Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Airbus Model A330 and A340 series airplanes. This proposal would require identifying the part and serial numbers of the pressure reducing valve on each air pressurization unit, testing pressure reducing valves and air pressurization units having affected serial numbers, and replacing faulty valves or units with new parts. This action is necessary to prevent the simultaneous failure of two air pressurization units, which could result in loss of three hydraulic circuits and consequent reduced controllability of the airplane. This action is intended to address the identified unsafe condition.