AC 29.481. § 29.481 TAIL-DOWN LANDING CONDITIONS.
a. Explanation. This s ection 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 landing gear in combination with vertical loads.
(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 29.725 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 criti cal attitude, both level landing and tail-down landing attitudes should be used in drop tests required by § 29.725.
AC 29.483. § 29.483 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 § 29.479(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. Large items of mass located radially from the center of gravity (aircraft centerline may be used) should also be structurally substantiated for the combined rolling (angular) and linear accelerations of this loading condition.
AC 29.485. § 29.485 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 § 29.479(b)(1).” These vertical loads are the level landing loads considering both contact and noncontact with the ground by the forward wheels.
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(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” 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 landing gear backup structure, as well as the landing gear itself, will be substantiated for these side load conditions.
AC 29.493. § 29.493 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.
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 assure that the landing gear structure and the attach and backup structure are adequately substantiated.
AC 29.497. § 29.497 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:
(i) All wheels (main and tail) contacting the ground simultaneously, as well as only forward main wheels contacting the ground.
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(ii) Maximum design vertical loads applied alone.
(iii) 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.
(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 operation.” The aircraft design load factors should not be exceeded during the evaluation. Section 29.235 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.
(ii) 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 nose-up landing procedures use § 29.481. The reference to “extremely remote” in § 29.497(d)(2) predates current §§ 25.1309, 29.1309, and AC 25.1309.1.
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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 29.481.
(4) Use § 29.483 for one-wheel landing procedures, paragraph AC 29.483.
(5) Use § 29.485 procedures for side load conditions, paragraph AC 29.485.
(6) Use § 29.493 procedures for braked roll conditions, paragraph AC 29.493.
(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.
(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 § 29.235, paragraph AC 29.235.
AC 29.501. § 29.501 (Amendment 29-3) 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 (plastic deformation) 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 § 29.475).
(4) Ground reactions are rationally distributed along the bottom of the skid unless otherwise specified. Paragraph (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 60-80 percent for wheeled gear.
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(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:
(i) Distributed among or between the skids;
(ii) Conc entrated at the forward end of the straight portion of the skid tube; and
(iii) Applied only to the forward end of the skid tube and its attachment to the rotorcraft.
(9) A concent rated 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.
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 are 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 § 29.303. 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 29.501A. § 29.501 (Amendment 29-30) GROUND LOADING CONDITIONS:
LANDING GEAR WITH SKIDS.
a. Explanation. Amendment 29- 30 relaxes previous requirements in two cases by:
(1) Allowing the total sideload of § 29.501(d)(3) to be distributed “equally between skids” rather than being “applied along the length of one skid only;” and, Page C - 40
(2) Allowing the concentrated load of § 29.501(f)(2)(ii) to be distributed over
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 29-19) continue to apply to Amendment 29-30 except for the use of the new load distributions.
AC 29.505. § 29.505 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.
(2) Up load of 1.33 P.
(3) Side load of 0.35 Pn.
(4) Torque load of 1.33 P (in foot-pounds), about vertical axis through the centerline of the pedestal bearings.
NOTE: Where P is the maximum static weight on each ski and n is the limit load factor obtained from drop tests. The load factor obtained from wheel or skid landing gear drop tests may be used.
b. Procedures. Structural substantiation may be accomplished by static test or analysis using the specified loads. Skis generally have a limit load rating. The design loads derived for this standard must not exceed the rating. TSO -c28 concerns, in part, standards for aircraft skis.
AC 29.511. § 29.511 (Amendment 29-3) GROUND LOAD: UNSYMMETRICAL LOADS ON MULTIPLE-WHEEL UNITS.
a. Explanation. Two loading conditions are provided to account for unsymmetrical loads on multiple-wheel units due to landing and normal operations over crowned runways and taxiways and to account for deflated tires. They are:
(1) Sixty percent of total ground reaction applied to one wheel of a dual wheel unit and 40 percent to the other.
(2) Sixty percent of the “specified load for the gear unit” is applied to the wheel with an inflated tire when the other tire is deflated (the 60 percent load may not be less than the 1g static load).
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NOTE: The 60:40 distribution also applies to nose wheel units as noted in § 29.479(b)(4).
b. Procedures. Structural substantiation may be accomplished by static test or analysis using the specified load. As provided by the standard, the total load on the gear units may neglect the transverse shift of the load centroid due to unsymmetrical load distribution; i.e., the external load for each gear may be calculated considering the same load centroid as with symmetrical wheel loads, and then the external load for each gear is divided in accordance with the distributions of § 29.511(a) and (b) between the wheels.
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SUBPART C - STRENGTH REQUIREMENTS
WATER LOADS AC 29.519. § 29.519 (Amendment 29-30) HULL TYPE ROTORCRAFT:
WATER-BASED, AMPHIBIAN.
a. Explanation.
(1) This regulation provides design criteria for amphibian rotorcraft with hull provisions.
(2) The most severe wave heights for which approval is desired are to be considered. A minimum of sea state 4 condition wave heights should be considered (reference paragraph AC 29.801 for a description of sea state 4 conditions).
(3) A rotor lift of two-thirds of the rotorcraft weight may be applied during landing impact.
(4) Vertical landing conditions are specified as:
(i) Zero forward speed.
(ii) Lik ely pitch and roll attitudes.
(iii) Vertical descent velocity ≥ 6.5 FPS.
(5) Forwar d speed landing conditions are specified as:
(i) Forward velocities of zero to 30 knots (a 30-knot limit may be reduced if it can be demonstrated that the maximum forward velocity selected would not be exceeded in a normal one-engine-out landing).
(ii) Likely pitch, roll, and yaw attitudes.
(iii) Vertical descent velocity ≥ 6.5 FPS.
(6) Auxiliary float immersion conditions are specified to be applied unless it can be shown that full immersion is unlikely. If full immersion is unlikely, the highest float buoyancy load is specified that considers loading of the float immersed to create restoring moments which compensate for upsetting moments caused by side wind, asymmetrical rotorcraft loading, water wave action, and rotorcraft inertia.
b. Procedures.
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(1) Tests should be conducted to establish procedures for water entry. These tests should include determination of optimum pitch attitude and forward velocity for landing in a calm sea as well as entry procedures for the highest sea state to be demonstrated (e.g., the recommended part of the wave on which to land and direction of landing relative to crest/trough direction).
(2) The landing structural design consideration should be based on water impact with a rotor lift of not more than two-thirds of the maximum design weight acting through the center of gravity under the following conditions:
(i) Vertical Landing Conditions.
(A) Zero forward velocity.
(B) The optimum pitch attitude as determined in paragraph AC 29.519b(1) with consideration for pitch attitude variations that would reasonably be expected to occur in service.
(C) Vertical descent velocity of 6.5 FPS or greater.
(D) Likely roll attitudes.
(ii) Forward Speed Landing Conditions.
(A) Forward velocities of zero to 30 knots (or a reduced maximum forward velocity if it can be demonstrated that a lower maximum velocity would not be exceeded in a normal one-engine-out landing).
(B) The optimum pitch attitude as determined in paragraph AC 29.519b(1) with consideration for pitch attitude variations that would reasonably be expected to occur in service.
(C) Vertical descent velocity of 6.5 FPS or greater.
(D) Likely roll and yaw attitudes.
(3) Landing load factors may be determined by--
(i) Landing gear drop tests for limited amphibian;
(ii) Water drop tests for amphibian; or
(iii) Analysis based on tests.
(4) Water load distribution should be determined by tests or analysis based on tests.
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(5) Auxiliary float loads should be determined by full immersion or restoring moments required to react upsetting moments caused by side wind, asymmetrical rotorcraft loading, water wave action, and rotorcraft inertia. Auxiliary float loads may be determined by analysis. Load distributions should be determined by tests or analysis based on tests.
AC 29.521. § 29.521 (Amendment 29-3) FLOAT LANDING CONDITIONS.
a. Explanation. This is an optional requirement for float operations, and it applies only when float operations are requested. The regulation specifies vertical loads, aft loads, and side loads to be applied to the float installations. The two loading conditions to be applied are:
(1) Up-load Condition.
(i) A vertical load appropriate to a landing load factor determined under § 29.473(b).
(ii) The resultant water reaction passes vertically through the aircraft CG.
(iii) An aft load equal to 25 percent of the vertical load.
(2) Side-load Condition.
(i) A vertical load equal to 75 percent of the vertical load for the up-load condition.
(ii) Vertical load equally divided among the floats.
(iii) A side load at each float equal to 25 percent of the vertical load at each float.
b. Proced ures.
(1) The vertical load factor is determined by drop tests in accordance with §§ 29.473(b) and 29.725. The floats may be drop tested, or they may be assumed to have the same load factor as wheeled gear which have been drop tested.
(2) Structural substantiation may be accomplished by either static tests or analysis using the specified loads. The load distribution on the floats may be realistically based on hydrostatic pressure distributions or conservative pressure distributions.
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SUBPART C - STRENGTH REQUIREMENTS
MAIN COMPONENT REQUIREMENTS AC 29.547. § 29.547 (Amendment 29-4) MAIN ROTOR STRUCTURE.
a. Explanation. This regulation requires the main rotor structure to be designed to the static load requirements of §§ 29.337 through 29.351 (vertical maneuvering loads, vertical and horizontal gust loads, and yawing maneuver loads). In addition, the main rotor blades, hubs, and flapping hinges are specified to be designed for impact forces of each blade against its stop during ground operation and for specified limit torque at any rotational speed including zero. The torque forces (from the drive system) are distributed to the rotor blades as specified.
b. Procedures.
(1) Substantiation in compliance with this standard is accomplished by application of the flight loads of §§ 29.337 through 29.351 and the torque loads of § 29.361 to the rotor structure by stress analyses and/or static tests. The use of wind tunnel data as well as flight loads survey data may be used to generate and/or check the external load magnitudes and distributions.
(2) Where new materials are used in the main rotor structure, such as composites containing plastics, the effects of temperature and humidity are to be considered in accordance with § 29.603, and the effects of uncertainties in manufacturing processes or inspection methods are to be considered in accordance with § 29.619.
(3) The design impact forces of each blade must be imposed against the blade stop or stops. Impact loads from 2 to 3 g’s have been commonly used to provide rotor structure protection against blades impacting against lower (droop) stops. Different values may be used for flapping and lag stops as determined by a rational basis.
Appropriate monitoring of the blades, hubs, flapping hinges, and stops during laboratory tests, ground endurance tests, and flight tests should ensure that the stops are sufficient for ground operation loads (taxiing, backing, etc.), training, and offshore platform landings. Taxiing should consider typical obstacles such as pavement edges, ropes, air lines, and so forth. The design torque loads are derived as prescribed.
AC 29.547A. § 29.547 (Amendment 29-40) MAIN ROTOR AND TAIL ROTOR STRUCTURE.
a. Explanation. Amendment 29- 40 revised § 29.547 to add requirements to perform a design assessment. Section 29.547 (a) and (b) set forth a definition of a rotor and its associated components and requires a design assessment to be performed.
The intent of these paragraphs is to identify the critical components and/or clarify their Page C - 46
design integrity to show that the basic airworthiness requirements which are applicable to the rotors will be met.
A design assessment of the rotors should be carried out in order to substantiate that they are of a safe design and that compensating provisions are made available to prevent failures classified as hazardous and catastrophic in the sense specified in paragraph b below. In carrying out the design assessment, the results of the certification ground and flight testing (including any failures or degradation) should be taken into consideration. Previous service experience with similar designs should also be taken into account (see also § 29.601(a)).
b. Definitions. For the purposes of this assessment, failure conditions may be classified according to the severity of their effects as follows:
(1) Minor. Failure conditions which would not significantly reduce rotorcraft safety, and which involve crew actions that are well within the crew capabilities. Minor failure conditions may include, for example, a slight reduction in safety margins or functional capabilities, a slight increase in crew workload, such as routine flight plan changes, or some inconvenience to occupants.
(2) Major. Failure conditions which would reduce the capability of the rotorcraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be, for example, a significant reduction in safety margins or functional capabilities, a significant increase in crew work load or in conditions impairing crew efficiency, or discomfort to occupants, possibly including injuries.
(3) Hazardous. Failure conditions which would reduce the capability of the rotorcraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be --
(i) A large reduction in safety margins or functional capabilities.
(ii) Physical distress or higher workload such that the flight crew cannot be relied upon to perform their tasks accurately or completely.
(iii) Serious or fatal injury to a relatively small number of the occupants.
(iv) Loss of ability to continue safe flight to a suitable landing site.
(4) Catastrophic. Failure conditions which would prevent a safe landing.
(5) Minimize. Reduce to the least possible amount by means that can be shown to be both technically feasible and economically justifiable.
(6) Health Monitoring. Equipment, techniques, and/or procedures by which selected incipient failure or degradation can be determined.
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