Caravan Undifferentiated
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Distraction; Other / Unknown; Fatigue; Time Pressure
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Departure Airport
Human Factors
At approximately XA17 local; I departed ZZZ airport for ZZZ1 on an IFR flight plan. I landed there without incident; loaded passengers on board; and then departed back for ZZZ at XB09. Upon reaching 5000 ft headed to ZZZ2; I was getting ready to perform the turbine trend for the day when I noticed that oil temperature was out of limit on the hot side; by 10 Celsius. After doing a quick scan; I found that all other engine parameters were normal. I notified ATC; canceled IFR. I stayed on their frequency while monitoring CTAF. At the time there was a broken cloud layer with multiple areas of scattered up ahead. I began a descent; to get to more solid VMC through one of the holes; but also in a bid to cool the engine. It did work; as the oil temp did slide to just above redline. We continued on to ZZZ and landed without incident. Upon exiting the aircraft I discovered what was left of the cowl plugs; the intake was gone; and the oil cooler cover was right where it had been left the night before. I had missed them on final walk-around; even after pulling the prop wind bungee off. To break down what happened after giving this some serious thought; here is what I believe happened. I actually did do the walk-around; but for some reason I didn't remove the plugs; while removing the prop bungee. While I did come to the nose of the plane; I didn't come all the way to the front. If I did; I know for certain I would have seen them. So doing a better 360 walk-around would help; especially when flying alone. Also I usually wrap the cowl plugs around the propeller for this very reason; so that instead of potential damage I just do a walk of shame. So being consistent in that would prevent it from ever happening again. And ultimately; having situational awareness on the ground as well as in the air is crucial. I take full responsibility for my imprecise piloting and this will not happen again.
C-208 Captain reported emergency air return due to an incomplete preflight.