change requires cooling fans to be designed and installed to enable safe landing of the rotorcraft following a fan blade failure. Compliance with the previous requirements could result in hazards to the rotorcraft with the loss of cooling air to critical powerplant components. A new paragraph was also added to the rule for cooling fans, which are part of the powerplant installation. It should be determined that no cooling fan blade resonant conditions exist within the operating limits of the rotorcraft unless a fatigue evaluation is conducted. These requirements will ensure that correct qualification procedures are used for rotorcraft engines and that all powerplant cooling fans are properly tested.
b. Procedures.
(1) Engine type certification. All engines installed in rotorcraft should have a type certificate. The specific certification requirements for reciprocating engines when installed in rotorcraft are found in the paragraph listed in Part 33. Engines certificated under other approved certification rules (CAR Part 13 and FAR § 21.29, for imported engines) are also eligible. If a component, system, or arrangement is certified under Part 33 or other requirement, the applicant is not relieved of the necessity to comply with the requirements of Part 27. If the component, system, or arrangement, supplied as a part of a certificated engine, meets the Part 33 and Part 27 requirements, subsequent changes to these components, systems, or arrangements could negate compliance with Part 27.
(2) The applicant may select §§ 27.903(b)(1)(i), (b)(1)(ii), or (b)(1) (III) to show compliance with this section.
For compliance with § 27.903(b)(1)(i), a demonstration should be conducted to show that at the expected maximum fan speed, a failed blade will be contained within a housing or shroud that is included in the proposed type design and designated as the containment shield. The maximum fan rotational speed may be related to an overspeed limiting device or to the expected maximum transient speed from analysis or test of the engine, system, or component which drives the fan. For fans driven directly by the engine, output shaft disconnect and the subsequent terminal speed of the engine may establish the maximum fan speed for the test condition. To conduct an overspeed blade failure containment demonstration, applicants have found it convenient to progressively weaken a blade to induce tailure at or above the required demonstration speed. Blade failure may be expected to subsequently occur on some or all of the remaining blades. This condition, provided all blades are contained, is acceptable for showing compliance with this rule. However, the corresponding loss of cooling may be unacceptable if it causes the loss of any function essential to continued safe flight and landing.
(ii) For § 27.903(b)(1)(ii) compliance, if containment protection is not installed, damage to any component or structure within the trajectory of the failed fan rotor should not cause the loss of any function essential to a controlled landing.
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(ili) For § 27.903(b)(1) (ili) compliance, a spin test should be conducted.
For fans driven directly by the engine, the test should be conducted at 122.5 percent of the terminal engine rotational speed that will occur under uncontrolled conditions, or at
122.5 percent of the maximum engine rotational speed that would be controlled by a reliable, approved engine overspeed limiting device. For fans driven by the rotor drive system, the test should be conducted at 122.5 percent of the maximum rotor drive system rotational speed expected in service, including transients.
(Note: Capability to withstand the ultimate load of 1.5 times the centrifugal force means that no failure should occur and distortion should not result in fan element contact with housings or other adjacent components during the 122.5 percent spin test which equates to 150 percent centrifugal force).
(3) Fatigue. If the cooling fan is not included in the fatigue evaluation under § 27.571, it should be shown that the cooling fan blades are not operating at resonant conditions within the normal operating limits of the rotorcraft.
399. § 27.907 ENGINE VIBRATION.
a. Explanation. Section 27.907 is intended to require the design of the rotor drive system, including the engine, to be free from harmful vibration. A vibration investigation is required.
b. Procedures. Review Order 8110.9, Handbook on Vibration Substantiation and Fatigue Evaluation of Helicopter and other Power Transmission Systems. Note that the mechanical coupling of the engines to the rotor drive system creates, for torsional vibration considerations, one, rather complicated, drive system which responds to any forced or resonant frequency. Antinodes or nodes and frequencies may exist in the engine shaft which are absent when the engine is operated on a test stand; therefore, the vibration investigation conducted under Part 33 is not conclusive with respect to torsionals. As noted in Order 8110.9, the engine manufacturers' assistance is necessary to find compliance. Section 27.571 was amended by Amendment 27-12 to include "rotor drive systems between the engines and the rotor hubs" as part of the flight structure. This rule supplements § 27.907 and requires coordination with the structures certification engineer to avoid duplication of effort by the rotorcraft manufacturer. Advisory Circular 20-95, which provides acceptable methods of compliance with § 27.571, may also be used to find compliance with § 27.907.
In addition to basic drive system components such as main and auxiliary rotor drive shafts, the vibratory evaluation should include couplings, gear teeth, gear cases and splines, and should consider, where appropriate, low cycle fatigue associated with ground-air-ground cycles.
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400.-420. RESERVED.
Intentionally Left Blank Par 400: 509 (thru 529)
SECTION 25. ROTOR DRIVE SYSTEM
421. § 27.917 (through Amendment 27-11) DESIGN.
a. § 27.917 (a):
(1) Explanation. This paragraph requires the design of the drive system to include a means to automatically disengage the engine(s) from the rotor drive system in order to prevent excessive drag from an inoperative engine from adversely affecting the pertormance of the rotor system.
(2) Procedures. The design objective usually is met by installing a freewheeling or overrunning clutch in the drive shaft between the engine and the first part of rotor drive system. If lubrication for these clutches is required, it should be provided by a means that continues to function after an engine is made inoperative except that for single-engine rotorcraft, clutch lubrication need only be provided for autorotation descent with the engine inoperative. A 15-minute demonstration of freewheeling or overrunning operation is usually acceptable.
b. § 27,917(b):
(1) Explanation. This paragraph requires that control rotors (tail rotors, for example) will continue to be driven by the main rotors when the rotorcraft is in autorotation.
(2) Procedures. Provide hard mechanical interconnect shafting between the rotors such that the main rotor will drive the control rotor (tail rotor). Note that this requirement must be met with all engines inoperative, thus, the driving force for the tail rotors must depend on the autorotative driving forces inherent in the main rotor(s).
c. § 27,917 (c):
(1) Explanation. This paragraph pertains to any device or feature designed into the rotor drive system intended to prevent damage in the event of excessive torque in the rotor drive system from high engine power or mechanical interference with normal rotation of the rotor drive system. The rule prohibits location of these devices in any part of the rotor drive system that is required to continue functioning to provide control of the rotorcraft.
(2) Procedures. Review the arrangement of the rotor drive system to determine that any intentionally designed weak links in the system, such as shear sections or slip clutches installed to relieve high torsional loads, are located so that their function will not compromise the interconnect mechanism between the main and auxiliary rotors of the rotorcraft.
530 Par 421
d. § 27.917 (d):
(1) Explanation. This paragraph sets forth a definition of the rotor drive system and its associated components.
(2) Procedures. Coordinate with other certification personnel to ensure that other rules pertaining to rotor drive systems are properly addressed.
422. $ 27.921 ROTOR BRAKE.
a. Background. Rotor brake safety requirements are intended not only to prevent adverse effects on aircraft performance due to brake drag but also to minimize the possibility of fire. These fires, caused by friction from a dragging rotor brake, have occurred both in flight and during ground operation with extremely hazardous consequences.
b. General. This rule requires (1) that any limitations on the use of the rotor brake must be established, and (2) that the control for the brake must be guarded to prevent inadvertent operation.
c. Limitations.
(1) The limitations on the use of the rotor brake should first be defined by the applicant and will normally consist of merely the maximum speed eligible for application of the brake. In some installations, other limitations associated with engine operation may be specified.
(2) Control guard mechanisms to prevent inadvertent operation may be conventional. A cockpit evaluation of the guard should be conducted by flight test personnel to affirm the function of the guard, that markings, if any, are adequate, and that both latched and unlatched positions of the guard do not interfere with other cockpit functions.
d. Other rules require both generalized and specific rotor brake qualification tests.
However, some significant aspects of brake safety tests are listed below for reference.
(1) Routine application of the brake at shutdown during the endurance test of § 27.923 and during the function and reliability tests of § 21.35.
(2) Torsional vibration loads in the rotor drive system and oscillatory loads in the brake components during a critical brake engagement procedure should be determined with appropriate consideration in the fatigue evaluation for these components. Brake engagements should be conducted with and without collective control displacement as authorized by the flight manual or a training manual.
Par 421 531
(3) Brake component temperature measurements during a critical brake application in conjunction with an evaluation of the general brake compartment for compliance with § 27.863.
(4) Placards, decals, and flight manual limitations and instructions appropriate to operate the rotor brake safely.
(5) An evaluation for hazardous failure modes as required by § 27.1309(b). If the brake hydraulic system is integral with the rotorcraft hydraulic system, failure modes of pressure regulators and control valves will be of interest. Mechanical cams, calipers, and levers may be prone to seize or fail to release the brake due, in part, to corrosion and lack of lubrication to be expected when brake components encounter high temperature cycling.
e. Maintenance manuals should be checked for completeness in the areas of wear limits for both pucks and disks, for disk warp limits, and for defects which induce brake chatter. Also, maintenance data to check for proper function of pressure modulating/relief devices should be included since misadjustments of this device can amplify the stresses and temperatures in the system.
423. § 27,923 (Amendment 27-12) ROTOR DRIVE SYSTEM AND CONTROL MECHANISM TESTS.
a. Explanation.
(1) This section is intended to require demonstration that the rotor drive system, as defined in § 27.917(d), is capable of normal operation within the limitations proposed, without hazard of failure from excessive wear or deterioration due to mechanical loads. The basic test is not designed and should not be expected to demonstrate safety from oscillatory stresses normally investigated under §§ 27.571 and
27.907, although any data generated by these tests applicable to showing compliance with §§ 27.571 and 27.907 may be used. Some variations in the endurance test plan to generate data applicable to the vibration substantiation effort or other qualification aspects may be acceptable if the basic requirements of the endurance test are preserved.
(2) This rule requires a series of runs consisting of a 60-hour, 30-hour, and 10-hour run for a total of (at least) 100 hours of testing, not including time required to adjust power or to stabilize operating conditions for those conditions that require stabilization. Extension of the total test time beyond 100 hours (or extension of any test run segment beyond the minimum) will occur if qualification for the 2 ½-minute one-engine-inoperative (OEI) optional rating is proposed by the applicant. The 30-minute OE rating qualification test will extend the test beyond 100 hours for rotorcraft equipped with three or more engines.
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(3) Section 27.923(b) requires the test to be conducted "on the rotorcraft." This means a rotorcraft in conformity to the design for which approval is requested.
However, many nonconformity features, such as doors, some cowling and instrumentation, fuel tanks (alternate external fuel supply may be utilized), interior features, fire detectors, extinguishers, inlet ducts, exhaust baffles, etc., may be acceptable provided each item is technically considered and found to be unimportant to the test results. Any significant deviations from the conformed rotorcraft configuration, such as using ground or flight test facilities instead of the rotorcraft, providing the conditions which exist on the rotorcraft can be accurately duplicated, should be defined in the test proposal and approved by the cognizant FAA/AUTHORITY engineering staff.
The restraint (tie-down) arrangement used during the test will necessarily be arranged to react rotor thrust loads in lateral as well as vertical directions. However, the restraint should permit normal deflections due to rotor thrust in the engine and drive system support arrangement. Safety cables may be installed normal to the tailboom at the tail rotor gearbox location; however, restraint may be provided to keep airframe deflections from exceeding those expected in normal and accelerated flight.
(4) The test torque requirements of § 27.923 mean the torque values for which approval is requested, but must not exceed the values approved for each respective limit for the engine being used. However, an applicant should be allowed to qualify the rotor drive system for torque values higher than those for which approval is requested if the engines actually used are capable of the torque and can be shown by an output shaft torsional investigation to be equivalent or conservative with respect to torsional vibration to the engines proposed for the initial certification configuration. Variations in rotational speed from the certification values should not be allowed except where careful evaluations of vibration aspects, bearing loads, centrifugal stiffening effects, and torque variations are conducted.
(5) The rotor configuration required by § 27.923(b) is intended to ensure that lift, torque, and vibration loads to be expected in service are introduced into the endurance test, although the presence of the vibration aspects does not normally satisfy the vibration evaluations required by §§ 27.571 and 27.907. In fact, vibration modes may be changed and amplified by the tie-down restraints and the increased thrust to be expected from in-ground-effects on the rotor system. These effects, although unquantified, are intended as a normal part of endurance testing.
Preproduction rotor blades have been successfully used in endurance tests but only after specific investigations of blade properties such as stiffness, inertia and inertial distribution, thrust and blade bending, and torsional frequency response have been carefully compared to ensure validity of the test. The endurance test includes testing the rotor control mechanism. Conformity of the rotors may be very significant to this aspect of the test.
(6) For approved designs, some drive system changes or mechanical power increases may only require partial testing to satisfy § 27.923 requirements provided an Par 423 533
equivalent level of safety finding can be made for the remaining requirements based on the previously approved data.
b. Procedures.
(1) Section 27.923(a) requires the rotor drive system and rotor control mechanism to be in a serviceable condition at the end of the test. Verification of this requirement requires a complete disassembly and examination of the entire rotor drive system and rotor control mechanism. The disassembly itself should be closely monitored for evidence of adequate breakaway torque on all bolted fasteners. Samples of lubrication from oil sumps and filters should be retained for spectrographic analysis, and seals should be examined for possible damage due to test requirements. Care should be taken to differentiate between seal damage and bearing damage due to disassembly procedures so that the direct results of the test may be properly considered. Close visual observation of each tooth on each gear is necessary to affirm proper load/contact patterns and absence of excessive surface stress or scrubbing motions. Bearings should be examined to verify that ball or roller paths are within limits, bearing cages are undamaged, and bearing balls or rollers and their races are free from pitting. Any evidence of bearing races turning or spinning in respective housing or bores probably indicates design or fit deficiencies. The applicant should have available wear limits data which include items such as distance across pins and tooth profile limits for gears. Many of these items require special, close tolerance inspection equipment and trained inspectors to determine compliance. In some instances, bearings, clutches, oil pumps, etc., should be returned to the original manufacturer for a finding of serviceability. Localized overheating usually exhibited by discolorations is an indication of an unsatisfactory condition. Should any of the items discussed above or other defects appear such that the component is unserviceable, a redesign which includes recognizable improvements should be required before authorizing a retest. To simply "try again" in hopes of success should not be accepted.
(2) Section 27.923(a) also prohibits intervening disassembly which might affect test results. Generally, this simply means no disassembly whatsoever. However, some very limited disassembly can usually be conducted provided care is used to ensure that items such as critical fastener torques or gear backlash controls are not disturbed.
(3) Section 27.923(b) requires that each rotor drive system and control mechanism be tested for not less than 100 hours. This endurance test is intended to demonstrate a minimum level of reliability and proper functioning of this system. The test should be conducted on the rotorcraft to provide the most realistic test environment. Exceptions can be made only if a ground or flight test facility is used that closely simulates the support and vibration conditions existing on the actual rotorcraft.
The rotor system installed on the ground test article should be the same as that used on the flight test vehicle. If significant productivity changes are made after completion of the tests, retesting may be required.
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(4) In § 27.923(c), (d), (e), and (f), the runs should be made with the proper torques, rotor speeds, and control positions specified by each paragraph. The controls discussed in each paragraph are the flight controls; i.e., cyclic and directional controls for rotorcraft with tail rotor and single main rotor. The collective control is normally used to set power and is not involved in the control cycling described in § 27.923(h). During control cycling the controls may be cycled from stop to stop, or a limited travel may be accepted if the travel produces the maximum fore and aft, left and right, and yaw thrust components of the rotors as measured in flight for a particular flight condition. One method of determining the required control displacement is to measure main rotor mast bending in level forward flight at maximum continuous power (or the power associated with the maximum rearward flight speed to be expected) for the aft control displacement limit. Using the same mast bending instrumentation with the rotorcraft in the ground tie-down situation and with collective control set for maximum continuous power, displace the cyclic fore and aft to obtain the same mast bending as measured in flight.
Similar measurements and control displacements may be used for sideward thrust components. Yaw control displacement should consider maneuver requirements in conjunction with sideward flight. Critical gross weight and center of gravity should be used to establish test conditions. Vertical thrust may be used during the takeoff run and the runs at 2 ½-minute power. OEI runs should be conducted with the cyclic set for maximum forward thrust for the 30-minute power run and at maximum vertical thrust for the 2 ½-minute power run. For these runs and any run that does not specity the position for the yaw control, that control should be set to react main rotor torque.
(5) Section 27.923(e) prescribes the takeoff portion of the endurance test. This is a 10-hour test that must be run at not less than the maximum torque and the maximum RPM to be approved for takeoff. For this test the main and auxiliary rotor controls should be in the normal position for vertical ascent. If the applicant elects (for a multiengine rotorcraft) to perform the 2 ½-minute OEI power test, a series of three 2 ½-minute repetitive runs should be conducted during the course of the 10-hour test.
For these tests, main and auxiliary rotor controls should be in the position for vertical ascent, and power settings for the operating engine should provide red line torque (or manifold pressure) and RPM for the 2 ½-minute OEI power rating. The nonoperating engine may be allowed to operate at idle or may be shut down.
(6) The torque and speed requirements in § 27.923(e) for the optional 2 ½-minute OEl tests should be interpreted as described above for the takeoff runs. If the test is conducted during warm ambient conditions, excessive engine gas temperatures may be required to achieve the torque and speed conditions required by this part of the test. Minor adjustments in the run schedule may be allowed to take advantage of cooler nighttime ambient temperatures. Addition of water/alcohol systems to increase engine hot-day power may be appropriate in some instances. Liquid nitrogen spray into engine inlets has also been used to depress inlet temperatures sufficiently to obtain test conditions.
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(7) The requirement in § 27.923(g) for declutching the engine may be difficult to achieve if engine decelerations and rotor system decelerations rates are similar. In some cases, the engine fuel control deceleration schedule may be adjusted to achieve clutch disengagement; otherwise, an engine shift brake mechanism may be needed.
(8) Tests described in § 27.923(h) should be conducted under the conditions of maximum continuous power and RPM as described in § 27.923(c).
(9) Section 27.923(1) requires 200 clutch engagements. This test is prescribed to establish a level of reliability of clutch components installed as a part of the rotor drive system of rotorcraft. The clutch tests apply to all clutches installed to comply with § 27.917(b), and each such clutch must be tested. A rotor brake is not required for certification, although a brake of some type may be installed temporarily to facilitate conducting the clutch testing required by this section. Clutch disengagement is also required by this section; thus, malfunction of the disengagement feature would be a basis for discontinuance. Some rotorcraft configurations (those with single-spool turbine engines or reciprocating engines) include an additional clutch to decouple the engine from the drive system to facilitate engine starting. These clutches should also be exercised at least 200 times during this test.
(10) Section 27.9230) sets forth the optional tests to be conducted if a 30-minute OEI rating is requested. Flight control positions should be set for level flight or climb, whichever produces the maximum forward thrust component, and the antitorque system control should be set to react the maximum rotor torque. The torque and rotational speed values should be the maximum for which approval is requested.
c. Additional Test Considerations.
(1) Pressure Lubricated Gearboxes. The endurance test hardware can be adjusted/modified to sustain high-limit oil temperature and low-limit oil pressure to provide a basis for approval of the values listed as limits. A minimum of 20 hours at maximum continuous torque and maximum continuous rotational speed should be involved in the test. Other parameters such as minimum oil temperature and maximum oil pressure may more appropriately be evaluated by bench test. The significant points here are effects of extremely high oil pressure (due to the high viscosity of cold oil) on any positive displacement oil pump, on filters for possible collapse, on oil coolers for possible rupture due to internal pressure, seals, bypass valves, and most important, adequate lubrication of gears, bearings, etc., under conditions of minimal oil flow.
Normally, an operational restriction against exceeding idle power/speed conditions until significant warm-up occurs is prescribed. Individual component qualification tests may provide data to meet some of these aspects.
(2) Asymmetric Power Inputs. The existing endurance test schedule does not necessarily provide for any asymmetric power inputs from multiengine drive system arrangements. For this situation, the drive system should at least be subjectively 536 Par 423
evaluated for possible hazards or excessive loads to be expected from asymmetric torque inputs. If required, additional testing should be considered.
(3) Accessory Drives. Normally, all accessory drives on a gearbox will be loaded during the endurance test. Electrical load banks or other suitable methods may be used to ensure that the generator drives are loaded and thus properly qualified.
Hydraulic pumps may be loaded by resetting hydraulic system relief valves to maintain limit pressure (load) continuously. If this condition is excessively severe, a method of load cycling may be appropriate. Note that accessory loads reduce the power available to the main rotor. Also, tail rotor loads are, insofar as the transmission is concerned, another large accessory. Care should be taken to ensure that in-flight unloading of these accessory drives, including the tail rotor, does not subject the main gearbox to loads significantly beyond those qualified by endurance tests.
(4) Gearbox Oil Tanks. Normally, gearbox oil is contained in an integral cast sump which, for other reasons, has sufficient strength to obviate the need for pressure tests. However, a subjective evaluation should be made to ensure that detail design features such as sight gauges, filler caps, etc., offer adequate strength.
423A. § 27.923 (Amendment 27-23) ROTOR DRIVE SYSTEM AND CONTROL MECHANISM TESTS.
a. Explanation. Amendment 27-23 revised § 27.923(c) to remove the references to "engine power" to avoid confusion. Previous wording could have been interpreted to mean tests prescribed by this section should be conducted at powers corresponding to engine ratings established under Part 33, rather than rotorcraft powers which may be lower than those established under Part 33, but selected by the applicant as a limit on their product. Section 27.923(d) was revised to remove the references to "engine power" and to clarify the test requirements for 30-minute and continuous OEI powers.
Previously, §§ 27.923(e) and I), as they relate to the 2 ½-minute and 30-minute power ratings, respectively, provided for only minimal testing of the capability of the rotor drive system to sustain these powers. Amendment 27-23 amended these sections to extend the testing to adequately assure valid qualification tests. These changes ensure the integrity of the rotor drive system so that it will safely sustain the higher stresses expected with actual, repeated use of these power ratings. A new § 27.923(k) was added that provides a qualification test schedule for the new, optional, continuous OEI rating.
b. Procedures. The policy material pertaining to this section remains in effect with the following additions:
(1) Section 27.923 requires a minimum of 100 hours of endurance testing.
(i) For single engine rotorcraft and others that will not have OEI ratings, the 100-hour test is comprised of 60 hours at not less than maximum continuous power, Par 423 537
30 hours at not less than 75 percent maximum continuous power, and 10 hours at not less than takeoff power.
(ii) For multiengine rotorcraft for which OEl ratings are requested, the test is comprised of 60 hours at not less than maximum continuous power, 25 hours at not less than 75 percent maximum continuous power, 10 hours at not less than takeoff power, and 5 hours at simulated OEI power conditions.
(2) The endurance time, cited in Paragraph b(1) above, excludes the time required to adjust power or to stabilize operating conditions. Extension of the total test time beyond 100 hours (or extension of any test run segment beyond the minimum) will occur if qualification for the 2 ½-minute, 30-minute, or continuous OEl optional ratings is proposed by the applicant for rotorcraft equipped with two or more engines.
(3) The requirements in § 27.923(f) stipulate that the endurance tests conducted at maximum continuous and 75 percent maximum continuous power should be conducted in intervals of not less than 30 minutes. These tests may be conducted on the ground or in flight. The takeoff power endurance test described in § 27.923(e) should be conducted in intervals of not less than 5 minutes.
(4) The new § 27.923(k) sets forth the tests to be conducted if a continuous OEl rating is requested. Flight control positions should be set for level flight or climb, whichever produces the maximum forward thrust component. The anti-torque system control should be set to react the maximum rotor torque. The torque and rotational speed values should be the maximum for which approval is requested.
423B. § 27.923 (Amendment 27-29) ROTOR DRIVE SYSTEM AND CONTROL MECHANISM TESTS.
a. Explanation. Amendment 27-29 added § 27.923(e)(2) that defines qualification tests for 30-second/2-minute OE ratings. This new paragraph also allows for the 30-second/2-minute OE portion of the endurance test to be accomplished on a representative bench test facility using the drive system components which can be adversely affected by these tests.
b. Procedures.
(1) For accomplishment of the endurance test for 30-second/2-minute OEl, Paragraph 27.923(e)(2) requires that 10 applications of 30-second/2-minute OEl power be demonstrated for each power section during the 10 hour takeoff power segment of Paragraph 27.923(e). Each 30-second/2-minute OEl application should be conducted immediately following a 5 minute stabilized takeoff power run. Following the 5 minute takeoff power run, one engine must simulate a power failure and each engine providing power after the failure must apply the maximum torque and maximum speed for use with 30-second OE power. This power level should be maintained for at least 538 Par 423A