(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
(ill) Applied only to the forward end of the skid tube and its attachment to the rotorcraft.
(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 tests).
180A. § 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:
232 Par 180
(1) Allowing the total sideload of Paragraph 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 Paragraph 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.
181. § 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.
(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 the 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.)
Par 180A 233
АС 27-1A 7/30/97
182.-192. RESERVED.
Intentionally Left Blank 234 (thru 244) Par 182
SECTION 11. WATER LOADS
193. RESERVED.
194. § 27.521 FLOAT LANDING CONDITIONS.
a. Explanation. This is an optional requirement for float operations. 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.
A vertical load appropriate to a landing load factor determined under § 27.473(b).
(її) A 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.
A vertical load equal to 75 percent of the vertical load for the up-load condition.
(її) A vertical load equally divided among the floats.
(ili) A side load at each float equal to 25 percent of the vertical load of Paragraph (a)(2)(i) at each float.
b. Procedures.
(1) The vertical load factor is determined by drop tests in accordance with §§ 27.473(b) and 27.725. 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 realistic, based on hydrostatic pressure distributions, or conservative.
195.-204. RESERVED.
Par 193 245 (thru 257)
SECTION 12. MAIN COMPONENT REQUIREMENTS
205. § 27.547 (Amendment 27-3) MAIN ROTOR STRUCTURE.
a. Explanation. This regulation requires the main rotor structure to be designed to the static load requirements of §§ 27.337 through 27.341 (vertical maneuvering loads and vertical and horizontal gust 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 §§ 27.337 through 27.341 and the torque loads of § 27.361 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 § 27.603, and the effects of uncertainties in manufacturing processes or inspection methods are to be considered in accordance with § 27.619. More experience is available for metallic materials, but § 27.603 requires that metallics be suitably protected against the effects of environmental conditions.
(3) The design impact forces of each blade must be imposed against its stop or stops. 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. The design torque loads are derived as prescribed.
206. § 27,549 (Amendment 27-3) FUSELAGE, LANDING GEAR, AND ROTOR PYLON STRUCTURE.
a. Explanation. This regulation requires that the fuselage, landing gear, and rotor pylon (including the tail fin, if any) be designed to withstand the flight loads of §§ 27.337 through 27.351, the ground loads of §§ 27.235, 27.471 through 27.497, skid loads of § 27.501, ski loads of § 27.505, water loads of § 27.521, and rotor loads of § 27.547 (d) and (e). The ski and water loads pertain to optional features. Consideration is also required of -- 258 Par 205
(1) Auxiliary rotor thrust;
(2) The torque reaction of each rotor drive system; and
(3) Balancing air and inertia loads.
b. Procedures. Compliance with this standard is accomplished by application of the specified aircraft loads including engine torque to the fuselage and rotor pylon structure by stress analyses and/or static tests. Drive system torque factors to be used are noted in § 27.547 for the main rotor structure as well as in Paragraph (e) of this standard.
Par 206 259
207.-217. RESERVED.
Intentionally Left Blank 260 (thru 270) Par 207
SECTION 13. EMERGENCY LANDING CONDITIONS
218. § 27.561 GENERAL.
a. Explanation.
(1) The occupants should be protected as prescribed from serious injury during an emergency/minor crash landing on water or land for the conditions prescribed in the standard. The standard states that each occupant should be given every reasonable chance of escaping serious injury in a minor crash landing.
(2) The standard in § 27.561(b)(3) specifies certain ultimate inertia load factors but allows a lesser downward vertical load factor by virtue of a 5 FPS ultimate rate of descent.
(3) In addition, the occupants shall be protected from items of mass inside the cabin as well as outside the cabin. For example, a cabin fire extinguisher must be restrained for the load factors prescribed in this section (reference § 27.1411(b)(2).) A transmission or engine must be restrained to the load factors in § 27.561(b)(3) if located above or behind the occupants.
(4) For aircraft equipped with retractable landing gear, the landing gear is retracted for compliance with the standards.
b. Procedures.
(1) The design criteria report or another similar report of the rotorcraft structural limits should contain the (ultimate) minor crash condition load factors.
(2) Section 27.785 (Paragraph 336 of this AC) concerns application of this design standard to seats, berths, belts, and harnesses.
(3) The ultimate design landing and maneuvering load factors may exceed the minor crash condition load factors. The highest load factor derived shall be used.
For example, for light weight conditions, the ultimate maneuvering load factor may be 5.25g as specified in § 27.337.
(il) The ultimate vertical landing load tactors derived trom 99 27.471 through 27.521, whichever is appropriate for the design, may exceed the 4.0g down load factor in this section. The rotorcraft landing case design limit contact velocity shall range from 6.5 to 8.3 FPS (see §§ 27.473 and 27.725).
Par 218 271
(4) As specified in (b)(3) (iv) of the standard, the downward load factor is 4.0 or a lower design load factor may be used.
The lower load factor relates to a rotorcraft impacting a flat, hard landing surface at 5 FPS (ultimate) vertical rate of descent. The load factor derived for each unique design is a tunction of the aircraft impact/crushing characteristics.
(i) The 4.0g down load factor case is related to either a fixed or retractable gear rotorcraft. This condition is not dependent on impact characteristics of the rotorcraft.
(ill) As noted in Paragraph (3) above, the design landing load factors may exceed each of the two previous cases and would then become the prominent design (vertical load) parameter for seats, transmissions, fire extinguishers, etc.
(5) Items of mass such as fire extinguishers, nav-com equipment, liferafts, engines, and/or transmissions shall be restrained for the appropriate load factors.
(6) Cargo/baggage compartments separated from the passenger compartment shall be designed for load factors specified in § 27.787. The conditions in § 27.561 are excepted from that standard.
218A. § 27.561 (Amendment 27-25) EMERGENCY LANDING CONDITIONS - GENERAL.
a. Explanation. Amendment 27-25 adds or increases the design static load factor of § 27.561 in two areas:
(1) The design static load factors for the cabin in § 27.561(b)(3) are increased in concert with the dynamic test requirements of new § 27.562.
(2) Design static load factors are added in § 27.561(c) for external items of mass located above and/or behind the crew and passenger compartment.
b. Procedures. The procedures of Paragraph 218, § 27.561, continue to apply except the new load factors of § 27.561 should be used. Penetration of any items of mass into the cabin or occupied areas should be prevented.
218B. § 27.561 (Amendment 27-30) EMERGENCY LANDING
CONDITIONS - GENERAL
a. Explanation.
(1) Amendment 27-30 adds Paragraph (d) which lists specific load factors for the fuselage structure in the area of internal fuel tanks located below the passenger 272 Par 218
floor level. For other locations, the fuselage structure is to be designed to resist crash impact loads prescribed in § 27.561(b)(3) for fuel tanks located within the cabin area; or § 27.561(c) for fuel tanks located behind or above the occupant area. These load factors are provided to prevent crash induced fuel tank ballistic hazards to occupants and to also protect the fuel tank from rupture as prescribed. The landing gear must be retracted if the rotorcraft is equipped with retractable gear.
(2) Section 27.952(b) provides specific load factors for the fuel tanks which are identical to the load factors stated in § 27.561. Paragraph 447 of this document provides information and guidance for § 27.952 and may be used in conjunction with this paragraph.
b. Procedures. The procedures of Paragraphs 218 and 218A continue to apply except new load factors are established for fuel tanks located below the passenger floor level. Each fuel tank and its installation are subject to the loads stated in the standard.
The load factors are determined by the fuel tank location.
218C. § 27,561 (Amendment 27-32) EMERGENCY LANDING CONDITIONS - GENERAL.
a. Explanation. Amendment 27-32 adds a new rearward emergency load factor of 1.5g to both §§ 27.561(b)(3)(v) and 27.561(c)(5). The addition of the 1.5g rearward load factor in § 27.561(b)(3)(v) is to provide an aft ultimate load condition for substantiation of the restraints required for retention of both occupants and significant items of mass inside the cabin that could otherwise come loose and cause injuries in an emergency landing. The addition of the 1.5g rearward load factor to § 27.561(c)(5) is to provide an aft ultimate load condition for substantiation of the support structure for retention of significant items of mass above and forward of the occupied volume(s) of the rotorcraft that could otherwise come loose and injure an occupant in an emergency landing. Amendment 27-32 also increases the forward, sideward, and downward emergency load factors of § 27.561(c)(2), (c)(3), and (c)(4), respectively; for retention of items of mass above and behind the occupied volumes) that could otherwise come loose and injure an occupant in an emergency landing.
b. Procedures. The procedures of Paragraphs 218, 218A, and 218B continue to apply except the newly specified load factors must be used. A list of the significant items of mass to be considered should be compiled by the applicant and approved by the certifying authority.
219. § 27.562 (Amendment 27-25) EMERGENCY LANDING DYNAMIC CONDITIONS.
a. Explanation. Amendment 27-25 adds new requirements for the dynamic testing of all seats in rotorcraft.
Par 218B 273
b. Procedures. AC 20-137, "Dynamic Evaluation of Seat Restraint Systems and Occupant Restraint for Rotorcraft (Normal and Transport)i provides procedures for complying with § 27. 562 using the 170-pound anthropomorphic test dummy specified in § 27.562(b). Those seats not occupied for takeoff and landing, and so placarded and identified in the rotorcraft flight manual (RFM), may be excluded from compliance.
220. § 27.563 (Amendment 27-11) STRUCTURAL DITCHING PROVISIONS.
a. Explanation. Amendment 27-11 included certification requirements for ditching approvals. The rotorcraft must be able to sustain an emergency landing in water as prescribed by § 27.801(e).
b. Procedures. Refer to Paragraph 338 of this AC for procedures.
220A. § 27.563 (Amendment 27-26) STRUCTURAL DITCHING PROVISIONS.
a. Explanation. Amendment 27-26 added specific structural conditions to be considered to support the overall ditching requirements of § 27.801. These conditions are to be applied to rotorcraft for which over-water operations and associated ditching approvals are requested.
(1) The forward speed landing conditions are specified as:
(i) The rotorcraft should contact the most critical wave for probable water conditions, in the likely pitch, roll, and yaw attitudes.
(її) The forward velocity relative to wave surface should be in a range of 0 to 30 knots with a vertical descent rate of not less than 5 FPS relative to the mean water surface.
NOTE: A forward velocity of less than 30 knots may be used for multiengine rotorcraft if it can be demonstrated that the forward velocity selected would not be exceeded in a normal one-engine-out touchdown.
(iii) Rotor lift of not more than two-thirds of the design maximum weight may be used to act through the CG throughout the landing impact.
(2) For floats fixed or deployed before water contact, the auxiliary or emergency float conditions are specified in § 27.563(b)(1). Loads for a fully immersed float should be applied (unless it is shown that full immersion is unlikely). If full immersion is unlikely, loads resulting from restoring moments are specified for sidewind and unsymmetrical rotorcraft landing.
(3) Floats deployed after water contact are normally considered fully immersed during and after full inflation. An exception would be when the inflation interval is long 274 Par 219
enough that full immersion of the inflated floats does not occur; e.g., deceleration of the rotorcraft during water impact and natural buoyancy of the hull prevent full immersion loads on the fully inflated floats.
b. Procedures.
(1) The rotorcraft support structure, structure-float attachments, and floats should be substantiated for rational limit and ultimate ditching loads.
(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 338 (§ 27.801) of this AC for a description of Sea State 4 conditions).
(3) 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) Forward velocities of 0 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).
The rotorcraft pitch attitude that would reasonably be expected to occur in service. Autorotation flight tests or one-engine-inoperative flight tests, as applicable, should be used to confirm the attitude selected. This information should be included in the Type Inspection Report.
(III) Likely roll and yaw attitudes.
(iv) Vertical descent velocity of 5 FPS or greater.
(4) Landing load factors and water load distribution may be determined by water drop tests or analysis based on tests.
(5) Auxiliary or emergency float loads should be determined by full immersion or the use of restoring moments required to react upsetting moments caused by sidewind, asymmetrical rotorcraft landing, water wave action, rotorcraft inertia, and probable structure damage and punctures considered under § 27.801. Auxiliary or emergency float loads may be determined by tests or analysis based on tests.
(6) Floats deployed after initial water contact are required to be substantiated by tests or analysis for the specified immersion loads (same as for (5) above and for the specified combined vertical and drag loads).
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