Official comments to the proposed consultation paper are to be filed through the EASA Comment-Response Tool (CRT).
Details
Identification of issue
-
Purpose and scope
The purpose of this Certification Memorandum is to provide specific guidance for CS-27 or CS-29 rotorcraft for the certification of hybrid bearings in the absence of a dedicated requirement and guidance material.
The scope of this Certification Memorandum is limited to hybrid bearings involved in CS-27 and CS-29 rotorcraft drive systems and rotor control mechanisms.
The scope is also limited to hybrid bearings rolling elements made of Silicon Nitride (Si3N4), which is a type of ceramic, as this is the only material that is currently used and studied by the industry for the aforementioned systems.
-
Background
The rotorcraft industry shows an increased interest of introducing hybrid bearings in rotorcraft rotor drive systems and/or rotor control mechanisms. In particular, the use of hybrid bearings in lieu of conventional bearings is expected to result in longer life, higher performance under degraded lubrication conditions, corrosion resistance, improved dynamic behaviour, and lower mass (which can be particularly advantageous under high-speed conditions).
Hybrid bearings are currently used on rotorcrafts on a limited number of applications within the drive system and rotor control mechanisms. These applications mainly include:
- Drive shaft hanger ball bearing and roller bearing (greased) on tail drive lines,
- Pre-loaded duplex ball bearing (greased) on main rotor swashplate,
- Pre-loaded duplex ball bearing (greased) on tail rotor control system.
It is also important to note that a hybrid bearing was involved in a fatal accident of a large rotorcraft for which Aircraft Accident Report AAR 1/2023 was published. Although the accident report associated with this event does not provide specific safety recommendations for the use of hybrid bearings, the following safety recommendations are considered by EASA in the “EASA Certification Policy” of this CM:
- Safety Recommendation 2023-020: It is recommended that the European Union Aviation Safety Agency amend Certification Specification 29.602 to define the airworthiness status of life limits on non-structural critical parts and how they should be controlled in service.
- Safety Recommendation 2023-025: It is recommended that the European Union Aviation Safety Agency amend the relevant requirements of Certification Specification 29 and their Acceptable Means of Compliance to emphasise that, where potentially catastrophic failure modes are identified, rather than rely solely on statistical analysis to address the risk, the wider system should also be reviewed for practical mitigation options, such as early warning systems and failure tolerant design, in order to mitigate the severity of the outcome as well as the likelihood of occurrence.
EASA considers that attention should be exercised in the certification of hybrid-bearing applications in rotorcraft rotor drive systems and rotor control mechanisms. Applicants need to pay particular attention when introducing a new type of hybrid bearings (e.g. tapered roller bearings) and/or when introducing this technology on new type of applications (e.g. inside gearboxes).