thanks to all for your answers.
thanks oracle, i think it will be caused by the cross-tube but there were one think that i canīt understand. i donīt understand why i continued bouncing flying (bigs bounces one-per-rev of SEVERE vertical oscillations)?.
the droop stop restriners i think there were right because i was flying three hours in fire-fighting when happened the first time (two hours of flyght, 40 minutes of rest and one hour of flyght in fire-fighting until i descend to recover the firefighters). i was flying only whith one hp engage in SAS mode (BJC). the landig zone was ten degrees of slope (left side), the collective was not full down because the slope, i need to apply collective to avoid slipping and the ciclyc styc was to the left to maintain the slope.but the helicopter was stable and without vibrations. When the firefighters open the basket to save the bambi bucket the helicopter started to severe verticals ascillations. i maintain in that position to
allow firefighters to derogate, and when i was close to dump i take of but the helicopter continued whit verticals oscilaciones and we lose the tail rotor control during 30 or 40 seconds. when i regain the tail rotor control the verticals continiun in all the time and i could disconnect the hp1 and all verticals disappeared. i go to my base and change hp1 box control, and we make a one hour test flight testing hp1.
maintenance not found anything abnormal and they thought that was due to a wrong behavior of hp1.
The next flight was a cruise flight and it was fine.
The next one was fine until i landed. helicopter at ground, collective full down, hp1 off (in caution to prevent a system fail), 100%NR, no vibration, one minute to descend all firefighters, and when they open the basket to draw the bambi bucket the helicopter sudenly become to jump, a big jumps. i take of fast, when the fire fighters broke the helicopter, to prevent ground resonance but it continued jumping, i had full control but the helicopter has several verticals vibrations one-per-rev. i try to take of but noises in the transmission were increasing (i think was the lift link) and when i was over 30 knots and one hundred feet the helicopter try to yaw 30š to the right in a jump, I fixed that position but it try to another jump to the right in 30š. i had to shutdown the engines and try to land in autorotation and crashed the helicopter.
two more helicopters in other company has similar problems and crashed the helicopters.(2005, 2006, 2006 and 2007).
I did not have communication maintenance.
i continue flying 412 with caution in ground.
i was sure that all verticals vibrations disappear with throttles in iddle posittion or shutdown engines. Now bell recommended 4š slopes.
thanks to all