CHAPTER 2. PART 27
AIRWORTHINESS STANDARDS
NORMAL CATEGORY ROTORCRAFT
SUBPART D - DESIGN AND CONSTRUCTION
DESIGN AND CONSTRUCTION - GENERAL AC 27.601. § 27.601 DESIGN.
a. Explanation.
(1) This rule requires that no design features or details be used that experience has shown to be hazardous or unreliable.
(2) Further, the rule requires that the suitability of each questionable design detail and part must be established by tests.
b. Procedures.
(1) This rule is met partially by a review of service history of earlier model rotorcraft, or for a new model, review of service experience of models with similar design features. Specifically, this rule covers "features or details" such as the following:
Seat track-to-seat interface fittings. These fittings should have adequate locking devices to prevent both premature structural failure and premature unlatching.
Seat belt and harness should be of a type and construction that service experience has shown to be easy to don and unlatch and remove. They should also be of a type that is reliable, does not interfere with egress, and does not sustain unnecessary wear and tear under normal operations.
(iii) Metallic parts less than a certain thickness gauge and composite materials less than a certain number of plies should not be used. The minimum thickness and number of plies should be based to a large degree on service (normal wear and tear) experience with similar designs.
(2) The effects of service wear on the loading of critical components should be considered. Flight testing, ground testing, and analyses may be used in these considerations.
(3) Tests are required for details and parts which the applicant chooses to use after questions have arisen concerning their suitability.
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AC 27-1B, Chg 2 4/25/06 AC 27.602 § 27.602 CRITICAL PARTS.
a. Explanation.
(1) Critical parts requirements apply to structural components, rotor drive systems, rotors, and mechanical control systems.
(2) The objective of identifying critical parts is to ensure that critical parts are controlled during design, manufacture, and throughout their service life so that the risk of failure in service is minimized by ensuring that the critical parts maintain the critical characteristics on which certification is based.
(3) Definitions with respect to § 27.602:
(i) The use of the word "could" in paragraph 27.602(a) of the rule means that this failure assessment should consider the effect of flight regime (i.e., forward flight, hover, etc.). The operational environment need not be considered.
(ii) With respect to this rule, the term "catastrophic" means the inability to conduct an autorotation to a safe landing, without exceptional piloting skills, assuming a suitable landing surface is available.
(iii) The use of the word "and" in paragraph 27.602(a) of the rule means the part must have both a catastrophic failure mode together with one or more critical characteristics.
(iv) With respect to this rule, the term "part" means one piece, or two or more pieces permanently joined together.
With respect to this rule, the term "critical characteristic" means any dimension, tolerance, finish, material, or any manufacturing or inspection process, or other feature which cannot tolerate variation from type design requirements and, if nonconforming, would cause failure of the critical part.
(4) Many rotorcraft manufacturers already have procedures in place within their companies for handling "critical parts." These plans may be required by their dealings with other customers, frequently military (e.g., US DOD, UK MoD, Italian MoD).
Although these plans may have slightly different definitions of "critical parts" which have sometimes been called "Flight Safety Parts," "Critical Parts," "Vital Parts," or "Identifiable Parts, they have in the past been accepted as meeting the intent of this requirement and providing the expected level of safety. It is acceptable for these plans to use alternative names and terminology provided they meet the intent of this requirement.
b. Procedures. The rotorcraft manufacturer should establish a Critical Parts Plan, which identifies and controls the critical characteristics. The policies and procedures which constitute that plan should be such as to ensure that- Page D - 2
(1) All critical parts of the rotorcraft are identified by means of an appropriate failure assessment and a Critical Parts List is established.
(2) Documentation draws the attention of the personnel involved in the design, manufacture, maintenance, inspection, and overhaul of a critical part to the special nature of the part and details the relevant special instructions. For example all drawings, work sheets, inspection documents, etc., could be prominently annotated with the words "Critical Part" or equivalent and the Instructions for Continued Airworthiness and Overhaul Manuals (if applicable) should clearly identity critical parts and include the needed maintenance and overhaul instructions. The documentation should:
(1) Contain comprehensive instructions for the maintenance, inspection and overhaul of critical parts and emphasize the importance of these special procedures;
(II) Indicate to operators and overhaulers that unauthorized repairs or modifications to critical parts may have hazardous consequences;
(iii) Emphasize the need for careful handling and protection against damage or corrosion during maintenance, overhaul, storage, and transportation and accurate recording and control of service life (if applicable);
(iv) Require notification of the manufacturer of any unusual wear or deterioration of critical parts and the return of affected parts for investigation when appropriate;
(3) Procedures should be established for identifying and controlling critical characteristics.
(4) To the extent needed for control of critical characteristics, procedures and processes for manufacturing critical parts (including test articles) are defined (for example material source, forging procedures, machining operations and sequence, inspection techniques, and acceptance and rejection criteria). Procedures for changing these manufacturing procedures should also be established.
(5) Any changes to the manufacturing procedures, to the design of a critical part, to the approved operating environment, or to the design loading spectrum are evaluated to establish the effects, if any, on the fatigue evaluation of the part.
(6) Materials review procedures for critical parts (i.e., procedures for determining the disposition of parts having manufacturing errors or material flaws) are in accordance with paragraphs (4) and (5) above.
(7) Critical parts are identified as required, and relevant records relating to the identification are maintained such that it is possible to establish the manufacturing history of the individual parts or batches of parts.
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AC 27-1B, Chg 2 4/25/06
(8) The critical characteristics of critical parts produced in whole or in part by suppliers are maintained.
AC 27.603. § 27.603 (Amendment 27-16) MATERIALS.
a. Explanation. The rule requires that the suitability and durability of materials, the failure of which could adversely affect safety, must be determined by three-fold considerations:
(1) Considerations based on experience or tests.
(2) By meeting approved specifications.
(3) By taking into account environmental conditions such as temperature and humidity.
b. Procedures.
(1) Where possible, materials that meet widely accepted specifications such as AISI, SAE, MIL, or AMS and alloys which have favorable experience or tests should be used. Where company developed materials are used, approved specifications are required to ensure the developed properties are duplicated in each lot of material.
(2) Environmental conditions may be taken into account by service experience, coupon testing, full-scale testing, or a combination of testing and experience, MIL-HDBK's -5, -17, and -23 include some environmental effects and contain reference to additional methods of testing for environmental effects.
(3) Section 27.613 concerns strength properties and design values. (See paragraph AC 27.613.)
AC 27.605. § 27.605 (Amendment 27-16) FABRICATION METHODS.
a. Explanation. The basic requirement of this rule is that the methods of fabrication must produce sound structure and produce it consistently.
(1) A process specification is required for fabrication processes requiring close control.
(2) A test program is explicitly required for each new aircraft fabrication method.
b. Procedures.
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(1) The approved specifications required by this rule may either be established government/industry specifications such as MIL, AISI, ASIM, or SAE; or the specifications may be company-developed proprietary specifications. Sufficient data should be provided to the FAA/AUTHORITY aircraft engineering offices to show that the desired features are provided by the process specification. In addition, sufficient process controls, inspections, and tests should be coordinated with FAA/AUTHORITY manufacturing inspection personnel to ensure that continued quality of the process is provided.
(2) In addition to the examples given by the rule; i.e., gluing, spot welding, and heat treating process, specifications should also be prepared for types of welding other than spot welding, for platings of metals, for protective finishes (other than decorative), for sealing, and for unique fabrication methods such as those used for composite materials.
(3) The required test programs should consider static strength effects, fatigue strength effects, and environmental effects as appropriate to the processes.
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AC 27-1B, Chg 4 3/31/2014 AC 27.607. § 27.607 (Amendment 27-4) FASTENERS.
a. Explanation. Section 27.607 of Amendment 27-4 requires dual locking removable fasteners in critical locations. A nonfriction locking device is specifically required in any bolt subject to rotation, as stated in the rules.
b. Procedures. Advisory Circular 20-71 contains information, procedures, and means of complying with § 27.607 of Amendment 27-4.
AC 27.609. § 27.609 PROTECTION OF STRUCTURE.
a. Explanation. The structure should be suitably protected as specified in the rule to maintain its design strength. Ventilation and drainage provisions must be provided as specified in the rule. Overboard drains should be furnished for corrosive or waste liquids. Drains for flammable fluids are specified in other rules such as §§ 27.999 and
b. Procedures.
(1) The structure may be preserved, painted, or treated with chemical films to protect it from strength deterioration. An approved process specification should be used for these types of treatments.
(2) Parts may be plated or chemically treated, such as anodized, for protection.
An evaluation and substantiation may be required to ensure the structure or parts are not adversely affected during, or as a result of, the plating or treatment process.
(§ 27.605 concerns approval of process specifications and fabrication methods.)
(3) Plating or material surface hardness or composition changes may require fatigue substantiation to ensure the fatigue strength is not altered or is otherwise properly assessed. An approved process specification should be used for these types of treatments.
(4) To prevent water accumulation, drain holes should be placed at possible dams such as bulkheads and at low points in the fuselage and in the stabilizing surfaces.
(5) Control tubes and tubes used as primary mount structures (i.e., transmission support structure and engine mount structure) should be designed to prevent entry and collection of corrosive fluids or vapor, including water.
A closed insert in each tube end may be used.
A sealant applied around the tube ends and around each rivet head may be used.
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) Overboard drains should discharge clear of the entire rotorcraft. Dye ater discharged in flight may be used to ensure fluids are properly draine
(1) Drains or vents which handle corrosive fumes (such as battery case vent line) may incorporate a container with an agent to neutralize the fumes prior to venting overboard.
(8) Welded tubes should be flushed and sealed after welding in accordance with an approved process specification.
(9) Refer to AC 43-4, "Corrosion Control for Aircraft," for further procedures.
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AC 27-1B, Chg 4 3/31/2014 AC 27.610. § 27.610 (Amendment 27-46) LIGHTNING AND STATIC ELECTRICITY PROTECTION.
Background. During the initial development and promulgation of airworthiness standards, rotorcraft operated primarily in a VFR and non-icing environment. A prudent pilot avoided thunderstorms where the possibility of encountering severe weather and a lightning strike was much greater. Now, many rotorcraft are authorized to fly under IFR and into known icing conditions. Because many rotorcraft now use the same advanced technologies in structures and systems as fixed-wing aircraft, a specific rule on lightning protection of rotorcraft was adopted in Amendment 27-21. Amendment 27-37 revised the title of § 27.610 to include protection from static electricity, and a new § 27.610(d) was added specifying requirements for electrical bonding and protection against lightning and static electricity. Amendment 27-46 removed the system lightning protection requirements for electrical and electronic equipment in § 27.610(d)(4) because those requirements were implemented in the new § 27.1316.
b. Explanation.
(1) The regulation requires protection of rotorcraft from the catastrophic effects of lightning. This means that lightning must not prevent the continued safe operation of the rotorcraft.
(2) Rotorcraft structural components, propulsion system, gearboxes, and mechanical and hydraulic control systems should be designed to ensure lightning will not prevent continued safe flight and landing of the rotorcraft (i.e., damage due to lightning currents that flow through any of these components must not result in catastrophic failure).
c. Procedures.
Certification Plan. A formal written certification plan is an acceptable means to ensure demonstration of regulatory compliance. This plan is also useful to identify and define an acceptable resolution to the critical issues early in the certification process. These are the usual steps to follow when utilizing a certification plan:
(1) Prepare a certification plan that describes the analytical procedures and qualification tests to be utilized to demonstrate lightning and static electricity protection effectiveness. Test proposals should describe the rotorcraft and system to be utilized, test drawing(s) as required, the method of installation that simulates the production installation, the lightning zone(s) applicable, the lightning simulation method(s), test voltage or current wavetorms to be used, diagnostic methods, and the appropriate schedules and location(s) of proposed tests).
(il) Obtain FAA/AUTHORITY concurrence that the certification plan is adequate.
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(iii) Obtain FAA/AUTHORITY detail part conformity of the test articles and installation conformity of applicable portions of the test setup. Obtain FAA/AUTHORITY approval of the test proposal. A comprehensive test proposal may be used.
(iv) Schedule FAA/AUTHORITY witnessing of the test or tests proposed.
(v) Submit a test report describing all results and obtain FAA/AUTHORITY approval of each report prepared.
(2) Lightning Environment and Zones. AC 20-155 refers to SAE documents ARP5412 (or EUROCAE ED-84) and ARP5414 (or EUROCAE ED-91), which provide acceptable definitions for the rotorcraft lightning environment and for rotorcraft lightning attachment zones.
(3) Testing. Tests may be required to check the adequacy of the lightning and electrostatic charge protection. Refer to SAE ARP5416 (or EUROCAE ED-105) for acceptable test methods to show that the lightning protection is effective and to SAE ARP5672 for acceptable test methods to show rotorcraft electrostatic charge control.
(4) Aircraft Lightning Protection Design Features. The Aircraft Lightning Protection Handbook (DOT/FAA/CT-89/22) provides information on aircraft lightning protection design. The following are examples where lightning protection should be considered.
(i) Rotors and Control Systems.
(A) It should be established that an adequate bonding path exists between the rotors and the airframe, such that a lightning strike to a rotor will not result in damage to or seizure of gearbox or swashplate bearings, control jacks, etc.
(B) Each hinge and bearing of rotor blades and control surfaces should either:
(1) be capable of withstanding lightning without damage or seizure leading to loss of function, or
(2) be provided with at least one bonding conductor, as flexible and short as possible and installed so that there is no danger of the conductor jamming the hinge or bearing following a lightning strike.
(II) External Non-Metallic Parts.
(A) Where non-metallic parts are fitted externally to the rotorcraft (e.g., rotors, radomes, composite skin panels) and may be subjected to lightning, they should be protected. The protection should consider disruption of the materials because of:
(1) rapid expansion of gases within them (e.g., water vapor), Page D - 9
AC 27-1B, Chg 4 3/31/2014
(2) rapid build-up of pressure in voids or in the enclosure provided by the parts resulting in mechanical disruption of the parts themselves or of the structure enclosed by them, and
(3) fire caused by the ignition of the materials themselves or of the materials contained within the enclosures.
(B) Materials used for external non-metallic parts should have low water absorption characteristics, should not occlude gases, and should be of high dielectric strength in order to encourage surface flashover rather than puncture.
(C) Rotors and other external parts of nonmetallic construction should be provided with effective lightning conductors that are capable of safely carrying lightning current, unless it can be shown that damage due to lightning will not endanger the rotorcraft or its occupants. Bonding straps and leads are not required for small gaps between metallic structure and diverters in non-conducting panels in order to comply with the lightning protection criteria. However, an electrical bonding path may be required to achieve static electricity protection.
(5) Protection Against the Effects of Static Electricity.
(i) General. Rotorcraft structure, rotor systems, and equipment should be electrically bonded together to minimize the accumulation and discharge of electrostatic charge, which could result in electrical shock, ignition of flammable vapors, or interference with essential equipment such as radio communications and navigational aids.
(Il) Intermittent Contact. The design should prevent random intermittent contact between metallic or metallized parts such that unwanted radio interference or degradation of the components due to sparking will not occur.
(III) Rotors and other external parts of nonmetallic construction should be provided with effective electrical conductors that are capable of safely conducting electrostatic charge.
(iv) High Pressure Refueling and Fuel Transfer. Where provision is made for high pressure refueling and/or high rates of fuel transfer, it should be established, by test, or by consultation with the appropriate fuel manufacturers, that dangerously high voltages will not be induced within the fuel system. If compliance with this requirement involves any restriction on the types of fuel to be used or on the use of additives, an appropriate operating limitation should be established under § 27.1501(a). The critical refueling rates are related to the rotorcraft refueling installations, and the designer should seek the advice of fuel suppliers on this problem.
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(A) With standard refueling equipment and standard aircraft turbine fuels, voltages high enough to cause sparking may be induced between the surface of the fuel and metal parts of the tank at refueling rates above approximately 250 gal/min. These induced voltages may be increased by the presence of additives and contaminants
(e.g., anti-corrosion inhibitors, lubricating oil, free water) and by splashing or spraying of the tuel in the tank.
(B) The static charge can be reduced as follows:
(1) By means taken in the refueling equipment such as increasing the diameter of refueling lines and designing filters to give the minimum of electrostatic charging, or
(2) By changing the electrical properties of the fuel by the use of antistatic additives and thus reducing the accumulation of static charge in the tank to a negligible amount.
(6) Fuel Systems. Requirements for lightning protection for fuel systems are in § 27.954. AC 20-53 provides guidance on compliance with § 27.954. Section 27.954 of this AC addresses the protection required for systems.
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AC 27-1B, Chg 4 3/31/2014 AC 27.611. § 27.611 INSPECTION PROVISIONS.
a. Explanation. The rotorcraft must have access panels or openings that will allow for proper maintenance and/or adjustment of the rotorcraft systems.
(1) The rule states: "There must be means to allow close examination of each part that requires recurring inspection, adjustment for proper alignment and functioning, or lubrication."
(2) "Structural" or load-carrying access panels may be used to comply with the rule. Structural panels should have stencils or permanent labels (§ 27.1541(a)(2)) stating the panels must be installed prior to ground or flight operation.
(3) Holes or "nonstructural" access panels should be used whenever possible.
b. Procedures.
(1) The determination of compliance can be accomplished in conjunction with the following activities:
(i) Reviewing type design drawings.
(ii) Conformity inspections accomplished during certification testing.
Be evaluated during the control system proof and operation tests (§§ 27.681 and 27.683).
(IV) During type inspection tests and functioning and reliability testing.
(2) Equipment requiring frequent inspections (at less than 25-hour intervals), lubrication, or adjustments should be accessible through "nonstructural" doors. Areas or items requiring daily attention should be accessible through "nonstructural" doors since properly rated maintenance personnel are required to "open and close" or reinstall structural panels, and special design features, such as multiple pins and latches, are generally necessary for structural doors.
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