A C150 PILOT LANDED SHORT OF THE AIRPORT WHEN HE LOST POWER ON APCH BECAUSE OF LOW FUEL STATE.

Narrative

LOSS OF ENG PWR ON 1/2 NM FINAL APCH INTO ZZZ. USE OF WIDER THAN NORMAL PATTERN WAS REQUIRED BECAUSE OF PREVIOUS LNDG TFC IN THE PATTERN; RESULTING IN DEEPER FINAL APCH THAN NORMAL. A QUICK CHK OF IGNITION AND FUEL CTL DID NOT REVEAL CAUSE FOR LOSS OF PWR AND SINCE GLIDE TO THE RWY WAS DOUBTFUL; PLT ELECTED TO LAND THE ACFT IN OPEN BEAN FIELD UNDER AND SLIGHTLY L OF THE APCH TO RWY XX WITHOUT DAMAGE OR INJURY. AFTER EXITING THE ACFT; THE SURPRISED PLT INSPECTED THE ACFT FOR CAUSE OF LOSS OF PWR. PLT CLBED THE WING STRUT AND FOUND THE L WING TANK FUEL CAP DISLODGED FROM THE TANK AND FUEL STAIN ON THE WING CONSISTENT WITH FUEL SIPHONING FROM THE TANK DURING FLT. FLT PLANNING AND TIME ENRTE SHOULD HAVE LEFT 9 GALLONS REMAINING UPON ARR (1 HR FLT TIME AT NORMAL CRUISE SPD). HOWEVER; VISUAL INSPECTION OF THE TANKS VERIFIED NO FUEL REMAINING. ACFT WAS PARTIALLY REFUELED AND AFTER SEEKING PERMISSION FROM THE OWNER OF THE BEAN FIELD; ACFT WAS FLOWN TO THE ARPT. PROB CAUSED BY FAILING TO ENSURE FUEL CAPS WERE SECURED PROPERLY PRIOR TO DEP; ALTHOUGH THE CAPS WERE INSPECTED DURING THE PREFLT AFTER REFUELING. A CONTRIBUTING FACTOR WAS THE DESIGN OF THE FUEL CAPS. THESE FUEL CAPS; MANDATED BY AIRWORTHINESS DIRECTIVE AND IN WIDE USE IN THE CESSNA FLEET BUT ARE DIFFERENT FROM THE ORIGINAL DESIGN USED ON THE MODEL AND ARE DIFFICULT TO SECURE PROPERLY; ESPECIALLY BECAUSE OF THE RETAINING CHAIN. PROB CAN BE AVOIDED WITH MORE DILIGENT INSPECTION OF THE FUEL CAPS; WHICH ARE KNOWN TO BE DIFFICULT TO SEAT PROPERLY.