A Mechanic reports a pitot static quick disconnect (QD) fitting for a #2 ADC; was not properly installed; and had pulled back; resulting in an Air Turnback for a First Officer's airspeed failure on a B737-300.

Narrative

I was assigned to Aircraft X. The 'A' autopilot was on MEL deferral for a flashing overspeed indication on the autopilot MCP. DFCS system BITE current status test #2 failed; indicating the left flap position transmitter was at fault. I removed and replaced the flap transmitter and performed the required tests per the Aircraft Maintenance Manual which included the system performance test for the stall warning system per aircraft Maintenance Manual. This test required the use of the pitot/static tester to input 120 KTS airspeed to the #2 ADC for the #2 stall warning system. While installing the pitot head adapters; I was having trouble getting the system leak test to check out. While hunting for the possible leak; I removed the pitot quick disconnect on the #2 ADC to inspect the seal. After reinstalling the quick disconnect; I subsequently found the leak was at the pitot head adapter. Once the leak was corrected; I completed the system test without failure; which verified that the fitting was installed. Apparently; the quick disconnect was not locked properly; since the fitting pulled back during the first flight. Following the air turn back for First Officer's airspeed failure; the Mechanic on duty discovered the pitot line not seated properly on the #2 ADC. This air turn back directly resulted from my being distracted and not ensuring to fully engage the pitot quick disconnect lock pins. Aircraft Maintenance Manual ADC installation procedure clearly explains the proper method for installing the quick disconnect; engaging the lock pins fully; and verifying the colored lock ring indicator showing a correct connection. In the future; I shall be more diligent in the proper connection of quick disconnect fittings. Reference discrepancies: ATA 22-1; ATA 34-15.