(2) The tail-down landing condition may be the critical condition for both landing load factor and for energy absorption by the main gear. Section 27.725 requires that “each landing gear must be tested in the attitude simulating the landing condition that is most critical.” Where questions exist as to the critical attitude, both level landing and tail-down landing attitudes should be used in drop tests required by § 27.725.
AC 27.483 § 27.483 ONE-WHEEL LANDING CONDITIONS.
a. Explanation. This section gives the condition to be used for one- wheel landing conditions. Only the vertical load condition of § 27.479(b)(1) is requi red.
b. Procedures. The one- wheel landing condition is generally critical for the landing gear-to-fuselage attachments and the landing gear elements between the attachments. Unbalanced external loads are reacted by rotorcraft inertia.
AC 27.485 § 27.485 LATERAL DRIFT LANDING CONDITIONS .
a. Explanation.
(1) This section provides the loading conditions which impose side (and vertical) loads on the landing gear. A level landing attitude is specified. Two main conditions required are--
(i) Only the aft wheels in contact with the ground; and
(ii) All wheels contacting the ground simultaneously.
(2) Loads. The vertical loads to be applied with the side loads are specified as “one-half of the maximum ground reactions of § 27.479(b)(1).” These vertical loads are the level landing loads considering both contact and noncontact with the ground by the forward wheels.
(i) One side load condition is specified as “0.8 times the vertical reaction acting inward on one side and 0.6 times the vertical reaction acting outward on the other side” when only the aft wheels contact the ground.
(ii) The other side load condition (for all wheels contacting the ground) specifies the 80 percent inward/60 percent outward distribution for the aft wheels and
0.8 times (80 percent) the vertical reaction for the forward wheels.
b. Procedures. The loading conditions, as specified, are applied to the landing gear and attaching structure. The loads are applied at the ground contact point, except for full swiveling gear which has the load applied at the center of the axle. In other words, full swiveling gear is considered to have swiveled to a static position under the side load before the design vertical and side loads are achieved. The rotorcraft as well as the landing gear itself will be substantiated for these side load conditions.
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AC 27.493 § 27.493 BRAKED ROLL CONDITIONS.
a. Explanation. This section provides two loading conditions for ground braking operations. Specific vertical loads in conjunction with drag loads (due to braking) are to be considered. The limit vertical load factor is 1.33 for condition of all wheels in contact with the ground and 1.0 for condition of aft wheels only in contact with the ground and nose wheel clear. The drag load on wheels with brakes is 0.8 times the vertical load or the drag load value based on limiting brake torque, whichever is less. The drag load value for limiting brake torque may be that determined in the performance testing to TSO C26 or equivalent, as required.
b. Procedures. The braking loads are calculated from the specified criteria with the shock absorbers in their static (normal) positions and with the drag loads applied at the ground contact point. Structural substantiation of the affected st ructure may be accomplished by test or analysis. If tests are used, the wheel and tire assembly is commonly replaced with a test fixture so the limit loads and static deflections specified can be more accurately controlled. The test specimen should be complete enough to ensure that the landing gear structure and the attach and backup structure are adequately substantiated.
AC 27.497 § 27.497 GROUND LOADING CONDITIONS: LANDING GEAR WITH TAIL WHEELS.
a. Explanation. This section provides the loading conditions for landing gear designs with tail wheels.
(1) Level landings are to consider the following:
(i) All wheels (main and tail) contacting the ground simultaneously, as well as only forward main wheels contacting the ground.
(ii) Maximum design vertical loads applied alone.
(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.
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(5) Braked roll conditions are specified for the level landing attitudes.
(6) Rear wheel turning loads are also specified for swiveling and locked tail wheels.
(7) Taxiway condition loads for the landing gear and rotorcraft are those that “occur when the rotorcraft is taxied over the roughest ground that may reasonably be expected in normal operation.” The aircraft design load factors should not be exceeded during the evaluation. Section 27.235 contains an identical standard that applies to all types of wheel landing gear.
b. Procedures.
(1) The specified loading conditions are to be used in load derivations.
(2) The critical center of gravity condition is used for each gear and gear support structure.
(i) The forward center of gravity condition with the tail gear clear will normally be critical for the forward gear and gear supports.
(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 tail-down landing procedures use § 27.481. The refer ence to “extremely remote” in § 29.497(d)(2) predates current §§ 25.1309, 29.1309, and AC 25.1309.1. This phrase has been used to require consideration of nose-up landings unless features of design are present which prevent nose-up landings or where such landings are unlikely during the life of the rotorcraft. (See paragraph AC 27.481.)
(4) Use § 27.483 for one-wheel landing procedures, paragraph AC 27.483.
(5) Use § 27.485 procedures for side load conditions, paragraph AC 27.485.
(6) Use § 27.493 procedures for braked roll conditions, paragraph AC 27.493.
(7) For rear wheel turning loads, swiveling of tail landing gears is allowed as in basic side load conditions. The side load is applied at the axle or, if the wheel is locked, the load is applied at ground contact. Rear wheels are loaded with the critical vertical static load in conjunction with an equal side load to substantiate the tail gear.
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(8) Since the rotorcraft is to be designed for load factors that will not be exceeded during taxi tests or other conditions, an instrumented taxi test program will be necessary. (Use § 27.235, paragraph AC 27.235.)
AC 27.501 § 27.501 (Amendment 27-2) GROUND LOADING CONDITIONS:
LANDING GEAR WITH SKIDS.
a. Explanation. This sectio n provides the ground loading conditions for landing gear with skids. The loading conditions are similar to those for wheeled gear except for the following criteria which are unique to skid gears:
(1) Structural yielding of elastic spring members under limit loads is allowed.
(2) Design ultimate loads for elastic spring members need not exceed the loads obtained in a drop test with a drop height of 1.5 times the limit drop height. The rotorcraft and the landing gear attachments are subject to the prescribed design ultimate loads.
(3) The gear must be in its most critically deflected position (similar to § 27.475).
(4) Ground reactions are rationally distributed along the bottom of the skid unless otherwise specified. Section 27.501(f) concerns specific “concentrated” and arbitrary load conditions.
(5) Drag loads are 50 percent of vertical reactions rather than the 25 percent for wheeled gear.
(6) Side loads are 25 percent of the total vertical reaction rather than the 60 to 80 percent for wheeled gear.
(7) Side loads are applied to one skid only (inward acting and outward acting) with resulting unbalanced moment resisted by angular acceleration.
(8) A ground reaction load of 1.33 times the maximum weight is to be applied at 45° from the horizontal axis:
(i) Distributed among or between the skids;
(ii) Concentrated 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.
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(9) A concentrated vertical load equal to one-half of the design limit vertical load is to be applied at a point midway between the skid tube attachments. This condition applies only to the skid tube and its attachment to the rotorcraft.
b. Procedures.
(1) The specified loading conditions are to be used in load derivations.
(2) The critical center of gravity conditions are to be used for each gear and gear support structure. Asymmetry of the skid tubes, cross tubes, and gear attachments is to be considered in determining the critical center of gravity condition.
(3) The rotorcraft and landing gear attachment must be substantiated for ultimate landing loads by either test or analysis utilizing an ultimate load factor of 1.5 in accordance with § 27.303. The elastic spring members may be analyzed or static tested for ultimate loads (and deflections) using either a factor of safety of 1.5 or one associated with an “ultimate” drop height of 1.5 times the limit drop height.
Substantiation by “ultimate” drop tests may be used provided all combinations of critical parameters are included in the total substantiation effort. This method will require a series of tests using several test specimens or a limited number of drop tests plus further substantiations by static tests or analyses for additional critical conditions not covered by the drop test(s).
AC 27.501A § 27.501 (Amendment 27-26) GROUND LOADING CONDITIONS:
LANDING GEAR WITH SKIDS.
a. Explanation. Amendment 27-26 relaxes the previous requirements in two cases by:
(1) Allowing the total sideload of § 27.501(d)(3) to be distributed “equally between skids” rather than being “applied along the length of one skid only;” and,
(2) Allowing the concentrated load of § 27.501(f)(2)(ii) to be distributed over the central 33.3 percent of the skid (between skid tube attachments) rather than being “concentrated at a point midway between the skid tube attachments.”
b. Procedures. The previous procedures (through Amendment 27-19) continue to apply to Amendment 27-26 except use the new load distributions.
AC 27.505 § 27.505 SKI LANDING CONDITIONS.
a. Explanation. This is an optional requirement for ski operations. The regulation specifies vertical loads, side loads, and torque loads (MZ) to be applied to ski installations. The four loading conditions to be applied at the pedestal bearings are:
(1) Simultaneous application of Pn, up load, and Pn/4, horizontal load.
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(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.)
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SUBPART C - STRENGTH REQUIREMENTS
WATER LOADS AC 27.521 § 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.
(i) A vertical load appropriate to a landing load factor determined under § 27.473(b).
(ii) 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.
(i) A vertical load equal to 75 percent of the vertical load for the up-load condition, equally divided among the floats.
(ii) A side load at each float equal to 25 percent of the vertical load specified for the side load condition.
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.
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SUBPART C - STRENGTH REQUIREMENTS
MAIN COMPONENT REQUIREMENTS AC 27.547 § 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. Appropr iate 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.
AC 27.549 § 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 – Page C - 38
(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(e) for the main rotor structure.
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AC 27-1B, Chg 4 3/31/2014
SUBPART C - STRENGTH REQUIREMENTS
EMERGENCY LANDING CONDITIONS AC 27.561 EMERGENCY LANDING CONDITIONS --GENERAL.
a. Explanation.
(1) Occupant protection. 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.
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.
(2) Load factor determination. 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) Retractable landing gear. For rotorcraft equipped with retractable landing gear only the retracted configuration must be considered.
(4) Door and exit protection. The minor crash conditions contained in § 27.561(b)(3) should also be considered in designing doors and exits (§ 27.807(b)(4)).
(5) External load considerations. The load factors of § 27.561 and the criteria of § 27.562 are not directly applicable to external load systems. This is because in emergency crash scenarios that involve external loads, the external load is neither typically subjected to the same minor crash loads (§ 27.561) as is the rotorcraft hull and its internal occupants nor are all of the occupant protection criteria (§ 27.562) needed or practicable to apply. Appropriate safety for external load carriage systems is provided by the criteria of § 27.865. Safety standards for external load attaching means are provided in § 27.865.
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 (section 27.785 of this AC) concerns application of this design standard to seats, berths, belts, and harnesses.
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(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.
(i) For example, for light weight conditions, the ultimate maneuvering load factor may be 5.25g as specified in § 27.337.
(ii) The ultimate vertical landing load factors derived from §§ 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).
(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.
(i) 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 function of the aircraft impact and crushing characteristics.
(ii) 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.
(iii) As noted in paragraph b.(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, life rafts, engines, and transmissions shall be restrained for the appropriate load factors.
(6) Cargo or 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.
AC 27.561A. § 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. The addition of these load factors eliminates the 5 FPS ultimate descent velocity criteria of unamended § 27.561(b)(3).
(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 or behind the crew and passenger compartment.
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AC 27-1B, Chg 4 3/31/2014
b. Procedures. The procedures in section 27.561 of this AC 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.
AC 27.561B. § 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 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. Section 27.952 of this AC provides information and guidance for § 27.952 and may be used in conjunction with this section.
(3) Door and exit protection. The minor crash conditions contained in § 27.561(d) should also be considered in designing doors and exits (§ 27.807(b)(4)). If any item of mass installed in the cabin can possibly interact with the fuselage and cause higher deformation, then § 27.561 (b) (3) loads factors should be applied to design of doors and emergency exits.
b. Procedures. The procedures in sections 27.561 and 27.561A of this AC 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.
(1) The crash impact load factors for the airframe structure surrounding the underfloor fuel tanks are specified in § 27.561(d). The fuselage structure must be designed to resist the specified crash impact loads and to help protect the fuel tank from rupture. If equipped with retractable landing gear, the effects of the landing gear on fuel system rupture should be considered in both the retracted and unretracted configurations for fuel cell hazard purposes, only, in accordance with section 27.952 of this AC.
(2) Section 27.952(b) (see section 27.952 of this AC) specifies the design load factors for crash resistant fuel systems in an otherwise survivable impact. This section relates to § 27.561(d) as follows. The § 27.952 load factors are for the fuel tanks, other significant mass items in the fuel system, and their attachment to the rotorcraft airframe for both occupant survivability and retention of fuel in a survivable impact; whereas, the Page C - 42