(1) This paragraph discusses the regulatory requirement contained in § 27.661.
That requirement is that there must be enough clearance between the rotor blades (main and tail rotor blades) and other parts of the structure to prevent the blades from striking any part of the structure during any operating condition.
(2) In the past, some rotorcraft that have been shown to comply with § 27.661 during the certification process have experienced subsequent accidents involving inflight contact between the main rotor and airframe (rotor/airframe contact). Completion of developmental and TIA flight testing without a rotor/airframe contact incident has proven not to be adequate demonstration of compliance with § 27.661 in all cases.
(3) Historically, in-flight rotor/airframe contact accidents have occurred as a result of mast bumping, rotor stall, or excessive rotor flapping due to control manipulation. For some rotorcraft, a more thorough examination may be required to ensure adequate clearances.
b. Procedures. Testing should be conducted by the applicant, prior to FAA/AUTHORITY participation, to ensure that the rotorcraft is in compliance with § 27.661 in all areas of the envelope during all operational maneuvers expected throughout the life of the aircraft. The tests should be performed concurrently with performance, flight characteristics, and flight loads testing. Tests should include:
(1) A blade flapping survey to determine flapping angles/margins, blade bending, and blade clearance from the entire airframe. Data may be gathered from instrumented flapping hinges, instrumented blades, high-speed video from airframe mounted cameras, a chase aircraft, or other acceptable means.
(2) Determine that margin exists between the minimum rotor RPM encountered during testing for compliance with § 27.143(d) and the RPM (power off) at which analysis shows that the rotor will experience a significant stall. A significant stall condition may be defined by the rotor reaching an RPM from which normal operating RPM is unrecoverable due to drag on the main rotor blades or, a stall that results in excessive main rotor flapping. The rotor RPM decay rate under the critical conditi ons of weight, density altitude, minimum approved power-on rotor RPM must provide a margin between the minimum rotor speed achieved during demonstration of compliance with §§ 27.79 and 27.143(d) and the analytically derived rotor stall RPM for the same conditions. For example, the minimum rotor RPM resulting during H-V tests must allow for a margin above the rotor stall value to allow for variations that may occur during operational flying.
(3) During parts of the certification flight test program, frangible devices (wood dowels) or other means of measuring clearance, may be requested to confirm that the clearances shown in the drawings and verified during company flight tests are adequate in all operating conditions. Balsa wood dowels or styrofoam pads may be clamped to the aft part of the fuselage and cabin roof within the rotor arc. Such devices may be Page D - 32
especially helpful in determining clearance during autorotation and controllability testing under FAR 27.143. If such measuring devices are used, the type inspection report should contain a record of clearance found during the tests. During TIA flight testing, it is not necessary to precisely determine the clearance but only necessary to determine "enough clearance" as stated in the rule.
AC 27.663 § 27.663 (Amendment 27-2) GROUND RESONANCE PREVENTION MEANS.
a. Explanation.
(1) This rule, adopted in Amendment 27-2 and revised in Amendment 27-26 requires reliability and damping action investigation for the ground resonance prevention means. The probable range of variations in service, not just the allowable range, must be established and investigated as prescribed. This probable range includes operation on the ground, and other appropriate landing surfaces applicable to the rotorcraft design shall be considered. Quantitative test data are generally obtained in compliance with this rule, but analysis or tests may be used.
(2) Appropriate maintenance information should be included in the maintenance manual (also called instructions for continued airworthiness).
(3) Paragraph AC 27.241 of this document concerns demonstrating freedom from ground resonance during certain applicant and TIA verification evaluations or tests of the rotorcraft. Section 27.241 complements the requirements of § 27.663. As noted in paragraph AC 27.241 of this document, a specific requirement for a ground vibration survey was removed from CAR Part 6. However Section 27.663 was adopted by Amendment 27-2 to investigate possible sources of ground resonance and to assure the reliability of the ground resonance prevention means, i.e., dampers, if necessary, to preclude occurrence of ground resonance. The total rotorcraft system is evaluated under this rule.
(4) Viscous dampers have been used for many years to prevent ground resonance. Modern rotorcraft designs may also use elastomeric dampers and may use elastomeric bearings in the rotor head and rotor pylon attachment to the airframe. The rule also requires investigation of the probable range of variations of these dampers, whether viscous or elastomeric, and these bearings to preclude ground resonance.
(5) Ground resonance can occur due to flexibility in the rotor pylon restraint system as well as with landing gear flexibilities. See paragraph b(2) of this guidance section (AC 27.663) for an explanation. An analysis may be done to show the effect of the rotor pylon mount stiffness on ground resonance stability. If the analysis shows that rotor pylon mount stiffness could affect ground resonance, the evaluation should include variations in stiffness and damping of the rotor pylon restraints that may occur in service (reference “Ground Vibrations of Helicopters,” M.L. Deutsch, JAS, Vol. 13, No. 5, May 1946).
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b. Procedures.
(1) The reliability of the means for preventing ground resonance may be substantiated as stated in the rule. An analysis report or a test proposal and subsequent test report may be used to show compliance. The probable ranges of damping restriction are an important part of the assessment. The test may be conducted in conjunction with the testing required by § 27.241. See paragraph AC 27.241.
(i) Analysis and tests may be used.
(ii) Reliable service history of identical or closely similar systems may be used. The materials and fluids used, clearance or fits, seals, and physical installation are important items to be evaluated and considered for “closely similar” systems.
(iii) Testing of the complete rotorcraft may be used to prove that malfunction of a single means or member of the damping system will not cause ground resonance. One method of demonstrating acceptable compliance is by removing all or most of the fluid from a damper and considering the allowable ranges of damping of the other parts of the rotorcraft damping system while operating the rotorcraft throughout the rotor speed range from start to maximum rotor speed. Investigation of elastomeric dampers may require innovative test procedures and preliminary discussions of these prior to preparation of a test proposal. The rotorcraft cyclic control should be displaced as noted in paragraph AC 27.241 of this document to assure that the possible rotorcraft resonance frequencies are excited. If vibrations are damped in all tests, the damping system is satisfactory. Each critical rotor damper and landing gear damper must simulate a malfunction to comply with the rule. The testing discussed, however, could be come very extensive if one were to attempt to test all combinations of all maintenance adjustments of all components which contribute to the prevention of ground resonance, while at the same time rendering each of the pertinent components ineffective in turn and then repeating all of the maintenance tolerance testing each time.
Fortunately, rational analytical methods are available which will permit the evaluation of such combinations so that only the combinations with the least amount of margin used are physically tested.
(2) The pylon damper variation can affect ground resonance. The variations in stiffness and/or damping of pylon mounts should be evaluated except the pylon mounts on contemporary conventional rotorcraft may have little influence on “classical” ground resonance stability. The dynamics of the rotorcraft on its landing gear is generally established by the airframe properties and the landing gear properties under the influence of the rotor system, with the “pylon” having little or no effect. For air or flight resonance, the rotor generally couples with the rigid body modes of the fuselage. For a specific design, a relatively simple analysis may be used to show the effect of the pylon mount system stiffness on air and ground resonance stability, and if not important, variations in the system may be omitted from the test program.
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(3) The probable ranges of damping must be established and investigated as prescribed and noted in paragraph AC 27.663(b). An approved test proposal and test results report should be used for complying with § 27.663(b). If wheel landing gear is used on the rotorcraft, the probable ranges of tire pressure or the lowest probable tire pressure should be stated in the test proposal and effects of the tire pressure investigated during the test. See paragraph AC 27.241, § 27.241, concerning tests and instrumentation of the test associated with complying with § 27.241. The instrumentation noted in paragraph AC 27.241 also applies to § 27.663(b).
(4) If the wheel landing gear is equipped with wheel brakes, the evaluation should include brakes “on” and “off.” The nose or tail wheel should be locked and unlocked if it swivels to evaluate any possible adverse effects of this feature.
(5) Any maintenance procedures should be included in the “recommended” part of that manual. See Appendix A, FAR Part 27.
AC 27.663A § 27.663 (Amendment 27-26) GROUND RESONANCE PREVENTION MEANS.
a. Explanation. Amendment 27-26 clarifies that analysis as well as tests may be used to show freedom from ground resonance after malfunction or failure of a single means of ground resonance prevention. This amendment primarily clarifies that the probable range of damping should be established as well as investigated.
b. Procedures. The procedures of paragraph AC 27.663 continue to apply with the addition of the need to document the establishment of the probable range of damping of ground resonance prevention means. Acceptable tire and oleo minimum and maximum pressures as well as other identified factors should be documented in maintenance instructions if necessary to maintain the desired characteristics.
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AC 27-1B, Chg 2 4/25/06
SUBPART D - DESIGN AND CONSTRUCTION
CONTROL SYSTEMS AC 27.671. § 27.671 CONTROL SYSTEMS --GENERAL.
a. Explanation.
(1) The rule requires basically that controls operate easily and smoothly and provide positive response of the rotorcraft from control input.
(2) In addition, the rule requires that incorrect assembly be prevented by special design features or special markings.
b. Procedures.
(1) Easy, smooth operations of controls are substantiated by the operations tests of § 27.683 and the FAA/AUTHORITY flight testing under TIA procedures.
Positive response of the rotorcraft to control inputs is also evaluated during company flight testing and FAA/AUTHORITY TIA flight testing to the requirements of §§ 27.141 through 27.175.
(2) To meet the requirement that incorrect assembly be prevented, the preferred method is providing design features which make incorrect assembly impossible. Typical design features which can be used are different lug thicknesses, different member lengths, or significantly different configurations for each system component. In the event that incorrect assembly is physically possible (because of other considerations), the rule may be met by the use of permanent, obvious, and simple markings. Permanent (durable) decals or stencils may be used.
(3) Design features of the control systems are checked when reviewing the type design drawings. During the proof and operation tests of §§ 27.681 and 27.683, the controls should be thoroughly reviewed for possible incorrect assembly and for any required markings supplied for compliance with this standard.
AC 27.672. § 27.672 (Amendment 27-21) STABILITY AUGMENTATION, AUTOMATIC, AND POWER-OPERATED SYSTEMS.
a. Explanation.
(1) This rule requires that the pilot be made aware of stability augmentation, automatic, or power-operated system failures which could lead to an unsafe condition.
It should be understood that this requirement applies to stability augmentation and supplementary controls and not the primary flight control system that is dealt with under § 27.695 and associated advisory material. Examples of clearly distinguishable Page D - 36
warnings include, but are not limited to, an obvious aircraft attitude change following the failure or an audio warning tone. A visual indication itself may not be adequate since detection of a visual warning would normally require special pilot attention. The use of devices such as stick pushers or shakers is not acceptable as a warning means since the automatic flight control systems (AFCS) may provide a hands-off capability or normal helicopter vibrations could mask a control shaker. However, this rule is not intended to eliminate the use of such devices for other purposes. Examples of automatic control systems other than a stability augmentation system would be a pitch axis actuator used for the purpose of demonstrating compliance with longitudinal static stability requirements or a fly-by-wire elevator. For control systems where a series actuator malfunction could degrade control authority, a means should be provided to the pilot to determine actuator alignment (see § 27.1329(b)).
(2) The corrective flight control input following a system failure should be in the logical direction. For example, a malfunction resulting in a nosedown pitch of the aircraft should require a corrective cyclic control input in the aft direction. The system deactivating means does not have to be located on the primary flight control grips;
however, it should be easily accessible to the pilot. Consideration should be given to the consequences of inadvertent de-selection of the automatic stabilization system, especially if the deactivation control is mounted on a primary control grip. Malfunctions and subsequent recoveries must be shown throughout the operating envelope of the aircraft. In a case where control authority is decreased following a malfunction, a practical flight envelope must be defined wherein compliance with controllability and maneuverability requirements can be demonstrated. This practical flight envelope must be presented in the flight manual. Compliance with trim and stability characteristics is not required following a malfunction; however, a pilot workload assessment should be made to show that these characteristics are not impaired below that needed for continued needed for continued safe flight and landing.
AC 27.673. § 27.673 (Amendment 27-21) PRIMARY FLIGHT CONTROL.
a. Explanation. This regulation basically defines primary flight controls as “those used by the pilot for immediate control of pitch, roll, yaw, and vertical motion of the rotorcraft.” This regulation was generated to clarify the application of § 27.1555 which requires markings for controls other than “primary flight controls or control(s) whose function is obvious.”
b. Procedures. The primary flight controls; i.e., cyclic stick, collective, and tail rotor pitch control pedals are excluded from the marking requirements of § 27.1555.
AC 27.674. § 27.674 (Amendment 27-26) INTERCONNECTED CONTROLS.
a. Explanation. A new § 27.674 is added by Amendment 27-26 which requires that the rotorcraft be capable of safe flight and landing after a malfunction, failure, or jam of any auxiliary interconnected control.
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b. Procedures.
(1) Section 27.674 requires that the rotorcraft be shown to be capable of safe flight and landing after a malfunction, failure, or jam of an auxiliary control interconnected with a primary control. The section does not apply to interconnected primary controls; e.g., cyclic and collective controls.
(2) Examples of auxiliary controls covered by this section may include certain autopilot or stability augmentation or trim system components. Section 27.1309 methods may be used in determining failure effects of autopilot and stability augmentation “system” components. For components whose purposes are solely mechanical functions, the procedures associated with § 27.571 for components such as the main rotor may be used.
(3) If an engine control could jam and result in a collective control jam, the controls should be designed to relieve that connection.
AC 27.675. § 27.675 (Amendment 27-16) STOPS.
a. Explanation.
(1) Stops are required to prevent unrestrained movements of pilot/autopilot inputs from causing interferences or overloads.
(2) The rule requires that the stop must be located to not appreciably affect the control system range of travel due to wear, slackness, or take-up adjustments.
(3) Each stop is required to withstand loads corresponding to design conditions.
(4) In addition, each main rotor blade, if appropriate for the design, must have stops to limit its travel about its hinge points. For rotors with hingeless design, stops may be provided as appropriate to limit blade travel. Loads which result from the blade hitting the stops (during starting or stopping the rotor or during any large but allowable pilot control inputs such as autorotation cyclic flares or when subjected to ground gusts, etc.) shall not overload the stops nor any rotor component.
b. Procedures.
(1) Stops are generally provided in the cockpit area and near any controllable surface end of the control system (i.e., main rotor hub, tail rotor hub, and stabilizer activators). For systems with control coupling or series actuators, stops have been located farther downstream (away from the cockpit) to permit increased control output during malfunction (hardover) or extreme control position cases.
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(2) Location of stops in close proximity to each end of a control system will allow the stop to provide its function most efficiently without undue deflections between the stop and its adjacent surface or its adjacent cockpit control lever or pedals.
The location of stops close to the control lever or surface will help meet the requirement that the stop (and its function) not be appreciably affected by wear, slackness, or takeup adjustments. Consideration should be given to limiting the total amount of take-up adjustments of both the stop and the control systems to preclude a hazardous adjustment of the control surface range of travel by either normal or extreme take-up adjustment.
(3) Each stop is to be substantiated for critical design conditions from either pilot effort, aerodynamic loads, hydraulic loads, and other critical loads, as applicable.
The stops can be substantiated for limit loads by the tests of § 27.681.
(4) The stops to limit the main rotor blade about its hinge points should be positioned to prevent the blades from striking any part of the structure, particularly during startup and shutdown operations. These stops should also limit the flapping of the static main rotor blades of the rotorcraft when they are subjected to ground gusts and rotor wash from nearby taxiing rotorcraft. Provisions should be made to prevent overloading the stops or the blade under conditions of ground gusts and rotor wash effects or during autorotational landing flares. The need for provisions to prevent possible overloads due to ground gusts and close taxiing by adjacent rotorcraft and by autorotational landing can be determined using the instrumented flight load survey aircraft by hover-taxiing another rotorcraft near the instrumented aircraft and by conducting autorotational landing flares with the instrumented aircraft. Substantiation for the final main rotor flapping stop design can be demonstrated by similar tests.
(5) If features of design are added to the main rotor stop assembly which activate certain portions of the stop assembly only on the ground to meet the requirement that the blade not hit the droop stop during any operation other than starting and stopping the rotor, such features of design must be substantiated to reliably operate by both ground tests and flight tests, as appropriate. Wear and rigging tolerances should be considered in these demonstration tests.
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AC 27-1B, Chg 3 9/30/2008 AC 27.679. § 27.679 CONTROL SYSTEM LOCKS.
a. Explanation. The rule requires that if control system locks are provided, means are necessary to prevent the rotorcraft from taking off with the locks engaged or, once airborne, to prevent the locks from engaging in flight.
b. Procedures. Three main approaches may be used to meet the requirements of this rule.
(1) The first approach is to provide a means to disengage the locks “automatically” as the pilot operates the controls. If this method is used, the means must disengage the locks in a manner so that they will not automatically re-engage during flight. The means may be physical removal of the locking device from close proximity to the control system interface, with deliberate crew action necessary to return the lock to the control system interface, or the means may be that the mechanism geometry or actions, or both, prevent the locking device from engaging in flight.
(2) The second approach is to provide locks that prevent take off with the locks engaged. Acceptable means are features which prevent engine startup or which restrict collective control operations to prevent sufficient lift for takeoff.
(3) The third approach is to provide a means that warns the pilot when a flight control lock system is engaged, such as through a non-cancelable warning message or through distinct control lock markings which are clearly visible to the pilot under all conditions (day and night).
(4) Unless it can be shown that the control locks cannot be inadvertently engaged (taking into account manufacturing tolerances and maladjustments), a means should be provided that allows, in flight, a single pilot to safely disengage any flight control lock. For example, the control lock design might include frangible fittings.
Further, the RFM should describe the appropriate pilot corrective action needed to disengage the control lock system while in flight.
(5) The rotorcraft Instructions for Continued Airworthiness should include appropriate maintenance checks and procedures to be completed following modification (for example, via STC or field approval), maintenance, alignment, or adjustment that affects the flight control system locks.
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AC 27.681. § 27.681 LIMIT LOAD STATIC TESTS.
a. Explanation.
(1) The rule requires static tests of the control system in showing compliance with limit load requirements.
(2) The tests are specified to include each fitting, pulley, and bracket of the control system being tested and to include the “most severe loading.”
(3) Also, the rule requires that compliance with bearing factors (reference § 27.623) be shown by individual tests or by analyses for control system joints subject to motion.
b. Procedures.
(1) Compliance with the requirements of this rule is obtained by static tests conducted on either a static test airframe or on a prototype flying ship. In either case, conformity of the control system and related airframe is necessary to validate the tests.
(2) The rotor blades or aerodynamic surfaces may be used to react pilot effort loads through the control system, or they may be replaced with fixtures. If fixtures are used, they should be evaluated for geometric and stiffness efforts to ensure test validity.
(3) The loads to be applied during the limit load static tests are specified in §§ 27.395, 27.397, and 27.399. The loads are applicable to collective, cyclic, yaw, and rotor blade control systems as well as any other flight control systems provided by the design.
(4) Although Part 27 does not explicitly specify the bearing factors to be used in control system rotating joint tests or analyses, the factors of § 29.685 have been used in past programs. These factors are 3.33 for push-pull systems and 2.0 for cable systems for joints with plain bearings and manufacturers’ ratings for ball and roller bearings.
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AC 27.683. § 27.683 OPERATION TESTS.
a. Explanation. The rule requires that the control system be free from jamming, excessive friction, and excessive deflection. An operational test is required in which specified loads are applied at the pilot controls and carried through an operating control system.
b. Procedures.
(1) Compliance with the requirements of this rule is obtained by use of a test setup similar to that used for the limit load tests of § 27.681, except the load reactions at the blades (or surfaces) must allow for movement of the blades (or surfaces) as the system is operated through its operating range.
(2) Fixtures are normally affixed to the surfaces (or replace the surfaces) to allow pulley arrangements which provide for movement under load. These fixtures should be evaluated to ensure that system loads up to limit will be applied during the full range of operations of each system.
(3) Each flight control system should be operated through its entire range under a light load and under limit load. As the controls are being operated, the system should be checked for jamming, excessive friction, and excessive deflection. Excessive deflection includes deflection sufficient to contact other systems or structure. Also, if under these limit load conditions the components deflect, the deflection would be considered excessive if there is permanent deformation of any component or supporting structure. Also any deflection that results in an uncorrected condition when the load is released, e.g., if a bellcrank is forced off-center or over-center during load and does not return to the normal position after load release is excessive deflection. Floor panels, wall panels, and other access panels may have to be removed to permit visual checks of the entire control system.
AC 27.685. § 27.685 (Amendment 27-11) CONTROL SYSTEM DETAILS.
a. Explanation. The rule requires that the control system be designed to prevent chafing, jamming, and interference from cargo, passengers, loose objects, or the freezing of moisture. Specifically, means are required in the cockpit to prevent the entry of foreign objects into places where they would jam the system, and means are required to prevent the slapping of cables or tubes against other parts.
b. Procedures.
(1) The geometry of the control system components and their installations are the primary control to prevent chafing, jamming, and interference. The control system from cockpit to surface should be checked for clearances both unloaded and loaded.
The control system should be checked under load during both the limit load static tests (reference § 27.681) and the operational tests of § 27.683. Location of guides or Page D - 41