LOW TIME PRIVATE PLT SEVERS A SMALL TREE WITH THE PROP OF HIS C172 WHILE TAXIING ON A GRASS TURF ARPT. DETERMINES IT IS SAFE TO FLY AND DOES SO WITHOUT CONSULTATION WITH MAINT. LATER LEARNS THE FLT WAS IN VIOLATION OF AN AD WHICH SPECIFIED A MORE STRINGENT INTERPRETATION OF THE SUDDEN STOPPAGE CRITERIA.

Narrative

I FLEW A RENTAL PLANE FROM MY HOME BASE ARPT TO ZZZ ON A PLEASURE FLT WITH A FRIEND (ALSO A PLT) AS THE ONLY PAX. THE FORECAST HAD BEEN FOR A PERFECT VFR DAY; AND I HAD CALLED FSS IN THE MORNING TO CONFIRM THE WX AND CHK ON ANY NOTAMS. I LANDED ON A GRASS TURF RWY; AND TAXIED ALONG THE RWY TOWARDS THE PARKING AREA ON THE FAR NW SIDE OF THE RWY. THERE WERE A FEW PLANES PARKED; FACING E. AS I APCHED THE PARKING AREA; I MOVED SLIGHTLY TO THE L (W) OF THE CTR OF THE RWY; SETTING UP FOR A R TURN FURTHER DOWN TO PARK NEXT TO THE PLANE CLOSEST TOWARDS ME. AS I CONTINUED TAXIING; THE PLANE SUDDENLY HAD SOME SORT OF IMPACT THAT FORCED IT INTO A SHARP 90 DEG TURN TO THE L; AND IT CONTINUED ON A PATH STRAIGHT TOWARDS THE BRUSH AND TREES THAT BORDER THE RWY. I HIT THE BRAKES; BUT WAS NOT ABLE TO STOP -- PERHAPS SLIPPING ON THE TURF -- BEFORE THE NOSE OF THE PLANE ENTERED JUST A LITTLE BIT INTO THE BRUSH; WITH THE PROP CHOPPING UP SMALL BRANCHES AND SPLITTING A SMALL TREE (APPROX 3 INCH DIAMETER). THE TREE BROKE AND FELL SIDEWAYS HITTING THE FAR LEADING EDGE OF THE L WING. EVEN THOUGH THE PLANE HAD STOPPED; THE ENG CONTINUED TO RUN UNTIL I STOPPED IT BY PULLING THE MIXTURE KNOB. MY PARTNER AND I COULD NOT DETERMINE THE CAUSE FOR THE SUDDEN TURN; WE COULDN'T FIND ANY OBVIOUS OBSTACLES IN THE TURF; AND WE COULDN'T IDENT ANY SIGNS OF IMPACT ON THE TREES ON THE L SIDE OF THE RWY; EXCEPT FOR THE TREE THAT FELL ON THE WING AFTER IT HAD BEEN SPLIT OFF BY THE PROP. AFTER PUSHING THE PLANE BACK; WE DID A THOROUGH INSPECTION. THE ONLY VISIBLE DAMAGE WAS A DENT ON THE FAR END OF THE LEADING EDGE OF THE PROP; AND FLAPS AND AILERONS FUNCTIONED NORMALLY. THE ENG STARTED NORMALLY; WITH ALL INSTS IN THE PROPER RANGE. I USED PWR TO PARK THE PLANE ALONG THE OTHER PLANES. NO MECHANICAL SVCS ARE AVAILABLE. I THEN CONSIDERED MY OPTIONS; IN CONSULTATION WITH MY PLT PAX. KEY FACTORS IN MY DECISION-MAKING PROCESS WERE: 1) THE ENG WAS NOT STOPPED FORCEFULLY BY ANY IMPACT ON THE PROP; BUT CONTINUED RUNNING UNTIL I STOPPED IT; 2) THE ENG HAD STARTED AND FUNCTIONED NORMALLY WHEN I PARKED THE PLANE; AND 3) NOSE AND PROP SHOWED NO SIGNS OF DAMAGE. THIS LED TO THE ASSESSMENT THAT THE ENG AND PROP WOULD OPERATE NORMALLY. FURTHERMORE; THE DENT IN THE L WING DID NOT APPEAR LARGE ENOUGH TO HAVE ANY SIGNIFICANT INFLUENCE ON THE PLANE'S FLYING CHARACTERISTICS; ESPECIALLY WITH ONLY 2 PERSONS ON BOARD (IE; WELL BELOW MAX WT OF THE 4 PERSON PLANE). BASED ON THIS ASSESSMENT; I MADE THE DECISION THAT I WOULD GO THROUGH A THOROUGH PREFLT; A NORMAL ENG START; THEN BACK-TAXI ON RWY; PERFORM A RUN-UP; AND IF ALL INSTS AND RUN-UP SHOWED ALL THE PROPER READINGS AND PASSED ALL CHKS; I WOULD TAKE OFF; CLB AND STAY CLOSE TO THE ARPT UNTIL I WOULD BE ASSURED THAT THE PLANE RESPONDED CORRECTLY TO ALL CTL INPUTS AND THAT IT KEPT A STABLE FLT ATTITUDE. SHOULD ANYTHING UNUSUAL BE NOTICED; I COULD LAND BACK ON RWY. MY PAX AGREED WITH MY DECISION AND FELT COMFORTABLE WITH THE PLAN. THIS PLAN WAS CARRIED OUT; EVERYTHING FUNCTIONED NORMALLY; AND I DECIDED TO HEAD BACK TO THE HOME BASE. THE FLT TO AND LNDG AT THE HOME BASE ARPT WERE SMOOTH AND UNEVENTFUL; WITHOUT ANY INDICATION WHATSOEVER THAT ANYTHING WAS OUT OF ORDER. UPON ARR AT THE HOME BASE; I IMMEDIATELY INFORMED THE STAFF OF THE FBO OF WHAT HAD HAPPENED. I ASKED TO SPEAK ON THE PHONE WITH THE MGR OF THE RENTAL OP AND I WAS ABLE TO BRIEF HIM ABOUT THE INCIDENT. ANOTHER PLT HAD RESERVED THE PLANE AND WAS AT THE FBO. I DESCRIBED TO HIM WHAT HAD HAPPENED AND EXPLAINED THAT THE PLANE HAD FUNCTIONED DURING THE RETURN FLT; WITHOUT SHOWING ANY SIGNS OF POSSIBLE PROBS. THE MGR WAS ASKED (ON THE PHONE) ABOUT APPLICABLE POLICIES OF THE RENTAL COMPANY IN THIS SITUATION. THE MGR; WHO I HAD JUST BRIEFED; REPLIED THAT THE DECISION TO FLY OR NOT TO FLY WOULD BE UP TO THE NEXT PLT; AND THAT EITHER DECISION WOULD BE FINE WITH HIM. THIS RESPONSE CONFIRMED TO ME THAT IT WAS WITHIN MY AUTH AS PIC TO HAVE MADE THE DECISION TO FLY BACK TO THE HOME BASE. AFTER A MECH HAD EXAMINED THE PLANE; I MET WITH THE MGR OF THE RENTAL COMPANY AND THE MECH; AND I WAS BRIEFED ON THE DAMAGES THE MECH HAD DETECTED. I ALSO WAS INFORMED AND GIVEN A COPY OF THE 2004 AIRWORTHINESS DIRECTIVE (04-10-14-C1); WHICH CONTAINS A CHANGE IN THE DEFINITION OF A 'PROP STRIKE OR SUDDEN STOPPAGE;' AND WHICH REQUIRES COMPLIANCE WITH THE AIRWORTHINESS DIRECTIVE (THAT INCLUDES AN INSPECTION/REPAIR OF THE CRANKSHAFT COUNTER BOARD RECESS; ETC; AND REPLACEMENT OF BOLT AND LOCK PLATE) BEFORE FURTHER FLT. IT WAS EXPLAINED TO ME THAT THE PROP STRIKE THAT OCCURRED FIT THIS CHANGE DEFINITION; AND THAT; THEREFORE; THE RETURN FLT TO THE HOME BASE WAS NOT IN COMPLIANCE WITH THE AIRWORTHINESS DIRECTIVE. THIS AIRWORTHINESS DIRECTIVE WAS A COMPLETE SURPRISE FOR ME. THE RENTAL COMPANY HAD NEVER INFORMED ME OF THE AIRWORTHINESS DIRECTIVE; EVEN THOUGH I HAD RENTED THIS PARTICULAR PLANE AS THE RENTING PLT. HAD I KNOWN OF THE AIRWORTHINESS DIRECTIVE; I CERTAINLY WOULD NOT HAVE FLOWN THE PLANE BACK. BECAUSE OF INCOMPLETE INFO; IT THUS APPEARS THAT I INADVERTENTLY FAILED TO COMPLY WITH THE AIRWORTHINESS DIRECTIVE AND; HENCE; COULD BE SUBJECT TO FAA ENFORCEMENT ACTIONS. AS PIC AND RENTER; I BELIEVE THAT THE DECISION-MAKING PROCESS IN SITUATIONS SUCH AS THE ONE RPTED HERE WOULD BENEFIT IF THE RENTAL COMPANY WOULD PROVIDE THE RENTING PLT WITH UP FRONT CLEAR; EXPLICIT; AND UP-TO-DATE INFO ABOUT CRITICAL AIRWORTHINESS DIRECTIVES; ESPECIALLY THOSE THAT DEFINE INSTANCES SPECIFIC TO THE PARTICULAR RENTAL AIRPLANE THAT WOULD PROHIBIT FLYING THE PLANE.