so that no local failures of the aircraft occur as a result of the limit or reserve energy drop tests.
Test data. Computations for the required drop height (h) and the effective drop weight (We) should be shown for each design level landing and tail down landing condition in compliance with §§ 27.479 and 27.481. The computations should be in accordance with § 27.725(a) for h and § 27.725(b) for We for the limit drop tests.
We and h are computed in accordance with § 27.725 for the limit drop test and with § 27.727 for reserve energy drop test. The computation of the static weight on the gear being dropped (W, WN, or WT) and used in the computation of We should be shown.
This static weight is defined as WM, WT, or WN for the main gears, tail gear, or nose gear, respectively, in § 27.725(d). It should be shown that the critical CG and proposed certificated maximum landing weight have been used in the computation of W, WT, or WN. The computation of the slope of the platforms required for the inclined reaction conditions should be presented also.
(iv) Test results. The results of the test are based on the values of We, h, d, W, and L used and obtained for each drop test and the value of N obtained from the accelerometer. These results should be summarized, and the method of computing the aircraft limit inertia load factor should be shown for each drop in accordance with § 27.725(d). A print or copy of the film or other recording trace from the accelerometer, if not a direct readout type of accelerometer, should be included in the test results.
Each critical condition should have several preliminary drops, as many times as required, to obtain reasonable correlation.
(2) Skid landing gear may be tested using similar procedures except a level landing attitude drop test is all that is required by § 27.501. The design load conditions specified in § 27.501(c) through (f) are derived from this level drop test condition.
Section 27.501(a)(2) and (3), contain special considerations for skid landing gear.
(ii) Section 27.501(a)(2) specifies that structural yielding of elastic spring members under limit load is acceptable. This yielding or deformation is a means of absorbing the landing impact. For skid landing gear that uses oleo or other types of shock absorbers, the standard does not allow structural yielding under limit load.
During the limit load and reserve energy (ultimate for skid landing gear with elastic spring numbers) drops, the yielding energy absorbing members will probably deform or yield. After a limit drop test, the gear may be used for a reserve energy drop at the discretion of the applicant, but a gear that has been subjected to a reserve energy drop should not be used unless it can be shown that no yielding has occurred in that gear.
(3) Wheel landing gear is tested in attitudes prescribed in paragraph AC 27.723a(3). Each unit, nose or main gear, is generally tested separately.
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(4) Skid landing gear is tested in attitudes prescribed in paragraph AC 27.723a(3). Due to the construction of skid landing gear, the complete skid landing gear is tested as a unit. Thus, the level landing with drag condition is probably the critical attitude for the forward cross-tube and its attachments. The level landing condition is probably the critical attitude for the aft cross-tube and its attachments.
(5) An FAA/AUTHORITY or FAA/AUTHORITY designated or delegated person need only witness the drop tests for "record" or "compliance." Preliminary or developmental drops do not require an FAA/AUTHORITY witness.
AC 27.725. § 27.725 LIMIT DROP TEST.
a. Explanation. Limit drop tests in the critical aircraft attitude or critical attitude of each gear are required for the landing gear. The drop height must be at least 8 inches, which equates to a 393-foot-per-minute (free fall) vertical descent speed. Rotor lift may be simulated, and an effective mass may be used in the drop test as prescribed.
b. Procedures. See paragraph AC 27.723, § 27.723.
AC 27.727. § 27.727 RESERVE ENERGY ABSORPTION DROP TEST.
a. Explanation.
(1) In addition to the limit drop tests, a reserve energy drop test is required.
The landing gear must not collapse in this test to the extent that the fuselage impacts the ground. Fracture (to separation) of landing gear parts is considered collapse of the landing gear. This test is not an ultimate load drop test for the landing gear, except as specified in § 27.501(a)(3) for certain skid landing gear designs using elastic spring members.
(2) All other types of landing gear must be substantiated for design ultimate loads in addition to this reserve energy drop test.
(3) Shock absorbing devices, such as oleos, must not "bottom" during the reserve energy drop test. "Bottoming" occurs when displacement of the device no longer occurs with increasing load.
(4) Requirements for proof of the landing gear and airframe structure are found in §§ 27.305, 27.307, and 27.473.
b. Procedures. See paragraph AC 27.723, § 27.723.
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AC 27.727A. § 27.727 (Amendment 27-26) RESERVE ENERGY ABSORPTION DROP TEST.
a. Explanation. Amendment 27-26 defines the word "collapse" as used in § 27.727(c). Collapse of the landing gear during reserve energy absorption drop tests occurs when:
(1) A member of the landing gear will not support the rotorcraft in the proper attitude; or,
(2) A landing gear member deforms sufficiently to allow the rotorcraft structure other than the landing gear and external accessories to impact the landing surface.
b. Procedures. The procedures of paragraph AC 27.727A continue to apply with the following supplemental guidance.
(1) The proper attitude for the rotorcraft after the reserve energy absorption drop test is an attitude which allows for permanent deformation of landing gear elements but provides for adequate egress from the rotorcraft. Refer to paragraph AC 27.807 for emergency exit standards that relate to attitudes after a crash, § 27.807(b)(2).
(2) External accessories that may not impact the level landing surface during drop testing (or equivalent gear deflections) include devices such as externally mounted fuel tanks or accessories likely to cause post-landing fires. Expendable accessories, such as cameras, loudspeakers, and search lights, may be damaged during landing gear deformations resulting from reserve energy drop tests if electrical connections are sufficiently protected to preclude electrical fires and the devices are not likely to penetrate a fuel compartment or occupied areas. The expendable accessories, if installed, should also be designed to not have "hard points" that would unacceptably damage the rotorcraft structure under landing impacts by penetration into the occupied areas or fuel tanks. Design features may be employed to preclude this penetration if possibly hazardous. The expendable accessories, if installed, should be designed with frangible fittings, frangible devices, or comparable design features. Also, these devices should be designed to not significantly alter the energy absorbing ability or design features of the landing gear.
AC 27.729. § 27.729 (Amendment 27-21) RETRACTING MECHANISM.
a. Explanation.
(1) This standard was added by Amendment 27-21.
(2) Structural substantiation is required for the gear, retracting mechanism, doors, gear supporting structure for landing loads, maneuvering, gusts, and yawing Page D - 51
flight condition loads. Design maximum airspeed for extension and retraction and for fully extended conditions are required conditions.
(3) An emergency means to extend the gear after failure of the retraction/extension system is required for all except solely manual mechanical systems.
(4) This regulation requires an indication to the pilot when the gear is secured in the extreme positions. This rule does not apply to rotorcraft that have fixed gear but does apply to amphibious rotorcraft with retractable gear.
(5) A "landing gear down" lock is required. An optional uplock may be used if it meets reliability requirements.
(6) A (ground) operation test should be conducted to ensure proper functioning of the system.
(7) Location and operation of the control lever or device must comply with § 27.777. This section includes identification of controls to prevent confusion and inadvertent operation. Amendment 27-21 added new § 27.779 for motion and effect of cockpit controls. Specifically, § 27.779(c) pertains to motion and effect of normal landing gear controls. Section 25.781 of Part 25 contains large airplane design requirements for motion, effect, and shape of cockpit controls and their knobs and should be consulted for further guidance.
(8) A landing gear warning is required as prescribed in § 27.729(g). Certain features are required. The landing gear shall be extended and locked.
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 § 27.729(a) (1), (a)(2), and (a) (3).
(2) Wheel well doors, if installed, should be designed for the aerodynamic loads, including loads from yawing conditions (angles selected by the applicant) 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.
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(3) The required "downlock" should be checked during the operation test. The design drawing should be reviewed for compliance prior to conducting an operation test.
(4) If an optional "uplock" is installed, the landing gear should be extended during the operation test after simulation of the critical failure mode of the retraction system.
(5) An "operation" test plan or proposal submitted for compliance with § 27.729 should include the items noted in 301b(3) and (4) above and should include a functional check of the position indicator system. Those ground 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 § 27.729(c), (e), (f), and
(g), respectively, will be verified or accomplished. In addition, the F&R test program plan (§ 21.35) will specify certain tests or evaluations for the retraction system.
(7) Position Indicator Evaluation.
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.
(¡i) 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.
(i¡i) 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" 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.
(8) Warning System.
A warning system to alert the crew if the landing gear has not been fully extended and locked is required.
(i) The landing gear warning system that is provided should be evaluated by a flight test pilot. A primary concern should be that the warning device provided is Page D - 53
distinctive in its operation from other warning devices incorporated into the rotorcraft cockpit design.
(ili) An acceptable method of interlocking a normal landing mode and the position of the landing gear would be through the selection of some appropriate speed that is less than VLE. The system would be instrumented such that if the gear is not down and locked and the rotorcraft goes below the selected airspeed, the landing gear warning device would be activated.
(iv) An acceptable manual shut off capability would be one that allows disabling the warning device and yet will automatically reset itself when the landing gear is cycled or retracted, or the rotorcraft's speed is increased above that speed selected to activate the warning device.
(v) The appropriate provisions of § 27.1309 should be used to evaluate the impact of system malfunctions.
AC 27.731. $ 27.731 WHEELS.
a. Explanation. This standard requires use of approved wheels, either approved under TSO-C26 or approved under the type certificate for the aircraft. Wheels must satisfy both a design static (1g) load and design limit landing or taxiing load determined under the applicable ground load requirements. Standards for a tire installed on a wheel are contained in § 27.733.
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 § 27.731(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-C26 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 TSO-C26, 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 27.
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 §§ 27.471 through 27.497 for the ground load conditions.
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(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 27.733. § 27.733 (Amendment 27-11) 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) Tire performance standards are contained in TSO-C62.
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 § 27.731(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.
(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 27.733b(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 paragraph d of § 27.729 adopted by Amendment 27-21, an operation test of any retractable landing gear should be performed. During this operation test, the tire clearances described in paragraph AC 27.733b(2) should be Page D - 55
determined and recorded. Only the least or minimal clearance found, if adequate, should be recorded in the type inspection report or other appropriate type design report.
(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) Aircraft Tires. Minimum performance standards for aircraft tires, excluding tail wheel tires are found in TSO-C62, Aircraft Tires. Tires meeting TSO-C62 are marked as prescribed in the standards. The load rating (reference § 27.733) 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.
AC 27.735. $ 27.735 (Amendment 27-21) 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 § 27.771.
(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.
(i) The brakes must be adequate to hold the rotorcraft parked on a 10° slope on dry, smooth pavement.
(4) In §§ 27.493(b)(2) and 27.497 (g)(2) (ii), limiting brake torque is one ground load standard for design of the landing gear.
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(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-C26 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 § 27.397.
b. Procedures.
(1) Wheel-brake assemblies approved under TSO-C26 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 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 TSO-C26. 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 § 27.493(b)(2).
(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-C26 brakes and to brakes approved as a part of the aircraft type design.
(4) If found necessary under the provisions of § 27.771, 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 27 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.
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(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 27.737. § 27.737 SKIS.
a. Explanation. This standard is derived from 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. These standards are conservative for rotorcraft ski installations.
(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 § 27.505, Ski landing conditions.
(3) Ski mounting or installation parts used in the particular application are subject to substantiation as any landing gear member is subject to substantiation.
(4) Ski installations are also subject to flight and ground operation evaluations.
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 § 27.505 shall not exceed the ski limit load rating.
(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) Pads or "bear paws" installed on skid or wheel landing gear to facilitate operations in snow conditions may be approved as a part of or as an alteration to the aircraft type design. Rational 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 27. 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. Resonant vibratory conditions should be avoided or highly damped.
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SUBPART D - DESIGN AND CONSTRUCTION FLOATS AND HULLS.
AC 27.751 § 27.751 (Amendment 27-2) MAIN FLOAT BUOYANCY.
a. Explanation.
(1) The 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 27-2) 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 at operating pressure of each float is used) should be a minimum of 50 or 60 percent greater than the maximum certificated weight of the rotorcraft.
(2) Capsizing.
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, that is ineffective.
(•) 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.
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AC 27.753 § 27.753 MAIN FLOAT DESIGN.
a. Explanation. Loads and load distributions are specified for float design as follows:
(1) Bag floats are to be designed for:
(1) The maximum pressure differential developed at the maximum design altitude.
(ii) The vertical loads prescribed in § 27.521(a) distributed over three-fourths of the bag's projected area.
(2) Rigid floats are to be designed for vertical, horizontal, and side loads prescribed in § 27.521 distributed along the length of the float.
b. Procedures. Structural substantiation may be accomplished by static tests or analyses using the specified loads. Substantiation should cover the float and float attachments.
AC 27.755 $ 27.755 HULLS.
a. Explanation.
(1) The section requires amphibious rotorcraft with a single hull (main float design) and with auxiliary floats (outriggers) to provide a margin of positive stability great enough to minimize the probability of capsizing when any single (usually the most critical) compartment is flooded. Landing gear wheel tires may be used for stability purposes as well.
(2) Limitations for water operation are not intended by this section, but information for water operation must be included in the rotorcraft flight manual.
(3) Wave height or sea state and buoyancy relative to fresh water is not specified but is encompassed in the objective statement of § 27.751(b).
(4) Section 27.751 specifies an excess buoyancy requirement of 50 percent for single main floats (hulls) and contains a capsize/stability standard also. This section complements § 27.755 for certain hull designs.
(5) Sections 23.751, 23.755, and 23.757 concern design standards for small airplanes and may provide insight into possible rotorcraft hull designs.
b. Procedures.
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