(1) Structural substantiation is required for the gear, retracting mechanism, doors, gear supporting structure for landing loads, maneuvering, gusts, and yawing flight condition loads.
(2) An emergency means to extend the gear after failure of the retraction/extension system is required for all except solely manual mechanical systems.
(3) This regulation requires an indication to the pilot when the gear is secured in the extreme positions. This rule does not apply to rotorcraft with fixed gear. The rule also applies to amphibious rotorcraft with retractable gear.
(4) A landing gear down-lock is required. An optional up- lock may be used if it meets reliability requirements.
(5) A (ground) operation test must be conducted to ensure proper functioning of the system.
(6) Location and operation of the control lever or device must comply with § 29.777. This section includes identification of controls to prevent confusion and inadvertent operation. Sections 25.779 and 25.781 of FAR Part 25 contain large airplane design requirements for motion, effect, and shape of cockpit controls and their knobs and should be consulted for further guidance.
b. Procedures.
(1) The design load factors and resulting loads should be derived from the design data. The landing gear, while retracted, operating, and extended, and its supporting structure should be substantiated for the critical aerodynamic and inertia loads. Yawed conditions should be considered. The specific conditions are noted in paragraphs (a)(1), (2), and (3) of § 29.729.
(2) Wheel well doors, if installed, should be designed for the aerodynamic loads, including loads from yawing conditions (angles proven under § 29.351) for airspeeds up to the design maximum landing gear extended speed. Aerodynamic effects on both open and closed doors must be considered in the door and door support substantiations. The applicant may choose to substantiate the rotorcraft for a “landing gear operating” and “extended” speed VLO and VLE, respectively, that is equal to the rotorcraft VNE. This option will alleviate an airspeed “structural limitation” because of the landing gear design substantiation. Any airspeed “structural limitation” should be listed in the structural limitations part of the TIA.
(3) The required “down-lock” should be checked during the operation test. The design drawing should be reviewed for compliance prior to conducting an operation test.
The “down-lock” system should be evaluated for § 29.1309 function and reliability requirements.
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(4) If an optional “up-lock” is installed (including hydraulic locking), the landing gear should be extended during the operation test after simulation of critical failure mode of the retraction system (reference § 29.1309).
(5) An “operation” test plan or proposal submitted for compliance with § 29.729(d) should include the items noted in the two previous subparagraphs and should include a functional check of the position indicator system. Those tests must be satisfactorily completed before issuing the TIA.
(6) During the official FAA/AUTHORITY flight tests, compliance with the emergency operation, position indicator, and control aspect of § 29.729(c), (e), and (f), respectively, will be verified or accomplished. In addition, the F and R test program plan (§ 21.35) will specify certain tests or evaluations for the retraction system.
(7) Position Indicator Evaluation.
(i) When evaluating the position indicator system, emphasis should be placed on the switches and their installations, and on the cockpit presentation. Each gear must have its own set of switches to indicate when it is secured in its extreme “up” position and its extreme “down” position. The switches must be located to give a valid indication of the arrival of the gear at its extreme position.
(ii) The reliability and environmental qualifications of the switches to be used should be carefully considered. An example of a condition that has potential for trouble is operation on wet areas. Trouble starts when water is picked up by the tires and deposited on the switches. During winter months the water can freeze, and the resulting ice may prevent the switch from functioning properly.
(iii) An acceptable cockpit presentation consists of two lights for each gear. One light is colored “green” and indicates when its gear is secured in the extreme “down” position. The other light is colored “amber” or “red” and indicates when its gear is in transit. When the gear is in either extreme position, the in transit light is “out.” For this presentation, the indication to the pilot that the gear is in the extreme “up” position is an all-gear, lights-out condition.
(iv) Some manufacturers have also included a warning system to alert the crew if the landing gear has not been extended prior to landing. If a warning system is presented, §§ 29.1301 and 29.1309 should be used to evaluate its functional characteristics and the impact of its failure modes.
AC 29.731. § 29.731 WHEELS.
a. Explanation. This standard requires use of approved wheels, either approved under TSO-C26 or a later revision or approved under the type certificate for the aircraft.
Wheels must satisfy both a design static (1g) load and design limit landing or taxiing Page D - 55
load determined under the applicable ground load requirements. Standards for a tire installed on a wheel are contained in § 29.733.
b. Procedures.
(1) The structural design loads data shall contain both a static load and a landing and taxiing load for each wheel. These loads are determined by virtue of compliance with the standards of § 29.731(b) and (c). The ratings of the wheel shall not be exceeded. TSO -C26c contains minimum performance standards for TSO approval of aircraft wheels and wheel-brake assemblies. Ratings are assigned in accordance with this performance standard.
(2) If a wheel selected for an aircraft design has TSO approval, the wheel manufacturer will supply the rating to the aircraft manufacturer. Each wheel shall be marked as prescribed which includes a listing of the TSO number. Even though a wheel is TSO approved, the application on the aircraft (loads imposed on the wheel) requires proof that the rating is not exceeded.
(3) If a wheel selected for an aircraft design is not approved under a TSO, the necessary data, both detail design and assembly drawings and qualification tests and test report data, will be required to comply with the standards contained in Part 29.
Design control and inspections will be accomplished as a part of the aircraft type design. Structural substantiation and any appropriate qualification tests shall be accomplished. See §§ 29.471 through 29.497 and § 29.511 for the ground load conditions.
(4) The Tire and Rim Association, Inc., generally issues a yearbook listing tire and rim sizes and ratings. The dimensions and contours for aircraft wheel rims are contained in Section 9 of this yearbook.
AC 29.733. § 29.733 (Amendment 29-12) TIRES.
a. Explanation.
(1) This standard specifies both design and performance criteria for tires. The tire must fit the wheel rim. The maximum static ground reaction for the condition specified must not exceed the maximum static load rating of each tire. In addition, any tire of retractable gear systems must have adequate clearance from surrounding structure and systems as specified.
(2) Main, nose, and tail wheel tires must comply.
(3) Rotorcraft design maximum weight shall be used. Static and “dynamic” conditions are specified for rotorcraft tires.
(4) Tire performance standards are contained in TSO-C62c.
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b. Procedures.
(1) The aircraft structural design loads should contain a maximum static load imposed on the tires. The load is derived for a static ground reaction assuming the design (maximum) weight and the critical center of gravity for each tire of the landing gear. The wheel loads are determined under § 29.731(b). Reduced weight but forward CG conditions may result in the highest static load on a nose wheel tire. Thus, combinations of weight and CG locations require investigation for the maximum tire load of each main, nose, and tail wheel tire. Nose wheel tires are subject to a specific dynamic condition.
(2) The maximum possible size of the tires considering appropriate temperatures, aging, and pressure should be obtained to check wheel well and cover clearances. Tire dimensions (for clearances) may be found in the yearbook noted in paragraph AC 29.733b(4). If the tire clearance is questionable, objects may be taped to the tire to simulate tire growth or oversize dimensions expected and the wheel retracted and rotated by hand to check for possible interferences. Minimum clearance, such as one-half inch, may be adequate as a design objective. The design drawings should be reviewed for information of correct systems installations and landing gear rigging within the wheel wells and wheel covers, if installed. If necessary to control tire sizes, specific manufacturer’s tires should be used as “required equipment” and the tire manufacturer and the part number should be specified in the design data and on the type certificate data sheet as “required equipment.”
(3) As specified in § 29.729(d), an operation test of any retractable landing gear should be performed. During this operation test, the tire clearances should be determined and recorded for the maximum tire size expected in service. Only the least or minimal clearance found, if adequate, should be recorded.
(4) The Tire and Rim Association, Inc., generally issues a yearbook listing tire and wheel rim sizes and ratings. This information is advisory as stated in the yearbook.
Section 9 concerns aircraft tires and rims. Table AP-5 in Section 9 of the yearbook concerns tires used on rotorcraft. The tire may be selected initially from the yearbook, but qualification data for the specific tires used shall be furnished with the type design data in compliance with the standards. Section 9 also contains tire size and tire growth dimensions.
(5) Minimum performance standards for aircraft tires, excluding tail wheel tires are found in TSO-C62c, Aircraft Tires. Tires meeting the TSO are marked as prescribed in the standards. The load rating (reference § 29.733) is marked on the tire.
TSO tires are not required but should be used whenever possible. The manufacturer’s information, such as load rating, should be included in the aircraft type design structural substantiation data.
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AC 29.735. § 29.735 (Amendment 29-24) BRAKES.
a. Explanation.
(1) Brakes are required for wheel landing gear aircraft. Minimum performance standards are contained in this section. During the course of the FAA/AUTHORITY flight test program and of any F&R program conducted under § 21.35, the brakes shall be used and evaluated.
(2) Design criteria are contained in this standard.
(i) The braking device must be controllable by the pilot. It is optional for the second pilot station except as may be specified under the provisions of § 29.771.
(ii) The braking device must be usable during power-off landings.
(3) Performance criteria are also contained in this standard.
(i) The brakes must be adequate to counteract any normal unbalanced torque when starting or stopping the rotor or rotors.
(ii) The brakes must be adequate to hold the rotorcraft parked on a 10° slope on dry, smooth pavement.
(4) In §§ 29.493(b)(2) and 29.497(g)(2)(ii), limiting brake torque is one ground load standard for design of the landing gear.
(5) Although not specifically noted in a standard, the position of the brake on the wheel is important. The brake should be positioned to avoid ground contact whenever the tire is deflated.
(6) TSO-C26c contains minimum performance standards for aircraft landing wheels and wheel-brake assemblies. For rotorcraft, a wheel -brake assembly design rating is established by the manufacturer. The TSO standard for rotorcraft brakes specifies a 20° slope standard (rather than a 10° slope) for an over-pressure hydraulic brake test.
(7) The brake application device at the pilot station is subject to other structure strength standards in this Part, such as the limit pilot forces or torque specified in § 29.397.
b. Procedures.
(1) Wheel-brake assemblies approved under TSO-C26 or a later revision will have various (rotorcraft) ratings as specified in the standard. One rating of TSO standard for a rotorcraft wheel-brake assembly is the kinetic energy capacity in Page D - 58
foot-pounds at the design landing rate of absorption. The design takeoff and landing weight and rotorcraft speed in knots for brake application are a part of the equation.
The brake manufacturer should furnish this rating and the two noted parameters for the selected design or designs. The ratings of selected brakes should be included in a structural design data report such as a design criteria report. The use or application of each brake design on the particular rotorcraft design should not exceed capacity of the brake or the ratings established under the TSO. If appropriate, the part number and manufacturer of each brake may be listed in the structural data reports as well as listed in the type design drawings.
(2) The limiting brake torque obtained from the brake manufacturer should be used in complying with §§ 29.493(b)(2) and 29.497(g)(2)(ii).
(3) Compliance with the brake standards should be confirmed, demonstrated, and recorded as a part of the flight test type inspection report. This applies to TSO brakes and to brakes approved as a part of the aircraft type design.
(4) If found necessary under the provisions of § 29.771, the second pilot station should have brake control devices. The brake control devices should be listed with the other required equipment that defines the equipment necessary for a second pilot station.
(5) A brake assembly may be evaluated and approved under Part 29 as a part of the aircraft type design. TSO -approved brakes are not specifically required but are recommended. For non- TSO-approved brakes, all detail and assembly drawings, required test proposals, and test results reports may be submitted and processed as a unique part of the particular aircraft type design.
(6) During an inspection of the landing gear, such as an engineering compliance inspection, the brake location should be checked to ensure the brake does not contact the ground when the tire is deflated. Type design drawings should control the proper location of the brake on the landing gear.
AC 29.737. § 29.737 SKIS.
a. Explanation. This st andard is, in part, derived from small airplane standards.
Aircraft skis approved under TSO-C28 may be used on rotorcraft. TSO -C28 for aircraft skis refers to Sections 4 and 5 of National Aircraft Standards Specification 808, dated December 15, 1951, for strength and performance standards. The standard also addresses flight/aerodynamic loads.
(1) A maximum limit load rating is assigned to each ski approved under TSO-C28.
(2) This limit load rating must not be exceeded by the maximum limit ground load determined under the standards of § 29.505, Ski landing conditions.
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(3) The ski installation is also subject to the maximum aerodynamic and inertia loads and to the ground rotation or torque load per § 29.505(c).
(4) Ski mounting or installation parts used in the particular application are subject to substantiation as any landing gear member is subject to substantiation.
(5) Ski installations are also subject to flight and ground operation evaluations.
(6) Pads or “bear paws” on skid or wheel landing gears for use in snow or soft soils are unique to rotorcraft. These shall be approved also. For new type certificate applications after November 27, 1989, § 29.571, Amendment 29-28 requires fatigue substantiation of the landing gear. The effect of pads, etc., shall be evaluated in compliance with the standard.
b. Procedures.
(1) The limit load rating for the ski selected shall be obtained from the ski manufacturer. This information shall be included in the design criteria and/or structural substantiation reports. The type design drawings will include the appropriate part number for the TSO-approved product and the necessary installation information.
(2) The design limit loads derived in compliance with § 29.505 shall not exceed the ski limit load rating. The skis shall be substantiated for the torque load in § 29.505(c) since the TSO standard does not contain a similar requirement.
(3) Skis that are not TSO approved may be approved as a part of the aircraft type design by complying with the strength and performance standards contained in TSO-C28 (NAS 808).
(4) The aerodynamic loads shall be based on a limit load design speed of
1.11 VNE. The maximum V NE used in design may be reduced only for a “ski configuration” airspeed limitation.
(5) Pads or “bear paws” installed on skid or wheel landing gear to facilitate operations in snow conditions or marsh lands may be approved as a part of or as an alteration to the aircraft type design. Rational flight and landing design loads applicable to the particular pad design must be developed and strength substantiating data submitted proving compliance with the strength and performance standards contained in Part 29. In addition, skid landing gear may be subject to excessive vibratory loads while in flight whenever the weight and mass distribution is altered by adding “bear paws.” The effect of additional weight should be investigated over the flight operating regimes, including the approved range of rotor speeds. Resonant vibratory conditi ons should be avoided or highly damped, thus avoiding a potential change in service life. In compliance with § 29.571, Amendment 29-28, stress measurement, etc., may be necessary, if the standard is applicable.
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SUBPART D - DESIGN AND CONSTRUCTION FLOATS AND HULLS.
AC 29.751. § 29.751 (Amendment 29-3) MAIN FLOAT BUOYANCY.
a. Explanation.
(1) This section specifies standards for single and multiple float buoyancy in fresh water. The standard does not apply to ditching/emergency flotation devices, but to amphibian rotorcraft devices.
(2) It is a design and a performance standard. Rigid or inflatable floats may be used. Enough water tight compartments (per Amendment 29- 3) rather than a specific number are required to minimize the probability of capsizing when one compartment is flooded or deflated.
b. Procedures.
(1) Excess buoyancy. A minimum of 50 or 60 percent in excess of the maximum certificated weight of the rotorcraft is required for single or multiple floats, respectively. The weight of fresh water (density 62.42 pounds per cu. ft.) displaced by fully submerged float or floats (total volume of each float at operating pressure is used) should be a minimum of 50 or 60 percent greater than the maximum certificated weight of the rotorcraft.
(2) Capsizing.
(i) Each float should have enough sealed, separate and approximately equal volume compartments to minimize the probability of capsizing when the critical compartment is flooded or deflated. Five or more compartments in each float are usually necessary to meet the standard. Ten compartments per float have been employed in certain designs.
(ii) An analysis or test or combination thereof may be used, if necessary, to prove a positive margin of stability with the most “critical” compartment in one float flooded or deflated.
(iii) The location of the floats, and the most critical compartment, the rotorcraft weight, mass moment of inertia, and center of gravity location are also important considerations for capsize stability.
AC 29.753. § 29.753 MAIN FLOAT DESIGN.
a. Explanation .
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(1) Strength or design load standards are encompassed in the standard for inflatable bag and rigid floats. Bag pressure loads are included. The standard applies to an amphibious rotorcraft.
(2) The float landing loads are derived from the drop test of the float landing gear, or the load may be derived from tests of the wheel (or skid) landing gear (reference § 29.521). Bag type floats are not subject to the side loads according to the standard. Rigid floats, whether single or dual, are subject to the side load in each direction.
(3) Inflatable bag type floats should also be designed for the maximum pressure differential developed for the maximum operating altitude difference requested. That is, the resulting pressure difference between an operational altitude and a take-off site elevation should be established, and proven and may become an operating limitation.
(4) Landing loads suffice for the aerodynamic loads for typical rotorcraft float designs. Nonetheless, design and/or support of the forward part of bag type floats should be evaluated for maximum design speeds to prevent collapse or significant distortion of the bag while in flight.
(5) Resistance to puncture and abrasion at attach/wear points is not in the standard but is an important design consideration. “Girt” or attachment design loads shall be sufficient to withstand the loads imposed by the standards.
(6) The water or sea conditions (wave heights) evaluated in §§ 29.231 and
29.239 tests are not limitations but should be noted in the procedures section of the flight manual.
(7) The standard does not apply to ditching/emergency floatation devices.
b. Procedures.
(1) Landing load factor.
(i) A drop test of the float landing gear may be conducted to obtain the limit landing load factor (reference § 29.725). Level landing attitude should be used for the float assembly.
(ii) The limit load factor for wheel or skid landing gear may be used (reference § 29.521) for the floats.
(iii) The float design ultimate load factor is 1.5 multiplied by the limit load factor.
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(2) Flight aerodynamic loads--bag type floats.
(i) Evaluate collapse or significant distortion of bag type floats for speeds up to VD (1.11 VNE) with the minimum operating bag pressure.
(ii) External tubes to support the bag may be employed.
NOTE: Design landing loads may exceed the flight loads.
(3) Altitude differential loads.
(i) Bag type floats should not rupture due to the change in absolute pressure from take-off to the operating altitude. The applicant should select and prove the maximum operating altitude differential desired. A 5,000 to 8,000 feet operating differential may be a sufficient limitation. That is the rotorcraft with bags properly inflated could not operate more than 5,000 to 8,000 feet above the take-off site elevation.
(ii) A proof and ultimate pressure test should be conducted for the design.
If operating or inflation pressure is 2.62 PSI (including a tolerance) and 5,000 feet (pressure) differential is desired (use sea level to 5,000 feet pressures), the proof or limit pressure should be 2.62 + 2.47 = 5.09 PSI. The pressure relief valves may be set at this value also. The change in size during inflation should be recorded. Significant changes may adversely affect flight characteristics and should be evaluated. The ultimate or burst free pressure should be proof pressure (5.09 PSI) multiplied by 1.5 or
7.635 PSI. A video or photographic record may be used as a reference of the change in size or shape for this test.
(iii) Each compartment should be equipped with a pressure relief valve to further protect the bag from excessive internal pressure.
(iv) At least one float should be subjected to a burst pressure test.
Record the gauge pressure of burst.
(4) Landing loads.
(i) Rigid float vertical and a combined vertical and aft load conditions. A vertical or up-load only and a vertical combined with an aft load component for a resulting vector angle of 14.03° from the vertical axis of the rotorcraft shall be used.
Reference § 29.521(a). The resulting design load is the same load in both cases.
(ii) Rigid float side and vertical load condition. For each rigid float, whether single or dual, a vertical load combined with a side load resulting in a vector angle of 26.6° from the vertical axis of the rotorcraft shall be used. The side load is applied to each float individually. Both inward and outward acting side load conditions shall be substantiated separately for the design of dual floats.
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(iii) Load distribution on rigid floats shall be appropriate for the critical conditions. ANC -3 or § 25.533 and FAR Part 25, Appendix B may be useful.
(iv) Bag type float. The loads and the distribution of the loads are rather simple according to the standards. Only vertical loads and vertical with aft (drag) component are specified in the standard. These shall be distributed along the length over 75 percent of the projected area of the bag. Side loads are not required.
(5) Operating limitations.
(i) VNE with floats installed is typically lower than the VNE for wheel or skid landing gear rotorcraft configurations.
fittings.
(ii) Bag inflation pressure shall be placarded or stenciled near inflation
(iii) The operating attitude differential proven for bag floats shall be an operating limitation. In addition, the flight manual should caution pilots about the effect of a significant decrease in altitude from the take off level which causes or reduces pressure in the bag. Placards may be employed as well.
(iv) Flight test results may dictate a further reduction in VNE or changes in other operating limitations.
AC 29.755. § 29.755 (Amendment 29-30) HULL BUOYANCY.
a. Explanation.
(1) This section contains performance standards for an integral fuselage hull and auxiliary (such as outrigger) floats. Water -based, amphibian and limited amphibian rotorcraft were encompassed in the standard.
(2) Amendment 29-3 added but Amendment 29-30 removed paragraph (b) which concerned Limited Amphibian Rotorcraft. Rotorcraft of that type used a “boat type hull” which is not desirable now and are certificated to the standards of § 29.801, Ditching, and § 29.563, Structural ditching provisions. (Limited amphibian rotorcraft were converted to the ditching configuration.)
(3) The worst combination of wave height and surface winds selected by the applicant shall be used in compliance with the standard.
b. Procedures.
(1) Capsizing.
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(i) The hull and auxiliary floats shall have enough sealed compartments to allow failure of the critical, single, compartment in either the hull or auxiliary float and minimize the probability of capsizing.
(ii) Location of the most critical compartment (whether hull, sponson, or auxiliary), rotorcraft weight, mass moment of inertia, and CG location are also important considerations to prove stability or not capsizing.
(iii) The lightweight rotorcraft configuration and wind and wave condition should be considered, as well as the heavy weight configuration.
(iv) The sea state (worst combination of wave height and surface winds) is selected by the applicant. The condition proven is included in the procedures or information section of the flight manual. (It is not an operating limitation.)
(2) Buoyancy.
(i) Excess buoyancy is necessary to comply with the standard but the amount is dependent on several factors, such as number, size, and location of the sealed, watertight, compartments.
(ii) Wheel tires may be used for buoyancy if appropriate to the design.
(iii) Fuel tanks, if properly located and protected from potential rupture and if the aircraft has a system to rapidly empty the tanks, may be used also for buoyancy.
(iv) Buoyancy may be determined using the displacement of fresh water, with 62.42 pounds per cubic ft. density.
(3) Tests.
(i) If necessary, scale models may be used to prove the stability of the rotorcraft design for the sea state and wind conditions selected by the applicant.
(ii) The rotorcraft is subject to water tests per § 29.231. Compliance with part of this standard may be demonstrated or proven for the sea state or wave height, and wind conditions selected in conjunction with the TIA flight test program. This information is not an operating limitation.
(iii) Proposals should be submitted for evaluation.
AC 29.757. § 29.757 (Amendment 29-3) HULL AND AUXILIARY FLOAT STRENGTH.
a. Explanation. The standard is an objective or performance strength standard.
The water loads in § 29.519 shall be imposed for the hull and auxiliary floats in a Page D - 65