Airworthiness Directives; Bombardier, Inc.

We are adopting a new airworthiness directive (AD) for all Bombardier, Inc. Model DHC-8-400 series airplanes. This AD was prompted by a report that a batch of main landing gear (MLG) door actuators with a certain part number having certain serial numbers could be assembled with the scraper installed backward. This AD requires repetitive functional checks of the MLG alternate extension system (AES) and eventual replacement of certain MLG door actuators with actuators that have either been reworked or do not have certain serial numbers. We are issuing this AD to prevent incorrectly installed scrapers, which could hinder the operation of the MLG AES, and result in failure of the MLG AES on one side, and consequent unsafe asymmetrical landing configuration.