Consultation expired with comments. The proposed Equivalent Safety Finding (ESF) has been updated based on the comments received, and the corresponding Comment-Response Document (CRD) has been completed by EASA.
Details
Identification of issue
EASA has received requests for an Equivalent Safety Finding to CS 29.923(a)(2) and CS 29.923(a)(3)(ii) for changes to type-certificates affecting the rotor drive system and/or rotor control mechanism of large rotorcraft. In addition, EASA has received from GAMA the Industry White Paper on Drive System Endurance testing (GAMA 22-49), requesting the approach described in this ESF to be considered as acceptable to meet CS 29.923.
The endurance tests prescribed in CS 29.923 require testing each rotor drive system and rotor control mechanism with the aim of demonstrating that they are capable of normal operation within the limitations proposed, without hazard of failure from excessive wear or deterioration due to mechanical loads.
In particular, the following requirements in CS 29.923 ‘Rotor drive system and control mechanism tests’ are applicable:
- CS 29.923(a) states: "[…] each rotor drive system and rotor control mechanism must be tested, as prescribed in sub-paragraphs […]”
- CS 29.923(a)(2) states: "The tests must be conducted on the rotorcraft."
- CS 29.923(a)(3)(ii) states: [The test torque and rotational speed must be] "Absorbed by the rotors to be approved for the rotorcraft."
Therefore, these tests shall be performed on the rotorcraft, with the rotorcraft engines and the rotors respectively providing and absorbing power.
In this context, the parts constituting each rotor drive system and rotor control mechanism should be considered as parts under test.
FAA AC 29.923A, which is accepted by EASA as means of compliance with CS 29.923 when complented by EASA AMC1 29.923 for applications including a 30-min power rating, states:
“This section[29.923(a)] also requires the test to be conducted ‘on the rotorcraft’. This means a rotorcraft that is in conformity to the 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.”
In the context of AC 29.923A, the applicant may propose nonconforming features in the test, limited to elements of the rotorcraft type design that would not impact the test conditions for the parts under test. In these cases, a direct compliance demonstration to CS 29.923 is expected without an ESF.
However, for changes to an existing rotor drive system and control mechanism, direct compliance with CS 29.923 would require having a complete rotorcraft in conformity to the type design available to perform the endurance tests. Typically, several changes are planned and introduced along the life of a type and in some cases, such availability may not be possible. In such cases, the applicant may propose to perform the endurance tests using laboratory test rigs (e.g. gearbox test rigs).
EASA considers that using laboratory test rigs as endurance test means cannot be simply considered as a non-conforming feature of the prescribed rotorcraft and must be considered as an alternative test means. This alternative test means should be adequately demonstrated not to impact the test results to substantiate an Equivalent Level of Safety to CS 29.923(a)(2) and CS 29.923(a)(3)(ii).
