PLT OF C172 FAILED TO REPLACE OIL FILLER CAP AFTER ADDING OIL FOR BFR FLT. DISCOVERED ENRTE; LANDED AT DEST.

Narrative

THE PURPOSE OF THE PART 91 FLT FROM OAK TO 0Q5 WAS A BIENNIAL FLT REVIEW IN ACCORDANCE WITH 14 CFR PART 61.56. THE DAY BEFORE THE FLT; I HAD CHANGED THE ENG OIL AND OIL FILTER; AND HAD RUN THE ENG TO CHK FOR LEAKS. THE MORNING OF THE FLT; I NOTICED THE ENG OIL LEVEL WAS 1 QUART LOW; AND REMEMBERED THAT I HAD NOT ACCOUNTED FOR THE QUART OF OIL IN THE FILTER WHEN I FILLED THE CRANKCASE THE DAY BEFORE. I REMOVED THE OIL FILLER CAP; AND PLACED IT ON THE TOP OF THE BATTERY BOX; ADDED A QUART OF OIL; DISPOSED OF THE EMPTY QUART CONTAINER. AND RETURNED TO THE AIRPLANE TO SECURE THE COWLING DOOR AND CONTINUE THE PREFLT INSPECTION. I WAS DISTR BY CONVERSATION WITH THE FLT INSTRUCTOR; AND NEGLECTED TO SECURE THE OIL FILLER CAP BEFORE SECURING THE COWLING DOOR. ABOUT 30 MI FROM THE DEST (0Q5); I NOTICED WHAT APPEARED TO BE INSECT SPLATTER GROWING IN SIZE AT THE LOWER L CORNER OF THE WINDSHIELD. IT OCCURRED TO ME THAT IT MIGHT BE ENG OIL; AND I REMARKED TO THE CFI THAT I COULDN'T REMEMBER SECURING THE OIL FILLER CAP AFTER ADDING THE QUART OF OIL DURING THE PREFLT INSPECTION. OIL PRESSURE AND TEMP INDICATIONS WERE NORMAL; AND SINCE THERE WERE NO ARPTS CLOSER THAN THE DEST; WE CONTINUED TO THE DEST ARPT 0Q5. IMMEDIATELY UPON LNDG AND SHUTTING DOWN THE ENG AT 0Q5; THE CFI AND I INSPECTED THE AIRPLANE AND FOUND THE OIL FILTER CAP RESTING ON THE TOP OF THE BATTERY BOX WHERE I HAD LEFT IT BEFORE DEP FROM OAKLAND. THE ENG OIL LEVEL WAS DOWN APPROX 1 QUART; AND THERE WAS A FILM OF ENG OIL THROUGHOUT THE ENG COMPARTMENT; AND ON THE LOWER PORT SIDE OF THE AIRFRAME EXTERIOR. THE DURATION OF THE FLT FROM OAK TO 0Q5 WAS 1.7 HRS. IN DISCUSSING THIS INCIDENT WITH THE CFI; WE AGREED THAT THE OVERSIGHT COULD HAVE BEEN PREVENTED BY HOLDING THE FILLER CAP INSTEAD OF PLACING IT ON THE BATTERY BOX WHILE ADDING THE OIL. BY HOLDING THE CAP IN MY HAND; IT WOULD HAVE BEEN DIFFICULT TO FORGET REPLACING AND SECURING IT AFTER ADDING THE OIL. SUPPLEMENTAL INFO FROM ACN 581602: THE XCOUNTRY FLT WAS TO INCLUDE A FLT REVIEW; WITH MYSELF AS THE INSTRUCTOR ACTING AS PIC IN THE OTHER PLT'S AIRPLANE (SINCE HIS 2-YR CURRENCY HAD JUST EXPIRED). I PAID LESS ATTN TO HIS PREFLT INSPECTION; AS HE FLIES ON A REGULAR BASIS; IS VERY THOROUGH AND MAINTAINS HIS PLANE IN IMPECCABLE CONDITION. IN THE FUTURE; WHEN ACTING AS PIC; AND TO THE EXTENT POSSIBLE; WHEN NOT THE PIC; I WILL PARTICIPATE MORE IN THE PREFLT INSPECTION; EVEN WHEN THE OTHER PLT IS HIGHLY EXPERIENCED AND CONSCIENTIOUS.