DA42 Twin Star
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument
Flight Crew Last 90 Days 320; Flight Crew Total 2300; Flight Crew Type 170
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; Flight Crew Landed As Precaution
Aircraft
On takeoff roll while training a Multi Engine Instructor (MEI) in a standardization course; Left Hand Engine RPM exceeded limits. Instructor Pilot-in-training reduced power lever and it went to normal (which is normal procedure). When he reestablished full power it returned to abnormal range but we reached Vr and rotated. Instructor Pilot conducting training took flight controls to check the condition and confirmed it was a malfunction. Thus requested a return to the field for landing. After a few minutes; when it was confirmed that even with significantly reduced thrust; the engine was still malfunctioning; [we advised ATC] and landed. Did not shut it down because the engine was still producing useful thrust. Landing was uneventful; and taxied back to the ramp.
Flight Instructor reported loss of engine thrust; causing a return to the departure airport for a precautionary landing.