Dash 8 Series Undifferentiated or Other Model
Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 71.0; Flight Crew Total 2335.8; Flight Crew Type 261.2
Aircraft Equipment Problem Critical
Flight Crew Inflight Shutdown; Flight Crew Returned To Departure Airport; Flight Crew Landed in Emergency Condition; General Maintenance Action; General Declared Emergency
Aircraft
Shortly after takeoff (approximately 1800 feet AGL; 2400 MSL; 145 KIAS) the aircraft's #1 engine malfunctioned resulting in zero torque.  The crew declared an emergency; secured the engine per the QRH and returned.  After landing; the crew turned off the runway and shut down on taxiway W-5.  Airfield emergency crews responded and secured the aircraft for shutdown.  Afterwards; ground crew towed the aircraft to the ramp.                The torque motor in the #1 Engine Hydro Mechanical Unit (HMU) malfunctioned as diagnosed through high power ground engine runs.  This failure was not realized by the crew inflight due to the nominal operation of the engine with Engine Control Unit (ECU) selector in TOP (Takeoff Power).  Once the ECU selector was switched from TOP to NORM (as per the QRH (pilot checklist) during after takeoff checks); the Enrichment Solenoid was de energized resulting in low fuel flow to the #1 engine. The low fuel flow caused a power reduction and a command to reduce torque to maintain 900 RPM.  The torque was reduced to zero resulting in the crew executing the Engine Failure QRH procedures.  Replacing the #1 HMU remedied the problem. This condition is serious and affects operators using this combination of PW120/121 / HMU's everywhere.  There were no indications of impending failure prior to moving the ECU selector from TOP to NORM.  This situation is dangerous because you have to create the dangerous situation before you know anything is wrong.  A safety message should go to all affected operators immediately. Possible workarounds (not verified).  If upon selecting ECU selector-NORM; if a loss of power is detected; go back to ECU-TOP and land as soon as possible.  Also disabling the affected ECU by selecting ECU-Manual may reestablish sufficient fuel flow through mechanical linkages.
The reporter states that the power loss occurred five to ten seconds after the ECU was switched from TOP to NORM and it was not obvious that the switch had caused the power loss. He believes that placing the switch back to TOP would have restored normal engine power and allowed a two engine landing. He flies multiple aircraft types for a maintenance facility and is not as familiar with the DHC8 as an airline crew would be and believes that the QRH and the flight training company should provide some guidance on this subject.
DHC8-100 Captain experiences a power loss after takeoff shortly after the ECU is switched from TOP to NORM. The engine is shut down in accordance with QRH procedures and the flight returns to the departure airport.