AC 27.903. $ 27.903 (Amendment 27-11) ENGINES.
[See new AC 27.903B (dated 9/17/2009) posted as separate document in RGL with this Master AC.]
a. Engine Type Certification.
(1) Explanation. Section 27.903(a) is intended to ensure that engines used in type certified aircraft are properly qualified and that the associated installation requirements are established.
(2) Procedures.
Compliance can be documented by verification that a type certificate data sheet has been issued by the FAA/AUTHORITY for the engine identified by the rotorcraft manufacturer as the engine planned for use in the rotorcraft. Reciprocating engines must have been qualified to a special test plan (§ 33.49(d)) to be eligible in rotorcraft. This eligibility should be verified by a note on the engine type certificate data sheet.
(i¡) On some occasions, the engine certification program is conducted concurrently with the rotorcraft certification program. This is technically acceptable provided the engine type certificate is issued prior to the rotorcraft type certificate.
However, practical considerations involving the use of unapproved engine installation data and the probability of engine design changes during the engine certification program that impact the rotorcraft certification program dictate that special procedures must be introduced to assure that the final rotorcraft certification program is satisfactory.
If the engine under consideration is merely a minor model change from a previously certificated engine and these changes are unlikely to cause rotorcraft certification problems and do not involve significant installation aspects, the rotorcraft project engineer need only to follow the engine certification program by routine checks with the FAA/AUTHORITY office responsible for engine certification and, as a final pre-type certification item, verify that the engine type certificate has been issued. Rotorcraft Type Board agenda/minutes should reflect the ongoing status of the engine TC program. For rotorcraft certification programs involving new or significantly changed engines, the powerplant certification engineer for the rotorcraft should become as familiar with the engine as practicable with particular attention to engine ratings, limitations, performance, engine/rotorcraft interface aspects, and any Part 27 certification requirement involved in the engine program (fuel/oil filters, fuel heaters, integral firewalls, etc.) and establish an appropriate working arrangement with the FAA/AUTHORITY engine certification office to monitor changes in the engine certification progress which may impact the rotorcraft certification program. In addition, any rotorcraft certification activity such as test plans, analysis, compliance inspections, etc., which involves the engine should be accepted on a conditional basis; i.e., pending confirmation of completion of the engine program without changes pertinent to these aspects of rotorcraft program. The rotorcraft applicant should be advised of any limitations in this procedure, and that normally, the engine certification program should be complete before authorizing formal FAA/AUTHORITY participation in the rotorcraft certification plan; i.e., TIA.
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AC 27-1B, Chg 3 9/30/2008
b. Engine cooling fan protection.
(1) Explanation. Section 27.903(b) is intended to provide safety to the rotorcraft in the event of an assumed cooling fan blade failure or to prescribe a test to show that the cooling fan blade retention means is sufficient that blade failure is not a consideration.
(2) Procedures. The applicant may select § 27.903(b)(1), (b)(2), or (b)(3) to show compliance with this section. If § 27.903(b)(1) is selected, a demonstration should be conducted to show that at the maximum fan speed to be expected, a failed blade is contained within a housing or shroud which is included in the proposed type design and designated by the applicant as the containment shield. The rotational speed required may be related to an overspeed limiting device or to the maximum transient speed to be expected from analysis or test of the system or component which drives the fan. For components driven directly by the engine, output shaft disconnect and the subsequent terminal speed of the engine may set the test condition. To conduct an overspeed blade failure containment demonstration, applicants have found it convenient to progressively weaken a blade to induce failure at or above the required demonstration speed. Blade failure may be expected to subsequently fail 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 a controlled landing.
(3) Section 27.903(b)(2) may be selected; however, without containment, damage to any component or structure in the plane of the fan rotor or any other trajectory to be expected should not cause the loss of any function essential to a controlled landing.
[The remainder of this page intentionally left blank.] [Section AC 27.903 continued on the next page.
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(4) If § 27.903(b)(3) is selected, a spin test at 122.5 percent of the maximum speed associated with either engine terminal speed or an overspeed limiting device would be acceptable to show compliance. No failure should occur and distortion should not result in fan element contact with housings or other adjacent components. (Note:
150 percent of the centrifugal force is achieved at 122.5 percent of the rotational speed.)
c. Turbine Engine Installation.
(1) Explanation. The certification of turbine engines and particularly, the qualification of turbine rotors, assumes that the limitations established during these certifications will be accurately and rigorously observed during ground and flight operations in an aircraft. This paragraph is intended to promote this concept.
(2) Procedures. Primary engine limitations in the form of time, gas temperature, torque, and rotational speed and their corresponding allowable transient values are defined in the approved engine installation manual. The rotorcraft manufacturer must provide reliable, accurate means to assure that these limitations are not exceeded. These means may be in the form of automatic limiters or by crew monitoring of appropriately marked instruments. The FAA/AUTHORITY powerplant certification engineer and the rotorcraft manufacturer's staff should verify these aspects by:
Evaluating all applicable instrument, indicator, or warning devices, including transmitters, and limiting devices, if any, for system tolerances.
Closely reviewing the component qualification reports of items in 398c(2)(i) above to verify that these devices are properly qualified and that any deviations are acceptable.
(iii) Assuring that maintenance data is provided for functional checks and calibration of instruments and devices which are used to monitor or protect critical turbine rotor limitations. Preflight checks for automatic limiter devices may be appropriate.
(iv) Verifying that instrument markings are clear and relatively simple, that corresponding flight manual instructions and descriptions are straightforward and complete, and instruments are located and orientated to minimize the probability of reading error.
AC 27.903A. $27.903 (Amendment 27-23) ENGINES.
a. Explanation. Amendment 27-23 adds a requirement to § 27.903(a) that requires reciprocating engines used in rotorcraft to be certified in accordance with the special rotorcraft engine test requirements in § 33.49(d). This change was needed to Page E - 7
ensure that certification requirements are not overlooked when reciprocating engines are installed in rotorcraft to be certified under Part 27 requirements. Section (b) was revised to prescribe tests and qualifications for powerplant area cooling fans. This rule 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 failure 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.
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(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.
(iii) For § 27.903(b)(1) iii) 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.
AC 27.903В. §27.903 (Amendment 27-44) ENGINES.
a. Explanation. Amendment 27-44, § 27.903(d) requires that any engine must have a restart capability that has been demonstrated throughout a flight envelope to be certificated for the rotorcraft.
b. Procedures.
and landing maximum altitude and temperature limits. Compliance is usually shown by conducting actual in-flight restarts during flight tests or other tests in accordance with an approved test plan. However, § 27.903(d)(1) does not require in-flight demonstration of restart capability for single-engine rotorcraft or for all-engine shutdown of multi-engine rotorcraft. In the past, engine relight capability for single engine rotorcraft has been demonstrated on the ground taking into account altitude effects, warm engine characteristics, depleted battery, etc. Restarts should be conducted at various altitudes, ambient temperatures, and fuel temperatures using the fuel type most critical, unless the applicant can show that this parameter is not pertinent. Other concerns involve the pilot station arrangement for flight controls and engine starting controls. It should be verified that the engine start can be accomplished without jeopardizing continued safe operation of the rotorcraft. Pilot workload for a preexisting one engine inoperative (OEI) situation, the location of the restart system controls, and the availability of a second pilot should be considered. The emergency and malfunction instruction sections of the rotorcraft flight manual (RFM) should present a detailed definition of the approved restart envelope and detailed instructions for the restart.
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AC 27-1B, Chg 4 3/31/2014 Eligible ambient atmospheric conditions, prestart requirements (to allow for waste fuel drainage), starter duty cycle (if different from the ground start duty cycle), and prestart situation analysis should be included. The prestart situation analysis should consider the following questions:
(i) Should a restart be attempted in view of the cause for initial shutdown?
(ii) Is inlet system ice ingestion a possibility?
(ili) Is re-ignition of fuel in the engine nacelle a possibility?
(iv) Is sufficient restart time available?
(v) Is power available?
(vi) Is altitude sufficient to maintain terrain clearance?
The restart capability can consider wind milling of the engine as part of this restart capability; however, most rotorcraft airspeeds and the locations of the engines do not support engine wind milling up to start speeds. Only electrical power requirements were considered for restarting; however, other factors that may affect this capability are permitted to be considered. Engine restart capability following an in-flight shutdown of all engines is the primary requirement, and the means of providing this capability is left to the applicant.
To minimize any potential height loss, engine restart should be available at the earliest opportunity. The engine certification should be checked to ensure that the flight manual instructions for in-flight restart are consistent with any specific engine restart requirements.
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AC 27-1B, Chg 9 6/23/23 AC 27.903C. § 27.903 (AMENDMENT 27-51) ENGINES.
a. Explanation. Amendment 27-51 changed the format of § 27.903(d) to match the format of § 29.903(e), which is the same requirement as § 27.903(d). The substantive requirements of § 27.903(d) for restart capability have not changed as a result of this reformatting.
b. Procedures. The policy pertaining to the procedures in this section remains unchanged.
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AC 27.907. § 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. AC 27 MG 11, 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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AC 27-1B, Chg 4 3/31/2014
SUBPART E - POWERPLANT
ROTOR DRIVE SYSTEM AC 27.917. § 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 performance 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 rotors).
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 no Page E - 10.2
compromise the interconnect mechanism between the main and auxiliary rotors of the rotorcraft.
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.
AC 27.921. $ 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 Page E - 11
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 § 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.
AC 27.923. § 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 equivalent level of safety finding can be made for the remaining requirements based on the previously approved data.
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