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.
Page E - 44
(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
type design for which approval is requested. However, many nonconforming 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 have no impact on the test results. Any significant deviations from the conformed rotorcraft type design should be coordinated with the cognizant FAA/AUTHORITY engineering staff and, if found acceptable, properly documented.
The restraint (tie-down) arrangement used during the test should be arranged to react rotor thrust loads in lateral as well as vertical directions. The restraint should permit normal deflections due to rotor thrust in the engine and drive system support arrangement.
(2) The test torque and speed requirements of § 29.923(a)(3)(i) refer to the torque/speed combination (or power) values for which approval is requested. The requested torque/speed combination should not exceed the limits approved for the respective engine(s) to be used. An applicant may qualify the rotor drive system for torque values higher than those approved for the engine.
(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 permissable “power-on” speed for paragraphs (e) and (f). Rotor control cycling should be accomplished during the “maximum continuous” portion of the endurance run. The controls of concern are the flight controls (cyclic and directional controls for typical rotorcraft). 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. The frequency for cycling the controls is defined in §§ 29.923(c)(1), (2), and (3), and specified in Note 3 of figure AC 29.923A-1. 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 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 flight 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, hover, and 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 Page E - 46
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 to main rotor torque.
(4) Section 29.923(k) sets forth the optional tests to be conducted if a 30-minute or a continuous OEI power rating is requested. Flight control positions should be set for level flight or climb (whichever produces the maximum forward thrust component) and 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.
Page E - 47
OE. RON 30 MIN O.E.I. OR CON'T O.E.I
EACH ENGINE
29.923 (K NO EI OVERSPEED AT M.C. TORQUE
29.923 (h)
MALFUNCTION RUN OF REPEAT T.O. RUN
9.923 ฿ JOTE ?
00 0W %09 ROTOR DRIVE SYSTEM ENDURANCE 10+ HOUR CYCLE - REPEAT 20 TIMES FELON (TOTAL OF 200+ HOURS) 80% M.C TIME - HOURS FIGURE AC 29 9234-1
29.923 (e) 90% M.C.
29.923 (d)
MAXIMUM CONTINUOUS RUN
29.923 (c) _ NOTE 1 TAKEOFF RUN 29.923(b),
2.5 MIN OEI T.O 30 MIN OEI M.C 90% M.C. 80% M.C 60% M.C. FIt. Idle Gr. Idle Page E - 48
#2 ENGINE 2 1/2 MIN. OEI #1 ENGINE 2 1/2 MIN. OEI TAKEOFF RUN PORTION OF ROTOR DRIVE SYSTEM ENDURANCE TEST 2 1/2 MINUTE OEI RATING IS REQUESTED NOTE 1 IF •AST 40 TIME - MINUTES FIGURE AC 29 923A-2 #2 ENGINE 2 1/2 MIN OFL #1 ENGINE 2 1/2 MIN. OEI T 2 1/2 MIN. OE T.O.
Page E - 49
TAKEOFF RUN PORTION OF ROTOR DRIVE SYSTEM ENDURANCE TEST 30 SECOND/2 MINUTE OEI RATING IS REQUESTED # 4211 · 777 722 NOTE #2 ENGINE 30 SEC & 2 MIN OEI * TIME - MINUTES FIGURE AC 29 923A-3 #1 ENGINE 30 SEC & 2 MIN OEI * NOTE 2 30 SEC OEI 2 MIN DEI 30 MIN OR CONT. T.O. GR. IDLE Page E - 50
Figures AC 29.923A-1, AC 29.923A-2, and AC 29.923A-3 Notes
1. If the 2½-minute OEI rating is requested, the following should be conducted for each engine: Demonstrate 2½-minute OEI power twice or 5 minutes per cycle for a total of 100 minutes at 2½-minute OEI power during the 200-hour endurance test. See figure AC 29.923A-2 for a graphi c description of the takeoff run for a two-engine rotorcraft using the 2½-minute OEI rating (Refer to § 29.923(b)(2)). If the 30-second/2-minute OEI rating is requested, the following should be conducted for each engine: Demonstrate 30-second OEI power followed by 2-minute OEI power. After 2 minutes reduce and stabilize power to 30-minute or continuous OEI level. Once the power is stabilized reappl y 2 minute OEI power. This should result in 4½ minutes per cycle for a total of 10 minutes at 30-second OEI power and 80 minutes at 2-minute OEI power during the 200-hour endurance test. See figure AC 29.923A-3 for a graphic description of the takeoff run for a two-engine rotorcraft (refer to § 29.923(b)(3)). If ei ther the 2½-minute or 30-second/2-minute OEI ratings are demonstrated, the takeoff run portion of figure AC 29.923A-1 will be longer than 1 hour as shown in figure AC 29.923A-2 or figure AC 29.923A-3, respectively.
2. Apply the rotor brake during the first minute of the 5-minute idle period. Conduct 400 brake applications during the 200-hour endurance test (§ 29.923(j)).
3. During the maximum continuous run, cycle the rotor controls 15 times per hour:
(Refer to §§ 29.923(c)(1) - (3)). The cyclic control should be cycled through maximum vertical thrust, maximum forward, maximum left, maximum right, and maximum rearward thrusts. The pedal controls should be cycled through maximum right, neutral, and maximum left positions. Each maximum cyclic and pedal control position should be held for at least 10 seconds. During the remainder of the test, set the yaw control to react to the main rotor torque, and set the flight controls to achieve:
Condition Portion of Test max vertical thrust 20 percent max forward thrust 50 percent max left thrust 10 percent max right thrust 10 percent max rearward thrust 10 percent Page E - 51
AC 29-2C, Chg 4 5/1/2014 Figures AC 29.923A-1, AC 29.923A-2, and AC 29.923A-3 Notes (continued)
4. The 60 percent maximum continuous run is 2 hours (Refer to § 21 29.923(f)), unless either 30-minute OEI or continuous OEI power is requested. In that case, the 60 percent maximum continuous run is 1 hour.
5. A 1-hour malfunction run (if deemed necessary) or the takeoff run is repeated (without OEI portions). Refer to § 29.923(g).
6. The OEI run defined in § 29.923(k) is not required unless an OEI power rating is requested. If a 30-minute OEI power rating is requested, each engine in sequence should be run at the 30-minute OEI condition for 30 minutes. If a continuous OEI power rating is requested, each engine in sequence should be run at the continuous OEI condition for 1 hour. T he total OEI run time may exceed the 1 hour shown in figure AC 29.923A-1.
AC 29.923B. § 29.923 (Amendment 29-31) ROTOR DRIVE SYSTEM AND CONTROL MECHANISM TESTS.
a. Explanation. Amendment 29-31 added § 29.923(p) that defines qualification tests for lubricants used in the rotor drive system and control mechanisms. Section
29.923(p) contains a requirement for a portion of the system qualification tests to be accomplished with specific lubricating oil temperatures and pressures. It also provides requirements for the qualification of additional or alternate lubricants by equivalent testing or by using comparative analysis of lubricant specifications and rotor drive and control system characteristics.
b. Procedures.
(1) When performing the ten-hour endurance test cycles prescribed in § 29.923(a), a minimum of three of the required test cycles should be accomplished with the main transmission and, if applicable, for gearboxes that are used for rotor phasing with the lubricating oil temperature set to the maximum operating temperature for which approval is requested. The oil temperat ure should be measured at the temperature sensor location that is used for flight crew indication of the oil temperature. For main transmission and gearboxes that are pressure lubricated, the testing should include setting the oil pressure to the minimum operating oil pressure for which approval is requested. The oil pressure should be measured at the pressure sensor location that is used for flight crew indication of the oil pressure. If approval of OEI ratings is sought, the ten hour test cycles must be extended to include the OEI endurance testing prescribed in § 29.923(b)(2) and (k).
(2) To be approved for use in the main transmission and gearboxes, lubricants must meet the specifications of lubricants that were used to satisfy the test requirements in § 29.923. Alternate or additional lubricants may be approved provided they are qualified by testing that is considered equivalent to the endurance testing Page E –52
required by § 29.923. A comparative analysis may be used to approve alternate or additional lubricants in lieu of testing, provided the analysis shows that the lubricant properties, specifications, and applications are equal to or better than those of a lubricant that has been previously approved for use in the main transmission or gearbox.
AC 29.923C. § 29.923 (Amendment 29- 34 and 29-40) ROTOR DRIVE SYSTEM AND CONTROL MECHANISM TESTS.
a. Explanation. This paragraph, AC 29.923C, reflects changes made by Amendment 29-34 and 29-40. Amendment 29-34 added § 29.923(b)(3) that defines qualification tests for 30-second/2-minute OEI ratings. This new paragraph also allows for the 30-second/2-minute OEI portion of the endurance test to be accomplished on a representative bench test stand using the drive system components which can be adversely affected by these tests. Amendment 29–40 doubles the endurance test time for 2-minute OEI for each power section.
b. Procedures.
(1) For accomplishment of the endurance test for 30-second/2-minute OEI, § 29.923(b)(3) requires that immediately following one of the 5-minute power on takeoff runs of § 29.923(b)(1) each 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 OEI power. This power level should be maintained for at least 30-seconds. The 30-second OEI power should then be followed by an application of the maximum torque and maximum speed for 2-minute OEI power for at least 2 minutes. After the 2-minute OEI power application the power should be reduced to and stabilized at 30-minute or continuous OEI, (whichever rating the rotorcraft will be certified with). After the power has been stabilized, the maximum torque and maximum speed for use with 2-minute OEI power should be reapplied for at least 2 minutes.
Figure AC 29.923A-1 shows a graphic representation of the ten-hour test cycle with the 30-second/2-minute OEI segment included for each engine presented in figure AC 29.923A-3. This figure shows the OEI test segment being accomplished immediately following a 5 minute takeoff run. The OEI test segment can be accomplished after any of the 5-minute takeoff run segments. Section 29.923(b)(3) also requires that one of the 30-second/2-minute OEI segments for each engine be accomplished from the flight idle condition.
(2) Additionally, due to the damage inflicted on the engines and the ensuing cost caused by operating the engine at these powers, the 30-second/2-minute portion of the endurance test can be accomplished on a bench test rig found to be representative of the rotorcraft. The representative bench test rig should have the ability to generate the torques, speeds, torsional vibration frequency, and engine acceleration rate generated by the actual installation. The pow er should have the same method/path of application as that used on the rotorcraft. T he test rig should be configured with the same components used for conducting the endurance test on the rotorcraft except that Page E – 53
AC 29-2C, Chg 4 5/1/2014 the test components not affected by asymmetric power application may not have to be installed (i.e., if a separate combining gearbox is used it may not be necessary to have the main transmission installed on the bench test rig).
(3) When conducting the bench test for 30-second/2-minute OEI, it is not necessary to reaccomplish the takeoff portion of the endurance test. The simulated power failure and application of 30-second/2-minute OEI power by the remaining power section should be accomplished after the input power has stabilized at takeoff power.
The takeoff portion of the endurance test should be accomplished on the rotorcraft.
Page E –54
AC 29.927. § 29.927 (Amendment 29-13) ADDITIONAL TESTS.
a. Section 29.927(a):
(1) Explanation. This paragraph is aut hority to require any special tests or investigations to establish that the rotor drive system is safe.
(2) Procedures. The certification engineer should review the design of the rotor drive system and its installation and intended operation for features or conditions that may not be adequately qualified in the tests prescribed by this Part. Additional qualification test programs should be developed and accomplished to ensure safe operation of the system. Items of intere st would include poorly defined load paths associated with redundant design features, flight deflections of structure, mounting arrangements which may not be properly qualified by ground tests, and special or unusual operating procedures which are anticipated by the applicant.
b. Section 29.927(b):
(1) Explanation. This paragraph prescri bes testing to qualify the rotor drive system for the power excursions to be expected with governor-controlled engines wherein the engine power changes automatically to maintain rotorspeed at preselected values. At high collective flight control displacements, the normal rotor speed droop will result in governor-controlled engines automatically accelerating to maximum fuel flow or to any other power, speed, temperature, or torque limiting device, regardless of crew action or artificially established limitations reflected by instrument markings. This high power condition can occur typically during a normal landing when the crew applies high collective to cushion ground contact or, for multiengine rotorcraft, during any flight regime when an engine fails and the corresponding loss of power results in drooping the rotor speed. Special tests are prescribed by this section to provide assurance that the rotor drive system can safely sustain these conditions. The tests of this section should be conducted without intervening disassembly, and all rotor drive system components should be in serviceable condition after the test. It is permissible but not required that these tests be performed on the same specimen of the rotor drive system used to show compliance with § 29.923.
(2) Procedures. These tests should be conducted on a ground-test rotorcraft conformed to the type design configuration similar to that required for endurance testing under § 29.923. Cyclic and collective control may be set to simulate vertical lift and antitorque control set and/or adjusted to react to main rotor torque. Rotation speed should be maximum normal for the test condition; i.e., for the all-engines test under § 29.927(b)(1), use the maximum RPM for takeoff power. For the one-engine-inoperative (OEI) test of § 29.927(b)(2), RPM droop, if any, that would occur in service, may be allowed. Since the OE I test of § 29.927(b)(2) usually requires the remaining engine(s) to produce power not usually available under normal atmospheric conditions, some supplemental method, such as refrigerating and/or ramming inlet air, or overfueling the engine, may be required. Alternatively, bench testing (transmission Page E – 54.1