Airworthiness Directives; Sandel Avionics Incorporated Model ST3400 Terrain Awareness Warning System/Radio Magnetic Indicator (TAWS/RMI) Units Approved Under Technical Standard Order(s) C113, C151a, or C151b; Installed on Various Small and Transport Category Airplanes

The FAA is revising an existing airworthiness directive (AD) that applies to Sandel Avionics Incorporated Model ST3400 TAWS/RMI units as described above. The existing AD currently requires installing a warning placard on the TAWS/RMI and revising the Limitations section of the airplane flight manual (AFM). The existing AD also requires installing upgraded software in the TAWS/RMI. This new AD allows installing later revisions of the software described in the existing AD. This AD results from a report that an in-flight bearing error occurred in a Model ST3400 TAWS/RMI configured to receive bearing information from a very high frequency omnidirectional range (VOR) receiver interface via a composite video signal, due to a combination of input signal fault and software error. We are issuing this AD to prevent a bearing error, which could lead to an airplane departing from its scheduled flight path, which could result in a reduction in separation from, and a possible collision with, other aircraft or terrain.