AC 29-2C, Chg 3 9/30/2008
(ii) VHM data can be used to improve the helicopter's monitoring practices to mitigate a major or a hazardous/severe failure. The VHM data can also be used to improve maintenance.
(iii) A VHM system can be used to monitor all or some critical components of the rotor drive systems. Critical components include the input driveshaft to the main gearbox from the engine, bearings, tail rotor drive shaft, hanger bearings, one way clutch, main rotor mast and tail rotor mast. The supplier should state the component coverage and the fault detection capability for all affected components. The health monitoring effectiveness should be validated by tests or analysis or both.
(iv) Typically, a VHM system consists of sensors (e.g., accelerometers and tachometer), signal acquisition, signal processing, data management, VHM alert generation and management, a pilot interface, and a maintenance interface.
(v) Signal Processing: The helicopter's rotors, its drive systems and engines are a mixture of complex and simple mechanical elements. Therefore, the sensors and signal processing and analysis techniques utilized should reflect the complexity of the mechanical elements and their vibratory modes.
(vi) AC 29 MG-15 provides airworthiness approval guidance for rotorcraft health usage monitoring systems. This guidance can be used for incorporating VHM.
d. Failure Analysis.
(1) The first stage of the design assessment should be the Failure Analysis, by which all the hazardous and catastrophic failure modes are identified. The failure analysis may consist of a structured, inductive bottom-up analysis, which is used to evaluate the effects of failures on the system and on the aircraft for each possible item or component failure. When properly formatted it will aid in identifying latent failures and the possible causes of each failure mode. The failure analysis should take into consideration all reasonably conceivable failure modes in accordance with the following:
(i) Each item/component function(s).
(ii) Item/component failure modes and their causes.
(iii) The most critical operational phase/mode associated with the failure mode.
(iv) The effects of the failure mode on the item/component under analysis, the secondary effects on the rotor drive system and on the rotors, on other systems and on the rotorcraft. Combined effects of failures should be analyzed where a primary failure is likely to result in a secondary failure.
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(v) The safety device or health monitoring means by which occurring or incipient failure modes are detected, or their effects mitigated. The analysis should consider the safety system failure.
(vi) The compensating provision(s) made available to circumvent or mitigate the effect of the failure mode (see also paragraph (1) below).
(vii) The failure condition severity classification according to the definitions given in paragraph (c) above.
(2) When deemed necessary for particular system failures of interest, the above analysis may be supplemented by a structured, deductive top-down analysis, which is used to determine which failure modes contribute to the system failure of interest.
(3) Dormant failure modes should be analyzed in conjunction with at least one other failure mode for the specific component or an interfacing component. This latter failure mode should be selected to represent a failure combination with potential worstcase consequences.
(4) When significant doubt exists as to the effects of a failure, these effects may be required to be verified by tests.
e. Evaluation of Hazardous and Catastrophic Failures.
(1) The second stage of the design assessment is to summarize the hazardous and catastrophic failures and appropriately substantiate the compensating provisions that are made available to minimize the likelihood of their occurrence. Those failure conditions that are more severe should have a lower likelihood of occurrence associated with them than those that are less severe. The applicant should obtain early concurrence of the cognizant certificating authority with the compensating provisions for each hazardous or catastrophic failure.
(2) Compensating provisions may be selected from one or more of those listed below, but not necessarily limited to this list.
(i) Design features; i.e., safety factors, part-derating criteria, redundancies, etc.
(ii) A high level of integrity: All parts with catastrophic failure modes and critical characteristics are to be identified as Critical Parts and be subject to a Critical Parts Plan (see AC 29.602.). Where a high level of integrity is used as a compensating provision, parts with a hazardous failure mode which would prevent continued safe flight may be included in a Critical Parts Plan or subjected to other enhancements to the normal control procedures for parts.
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AC 29-2C, Chg 3 9/30/2008
(iii) Fatigue tolerance evaluation.
(iv) Flight limitations.
(v) Emergency procedures.
(vi) An inspection or check that would detect the failure mode or evidence of conditions that could cause the failure mode.
(vii) A preventive maintenance action to minimize the likelihood of occurrence of the failure mode, including replacement actions and verification of serviceability of items which may be subject to a dormant failure mode.
(viii) Special assembly procedures or functional tests for the avoidance of assembly errors which could be safety critical.
(ix) Safety devices or use of vibration health monitoring systems are recommended in addition to those provisions identified in paragraphs e.(2)(vi) and
e.(2)(vii).
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AC 29.921. § 29.921 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 fires. These fires, caused by friction from a dragging rotor brake, have occurred both in flight and during ground operation with extremely hazardous consequences.
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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 rotor speed eligible for application of the brake. In some installations, limitations associated with engine operation may be specified. For example, some “free power section” type turbine engines can be safely operated within certain low limits with the rotor brake engaged, while other engines cannot tolerate this condition. At least one manufacturer has included a maximum rotor speed for emergency rotor brake application. This is considered an enhancing safety consideration and is recommended.
(2) Control guard mechanisms to prevent inadvertent operation may be conventional. A cockpit evaluation should be conducted by flight test personnel to affirm the function of the guard and the brake, and that markings, if any, are adequate and that both latched and unlatched positions of the control do not interfere with other cockpit functions.
d. General qualification aspects should include:
(1) The 400 applications required by § 29.923(j) conducted as a part of the § 29.923 endurance test.
(2) Torsional vibration measurements of the loads in the brake components and the rotor drive system during a critical brake engagement procedure, 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.
(3) Brake component temperature measurements during a critical brake application in conjunction with an evaluation of the general brake compartment for compliance with §§ 29.863 and 29.1183.
(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 § 29.901(c). If the brake hydraulic system is integral with the rotorcraft hydraulic system, failure modes of pressure regulators and control valves, including valve leakage, 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 Page E - 38
NOTE: Most rotor brakes include nonmetallic pucks or liners, usually included in nonrotating brake components, which are subject to wear in proportion to the number of applications. Replacement of these pucks during the § 29.923 endurance test has been found acceptable provided the reason for replacement is simply wear and not because of any change in brake loading, disk temperature, or vibratory characteristics which can be expected in service. Verify that the maintenance manual includes a routine check for excessive puck or liner wear.
e. Other comments. Rotor brakes may be added to the basic design as a postcertification program without necessarily reconducting the complete § 29.923 endurance test provided:
(1) Steady and vibratory stresses in brake components, the rotor drive system, and in the rotor system itself are determined and shown to be acceptable.
NOTE: Moments, stresses, etc., from brake operation apply loads to the drive system in the reverse direction to normal powered flight. Advise the airframe engineer to require evaluation of chordwise bending loads in the hub and blade components of the main rotor system.
(2) The 400 brake engagements of § 29.923(j) should be accomplished with a complete rotor and rotor drive system, followed by disassembly sufficient to verify that all components subject to loads from the brake remain serviceable. Since this test may be so short as not to cause appreciable wear patterns to appear, special pretest coatings such as black oxide or Du-Lite may be needed on gear teeth and bearing races to distinguish and evaluate the contact patterns. Information on maximum deceleration rates should be supplied to the manufacturer of the engines to be used in the rotorcraft for evaluation of the acceptability of backloading or motoring of turbines, fuel control components, torque meters, etc.
AC 29.923. § 29.923 (Amendment 29-17) ROTOR DRIVE SYSTEM AND CONTROL MECHANISM TESTS.
a. Explanation.
(1) This rule is intended to require demonstration that the rotor drive system, as defined in § 29.917(a), 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 §§ 29.571 and 29.907, although any data generated by these tests, which are in fact applicable to showing compliance with §§ 29.571 and 29.907, may be used. Some variations in the endurance test plan to generate data applicable to the vibration substantiative effort or other qualification aspects may be acceptable if the basic requirements of the endurance test are preserved.
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(2) The construction of this rule is such that a series of runs, each at least (but not limited to) 10 hours in length must be repeated 20 times, for a total of (at least) 200 hours of test, not including time required to adjust power or to stabilize operating conditions for those conditions that require stabilization. Extension of the total test beyond 200 hours (or extension of test runs beyond 10 hours) will occur if qualification for the 2½-minute one-engine-inoperative (OEI) optional rating is proposed by the applicant. The 30- minute OEI rating qualification test will extend the test beyond 200 hours for rotorcraft equipped with three or more engines. Also, compliance with § 29.923(g) may result in extended endurance tests if dynamic or malfunction conditions exist which adversely affect the endurance tolerance of the rotor drive system.
Section 29.923(a) should be interpreted as requiring test runs or cycles to be repeated in essentially the same sequence, although more than 10 hours may be needed to complete a run or cycle. This section also 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 should be coordinated with the cognizant FAA/AUTHORITY engineering staff and if found acceptable, documented as such. 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.
(3) Safety cables may be installed normal to the tail boom 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 § 29.923(a)(3)(i) mean the torque values for which approval is requested but not to exceed the values approved for the respective limits for the engine to be 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 evaluation of vibration aspects, bearing loads, centrifugal stiffening effects, and torque variations are conducted.
(5) The rotor configuration required by § 29.923(a)(3)(ii) is intended to assure 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 §§ 29.571 and 29.907. In fact, vibration modes may be changed and amplified by the tie-down restraints and the increased thrust to be Page E - 40
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 inertia distribution, thrust and blade bending, and torsional frequency response have been carefully compared to assure validity of the test. The endurance test includes testing of the rotor control mechanism. Conformity of the rotors may be very significant to this aspect of the test.
b. Procedures.
(1) Section 29.923(b)(1) prescribes the takeoff portion of the endurance test.
This test involves a series of 5-minute repetitive runs at the torque and at the engine/rotor rotational speed selected by the applicant for the takeoff limit for the rotorcraft. These values of torque (manifold pressure, for reciprocating engines) and RPM should correspond to the red radials on the corresponding powerplant instruments, except on installations where uncompensated engine governor “droop” results in a higher rotational speed for lower powers. The requirement in this section for declutching the engine may be difficult to achieve if engine deceleration and rotor system deceleration rates are similar. In some cases, the engine fuel control deceleration schedule may be adjusted to achieve clutch disengagement, otherwise, an engine shaft brake mechanism may be needed.
(2) The torque and speed requirements for the optional 2½-minute one-engine-inoperative (OEI) 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.
(3) In § 29.923(c), (d), (e), and (f), the torque requirements should be interpreted as above; i.e., the run should be made with maximum continuous torque or percentage thereof, as specified by the subparagraph, and the rotational speed should be maximum continuous for paragraph (d) and the lowest permissible “power-on” speed for paragraphs (e) and (f). Rotor control cycling must be accomplished during the “maximum continuous” portion of the endurance run. The controls of concern 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 control cycling. 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. One method of determining the required control displacement is to measure main rotor mast bending in level forward flight at maximum continuous power for the forward control displacement limit, and in level rearward flight at maximum continuous power (or the power Page E - 41
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. These same procedures may be used to establish limited control positions required to comply with § 29.923(i) except that typical flight conditions to be used would be stabilized level flight at maximum continuous power, climb at maximum continuous power, and hovering, including stabilized sideward and rearward flight. Note that for § 29.923(i)(1) vertical thrust is required. Depending on the mast angle and center of gravity, this condition may not necessarily involve zero mast bending loads. Vertical thrust may be used during the takeoff run, including the runs at 2½-minute power and the overspeed run of § 29.923(h). One- engine-inoperative runs (§ 29.923(k)) should be conducted with the cyclic set for maximum forward thrust. For these runs and any run that does not specify the position for the yaw control, that control should be set to react main rotor torque.
(4) Section § 29.923(g) provides for introducing special tests into the endurance tests to demonstrate that the transmission and drive system can tolerate certain engine malfunctions to be expected. This was originally directed at demonstrating safety in the event of spark plug or magneto failures of reciprocating engines. Turbine engines normally do not exhibit failure modes suitable for substantiation by endurance testing; however, severe or abusive operating conditions which must be expected to occur in service should be defined and included in this test.
Conditions or phenomena to be considered should include but not be limited to moderate engine surge, abusive clutch engagements, torque mismatching, anticipated control mishandling, and so forth. Alternatively, repeating the takeoff run of § 29.923(b) may be appropriate. It is not intended that the special testing for 2½ -minute power be repeated if a rerun of the takeoff power run is required by § 29.923(g).
(5) Section 29.923(h) requires overspeed testing at the torque which will produce maximum continuous power and at the maximum rotational speed to be expected. Normally this would be the maximum transient, power -on rotor speed available with speed controls operating. Special control adjustments for test purposes may be needed to achieve the required test conditions.
(6) Section 29.923(i) requires stabilized flight control displacement according to a prescribed schedule. The control displacement should be the same as derived to show compliance with § 29.923(c)(2).
(7) Section 29.923(j) requires 400 clutch and brake engagements. These tests are prescribed to establish a level of reliability of clutch and brake components installed as a part of the rotor drive system of rotorcraft. The clutch tests apply to all clutches installed to comply with § 29.917(b)(3), and each such clutch must be tested. A rotor Page E - 42
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 400 times during this test.
(8) Section 29.923(k) sets forth the optional tests to be conducted if a 30-minute OEI rating is requested. It may be noted that the time for conducting this test replaces time deducted from the run of § 29.923(f). 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.
(9) Section 29.923(m) normally is satisfied by the requirements of §§ 29.571 and 29.907.
(10) Section 29.923(n) requires special tests for rotor drive systems designed to operate at two or more gear ratios. Depending on the limitations and instructions proposed for operating at other gear ratios, additional tests (beyond the normal 200-hour schedule) or substitutions into the basic test should be conducted to qualify the rotor drive system for operations at other gear ratios. The length of testing, torque and speed requirements, overspeed tests, and control positions for these tests should parallel the requirements of the basic endurance test.
(11) Section 29.923(o) 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 l imits, 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 Page E - 43
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.
(12) This section 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 assure that items such as critical fastener torques or gear backlash controls are not disturbed.
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 in order 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 operation 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) 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 evaluated for possible hazards or excessive loads to be expected from asymmetric torque inputs and additional testing prescribed under the authority of § 29.923(g). The extent and severity of these tests should be established in consideration of the design peculiarities, the recommended operating procedures, and any OEI tests included in this test schedule.
(3) Accessory Drives. Nor mally, all accessory drives on a gearbox will be loaded during the endurance test. Electrical load banks or other suitable methods may be used to assure 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 assure 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.
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(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 assure that detail design features such as sight gauges, filler caps, etc., offer adequate strength.
AC 29.923A. § 29.923 (Amendment 29-26) ROTOR DRIVE SYSTEM AND CONTROL MECHANISM TESTS.
a. Explanation. Amendment 29- 26 includes additional endurance test criteria for a new continuous OEI rating, and clarifies the torque and RPM relation intended for the various power ratings involved in the tests prescribed by this section.
(1) Section 29.923(a)(1) was amended to require that the test cycle be extended beyond 10 hours if OEI rating tests are included in the test program. This change was needed to maintain the cycle aspect of the test if OEI ratings are included.
(2) Section 29.923(a)(3) was amended to include rotational speed as a part of the test because the term “torque” by itself does not adequately define the test requirements.
(3) Section 29.923(b)(2), (f), and (k) were amended to add the test requirements for the new continuous OEI rating and retain, as an alternate, the 30-minute OEI rating tests for those applicants who may request this rating. This change provided a regulatory test basis for qualifying the rotor drive system for optional OEI ratings.
(4) Section 29.923(g) was amended to remove the inference that the 2½-minute OEI runs should be repeated if the takeoff run is reconducted. Under these circumstances, additional testing for the 2½-minute rating is unnecessary.
b. Procedures.
(1) The construction of this amendment is such that a series of runs, each at least (but not limited to) 10 hours in length, should be repeated 20 times for a total of at least 200 hours of test. The time required to adjust power or to stabilize operating conditions for those conditions that require stabilization is not included.
Figure AC 29.923A-1 shows a graphic representation of the 10-hour test cycle.
Extension of the total test time beyond 200 hours (or extension of test runs beyond 10 hours) will occur if qualification for the 2½-minute, 30-minute, or continuous OEI optional ratings is proposed by the applicant for rotorcraft equipped with two or more engines. Also, compliance with § 29.923(g) may result in extended endurance tests if dynamic or malfunction conditions exist which adversely affect the endurance tolerance of the rotor drive system. Section 29.923(a) should be interpreted as requiring test runs or cycles to be repeated in essentially the same sequence, although more than 10 hours may be needed to complete a run or cycle. This section also requires the test to be conducted “on the rotorcraft.” This means a rotorcraft that is in conformity to the Page E - 45