Summary
We propose to supersede an existing airworthiness directive (AD) that applies to certain Bombardier, Inc. Model DHC-8-400 series airplanes. The existing AD currently requires a free-play check for excessive free-play of the shaft swaged bearing installed in the tailstock end of each elevator power control unit (PCU), and replacing any PCU on which the bearing exceeds allowable limits with a serviceable PCU. Since we issued that AD, we have determined that additional airplanes are affected by the identified unsafe condition. This proposed AD would add airplanes to the applicability in the existing AD. We are proposing this AD to detect and correct excessive freeplay of the swaged bearings, which could lead to excessive airframe vibrations and difficulties in pitch control, and consequent loss of controllability of the airplane.