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 of failure of the high pressure shutoff valves (HPSOVs) causing the timer and monitor unit (TMU) to become inoperative since the HPSOV and the TMU are on the same circuit breaker. This proposed AD would require a wiring modification to segregate the HPSOV power supply from the TMU. We are proposing this AD to prevent an inoperative TMU, which could result in the loss of the automatic de-icing mode, and lead to an increased workload for the flight crew and loss of control of the airplane.