Narrative:

Conducted a normal preflight inspection and found no abnormalities with the aircraft. After a normal takeoff; climb; and approximately 20 minutes of cruise flight; I noticed that the bottom cowling on the left engine had come loose and was now only secured by one fastener instead of the normal three. I elected to air return to [departure airport] as it was the closest suitable airport with facilities to make necessary repairs and given that the cowling was relatively secure given that two fasteners were no longer secure (i.e it didn't appear to be getting any worse); I elected to not land at one of the closer; smaller airports. After an uneventful approach to landing; maintenance personnel discovered that the cowling had been secured improperly with three of the six fasteners improperly locked. In-flight departure of a cowling could be potentially catastrophic to engine/flight control components as well as persons and objects on the ground and greater care should be taken by all parties involved to ensure they are properly secured. Pilots should take extra time during pre-flight inspections to not only ensure that fasteners are present as many do/have been trained to do; but also ensure they are installed properly; which is something I wasn't trained to look for on this aircraft type (there is a very specific way the fasteners need to be locked as I now know). Maintenance personnel should always check and double check that the cowling has been properly secured following any maintenance action that involves opening the cowling.

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Original NASA ASRS Text

Title: Piper PA31 pilot noticed in cruise that one of the engine cowls was not fully attached and elected to return to the departure airport. It was reported that the cowling had not been properly locked following maintenance activity.

Narrative: Conducted a normal preflight inspection and found no abnormalities with the aircraft. After a normal takeoff; climb; and approximately 20 minutes of cruise flight; I noticed that the bottom cowling on the left engine had come loose and was now only secured by one fastener instead of the normal three. I elected to air return to [departure airport] as it was the closest suitable airport with facilities to make necessary repairs and given that the cowling was relatively secure given that two fasteners were no longer secure (i.e it didn't appear to be getting any worse); I elected to not land at one of the closer; smaller airports. After an uneventful approach to landing; maintenance personnel discovered that the cowling had been secured improperly with three of the six fasteners improperly locked. In-flight departure of a cowling could be potentially catastrophic to engine/flight control components as well as persons and objects on the ground and greater care should be taken by all parties involved to ensure they are properly secured. Pilots should take extra time during pre-flight inspections to not only ensure that fasteners are present as many do/have been trained to do; but also ensure they are installed properly; which is something I wasn't trained to look for on this aircraft type (there is a very specific way the fasteners need to be locked as I now know). Maintenance personnel should always check and double check that the cowling has been properly secured following any maintenance action that involves opening the cowling.

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.