Date: 201705
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 50
Light: Daylight
Aircraft: DA42 Twin Star
Operator: FBO
Operating under: Part 91
Flight plan: None
Mission: Training
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Propeller Pitch Change Mechanism
Component problem: Malfunctioning
Reporter organization: FBO
Reporter function: Instructor; Pilot Flying
Reporter qualification: Flight Crew Commercial; Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Human factors: Distraction; Situational Awareness; Troubleshooting; Workload
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: During a routine Part 91 Multi-engine training flight in a DA-42NG; we were practicing normal in-flight engine shutdown; troubleshooting and securing checklist scenarios. We were daytime; VMC with unlimited visibility and no ceiling; smooth air; Class E airspace underneath Class B airspace at approximately 4;500 ft. After doing an intentional right engine shutdown/secure checklist training event; we attempted to restart the engine using the appropriate checklist restart sequence. Upon initiating the restart sequence (starter-assisted type versus wind milling start type) per the DA-42NG aircraft checklist and turning on the right engine ECU (Engine Control Unit); the propeller blades failed to 'unfeather'. The blades normally immediately unfeather upon turning the ECU to the 'ON' position; so we immediately knew something was not proper. While under perfect flight control of the aircraft with the right engine in the fully feathered condition; we began to troubleshoot the situation using the restart checklist to ensure we completed it in proper sequence. We checked/analyzed circuit breakers; electrical systems; and cycled the right engine ECU switch to see if this would assist and it did not. We attempted a starter-assisted engine start in an attempt to see if this would allow the propeller blades to unfeather. The engine would not start and the blades continued to be stuck in the 100% feathered position. At that time; we immediately turned towards the nearest large airport to accommodate our planned precautionary single-engine landing. We trimmed the aircraft for single engine ops; fully secured the right engine per the securing checklists; and ran all descent and landing checklists while simultaneously notifying Approach of the situation. I advised them of the issue and condition of the situation as being fully under control; and I advised them I would complete a precautionary landing. The aircraft maintained altitude perfectly as well as directional control with no issues. Approach advised the Tower of our situation and our intent to land and then passed us to Tower. Tower provided us immediate clearance to land; asked if we required any services on the ground; and provided us excellent support clearing us for left base and #1 for clearance to land. We advised them we did not require any services at that time. As the aircraft performed excellent during the single engine descent; we executed a normal landing and cleared the runway. We then taxied to parking with no issues. Upon post-flight review of the systems and discussion with maintenance personnel; it appears that there might have been an issue with a bad valve or accumulator system versus an electrical issue. This report was submitted to help assist other DA-42NG pilots by increasing their awareness of this type of situation; and to advise them on the excellent single-engine performance and controllability of the Diamond DA-42NG.




Synopsis: DA42 instructor reported an engine would not unfeather during an attempted restart after an engine shutdown for training.