Airworthiness Directives; Bombardier, Inc. Airplanes

We propose to adopt a new airworthiness directive (AD) for certain Bombardier, Inc. Model DHC-8-102, -103, -106, -201, -202, -301, -311, and -315 airplanes. This proposed AD was prompted by reports of un-annunciated failures of the direct current (DC) starter generator, which caused caution indicators of the affected systems to illuminate and prompted emergency descents and landings. This proposed AD would require, for certain airplanes, replacing the DC generator control units (GCUs) with new GCUs and replacing the GCU label. We are proposing this AD to prevent a low voltage condition on the left main DC bus which, during critical phases of flight, could result in the loss of flight management, navigation, and transponder systems, and could affect continued safe flight.