(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).
(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.
(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. 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.
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.
(IlI) 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
§ 27.561(d) load factors only apply to the rotorcraft airtrame surrounding the underfloor fuel tanks and their installation for the same reasons. These two sets of load factors are not additive. They are applied separately (as design ultimate load factors) to the portions of the rotorcraft to which they are specified to apply. The application of the § 27.561(d) load factors is described as follows. The loads generated by § 27.561(d) are intended to be applied to the airframe structure surrounding the fuel cell to ensure that the entire airframe structure provides the appropriate level of crash resistance (i.e., stiffness, crushability, crushing rate, energy absorption capability, etc.) and to ensure that the airframe structure's failure modes (e.g., buckling, creation of sharp edges, structural spears, etc.) are such that fuel cell rupture (and the resultant post crash fire potential) is mitigated to the maximum practicable extent in a otherwise survivable emergency landing. Each fuel cell (and major fuel cell component) creates an applied load on the airframe in an emergency landing condition. These loads are determined by multiplying the worst case mass of the fuel cell (i.e., a full fuel cell) by the load factors of § 27.561(d). These loads are then applied (utilizing the appropriate design load paths) to the airframe structure surrounding the fuel cell to help design the structure for optimal crash resistance. Added stiffness effects for both a full and less than full fuel cell should be considered in the design process. A significantly less than full fuel cell will typically not have any significant stiffness effects, since in a less than full condition the fuel cell cannot typically transfer load hydraulically.
AC 27.561C. § 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 volumes) 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 in sections 27.561, 27.561A, and 27.561B of this AC 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.
Note: For doors and emergency exit design, when applicable, the rearward load factor to consider is in § 27.561(b)(3)(v).
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AC 27-1B, Chg 4 3/31/2014 AC 27.562§ 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. This paragraph is rewritten to incorporate the guidance previously documented in AC 20-137 dated 3/30/92.
b. Background. Improved occupant restraint in civil rotorcraft is addressed in Amendments 27-25 to the airworthiness standards, which add two dynamic crash impact design conditions for seat and occupant restraint systems. This amendment also prescribes a shoulder harness for each occupant and adopts human impact injury criteria as a measure for occupant protection for the dynamic crash impact conditions.
In addition, these amendments significantly improve occupant protection for normal category rotorcraft in a survivable emergency landing. This advisory material addresses the dynamic test conditions and the related pass-fail injury criteria for the dynamic test conditions. This material pertains to single as well as multiple seats and tandem arrangements of the seats in rotorcraft.
(1) Dynamic test methods. This guidance focuses on the use of dynamic test methods for evaluating the performance of rotorcraft seats, restraint systems, and certain related interior systems for demonstrating structural strength and the ability of those systems to protect an occupant from possible injuries in an emergency landing environment represented by the standard. These test methods differ from static test methods, which are limited to demonstrating only the structural strength of the seat or restraint system under ultimate load for at least 3 seconds. This guidance contains sources for appropriate test procedures and provides some insight into the logic of these procedures. It also defines, in part, test facility and equipment characteristics necessary for conducting these tests.
(2) Standardized test methods. Dynamic tests are often conducted at a specially equipped facility, one other than that owned by the designer or manufacturer of the test article. To obtain consistent test results, it is necessary to specify the critical test procedures in detail in the test plan, and then carefully follow these procedures when conducting the tests. This guidance defines certain critical procedures for accomplishing the tests of the seat and restraint systems and assessing the data obtained in the tests. Many of these procedures are accepted as standards by government and commercial test facilities and have been modified in this guidance only as necessary for the specific testing of rotorcraft systems.
(3) Relationship of dynamic tests to design standards. This guidance describes test procedures useful in assessing the performance of a rotorcraft seat, restraint system, and interior system. However, it is impractical to conduct sufficient tests for assessing the performance of the system throughout its entire range of possible uses in Page C - 44
unique interior arrangements. The seat, restraint system, and related interior system should be designed for the range of occupants and environments for which it is expected to perform, not just for the dynamic test conditions described in this guidance.
(i) Occupant size. The dynamic tests are conducted with a specific, acceptable, standard anthropomorphic test dummy (ATD) representing a 50" percentile male occupant. Energy absorbing systems, restraint system loads and anchorage locations, seat adjustments, seat pitch (for multiple seat rows), head strike envelopes, etc., are typical factors directly influenced by occupant size.
Seat position and location. The tests should be sufficient to represent the range of performance expected of a seat and restraint system. A seat, especially an adjustable flight crew seat, should be qualified for those positions approved for take-off and landing. As with static test procedures the seat is also tested to the most critical condition for the dynamic tests. For an adjustable flight crew seat, as an example, the full-up position and longitudinal impact case are expected to be the critical condition.
But these dynamic tests and occupant injury assessment provide a systems approach to qualification. It is therefore necessary to test adjustable seats at the design position for the ATD. Two tests would be required to demonstrate compliance with the strength standards and with the occupant injury criteria. Alternatively adjusting the flight crew seat to its highest position with the interior features, such as an instrument panel shield, raised to maintain the proper perspective or relation to the ATD, is considered an acceptable test procedure for demonstrating compliance with the structural and occupant injury requirements for the seat and its location in a particular cockpit arrangement.
(i¡i) Test conditions. Only two minimum impact tests are described in the dynamic test procedures discussed in this guidance. These procedures address the tests needed to demonstrate compliance for a typical seat and restraint system installation. Additional tests may be necessary to demonstrate compliance for other types or variations of seat and restraint system installations. For example, while only one lateral load direction is specified in the tests, the system should perform properly when similarly loaded from either side.
(iv) Floor deformation. The test procedures require evaluating the effect of certain sidewall or floor deformation. The seat and its attachments and the restraint system should also perform properly if no floor deformation is present.
(V) Head impact. Should such contact occur, head impact with a seat back or the interior of the rotorcraft is evaluated by using a Head Injury Criterion (HIC), which can be measured directly in the tests discussed in this guidance or in supplementary tests of the interior. The design of the interior should protect the head from serious injury throughout the head strike envelope, not just along the head strike paths demonstrated in the test conditions discussed in this guidance.
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AC 27-1B, Chg 4 3/31/2014
(vi) Emergency egress. Standards for emergency evacuation of the rotorcraft are contained in FAR Part 27. The objective is to allow each occupant to leave the seat and rapidly evacuate the rotorcraft using an exit on either side of the rotorcraft.
c. Dynamic Test Methods and Facilities.
(1) General. A minimum of two dynamic tests are used to assess the performance of the rotorcraft seat, restraint system, and related interior system. The seat, the restraint, and the nearby interior all function together as a system to protect the occupant during emergency landing. The specific test conditions are shown in Figure AC 27.562-1. Explanations of the test conditions are as follows:
Test 1. The test determines the protection provided when the impact environment is such that the resulting predominant impact load component (vertical) is directed along the spinal column of the occupant in combination with a horizontal (longitudinal) component. Protection against spinal injury is important and it may be necessary to provide energy absorbing (load limiting) or attenuation capability in the seat system in order to comply with the human injury criteria specified in § 27.562 (c)(7).
Test 2. The test determines the protection provided in an impact where the predominant impact load component is in the longitudinal direction in combination with a lateral component. Evaluation of head injury protection is important in this test if the head could strike some interior portion of the rotorcraft or a forward seat. Chest or spinal column injury, which might result from the upper torso restraint, is also evaluated in this test.
Tests 1 and 2. These test conditions are also significant for the structural strength of the system. Both tests should be used to assess submarining (where the seat belt slips above the ATD pelvis) and rollout of the upper torso restraint system particularly with single, diagonal torso restraint belts. Since external crash forces frequently cause significant structural deformation, simulated floor deformation is specified for the tests to prove the seat design can accommodate the relative deformation between the seat and the floor or sidewall and still function without imposing excessive loads on the seat, the attachment fittings, or floor tracks.
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Illuscracion shovs a forward facing Test 1 Test 2 seat ATO Inertial load shown by acrow:
10° H730° Min. Vi. Ips 30 Max. tr sec. - 031 - 071 Min. G 10 18. 4 Deform floor:
degrees roll 10 10 degrees pitch 10 10 decelerarion Vi = Impact Velocity C FIGURE AC 27,562-1, Seal RestrainI System Dynamic Tests Normal and Transport Calegory Rotorcraft Page C - 47