A Blast from the past
Here is General Operation Safety Notice 76-6:
OSN-GEN- 76-6
TO: ALL 206A/B/L, 204B, 205A1 AND 212 OPERATORS
SUBJECT: COLLECTIVE MINIMUM FRICTION
SOME OPERATORS ARE FAILING TO MAINTAIN THE REQUIRED COLLECTIVE MINIMUM FRICTION. ADJUSTMENT FOR COLLECTIVE MINIMUM FRICTION (NOT TO BE CONFUSED WITH COLLECTIVE FRICTION THAT IS AFFECTED BY ADJUSTMENT OF THE KNURLED NUT ON THE COLLECTIVE STICK) MUST BE ACCOMPLISHED ON THE GROUND BY QUALIFIED MAINTENANCE PERSONNEL AS SET FORTH IN EACH MODEL'S MAINTENANCE MANUAL*. MAINTENANCE OF COLLECTIVE MINIMUM FRICTION IS A CERTIFICATION REQUIREMENT BY THE FAA AND IS DESIGNED TO PRECLUDE "COLLECTIVE BOUNCE" BY:
1. REDUCING THE EFFECTS OF PILOT OVER-CONTROL.
2. REDUCING EFFECTS OF WIND GUSTS AND TURBULENCE THROUGH THE ROTOR INTO THE AIRFRANE.
3. PREVENTING FEEDBACK FROM EXTERNAL SLING LOADS INTO THE AIRFRAME.
IF "COLLECTIVE BOUNCE" SHOULD OCCUR WITH THE PROPER COLLECTIVE MINIMUM FRICTION, THE CAUSE MAY BE WORN LINKAGES, PILOT OVERCONTROL, EXTREME TURBULENCE OR HYDRAULIC CYLINDER IN NEED OF REWORK, WHICH SHOULD BE CORRECTED INMEDIATELY.
DO NOT FLY WITHOUT PROPER COLLECTIVE MINIMUM FRICTION.
The only reason the 412 is not listed at the top is because it wasn't invented yet (1976 versus 1980) but same logic applies.
Me agrees with SASless; collective bounce is the prime suspect
