Narrative:

While departing from the terminal area I had just climbed and maintained 10;000 feet MSL as assigned on my IFR clearance. I was still in the cruise climb configuration while I was in level off attempting to gain airspeed when a large deep surge happened on the left engine feeling like it was tugging the structure of the aircraft left yaw about 5-10 degrees. I looked outside immediately to identify if there was any signs of failure after outside inspection I referenced the engine instruments for both engines and found nothing wrong. I referenced my position and notified operations control of the abnormality and asked for a suggestion based off the situation. They said that they would not be able to send anyone out to inspect due to shortage of flight crew and that if there was going to be a mechanical write up the aircraft should be returned to ZZZ. I notified ATC that I would need to divert to ZZZ due to a mechanical irregularity and that I did not need any assistance as the engine was preforming normally for now. With initial contact with tower I advised them that I was doing a precautionary return for a mechanical irregularity of the left engine. While I was in descent I noticed a surge again on the left engine but far less pronounced then from before. At this point I elected to cancel IFR and proceed straight in. I applied full cruise climb power to increase available power and advanced mixtures to full as the left engine was producing less than normal fuel flow and was approximately 15-24 pounds different from the right on the fuel flow indicator. I grabbed the QRH and flipped to the engine failure in flight page and briefed myself on the procedures while I was proceeding for the airport to be fully prepared for a left engine failure. I set the QRH down to my right in the empty seat so I could reference it quickly should the engine fail. When I reduced engine power to go in range I noticed a grinding feeling coming out of the left engine as the power was reduced. When I landed I requested to taxi to the maintenance hangar where I was followed by a fire truck and spoke with airport rescue and firefighting and helped them fill out a report of the situation. I called operations control; maintenance control and spoke with the duty manager about the situation as well as debriefed maintenance about the abnormality. I wrote up the airplane and was relieved of flying for the rest of the day due to two engine abnormality for the flight day to comply with imsafe checklist. After speaking with the duty manager and maintenance is was likely a fuel contamination leading so surging and rough running due to fuel abnormalities. Closer inspection of fuel sumps for possible contamination of fuel line.

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Original NASA ASRS Text

Title: C402C Captain reported a left engine surge requiring a return to departure airport.

Narrative: While departing from the terminal area I had just climbed and maintained 10;000 feet MSL as assigned on my IFR clearance. I was still in the cruise climb configuration while I was in level off attempting to gain airspeed when a large deep surge happened on the left engine feeling like it was tugging the structure of the aircraft left yaw about 5-10 degrees. I looked outside immediately to identify if there was any signs of failure after outside inspection I referenced the engine instruments for both engines and found nothing wrong. I referenced my position and notified Operations Control of the abnormality and asked for a suggestion based off the situation. They said that they would not be able to send anyone out to inspect due to shortage of flight crew and that if there was going to be a mechanical write up the aircraft should be returned to ZZZ. I notified ATC that I would need to divert to ZZZ due to a mechanical irregularity and that I did not need any assistance as the engine was preforming normally for now. With initial contact with Tower I advised them that I was doing a precautionary return for a mechanical irregularity of the left engine. While I was in descent I noticed a surge again on the left engine but far less pronounced then from before. At this point I elected to cancel IFR and proceed straight in. I applied full cruise climb power to increase available power and advanced mixtures to full as the left engine was producing less than normal fuel flow and was approximately 15-24 pounds different from the right on the fuel flow indicator. I grabbed the QRH and flipped to the engine failure in flight page and briefed myself on the procedures while I was proceeding for the airport to be fully prepared for a left engine failure. I set the QRH down to my right in the empty seat so I could reference it quickly should the engine fail. When I reduced engine power to go in range I noticed a grinding feeling coming out of the left engine as the power was reduced. When I landed I requested to taxi to the maintenance hangar where I was followed by a fire truck and spoke with airport rescue and firefighting and helped them fill out a report of the situation. I called Operations Control; Maintenance Control and spoke with the Duty Manager about the situation as well as debriefed Maintenance about the abnormality. I wrote up the airplane and was relieved of flying for the rest of the day due to two engine abnormality for the flight day to comply with IMSAFE checklist. After speaking with the Duty Manager and Maintenance is was likely a fuel contamination leading so surging and rough running due to fuel abnormalities. Closer inspection of fuel sumps for possible contamination of fuel line.

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.