Summary
The FAA proposes to supersede an existing airworthiness directive (AD) that applies to certain Airbus Model A319, A320, and A321 series airplanes equipped with certain Litton air data inertial reference units (ADIRUs). The existing AD currently requires modifying the shelf (floor panel) above ADIRU 3, modifying the polycarbonate guard that covers the ADIRUs for certain airplanes, and modifying the ladder located in the avionics compartment for certain airplanes. This proposed AD would require those modifications on additional airplanes. This proposed AD would also require replacing all three ADIRUs with improved ADIRUs. This proposed AD also adds Model A318 series airplanes to the applicability. This proposed AD results from reports that "NAV IR FAULT'' messages have occurred during takeoff due to failure of an ADIRU and subsequent analysis showing that the shelf modification has not sufficiently addressed failure of an ADIRU. We are proposing this AD to prevent failure of anADIRU during flight, which could result in loss of one source of critical attitude and airspeed data and reduce the ability of the flightcrew to control the airplane.