CS 27.1503  Airspeed limitations: general

ED Decision 2003/15/RM

(a) An operating speed range must be established.

(b) When airspeed limitations are a function of weight, weight distribution, altitude, rotor speed, power, or other factors, airspeed limitations corresponding with the critical combinations of these factors must be established.

CS 27.1505 Never-exceed speed

ED Decision 2023/001/R

(a) The never-exceed speed, V_NE, must be established so that it is:

(1) Not less than 74 km/h (40 knots) (CAS); and

(2) Not more than the lesser of:

(i) 0.9 times the maximum forward speeds established under CS 27.309;

(ii) 0.9 times the maximum speed shown under CS 27.251 and 27.629; or

(iii) 0.9 times the maximum speed substantiated for advancing blade tip mach number effects.

(b) V_NE may vary with altitude, rpm, temperature, and weight, if:

(1) No more than two of these variables (or no more than two instruments integrating more than one of these variables) are used at one time; and

(2) The ranges of these variables (or of the indications on instruments integrating more than one of these variables) are large enough to allow an operationally practical and safe variation of V_NE.

(c) For helicopters, a stabilised power-off V_NE denoted as V_NE (power-off) may be established at a speed less than V_NE established pursuant to sub-paragraph (a), if the following conditions are met:

(1) V_NE (power-off) is not less than a speed midway between the power-on V_NE and the speed used in meeting the requirements of:

(i) CS 27.65(b) for single-engine helicopters; and

(ii) CS 27.67 for multi-engine helicopters.

(2) Unless it is automatically displayed to the crew, the V_NE (power-off) is –

(i) A constant airspeed; or

(ii) A constant amount less than power-on V_NE; or

(iii) A constant airspeed for a portion of the altitude range for which certification is requested, and a constant amount less than power-on V_NE for the remainder of the altitude range.

AMC1 27.1505 Never-exceed speed

ED Decision 2023/001/R

This AMC replaces FAA AC27-1B, § AC 27.1505 and should be used when demonstrating compliance with CS 27.1505.

(a) Explanation

(1) General

CS 27.1505 requires the never-exceed speed (V_NE) for both Power-ON and Power-OFF flight to be established as operating limitations. The rule specifies how to establish and substantiate these limits.

(2) Power-ON limit

(i) All engines operative (AEO)

(A) The all-engines-operating V_NE is established by design and substantiated by flight tests. The V_NE limit is the most conservative value that demonstrates compliance with the structural requirements (CS 27.309), the manoeuvrability and controllability requirements (CS 27.143), the stability requirements (CS 27.173 and CS 27.175), or the vibration requirements (CS 27.251). The Power-ON V_NE will normally decrease as density altitude or weight increases. A variation in rotor speed may also require a variation in the V_NE. The regulation restricts to two the number of variables that are used to determine the V_NE at any given time so that a single pilot can readily ascertain the correct V_NE for the flight condition with a minimum of mental effort. Helicopter manufacturers have typically presented never-exceed-speed limitation data as a function of pressure altitude and temperature. This information was placarded as well as contained in the flight manual. As the weight of some derivative models was increased, EASA and the FAA accepted altitude/temperature/V_NE limitations that were categorised or contained within a weight range. Literal compliance with the regulation then required that the take-off weight be calculated and then the indicated, appropriate airspeed limitation chart or placard be used for the entire flight. However, V_NE charts or placards based on longitudinal centre of gravity have been found to be unacceptable, since the same chart would potentially not be used throughout the flight and the pilot would thus be dealing with more than two variables to determine the V_NE. Alternatively, rotorcraft that are equipped with modern avionics systems may be able to automatically calculate and display the V_NE in an unambiguous manner as a function of the different parameters upon which it depends. For these designs, the applicant is expected to appropriately address the criticality associated with the loss and misleading presentation of the V_NE when compliance of such systems with 27.1309 is assessed. These rotorcraft should also have a method for determining the V_NE that complies with the regulation for all failure conditions or combinations of failure conditions that are not extremely improbable. This method is usually more conservative than the automatic system because of the limitation in the number of parameters that can be varied. A placard may be used or appropriate RFM instructions.

(B) To ensure compliance with the structural requirements (CS 27.309), vibration requirements (CS 27.251), and flutter requirements (CS 27.629), the all-engines-operating V_NE should be restricted so that the maximum demonstrated main rotor tip Mach number will not be exceeded at 1.11 V_NE for any approved combination of altitude and ambient temperature. Previous rotorcraft cold weather tests have shown that the rotor system may exhibit several undesirable and possibly hazardous characteristics due to compressibility effects at high advancing blade tip Mach numbers. As the centre of pressure of the advancing rotor blade moves aft near the blade tip due to the formation of localised upper surface shock waves, rotor system loads may increase, the rotor system may exhibit an aerodynamic instability such as rotor weave, rotorcraft vibration may increase substantially, and rotorcraft static or dynamic stability may be adversely affected. Which, if any, of these adverse characteristics are exhibited at high rotor tip Mach numbers is dependent on the design of each particular rotor system. EASA and the FAA experience with high advancing blade tip Mach number has shown that different types of rotor systems (articulated, semi-rigid, rigid, etc.) have various adverse characteristics. Therefore, it has been EASA and the FAA policy to establish V_NE so that it is not more than 0.9 times the maximum speed substantiated for advancing blade tip Mach number effects for the critical combination of altitude, approved Power-ON rotor speed, and ambient temperature conditions. This policy was incorporated as a specific regulatory requirement with Amendment 27-21 to § 27.1505. High main rotor tip Mach numbers obtained power off at higher-than-normal main rotor rotational speeds should not be used to establish the maximum Power-ON tip Mach number V_NE limit. In addition, since the onset of adverse conditions associated with high tip Mach numbers can occur with little or no warning and amplify very rapidly, no extrapolation of the maximum demonstrated main rotor tip Mach number V_NE limitation should be allowed.

(C) A maximum speed for use of power in excess of maximum continuous power (MCP) should be established unless structural requirements have been substantiated for the use of take-off power (TOP) at the maximum approved V_NE airspeed. TOP is intended for use during take-off and climb for not more than 5 minutes at relatively low airspeeds. However, EASA and the FAA experience has shown that pilots will not hesitate to use TOP at much higher than best-rate-of-climb airspeeds unless a specific limitation against TOP use above a specified airspeed is included in the RFM. Structural and fatigue substantiations have not normally included loads associated with the use of TOP at V_NE. Thus, a TOP airspeed limitation should be established from the structural substantiation data to preclude the accumulation of damaging rotor system and control mechanism loads through intentional use of the TOP rating at high airspeeds.

(ii) One engine inoperative (OEI)

A one-engine-inoperative (OEI) V_NE is generally established through flight test and is usually near the V_H or V_NE of the rotorcraft. It is the highest speed at which the failure of the remaining engine must be demonstrated. For rotorcraft with more than two engines, the appropriate designation would be ‘one-engine-operating’ V_NE and would be that speed at which the last remaining engine could be failed with satisfactory handling qualities. It is possible, although believed improbable, that a rotorcraft with more than two engines could have different V_NE depending upon the number of engines still operating. It is recommended that the OEI V_NE not be significantly lower than the OEI best range airspeed. A multiengine rotorcraft may require an OEI V_NE if the handling qualities following the last remaining engine failure are not satisfactory or if the rotor speed decays below the Power-OFF transient limits at the all-engine-operating V_NE.

(3) Power-OFF limits

(i) A Power-OFF V_NE may be established either by design or flight test and should be substantiated by flight tests. A Power-OFF V_NE is generally required if the handling qualities or stability characteristics at high speed in autorotation are not acceptable. A limitation of the Power-OFF V_NE may also be used if the rotorcraft has undesirable or objectionable flying qualities, such as large lateral-directional oscillations, at high autorotational airspeeds. The Power-OFF V_NE must meet the same criteria for control margins as the Power-ON V_NE. The regulation requires that the Power-OFF V_NE be no less than the speed midway between the Power-ON V_NE and the speed used to comply with the rate of climb requirements for the rotorcraft. When the regulation was written, rotorcraft V_NE speeds were significantly lower than those of recently certificated rotorcraft. The high V_NE speeds of current rotorcraft result in relatively high values for Power-OFF V_NE. Speeds lower than those specified in the regulation have been found acceptable through a finding of equivalent safety if the selected Power-OFF V_NE is equal to or greater than the Power-OFF speed for best range. In any case, the Power-OFF V_NE must be a high enough speed to be practical. A demonstration is required of the deceleration from the Power-ON V_NE or OEI V_NE to the Power-OFF V_NE. The transition must be made in a controlled manner with normal pilot reaction and skill.

(ii) In addition to the minimum speed requirements for Power-OFF V_NE, the rule restricts the manner in which Power-OFF V_NE can be specified when it is not automatically calculated and displayed to the crew. To reduce the crew workload, in all the cases where the Power-OFF V_NE is not automatically calculated and displayed, the Power-OFF V_NE may be a constant airspeed which is less than Power-ON V_NE for all approved ambient conditions/gross weight combinations; a series of airspeeds varying with altitude, temperature or gross weight that is always a constant amount less than the Power-ON V_NE for the same ambient condition/gross weight combination; or some combination of a constant airspeed for a portion of the approved altitude range and a constant amount less than Power-ON V_NE for the remainder of the approved altitude range.

(b) Procedures

The tests to substantiate the different V_NE speeds are ordinarily conducted during the flight characteristics flight tests. The flight test procedures are discussed for the various limiting areas in earlier paragraphs of this document. Static stability test techniques are covered in § AC 27.175 and the vibration test techniques in § AC 27.251.

[Amdt 27/10]

CS 27.1509  Rotor speed

ED Decision 2003/15/RM

(a) Maximum power-off (autorotation). The maximum power-off rotor speed must be established so that it does not exceed 95% of the lesser of –

(1) The maximum design rpm determined under CS 27.309(b); and

(2) The maximum rpm shown during the type tests.

(b) Minimum power-off. The minimum power-off rotor speed must be established so that it is not less than 105% of the greater of:

(1) The minimum shown during the type tests; and

(2) The minimum determined by design substantiation.

(c) Minimum power-on. The minimum power-on rotor speed must be established so that it is:

(1) Not less than the greater of:

(i) The minimum shown during the type tests; and

(ii) The minimum determined by design substantiation; and

(2) Not more than a value determined under CS 27.33(a)(1) and (b)(1).

CS 27.1519  Weight and centre of gravity

ED Decision 2003/15/RM

The weight and centre of gravity limitations determined under CS 27.25 and 27.27, respectively, must be established as operating limitations.

CS 27.1521 Powerplant limitations

ED Decision 2012/021/R

(a) General. The powerplant limitations prescribed in this paragraph must be established so that they do not exceed the corresponding limits for which the engines are type certificated.

(b) Take-off operation. The powerplant take-off operation must be limited by:

(1) The maximum rotational speed, which may not be greater than:

(i) The maximum value determined by the rotor design; or

(ii) The maximum value shown during the type tests;

(2) The maximum allowable manifold pressure (for reciprocating engines);

(3) The time limit for the use of the power corresponding to the limitations established in sub-paragraphs (b)(1) and (2);

(4) If the time limit in sub-paragraph (b)(3) exceeds 2 minutes, the maximum allowable cylinder head, coolant outlet, or oil temperatures;

(5) The gas temperature limits for turbine engines over the range of operating and atmospheric conditions for which certification is requested.

(c) Continuous operation. The continuous operation must be limited by:

(1) The maximum rotational speed which may not be greater than:

(i) The maximum value determined by the rotor design; or

(ii) The maximum value shown during the type tests;

(2) The minimum rotational speed shown under the rotor speed requirements in CS 27.1509(c); and

(3) The gas temperature limits for turbine engines over the range of operating and atmospheric conditions for which certification is requested.

(d) Fuel grade or designation. The minimum fuel grade (for reciprocating engines), or fuel designation (for turbine engines), must be established so that it is not less than that required for operation of the engines within the limitations in sub-paragraphs (b) and (c).

(e) Turboshaft engine torque. For rotorcraft with main rotors driven by turboshaft engines, and that do not have a torque limiting device in the transmission system, the following apply:

(1) A limit engine torque must be established if the maximum torque that the engine can exert is greater than:

(i) The torque that the rotor drive system is designed to transmit; or

(ii) The torque that the main rotor assembly is designed to withstand in showing compliance with CS 27.547(d).

(2) The limit engine torque established under sub-paragraph (e)(1) may not exceed either torque specified in sub-paragraph (e)(1)(i) or (ii).

(f) Ambient temperature. For turbine engines, ambient temperature limitations (including limitations for winterization installations, if applicable) must be established as the maximum ambient atmospheric temperature at which compliance with the cooling provisions of CS 27.1041 to 27.1045 is shown.

(g) Two and one-half minute OEI power operation. Unless otherwise authorised, the use of 2½-minute OEI power must be limited to engine failure operation of multi-engine, turbine-powered rotorcraft for not longer that 2½ minutes after failure of an engine. The use of 2½-minute OEI power must also be limited by:

(1) The maximum rotational speed, which may not be greater than:

(i) The maximum value determined by the rotor design; or

(ii) The maximum demonstrated during the type tests;

(2) The maximum allowable gas temperature; and

(3) The maximum allowable torque.

(h) Thirty-minute OEI power operation.

Unless otherwise authorised, the use of 30-minute OEI power must be limited to multi-engine, turbine-powered rotorcraft for not longer than 30 minutes after failure of an engine. The use of 30-minute OEI power must also be limited by:

(1) The maximum rotational speed which may not be greater than –

(i) The maximum value determined by the rotor design; or

(ii)  The maximum value demonstrated during the type tests;

(2) The maximum allowable gas temperature; and

(3) The maximum allowable torque.

(i) Continuous OEI power operation. Unless otherwise authorised, the use of continuous OEI power must be limited to multi-engine, turbine-powered rotorcraft for continued flight after failure of an engine. The use of continuous OEI power must also be limited by:

(1) The maximum rotational speed, which may not be greater than –

(i) The maximum value determined by the rotor design; or

(ii) The maximum value demonstrated during the type tests;

(2) The maximum allowable gas temperature; and

(3) The maximum allowable torque.

(j) Rated 30-second OEI power operation. Rated 30-second OEI power is permitted only on multi-engine, turbine-powered rotorcraft, also certificated for the use of rated 2-minute OEI power, and can only be used for continued operation of the remaining engine(s) after a failure or precautionary shutdown of an engine. It must be shown that following application of 30-second OEI power, any damage will be readily detectable by the applicable inspections and other related procedures furnished in accordance with paragraph A27.4 of Appendix A of this CS-27. The use of 30-second OEI power must be limited to not more than 30 seconds for any period in which that power is used, and by:

(1) The maximum rotational speed which may not be greater than:

(i) The maximum value determined by the rotor design: or

(ii) The maximum value demonstrated during the type tests:

(2) The maximum allowable gas temperature; and

(3) The maximum allowable torque.

(k) Rated 2-minute OEI power operation. Rated 2-minute OEI power is permitted only on multi-engine, turbine-powered rotorcraft, also certificated for the use of rated 30-second OEI power, and can only be used for continued operation of the remaining engine(s) after a failure or precautionary shutdown of an engine. It must be shown that following application of 2-minute OEI power, any damage will be readily detectable by the applicable inspections and other related procedures furnished in accordance with A27.4 of appendix A of this CS-27. The use of 2-minute OEI power must be limited to not more than 2 minutes for any period in which that power is used, and by:

(1) The maximum rotational speed, which may not be greater than:

(i) The maximum value determined by the rotor design; or

(ii) The maximum value demonstrated during the type tests;

(2) The maximum allowable gas temperature; and

(3) The maximum allowable torque.

[Amdt 27/3]

AMC1 27.1521 Powerplant limitations

ED Decision 2023/001/R

(a) Introduction

This AMC supplements FAA AC 27-1B, § AC 27.1521 and should be used in conjunction with that AC when demonstrating compliance with CS 27.1521.

(b) 30-minute power rating

(1) Explanation

The 30-minute power rating may be set at any level between the maximum continuous up to and including the take-off rating, and may be used for multiple periods of up to 30 minutes each, at any time between the take-off and landing phases in any flight.

This rating is associated with some limitations which should be adequately established and declared.

(2) Procedure

CS 27.1521(a) refers to the limits for which the engines are type certificated. This should include the 30-minute power rating usage and:

—              the associated usage limit:

—              maximum duration in one single shot up to 30 minutes;

—              cumulative limit, if any, in one flight; and

—              any other limits associated with the usage of the 30-minute power rating declared in the installation and/or operating manual of the engine.

[Amdt 27/10]

CS 27.1523  Minimum flight crew

ED Decision 2003/15/RM

The minimum flight crew must be established so that it is sufficient for safe operation, considering:

(a) The workload on individual crew members;

(b) The accessibility and ease of operation of necessary controls by the appropriate crew member; and

(c) The kinds of operation authorised under CS 27.1525.

CS 27.1525  Kinds of operations

ED Decision 2003/15/RM

The kinds of operations (such as VFR, IFR, day, night, or icing) for which the rotorcraft is approved are established by demonstrated compliance with the applicable certification requirements and by the installed equipment.

CS 27.1527  Maximum operating altitude

ED Decision 2003/15/RM

The maximum altitude up to which operation is allowed, as limited by flight, structural, powerplant, functional, or equipment characteristics, must be established.

CS 27.1529  Instructions for Continued Airworthiness

ED Decision 2003/15/RM

Instructions for Continued Airworthiness in accordance with Appendix A must be prepared.

Appendix A – Instructions for Continued Airworthiness

ED Decision 2020/006/R

A27.1 General

(a) This appendix specifies requirements for the preparation of instructions for continued airworthiness as required by CS 27.1529.

(b) The instructions for continued airworthiness for each rotorcraft must include the instructions for continued airworthiness for each engine and rotor (hereinafter designated ‘products’), for each appliance required by any applicable CS or operating rule, and any required information relating to the interface of those appliances and products with the rotorcraft. If instructions for continued airworthiness are not supplied by the manufacturer of an appliance or product installed in the rotorcraft the instructions for continued airworthiness for the rotorcraft must include the information essential to the continued airworthiness of the rotorcraft.

A27.2 Format

(a) The instructions for continued airworthiness must be in the form of a manual or manuals as appropriate for the quantity of data to be provided.

(b) The format of the manual or manuals must provide for a practical arrangement.

A27.3 Content

The contents of the manual or manuals must be prepared in a language acceptable to the Agency. The instructions for continued airworthiness must contain the following manuals or sections, as appropriate, and information:

(a) Rotorcraft maintenance manual or section

(1) Introduction information that includes an explanation of the rotorcraft’s features and data to the extent necessary for maintenance or preventive maintenance.

(2) A description of the rotorcraft and its systems and installations including its engines, rotors, and appliances.

(3) Basic control and operation information describing how the rotorcraft components and systems are controlled and how they operate, including any special procedures and limitations that apply.

(4) Servicing information that covers details regarding servicing points, capacities of tanks, reservoirs, types of fluids to be used, pressures applicable to the various systems, location of access panels for inspection and servicing, locations of lubrication points, the lubricants to be used, equipment required for servicing, tow instructions and limitations, mooring, jacking, and levelling information.

(b) Maintenance instructions

(1) Scheduling information for each part of the rotorcraft and its engines, auxiliary power units, rotors, accessories, instruments and equipment that provides the recommended periods at which they should be cleaned, inspected, adjusted, tested, and lubricated, and the degree of inspection, the applicable wear tolerances, and work recommended at these periods. However, it is allowed to refer to an accessory, instrument, or equipment manufacturer as the source of this information if it is shown that the item has an exceptionally high degree of complexity requiring specialised maintenance techniques, test equipment, or expertise. The recommended overhaul periods and necessary cross references to the Airworthiness Limitations section of the manual must also be included. In addition an inspection program that includes the frequency and extent of the inspections necessary to provide for the continued airworthiness of the rotorcraft must be included.

(2) Troubleshooting information describing probable malfunctions, how to recognise those malfunctions, and the remedial action for those malfunctions.

(3) Information describing the order and method of removing and replacing products and parts with any necessary precautions to be taken.

(4) Other general procedural instructions including procedures for system testing during ground running, symmetry checks, weighing and determining the centre of gravity, lifting and shoring, and storage limitations.

(c) Diagrams of structural access plates and information needed to gain access for inspections when access plates are not provided.

(d) Details for the application of special inspection techniques including radiographic and ultrasonic testing where such processes are specified.

(e) Information needed to apply protective treatments to the structure after inspection.

(f) All data relative to structural fasteners such as identification, discard recommendations, and torque values.

(g) A list of special tools needed.

A27.4 Airworthiness Limitations Section

The instructions for continued airworthiness must contain a section titled airworthiness limitations, that is segregated and clearly distinguishable from the rest of the document. This section must set forth each mandatory replacement time, structural inspection interval, and related structural inspection procedure required for type-certification. If the instructions for continued airworthiness consist of multiple documents, the section required by this paragraph must be included in the principal manual. This section must contain a legible statement in a prominent location that reads: ‘the airworthiness limitations section is approved and variations must also be approved.’

A.27.5 Information system security Instructions for Continued Airworthiness

The applicant must prepare Instructions for Continued Airworthiness (ICA) that are applicable to aircraft information system security protection as required by CS 27.1319 (see AMC 20-42 Section 9).

[Amdt No: 27/2]

[Amdt No: 27/3]

[Amdt No: 27/7]

AMC1 27.1529 Instructions for continued airworthiness

ED Decision 2023/001/R

(a) Introduction

This AMC supplements FAA AC 27-1B, § AC 27.1529 and should be used in conjunction with that AC when demonstrating compliance with CS 27.1529.

(b) Abnormal events

The ICA should include instructions that ensure that operators conduct appropriate inspections or other actions following abnormal events in operation, maintenance or during transportation of components.

Abnormal events that should be considered include hard landings, severe gust encounters, lightning strike, exposure to high winds when parked and dropping components during maintenance or transport.

The instructions should consider the nature of the components, including but not limited to critical parts, and in particular the possibility of damage that can occur during impact or overload events that may not be detectable but could subsequently lead to premature failure in operation. In such cases, scrapping the component or parts of it may be the only appropriate action to take.

(c) Time between overhaul (TBO) development

(1) Explanation

The purpose of this AMC is to provide guidance for establishing an appropriate TBO for rotorcraft drive system gearboxes at type certificate approval and to increase it during the service life of the product.

A rotorcraft rotor drive system gearbox is usually a complex assembly composed of many parts of which a significant proportion can be critical parts. Many are rotating parts which are subject to high torque and fatigue loads, such as bearings, shafts, gears, and free wheels with the primary function of transmitting power from the engine to the rotors. Non-rotating components have other functions such as support, lubrication, load transfer or condition monitoring.

Most gearbox components are enclosed inside the housings, which prevents the possibility of detailed maintenance inspections without disassembly. As a result, to ensure that the internal gearbox components remain in serviceable condition, periodic overhauls of the assembly are typically scheduled. Overhaul allows an in-depth and periodic inspection of gearbox components, controlling and limiting the development of degradation and build-up of debris, as well as checking for cracks and other damages that may be developing. In addition, the inspection findings can determine whether parts are sufficiently protected and whether they remain in serviceable condition. In summary, the overhaul of the gearbox is intended to verify the condition of its elements, restore them to a serviceable condition or replace them where needed, and ensure that the gearbox will be safe for operation until the following overhaul. The TBO is the periodic interval between two overhauls and is traditionally defined in flight hours and calendar time.

During the type-certification process, rotorcraft drive system gearbox components are subject to various forms of analyses and tests, which assess their criticality, integrity and reliability. These assessments rely on a number of assumptions regarding the condition of the components during their service life and have an impact on aspects such as contact conditions between elements, fretting, wear, loads and environmental deterioration. The applicant should consider that the continued validity of these assumptions is typically linked to an appropriate TBO. As a result, the validation of these assumptions and the development of the TBO are processes that should be progressed in parallel after entry into service (EIS).

The final and mature TBO should normally be based on the results of investigations from in-service aircraft, overhauled gearboxes and data acquired during development, certification, and maturity tests substantiating the reliability of the parts and their capability to operate safely. However, until this data becomes available, the applicant should maintain a conservative TBO, extending it throughout the life of the product as positive supporting data from service becomes available.

(2) Guidance

For drive system gearboxes that are essential to drive the rotors, EASA considers that the initial TBO at EIS and the plan to increase it in service should be justified. For this purpose, the following should be considered by the applicant:

—              Initial TBO (applicable at EIS)

At EIS, the available data supporting the justification of the TBO of a rotor drive system gearbox is typically limited. The applicant should, therefore, propose a conservative initial TBO supported by the data coming from:

—              the endurance test,

—              flight tests,

—              other relevant tests, and

—              experience on similar design having the same characteristics.

The applicant should take into account that, in general, only limited experience of the real operating environment and conditions for a new gearbox is available at EIS.

This initial TBO should ensure enough opportunities to verify the condition of internal gearbox components in order to validate the assumptions made at the time of certification, preventing that any compromised assumption may lead to an in-service catastrophic or hazardous failure.

—              TBO step increase

The increase of a gearbox TBO in service should be accomplished in steps providing confidence progressively in the validity of the certification assumptions. Each TBO step increase should:

—              only be proposed when the current TBO is supported by a sufficient number of gearbox overhaul inspection results;

—              be based on a sufficient number of gearboxes from the fleet to be inspected, and take into account the representativeness of operational and environmental aspects of the selected samples to represent the full spectrum of gearbox usage;

—              be based on technical justifications from overhauled gearboxes (e.g. condition of inspected parts, evidence from similar designs, etc.), maturity testing and in-service feedback (incidents, health and usage monitoring system (HUMS) data, etc.); and

—              be completed prior to formally increasing the TBO to verify acceptable behaviour and condition of the gearbox components prior to starting a new increase phase.

—             Management of TBO steps

The process for managing the evolution of the TBO of drive system gearboxes should be documented in a TBO maturity plan. This should include:

—              planned increase steps and target TBO, technical criteria for the validation of the steps planned and justification of the proposed plan (see note 1);

—              definition of the number of gearboxes and selection criteria considering operation and environment (see note 1);

—              definition of responsible parties for performing the TBO step increase validation inspections, activities involved and information to be reported;

—              proposed analysis process of the inspection results, responsible parties and methods of analysis; and

—              the TBO step increase validation process and associated deliverables (see note 2).

Any findings arising from the TBO development process which might bring into question the suitability of the current TBO or impair the capability of the gearbox to reach the planned increase in TBO should be reported to the Agency.

Finally, if a major change is introduced to or affecting a drive system gearbox, the applicant should evaluate the need to revise the TBO and incorporate additional steps in the gearbox TBO maturity plan.

Note 1: The TBO maturity plan and the associated TBO increase validation criteria should be defined by the applicant and provided to the Agency during the certification process. The results of the process of validation of each step might lead to revisions of the maturity plan.

Note 2: The acceptance of each individual step as well as the closure of the maturity plan should be formally endorsed by the applicant and duly documented.

[Amdt 27/10]