AD FAA-2013-0966

We are adopting a new airworthiness directive (AD) for certain Rockwell Collins TPR-720 and TPR-900 Mode select (S) transponders that are installed on airplanes. This AD was prompted by the identification that the TPR-720 and TPR-900 Mode S transponders respond intermittently to Mode S interrogations from both ground-based and traffic collision avoidance system (TCAS-) equipped airplanes. This AD requires testing and calibration of the alignment of the transponders. We are issuing this AD to correct the unsafe condition on these products.