Narrative:

We took off for a routine multi-engine stage check. About 30 minutes into the flight; we ran through our engine failure procedure where I failed the left engine by use of throttle. The student showed his ability to control the aircraft. We proceeded to do the engine shut down checklist - in practice. The way we teach it here is to actually feather the propeller and shut off the mixture. Let the student see the single engine performance and handling characteristics of the plane on one engine and then run through the engine air start checklist. We did the air start checklist but the prop did not come out of feather. We tried 4 or 5 times and decided at that point to divert to the nearest suitable airport. I took controls and the radio as my student ran through the checklist aloud. We landed without incident on [the] longest runway. The aircraft was found to have a faulty accumulator and/or settings. Before we departed; I checked the maintenance logs as usual and saw both engines had been replaced after prop strikes. I've worked at [company] for 3 years now and have done maintenance test flights for them following maintenance. So I was under the impression they did a maintenance test flight after the new engines were installed. When I returned to [home base]; I did some investigating and found out they did not; in fact; do any maintenance test flights before returning it to service. As our manual says they are not required to do test flights for new engines that come from the factory broken in. I looked up far 91.407 and it states after maintenance; the plane has to have a test flight. I believe a test flight after the new engines were installed may have caught the accumulator issue and would have been a much safer route.

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Original NASA ASRS Text

Title: DA-42 flight instructor reported diverting to an alternate after an intentionally feathered prop would not unfeather.

Narrative: We took off for a routine multi-engine stage check. About 30 minutes into the flight; we ran through our engine failure procedure where I failed the left engine by use of throttle. The student showed his ability to control the aircraft. We proceeded to do the engine shut down checklist - in practice. The way we teach it here is to actually feather the propeller and shut off the mixture. Let the student see the single engine performance and handling characteristics of the plane on one engine and then run through the engine air start checklist. We did the air start checklist but the prop did not come out of feather. We tried 4 or 5 times and decided at that point to divert to the nearest suitable airport. I took controls and the radio as my student ran through the checklist aloud. We landed without incident on [the] longest runway. The aircraft was found to have a faulty accumulator and/or settings. Before we departed; I checked the maintenance logs as usual and saw both engines had been replaced after prop strikes. I've worked at [company] for 3 years now and have done maintenance test flights for them following maintenance. So I was under the impression they did a maintenance test flight after the new engines were installed. When I returned to [home base]; I did some investigating and found out they did not; in fact; do any maintenance test flights before returning it to service. As our manual says they are not required to do test flights for new engines that come from the factory broken in. I looked up FAR 91.407 and it states after maintenance; the plane has to have a test flight. I believe a test flight after the new engines were installed may have caught the accumulator issue and would have been a much safer route.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.