R44 HELI ENTERED UNCTLABLE DIVERGENT PITCH OSCILLATIONS DURING LNDG.

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CIRCUMSTANCES SURROUNDING R44 INCIDENT AT XXXX NEAR ZZZ. ENVIRONMENT: 1 PLT; 170 LBS; ABOUT 3/4 FUEL; WIND FROM THE E 4 KTS; HYD CTLS. PLT EXPERIENCE: 30000 HRS FIXED WING; 350 HRS ROTOR WING -- MOSTLY IN HUGHES TYPES 300/500 WITH SFAR CHKOUT IN R44; RHC SAFETY COURSE AND ABOUT 20 HRS IN R44'S WITH 2+ PAX. SOLO -- ONLY 1 LNDG. APCHED NORMALLY INTO WIND; CAME TO HOVER AND LET DOWN GENTLY. IMMEDIATELY AFTER TOUCHDOWN; ACFT BEGAN MOTION FORE AND AFT -- HARD TO SAY WHICH WAS FIRST. OSCILLATIONS INCREASED IN PITCH. I LOWERED THE COLLECTIVE ALL THE WAY SMOOTHLY BUT WITHOUT DELAY. THIS HAD NO EFFECT. IT IS ASSUMED THAT I WAS ATTEMPTING TO QUIET THE FORE AND AFT MOTION. WITH NO JOY IN AMPLITUDE DAMPENING; I THEN PICKED UP ON THE COLLECTIVE TO LOAD THE BLADES AND STOP THE ROCKING BEFORE THE TAIL BOOM WAS CONTACTED. AT THIS POINT; I LIFTED OFF SLIGHTLY AND BEGAN TO MOVE FORWARD AS I CERTAINLY DID NOT WISH TO GET HIGHER WHILE OUT OF CTL. I CONTACTED THE R FRONT SKID UNDER SOME TURF WHICH CAUSED A 15 DEG CLOCKWISE ROTATION AND AN APPROX 45 DEG PITCH UP. I IMMEDIATELY LOWERED THE COLLECTIVE AND PULLED THE MIXTURE CTL AND DISENGAGED THE CLUTCH AS THE SHIP CONTINUED TO ROCK BACK AND FORTH; HOWEVER; DAMPENING IN AMPLITUDE. I USED THE ROTOR BRAKE TO STOP THE BLADES. AT THIS TIME MY FRIENDS RIGHT BEHIND ME LANDED IN A HUGHES 500 HAVING WATCHED IT ALL FROM ABOVE. I THEN GOT OUT AND ALONG WITH 3 OTHERS; INSPECTED FOR DAMAGE. THERE WAS NO EVIDENCE OF A MAIN ROTOR BLADE STRIKE EITHER UPON THE GND OR THE TAIL BOOM. THE ONLY APPARENT DAMAGE WAS TO THE FIBERGLASS ON THE NOSE. AFTER STARTING THE ENG AND ENGAGING THE CLUTCH WITH NO VIBRATION AT ALL; WE SHUT IT DOWN AND AGAIN INSPECTED FOR DAMAGE OR LEAKAGE. FINDING NONE; I STARTED AGAIN AND CAME TO A SMOOTH HOVER. AFTER A COUPLE OF MINS I DID A HOVER TAXI AND INCREASED TO FLYING SPD -- AGAIN WITHOUT VIBRATION BEYOND NORMAL. I CONTINUED TO FLY THE ACFT BACK HOME WITHOUT INCIDENT OR ANY UNUSUAL CHARACTERISTICS OF ANY KIND. 2 DAYS LATER; 2 MECHS CAME FROM THE SHOP AND ON THEIR INSPECTION THEY OBSERVED THAT THE TAIL BOOM WAS TOO LOW -- INDICATING A POSSIBLE SPREADING OF THE SKIDS AND THAT THE MAST HAD CONTACTED THE FUSELAGE; AMONG OTHER THINGS. CONTRIBUTING FACTORS: NO FEEL ON THE HYD CTLS AT ALL. NO NEUTRAL POINT AS A GUIDE EVEN WITH CHANGING CTR OF GRAVITY -- WIDE VARIATION. MAX AFT CTR OF GRAVITY FOR THIS LNDG. SOFT GND -- EVEN WITH EVEN DEPRESSION OF SKID ALONG FULL LENGTH; HEEL AND TOE WENT IN 2-3 INCHES DURING OSCILLATIONS. NO SIDE TO SIDE MOVEMENT AT ALL. ADMONITION BY INSTRUCTOR TO 'JUST KEEP MOVING THE COLLECTIVE DOWN' WAS IN MY MIND AS THE FIRST ROCKING MOTION BEGAN; WHICH WITH THE 'BENT ARM' SKID PRODUCES A FORWARD ROCK WHEN AFT CTR OF GRAVITY CONDITION PRESENT AND RETURNING THE NON FEEL HYDS TO THE PREVIOUS NEUTRAL WOULD BE AN AFT POS THUS MOVING THE ACFT BACK ON THE ROCKING PART OF THE SKIDS AND EXACERBATING THE WHOLE PROCESS. I AM REMINDED OF THE MOVIE. NO WAY OUT. MY RECOMMENDATIONS AND ANALYSIS: I AM NOT A HAM HANDED PLT. I DO NOT OVER CTL AIRSHIPS OF ANY KIND. I HAVE ABOUT 350 HRS IN HELIS BUT AM NEW TO THE R44; BUT NOT BRAND NEW. WITH 20 HRS OR SO; AND THE SFAR AND FACTORY COURSE; NO TENDENCY HAS EVER BEEN NOTED THAT WOULD EXPLAIN THIS OCCURRENCE TO MY SATISFACTION THUS I SEARCH FOR REASONS WHY; AND HOW IT COULD HAVE BEEN AVOIDED. I AM SURELY MORE AWARE NOW ABOUT THE DIFFERENCE WITH THE CHANGE IN CTR OF GRAVITY AND IN PARTICULAR OF THE CLIPPER SKIDS SINCE I HAVE NOW FLOWN IN THE SAME EXACT CONFIGN AN ADDITIONAL 20 HRS EXPERIENCING A SIMILAR ROCK BUT OF COURSE NOW I EXPECT TO TOUCH DOWN ON THE BACK OF THE SKIDS AND LOWER IT MORE GENTLY -- BELIEVE ME! I WOULD RECOMMEND THAT THE MANUAL AND THE TRAINING BE MODIFIED TO INCLUDE WARNINGS ABOUT THE ROCKING TENDENCY WHEN CONTACTING THE AFT PORTIONS OF THE SKIDS FIRST -- PARTICULARLY IN THE CLIPPER SINCE IT IS SHAPED LIKE A ROCKING CHAIR TO ACCOMMODATE THE FLOATS. IF THE COLLECTIVE IS LOWERED TOO FAST; A ROCKING TENDENCY IS SET UP; AND THE NEUTRAL ON THE CYCLIC IS FARTHER FORWARD IN THE AFT CTR OF GRAVITY CONDITION; THUS IF ONE RETURNS TO THE 'OLD NEUTRAL;' LEARNED AND IN FEEL MEMORY FROM FLYING WHILE TRAINING WITH LESS THAN FULL TANKS AND 2 PERSONS IN THE FRONT; THEN AFTER THE FIRST ROCK FORWARD; AN AFT ROCK COULD BE INDUCED STARTING THE PITCH OSCILLATIONS I EXPERIENCED EVEN AFTER LOWERING THE COLLECTIVE. ANYONE WOULD NATURALLY TRY TO STOP THE OSCILLATIONS BUT WITH NO FEEL; A MODIFIED NEUTRAL AND A BENT TUBE SKID; IT WAS VERY NEARLY A HULL LOSS. FROM LOOKING AT THE NOSE POS; THE ONLY WAY THE BLADES DID NOT HIT WAS CONING FROM THE COLLECTIVE BEING PULLED IN AND AFT CYCLIC -- TRULY; AMAZING. I WOULD SAY THE LUCKIEST THING I HAVE SEEN. AN UGLY RIDE AND NO CLR BEGINNING REASON FROM A COMPLETELY NORMAL APCH AND TOUCHDOWN INTO A LIGHT WIND EXCEPT FOR THE SINGLE 170 LB OCCUPANT AND ABOUT 3/5 TANKS FUEL AND THE CLIPPER SKIDS PLUS SOFT BUT NOT MUSHY GND. THE ONLY THING I CAN THINK OF THAT I ADDED WAS PERHAPS LOWERING THE COLLECTIVE TOO FAST -- BUT ONLY A MINOR AMOUNT -- JUST TO CONTINUE THE TOUCHDOWN -- CERTAINLY NOT A QUICK SHOVE DOWN. NO RAPID INPUTS WERE PUT IN UNTIL THE ROCKING BEGAN AND GOT WORSE RAPIDLY -- PERHAPS 3 FULL AND INCREASING FORE AND AFT MOTIONS OR MORE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PLT STILL DOES NOT KNOW WHY THE INCIDENT OCCURRED. HE THINKS CTR OF GRAVITY AND SOFT GND CONTRIBUTED. HE HAS NOT TALKED TO THE ACFT FACTORY TEST PLTS TO FIND OUT WHETHER THIS IS A PROB ON THIS ACFT TYPE. HE HAS FLOWN OTHER ACFT OF THE SAME TYPE WITHOUT THE PROB.