C340A Maintenance Technician reported a repair to the vacuum pump and the following engine test run was incorrectly completed. The pilot flew the aircraft after the first repair and oil began to stream from the cowling and the pressure fluctuated; which led to the discovery of the problem.

Narrative

Vacuum pump was changed due to failure; left bottom stud threaded out when nut stuck on stud. Pump and new gasket installed and stud threaded back into case and tightened as well as other 3 nuts. Work completed by a new apprentice; 2 months on the job; under supervision of A&P / IA (Inspection Authorization). Work inspected by A&P and ground run; idle to approximately 1500 RPM; completed for approximately 4 to 5 minutes. No leak noted and vacuum functioned normally. Aircraft departed and approximately 15 minutes into the flight; pilot noticed oil streaming from cowling and pressure fluctuating. Engine was shut down and aircraft landed without incident. Cowling was removed and discovered the lower left stud for vacuum pump was loose and oil was streaming from gasket area where gasket was blown out. Removed pump and removed loose stud. Cleaned and inspected case and stud. Reinstalled stud with sealant to proper depth. Reinstalled pump with new gasket and tightened hardware to specifications. Ground run at power for several minutes and taxi back to hangar for a total of approximately 15 minutes run time. No leaks noted.Suspect the stud may not have been fully seated and was not allowing the pump flange to sit flat and seal gasket. Checked stud height against the others on the stud repair and confirmed it was seated to the same depth. Studs did not touch pump housing. Suspect that during inspection of work the IA saw the 4 nuts and studs but not the stud height and the engine run was not long enough to allow the leak to progress to a point it could blow the gasket out. Had engine been run at power for a longer period of time the gasket may have blown out on the ground run and the leak noted. Full power runs for an extended - temperature permitting - period of time should be conducted to verify leak checks and a closer inspection of hardware would have possibly discovered stud height issue. Log entry mistake was made for date and Hobbs time. Aircraft has a Garmin EIS (Engine Indication System) and records total; flight; [and] left and right engine times. The wrong time was entered in the program and printed with log entries with incorrect dates. The program is new to the company and all the subtle differences and processes have not been fully learned. Corrected log entries were generated and provided to the customer.