Airworthiness Directives; Bombardier Model DHC-8-400 Series Airplanes

The FAA is superseding an existing airworthiness directive (AD) that applies to certain Bombardier Model DHC-8-400 series airplanes. The existing AD currently requires revising the Limitations section of the airplane flight manual (AFM) to include procedures for pulling the "HYD PWR XFER" circuit breaker in the event of the loss of all hydraulic fluid in the No. 1 or No. 2 hydraulic system. This AD requires a revision to the AFM to include additional procedures for ensuring that the "PTU CNTRL" switch is Normal, the "PTU CNTRL ON" advisory light is out, and the "HYD PWR XFER" circuit breaker is pulled in the event of the illumination of the "<greek-i>2 HYD ISO VALVE" caution light. This AD resulted from low No. 2 hydraulic pressure in-flight, which caused the power transfer unit to overspeed, and the fluid flow within the No. 1 hydraulic system to increase. We are issuing this AD to prevent possible loss of both the No. 1 and No. 2 hydraulic systems, resulting in the potential loss of several functions essential for safe flight and landing of the airplane.