Narrative:

After making 4 traffic patterns and landings to a full stop (full stop this case being from on the ground with full down collective; to a hover; normal takeoff; traffic pattern; approach to a hover; touchdown; and lowering the collective to the full down position); the helicopter was on the ground after the final approach and landing. The collective was full down and the situation was normal. The helicopter was facing into the wind; and there were no obstructions or unusual topographical features that would make wind a significant factor. I had been on the ground with the collective down for between 10 and 15 seconds. I then moved the engine condition lever from flight position to ground idle. After the engine had spooled down a bit; the helicopter began to shake violently. It went from on the ground and stable to bouncing back and forth between the left and right landing gear in what seemed to be a second or two. The controls did nothing to dampen the vibration. I then applied the rotor brake in with the hope of preventing a roll over; as I knew this would subject the helicopter to severe damage. The rotor brake did its job well. Within a split second after engagement; the vibration began to decrease in amplitude. The landing gear had already broken at this point. The inertia of the rotor blades was absorbed by the rotor brake. The helicopter remained upright; resting on its belly. This did; however result in a 270 degree turn to the right before the helicopter came to rest. The rotor blades did not make contact with the ground or any part of the aircraft; and are undamaged. The fiberglass fairing on the bottom of the tail boom was destroyed; but the tail boom did not sustain any other damage. I was the sole occupant of the aircraft. I was not injured. There was no damage to persons or property on the surface.callback conversation with reporter revealed the following information: the reporter stated that this event was the result of a design problem known to the mfg for some time. The uk military first discovered the ground resonance problem and would not allow unmodified aircraft to fly. Landing shock absorbers needed to dampen any ground resonance were omitted from the aircraft design on early model aircraft. Without the two main strut shock absorbers; ground resonance can be transmitted back up to the rotors. Newer aircraft have the modification installed during manufacturing and at this point there are very few of the earlier unmodified aircraft remaining. The modification costs about $50;000. The reporter stated that at very least the mfg should be required to publish a pilot handbook revision noting this problem. As it is; users are left to discover it on their own often with dire consequences.

Related reports:

Google
 

Original NASA ASRS Text

Title: AN SA-341G EXPERIENCED AN UNCONTROLLABLE GND RESONANCE PROBLEM AFTER LNDG; EVEN WITH FULL DOWN COLLECTIVE.

Narrative: AFTER MAKING 4 TRAFFIC PATTERNS AND LANDINGS TO A FULL STOP (FULL STOP THIS CASE BEING FROM ON THE GROUND WITH FULL DOWN COLLECTIVE; TO A HOVER; NORMAL TAKEOFF; TRAFFIC PATTERN; APPROACH TO A HOVER; TOUCHDOWN; AND LOWERING THE COLLECTIVE TO THE FULL DOWN POSITION); THE HELICOPTER WAS ON THE GROUND AFTER THE FINAL APPROACH AND LANDING. THE COLLECTIVE WAS FULL DOWN AND THE SITUATION WAS NORMAL. THE HELICOPTER WAS FACING INTO THE WIND; AND THERE WERE NO OBSTRUCTIONS OR UNUSUAL TOPOGRAPHICAL FEATURES THAT WOULD MAKE WIND A SIGNIFICANT FACTOR. I HAD BEEN ON THE GROUND WITH THE COLLECTIVE DOWN FOR BETWEEN 10 AND 15 SECONDS. I THEN MOVED THE ENGINE CONDITION LEVER FROM FLIGHT POSITION TO GROUND IDLE. AFTER THE ENGINE HAD SPOOLED DOWN A BIT; THE HELICOPTER BEGAN TO SHAKE VIOLENTLY. IT WENT FROM ON THE GROUND AND STABLE TO BOUNCING BACK AND FORTH BETWEEN THE LEFT AND RIGHT LANDING GEAR IN WHAT SEEMED TO BE A SECOND OR TWO. THE CONTROLS DID NOTHING TO DAMPEN THE VIBRATION. I THEN APPLIED THE ROTOR BRAKE IN WITH THE HOPE OF PREVENTING A ROLL OVER; AS I KNEW THIS WOULD SUBJECT THE HELICOPTER TO SEVERE DAMAGE. THE ROTOR BRAKE DID ITS JOB WELL. WITHIN A SPLIT SECOND AFTER ENGAGEMENT; THE VIBRATION BEGAN TO DECREASE IN AMPLITUDE. THE LANDING GEAR HAD ALREADY BROKEN AT THIS POINT. THE INERTIA OF THE ROTOR BLADES WAS ABSORBED BY THE ROTOR BRAKE. THE HELICOPTER REMAINED UPRIGHT; RESTING ON ITS BELLY. THIS DID; HOWEVER RESULT IN A 270 DEGREE TURN TO THE RIGHT BEFORE THE HELICOPTER CAME TO REST. THE ROTOR BLADES DID NOT MAKE CONTACT WITH THE GROUND OR ANY PART OF THE AIRCRAFT; AND ARE UNDAMAGED. THE FIBERGLASS FAIRING ON THE BOTTOM OF THE TAIL BOOM WAS DESTROYED; BUT THE TAIL BOOM DID NOT SUSTAIN ANY OTHER DAMAGE. I WAS THE SOLE OCCUPANT OF THE AIRCRAFT. I WAS NOT INJURED. THERE WAS NO DAMAGE TO PERSONS OR PROPERTY ON THE SURFACE.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT THIS EVENT WAS THE RESULT OF A DESIGN PROBLEM KNOWN TO THE MFG FOR SOME TIME. THE UK MILITARY FIRST DISCOVERED THE GND RESONANCE PROBLEM AND WOULD NOT ALLOW UNMODIFIED ACFT TO FLY. LNDG SHOCK ABSORBERS NEEDED TO DAMPEN ANY GND RESONANCE WERE OMITTED FROM THE ACFT DESIGN ON EARLY MODEL ACFT. WITHOUT THE TWO MAIN STRUT SHOCK ABSORBERS; GND RESONANCE CAN BE TRANSMITTED BACK UP TO THE ROTORS. NEWER ACFT HAVE THE MODIFICATION INSTALLED DURING MANUFACTURING AND AT THIS POINT THERE ARE VERY FEW OF THE EARLIER UNMODIFIED ACFT REMAINING. THE MODIFICATION COSTS ABOUT $50;000. THE REPORTER STATED THAT AT VERY LEAST THE MFG SHOULD BE REQUIRED TO PUBLISH A PILOT HANDBOOK REVISION NOTING THIS PROBLEM. AS IT IS; USERS ARE LEFT TO DISCOVER IT ON THEIR OWN OFTEN WITH DIRE CONSEQUENCES.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.