BE58 PILOT ON APPROACH REPORTS FLYING UNDER A C172 ON DEPARTURE IN THE OPPOSITE DIRECTION AFTER ANNOUNCING HIS INTENTIONS TO DO SO ON CTAF.

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I WAS CLRED FOR AN ILS APCH TO RWY 5 AT ZZZ. VISIBILITY WAS ABOUT 6 MI; NO CLOUDS. I WAS LISTENING TO UNICOM WHILE NEARING THE FINAL FIX. DETERMINED THAT THERE WAS A C172 (OR SIMILAR) DOING TOUCH AND GOES ON RWY 23. I BELIEVED THAT I COULD LAND ON RWY 5 SAFELY AFTER HE HAD COMPLETED HIS TKOF FROM RWY 23. I MADE SEVERAL ANNOUNCEMENTS OF MY LOCATION AND INTENTIONS; AND I COULD BOTH HEAR HIM RPT HIS POS AND SEE HIM ON THE RWY AND AFTER TKOF. I EASILY FLEW UNDER THE OTHER ACFT BY AT LEAST 500 FT. AFTER LNDG; THE OTHER ACFT CALLED ON UNICOM TO THE 'PLANE WHICH LANDED' AND SAID HE SUSPECTED WE HAD FLOWN UNDER HIM. I ACKNOWLEDGED THAT I HAD DONE THE SAME AND THAT I HAD BROADCAST MY INTENTIONS SEVERAL TIMES AND HEARD HIS BROADCASTS. HE SAID HE HAD NOT HEARD MY BROADCASTS. AFTER LNDG I COULD SEE THAT THIS WAS AN INSTRUCTOR WITH A STUDENT; MOST LIKELY PREPPING FOR SOME UNSUPERVISED SOLO TOUCH AND GOES; BECAUSE THE INSTRUCTOR GOT OUT AND WATCHED THE OTHER PLT DO THAT. IT DAWNED ON ME THAT I HAD GONE AGAINST MY USUAL PROC OF ENTERING PATTERN WITH PREVAILING TFC; ALL IN THE INTEREST OF SAVING A FEW MINS. I HAD NOT ENDANGERED ANOTHER PLT; BUT I HAD SUBJECTED THEM TO A SCENARIO IN WHICH THEY WERE UNWILLING PARTICIPANTS. FOR ALL I KNEW; THE PF COULD HAVE BEEN A STUDENT PLT ON FIRST SOLO; AND I WOULD HAVE BEEN THROWING IN UNNEEDED VARIABLES TO THEIR FLT. I THOUGHT BACK TO MY TIME AS A FLT INSTRUCTOR IN SIMILAR SITUATIONS AND MY CHAGRIN AT SEEING A CPR OR CHARTER PLANE COME IN AND TAKE SHORTCUTS INSTEAD OF GOING WITH THE FLOW. I HAVE DETERMINED TO SUPERVISE MYSELF MORE CLOSELY AND SEE THAT I STICK TO STANDARD PROCS RATHER THAN DO SOMETHING I MIGHT EVEN BE AGHAST AT OBSERVING ANOTHER PLT DOING.