Airworthiness Directives; Bombardier Model DHC-8-400 Airplanes

This amendment supersedes an existing airworthiness directive (AD), applicable to certain Bombardier Model DHC-8-400 airplanes. That AD currently requires revising the Normal and Abnormal sections of the airplane flight manual (AFM) to include procedures that enable the flightcrew to determine if the main landing gear (MLG) is extended before landing, and to take appropriate actions if necessary. This amendment adds an airplane to the applicability, and requires replacing the existing MLG downlock proximity sensors with new, improved sensors. After the replacement, this action also requires removing from the AFM the revision to the Normal and Abnormal sections required by the existing AD. The actions specified by this AD are intended to prevent failure of the MLG downlock proximity sensors on the same MLG at the same time, which could result in the MLG's failure to extend during landing, and cause injury to flightcrew and passengers. This action is intended to address the identified unsafe condition.