A B737-300 FIRST FLT AFTER RELEASE FROM A HVY MAINT CHK HAD A L PYLON ACCESS PANEL DEPART THE ACFT.

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ON MON/AUG/06; I WAS ON THE #1 ENG DURING THE B/C CHK. I OPENED THE REQUIRED PANELS FOR INSPECTION. AFTER THE INSPECTION; I CLOSED THE PANELS. THE INSPECTOR INSPECTED THE PANELS THAT THEY WERE PROPERLY INSTALLED AND INDICATED MARKS FOR ALIGNMENT. SOME TIME DURING THE FIRST FLT; PANEL #5727L LEFT THE ACFT. AT THIS TIME I HAVE NO IDEA WHY THIS PANEL CAME OFF. TO PREVENT THIS FROM HAPPENING AGAIN IS SOME WAY TO ADD AN ANGLE TO THE STRUCTURE OF THE PYLON WITH NUT PLATE FOR SCREW ATTACHMENT.SUPPLEMENTAL INFO FROM ACN 707489: MY RESPONSIBILITY IN THIS MATTER IS TO VERIFY INSTALLATION OF THIS PANEL. AFTER WITNESSING THE MECHANIC INSTALL THE PANEL; I USED A WORK LADDER TO VERIFY ALL LOCK INDICATORS WERE ALIGNED PROPERLY AND ALL FASTENERS WERE FLUSH. I FEEL THE MECHANIC PROPERLY INSTALLED THE PANEL PER THE PROCS GIVEN; HOWEVER; I CANNOT EXPLAIN WHY THE PANEL CAME OFF.CALLBACK CONVERSATION WITH RPTR ACN 707488 REVEALED THE FOLLOWING INFO: THE RPTR STATED THE FASTENER IS LOCKED WHEN THE ROTATING WASHER UNDER THE HEAD OF THE SCREW ALIGNMENT SLOT IS ALIGNED WITH THE TANG BEHIND THE PANEL. THIS PUTS THE LOCKING TANG ON THE INNER PYLON SURFACE LOCKING THE PANEL. THERE ARE ABOUT SIX FASTENERS TO EACH PANEL.CALLBACK CONVERSATION WITH RPTR ACN 707489 REVEALED THE FOLLOWING INFO: THE PYLON PANEL'S FASTENERS HAD ALL THE ALIGNMENT SLOTS IN THE LOCK POSITION AND THE RPTR IS CERTAIN THE PANEL FASTENERS WERE ALL LOCKED.