Summary
We propose to adopt a new airworthiness directive (AD) for the products listed above. This proposed AD results from mandatory continuing airworthiness information (MCAI) originated by an aviation authority of another country to identify and correct an unsafe condition on an aviation product. The MCAI describes the unsafe condition as:
In accordance with design regulation, the THSA [trimmable horizontal stabilizer actuator] has a failsafe design. Its upper attachment to the aeroplane has two load paths, a Primary Load Path (PLP) and a Secondary Load Path (SLP), which is only engaged in case of PLP failure. Following the design intent, engagement of the SLP leads to jam the THSA, indicating the failure of the PLP. Tests carried out under the loads-measured during representative flights have demonstrated that, when the SLP is engaged, it does not systematically jam the THSA. In addition, laboratory tests have confirmed that the SLP will only withstand the loads for a limited period of time. This condition of PLP failure during an extended period of time, if not detected and corrected, would lead to the rupture of the THSA upper attachment and consequent THSA loss of command, resulting in reduced control of the aeroplane.
* * * * * The proposed AD would require actions that are intended to address the unsafe condition described in the MCAI.