CAP MISSING ON SINGLE POINT FUELING SYS.

Narrative

FO RPTED R UNDERWING SINGLE POINT FUEL CAP MISSING. UNABLE TO LOCATE CAP IN IMMEDIATE AREA. CONTACTED MAINT CTL; WHO FURNISHED AND SECURED ANOTHER CAP. ACFT HAD PREVIOUSLY BEEN PREFLTED AFTER REFUELING ON PREVIOUS FLT; AND FUEL CAP OBSERVED TO BE IN PLACE; ALIGNED AND SECURE. NO LESS THAN 2 VISUAL INSPECTIONS BY CAPT. ACFT HAD ALSO BEEN PUSHED BACK; TURNED AND LINED UP FOR TAXI AT GATE IN ORD. WITH NO LESS THAN 3 WING WALKERS AND 2 RAMP PERSONNEL IN IMMEDIATE VICINITY OF ACFT. NO DISCREPANCIES OR PROBS RPTED BY ANY OF THESE PERSONS WHEN QUERIED REGARDING EQUIP MISSING OR NOT APPEARING SECURED ON ACFT. IN LATER DISCUSSION WITH CHIEF PLT AND MAINT CTL; IT WAS POINTED OUT THAT THIS ACFT HAD IDENTICAL PROBS EARLIER RPTED WITH SAME FUEL CAP. FLT INTO FORT WAYNE PRIOR TO DISCOVERY OF MISSING CAP; AND AFTER CAP LAST SEEN IN PLACE WAS NORMAL. NO UNUSUAL INDICATIONS OR OCCURRENCES. NOTE: LATER FLT THAT EVENING ON SAME ACFT DURING POSTFLT INSPECTION SHOWED CAP HAD WORKED LOOSE AND ROTATED ITSELF; READY TO FALL OFF AT SHARP JOLT. THIS WAS AFTER MAINT PERSONNEL INSPECTED AND OK'ED CAP FOR FLT.