Summary
We propose to adopt a new airworthiness directive (AD) for certain Bombardier Inc. Model DHC-8-400 series airplanes. This proposed AD was prompted by reports that the automatic de-icing mode became unavailable due to a failure of the timer and monitor unit (TMU). This proposed AD would require replacing the TMU. We are proposing this AD to prevent loss of the automatic de-icing mode and consequent increased workload for the flight crew, which, depending on additional failures, could lead to loss of control of the airplane.