C172 PILOT REPORTS SEAT SLIDING AFT DURING VISUAL APPROACH TO RWY 16L AT VNY CAUSING PITCH UP AND DISORIENTATION; THEN A TOUCH AND GO ON RWY 16R WITHOUT CLEARANCE.

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I WAS DOING 'TOUCH & GOES' ON RWY 16L AT VNY ARPT. ON DOWNWIND THE TWR ASKED ME TO DO A 'SHORT APCH' TO RWY 16L. AS I WAS DOING THE APCH; THE PLT'S SEAT SLID BACK ON THE RAILS. WHEN I FINALLY REALIZED WHAT HAD HAPPENED AND REGAINED MY COMPOSURE; I DECIDED THAT I NEEDED TO GET IT ON THE GND ASAP. UNFORTUNATELY; I WAS NOW IN A POS WHEREBY A TURN TO RWY 16L WOULD HAVE REQUIRED TOO STEEP A TURN AND A POSSIBLE STALL/SPIN CRASH. I CONTINUED TO COMPLETE THE APCH AS RWY 16R WAS AHEAD OF ME. AS I WAS LNDG; I REALIZED THAT I HAD JUST SOLVED ONE PROB AND NOW HAD CREATED ANOTHER PROB. IF AN ACFT WAS TAKING OFF OR LNDG ON RWY 16R; I WAS ABOUT TO BE HIT FROM THE REAR BY ANOTHER ACFT! THE SEAT HAD STAYED IN ITS POS AND HADN'T MOVED ANYMORE DURING THE LNDG AND SO I DECIDED THAT THE QUICKEST WAY OUT OF HARM'S WAY WAS TO GET AWAY FROM RWY 16R ASAP TO REDUCE THE TIME EXPOSED TO DANGER. A GAR WAS OUT OF THE QUESTION BECAUSE THE ACFT HAD A SPRING LOADED ELECTRIC FLAP SWITCH; WHICH HAD TO BE HELD FOR SEVERAL SECONDS TO RETRACT THE FLAPS WHILST DOING THE GAR. THIS MEANS I COULDN'T HAVE MY HAND ON THE THROTTLE TO CTL THE PITCH BY PWR DURING THE TRANSITION. IN ADDITION; ADDING PWR FOR A GAR WITH FULL FLAPS CREATES A STEEP PITCH UP ATTITUDE WHICH WOULD PUT EXTRA REARWARD PRESSURE ON THE SEAT; WHICH I DIDN'T WANT TO DO. I DECIDED TO PUT THE ACFT ON THE RWY; CLEAN UP THE FLAPS AND TKOF AGAIN (COMPARED TO A MORE TIME CONSUMING ROLLOUT) AND MOVE BACK OVER TO RWY 16L. I USED THE TRIM WHEEL TO ROTATE THE ACFT AND CLB AND HELD THE YOKE WITH ONLY MY FINGERTIPS SO THAT IF THERE WERE ANYMORE SEAT SLIPPAGE MY HAND WOULD SLIP OFF THE YOKE AND NOT CAUSE A STALL/SPIN CRASH. MY NEXT DECISION WAS WHETHER OR NOT TO STAY IN THE L SEAT FOR LNDG OR SWITCH TO THE R SEAT. AFTER SOME SERIOUS TESTING OF THE SEAT WHILE I WAS IN THE PATTERN; I DECIDED THAT IT WOULD BE BETTER TO LAND USING THE L SEAT BECAUSE OF LESS FLYING EXPERIENCE IN THE R SEAT. I LANDED ON RWY 16L BUT IT WAS NOT A GOOD LNDG AS I LANDED A LITTLE LONG. I REPEATED MY TKOF PROC THAT I HAD JUST USED ON RWY 16R AND CAME BACK FOR ANOTHER LNDG WHICH WAS SUCCESSFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE ACFT WAS NOT OWNED BY THE REPORTER AND WAS QUITE OLD. WEAR IN THE SEAT LOCKING MECHANISM ALLOWED IT TO SLIP IF IT WAS NOT MANUALLY HELPED INTO THE LOCKED POSITION. HE BELIEVES THAT HIS DECISION TO MAKE A TOUCH AND GO ON RWY 16R WAS REASONABLE; GIVEN THE TIME CONSTRAINTS AND THE PERCEIVED NEED TO 'GET OUT OF THE WAY.'