Narrative:

I was flying several photo missions for my company with [several] locations. I was flying enroute to ZZZ at 7500 ft. MSL. I was using flight following services with TRACON. I cleared the terrain at 7500 ft. MSL; initiated a planned descent to 3500 ft. Slowly. At about 6500 ft. MSL; I experienced manifold drop of 4-5 inches with associate loss in RPM to roughly 2100 (from 2400). I pushed throttle; propeller; and mixture in and verified engine temperatures; oil pressure; and fuel flow were in the green. (Noted oil pressure of 41psi which is low for cruise power but still within normal limits). The power remained consistently low for 5-10 seconds then recovered to full power and 2600+ RPM without other adjustments. Adjusting settings back to cruise (27 in.; 2400 RPM) worked as expected. Then; another power loss occurred... Same result. 27 in. Down to 21 in. I worked the engine controls; my operator/co pilot changed fuel tank from left to right.the power did not recover to full power immediately; but held steady (at 20-21 in.) then recovered to full. On that 2nd power drop; I declared to [TRACON] that we are experiencing partial engine failure and would be heading to ZZZ1 for landing. We were about 6-8 miles north of ZZZ1. I switched over to tower; told them we are experiencing partial engine failure and coming in to land. They cleared us to do one circling descent; so I did. They suggested to do one more circle or circle to land xx but I decided to do straight in xxr. They cleared us to land xxr. During circling to lose altitude; we experienced engine power responding to control inputs as expected. Not knowing what has caused the partial reduction of power; I still made a decision to land at ZZZ1. I landed at xxr. We requested taxi to run up and performed quick magneto checks. We did not see anything out of ordinary. We taxied to [FBO] and notified our company and mechanic. Since we experienced the engine roughness on the left tank; we asked [FBO] to top off only the left tank. That was when I was told the left tank fuel cap is missing. I was the one who fueled the plane and I did not remember securing the left fuel. We relayed that info to our hub and my coworkers learned from a maintenance shop a fuel cap had been recovered on the taxi. One of my coworkers verified it was ours and drove it up to ZZZ1.after inspecting/installing the fuel cap; we; then; asked to get the tank topped off again. The right tank took about 20 gallons and the left tank took about 40 gallons (proving it was empty). Knowing that we flew about an hour on the right tank and experienced partial engine roughness about 30 minutes in on the left tank; we preliminarily decided that cause of partial engine roughness was due to fuel starvation on the left tank. After getting fuel; we did extensive run ups at low and high power settings and also high speed taxi on the runway. Since we did not see any problems with the engine; we requested a circling up to 2900 ft. MSL and stayed in the vicinity of the airport. Not experiencing any problems; we continued the flight and finished several of the left over photo missions. Fuel consumption was proven to be consistent with normal operation. As a precaution; we ordered a new fuel cap to replace the one that got dropped.

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Original NASA ASRS Text

Title: A Cessna 206 pilot reported experiencing engine roughness while feeding from the left fuel tank; which subsequently was found to be missing a fuel cap.

Narrative: I was flying several photo missions for my company with [several] locations. I was flying enroute to ZZZ at 7500 ft. MSL. I was using flight following services with TRACON. I cleared the terrain at 7500 ft. MSL; initiated a planned descent to 3500 ft. slowly. At about 6500 ft. MSL; I experienced manifold drop of 4-5 inches with associate loss in RPM to roughly 2100 (from 2400). I pushed throttle; propeller; and mixture in and verified engine temperatures; oil pressure; and fuel flow were in the green. (Noted Oil Pressure of 41psi which is low for cruise power but still within normal limits). The power remained consistently low for 5-10 seconds then recovered to full power and 2600+ RPM without other adjustments. Adjusting settings back to cruise (27 in.; 2400 RPM) worked as expected. Then; another power loss occurred... same result. 27 in. down to 21 in. I worked the engine controls; my operator/co pilot changed fuel tank from left to right.The power did not recover to full power immediately; but held steady (at 20-21 in.) then recovered to full. On that 2nd power drop; I declared to [TRACON] that we are experiencing partial engine failure and would be heading to ZZZ1 for landing. We were about 6-8 miles north of ZZZ1. I switched over to Tower; told them we are experiencing partial engine failure and coming in to land. They cleared us to do one circling descent; so I did. They suggested to do one more circle or circle to land XX but I decided to do straight in XXR. They cleared us to land XXR. During circling to lose altitude; we experienced engine power responding to control inputs as expected. Not knowing what has caused the partial reduction of power; I still made a decision to land at ZZZ1. I landed at XXR. We requested taxi to run up and performed quick magneto checks. We did not see anything out of ordinary. We taxied to [FBO] and notified our company and mechanic. Since we experienced the engine roughness on the left tank; we asked [FBO] to top off only the left tank. That was when I was told the left tank fuel cap is missing. I was the one who fueled the plane and I did not remember securing the left fuel. We relayed that info to our hub and my coworkers learned from a maintenance shop a fuel cap had been recovered on the taxi. One of my coworkers verified it was ours and drove it up to ZZZ1.After inspecting/installing the fuel cap; we; then; asked to get the tank topped off again. The right tank took about 20 gallons and the left tank took about 40 gallons (proving it was empty). Knowing that we flew about an hour on the right tank and experienced partial engine roughness about 30 minutes in on the left tank; we preliminarily decided that cause of partial engine roughness was due to fuel starvation on the left tank. After getting fuel; we did extensive run ups at low and high power settings and also high speed taxi on the runway. Since we did not see any problems with the engine; we requested a circling up to 2900 ft. MSL and stayed in the vicinity of the airport. Not experiencing any problems; we continued the flight and finished several of the left over photo missions. Fuel consumption was proven to be consistent with normal operation. As a precaution; we ordered a new fuel cap to replace the one that got dropped.

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.