structure are adequately substantiated.
AC 27.497 § 27.497 GROUND LOADING CONDITIONS: LANDING GEAR WITH TAIL WHEELS.
a. Explanation. This section provides the loading conditions for landing gear designs with tail wheels.
(1) Level landings are to consider the following:
All wheels (main and tail) contacting the ground simultaneously, as well as only forward main wheels contacting the ground.
(ii) Maximum design vertical loads applied alone.
(ili) The maximum design vertical loads combined with a drag load of at least 25 percent of the vertical loads for both conditions.
(2) Nose-up landings with only the rear wheel or wheels initially contacting the ground must be considered unless shown to be extremely remote.
(3) Level landings on one forward wheel only are to be considered. Drag loads are not required.
(4) Side load conditions are imposed on the main wheels and tail wheels for level landing attitudes. Criteria for full swiveling and locked tail wheels are included in this standard.
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(5) Braked roll conditions are specified for the level landing attitudes.
(6) Rear wheel turning loads are also specified for swiveling and locked tail wheels.
(7) Taxiway condition loads for the landing gear and rotorcraft are those that "occur when the rotorcraft is taxied over the roughest ground that may reasonably be expected in normal operationi The aircraft design load factors should not be exceeded during the evaluation. Section 27.235 contains an identical standard that applies to all types of wheel landing gear.
b. Procedures.
(1) The specified loading conditions are to be used in load derivations.
(2) The critical center of gravity condition is used for each gear and gear support structure.
(i) The forward center of gravity condition with the tail gear clear will normally be critical for the forward gear and gear supports.
The aft center of gravity condition with the tail gear clear should be checked for criticality of security of large mass items located forward of the center of gravity. Vertical and angular accelerations are additive under this landing condition.
(iii) The aft center of gravity condition with each gear contacting the ground simultaneously will generally design tail gear elements critical for vertical loads.
The other conditions are generally less severe but must be proven.
(3) For tail-down landing procedures use § 27.481. The reference to "extremely remote" in § 29.497(d)(2) predates current §$ 25.1309, 29.1309, and AC 25.1309.1. This phrase has been used to require consideration of nose-up landings unless features of design are present which prevent nose-up landings or where such landings are unlikely during the life of the rotorcraft. (See paragraph AC 27.481.)
(4) Use § 27.483 for one-wheel landing procedures, paragraph AC 27.483.
(5) Use § 27.485 procedures for side load conditions, paragraph AC 27.485.
(6) Use § 27.493 procedures for braked roll conditions, paragraph AC 27.493.
(7) For rear wheel turning loads, swiveling of tail landing gears is allowed as in basic side load conditions. The side load is applied at the axle or, if the wheel is locked, the load is applied at ground contact. Rear wheels are loaded with the critical vertical static load in conjunction with an equal side load to substantiate the tail gear.
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(8) Since the rotorcraft is to be designed for load factors that will not be exceeded during taxi tests or other conditions, an instrumented taxi test program will be necessary. (Use § 27.235, paragraph AC 27.235.)
AC 27.501 § 27.501 (Amendment 27-2) GROUND LOADING CONDITIONS:
LANDING GEAR WITH SKIDS.
a. Explanation. This section provides the ground loading conditions for landing gear with skids. The loading conditions are similar to those for wheeled gear except for the following criteria which are unique to skid gears:
(1) Structural yielding of elastic spring members under limit loads is allowed.
(2) Design ultimate loads for elastic spring members need not exceed the loads obtained in a drop test with a drop height of 1.5 times the limit drop height. The rotorcraft and the landing gear attachments are subject to the prescribed design ultimate loads.
(3) The gear must be in its most critically deflected position (similar to § 27.475).
(4) Ground reactions are rationally distributed along the bottom of the skid unless otherwise specified. Section 27.501(f) concerns specific "concentrated" and arbitrary load conditions.
(5) Drag loads are 50 percent of vertical reactions rather than the 25 percent for wheeled gear.
(6) Side loads are 25 percent of the total vertical reaction rather than the 60 to 80 percent for wheeled gear.
(7) Side loads are applied to one skid only (inward acting and outward acting) with resulting unbalanced moment resisted by angular acceleration.
(8) A ground reaction load of 1.33 times the maximum weight is to be applied at 45° from the horizontal axis:
Distributed among or between the skids;
(ii) Concentrated at the forward end of the straight portion of the skid tube; and (¡i) Applied only to the forward end of the skid tube and its attachment to the rotorcraft.
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(9) A concentrated vertical load equal to one-half of the design limit vertical load is to be applied at a point midway between the skid tube attachments. This condition applies only to the skid tube and its attachment to the rotorcraft.
b. Procedures.
(1) The specified loading conditions are to be used in load derivations.
(2) The critical center of gravity conditions are to be used for each gear and gear support structure. Asymmetry of the skid tubes, cross tubes, and gear attachments is to be considered in determining the critical center of gravity condition.
(3) The rotorcraft and landing gear attachment must be substantiated for ultimate landing loads by either test or analysis utilizing an ultimate load factor of 1.5 in accordance with § 27.303. The elastic spring members may be analyzed or static tested for ultimate loads (and deflections) using either a factor of safety of 1.5 or one associated with an "ultimate" drop height of 1.5 times the limit drop height.
Substantiation by "ultimate" drop tests may be used provided all combinations of critical parameters are included in the total substantiation effort. This method will require a series of tests using several test specimens or a limited number of drop tests plus further substantiations by static tests or analyses for additional critical conditions not covered by the drop test(s).
AC 27.501A § 27.501 (Amendment 27-26) GROUND LOADING CONDITIONS:
LANDING GEAR WITH SKIDS.
a. Explanation. Amendment 27-26 relaxes the previous requirements in two cases by:
(1) Allowing the total sideload of § 27.501(d)(3) to be distributed "equally between skids" rather than being "applied along the length of one skid only;" and,
(2) Allowing the concentrated load of § 27.501(f)(2)(ii) to be distributed over the central 33.3 percent of the skid (between skid tube attachments) rather than being "concentrated at a point midway between the skid tube attachments."
b. Procedures. The previous procedures (through Amendment 27-19) continue to apply to Amendment 27-26 except use the new load distributions.
AC 27.505 § 27.505 SKI LANDING CONDITIONS.
a. Explanation. This is an optional requirement for ski operations. The regulation specifies vertical loads, side loads, and torque loads (Mz) to be applied to ski installations. The four loading conditions to be applied at the pedestal bearings are:
(1) Simultaneous application of Pn, up load, and Pn/4, horizontal load.
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