I have heard of at least four aircraft in the last six months having hard landings some severe, luckily with no fatalities.
In each event the pilot stated that they did not stiff arm the collective. I am sure they probably did not move their arm in relation to their body. If their body was moving without moving their arm they were inducing biomechanical feedback into the system.
In one occasion the bounce was so severe that the pilots feet came off the pedals. This situation of biomechanical feedback is also mentioned in the NTSB report DCA16FA199 of the July 06, 2016 accident of the Bell 525 test aircraft as a major factor.
Possible Mechanical factors The connection where the input rod connects to the servo all the bolts to include those on the servo should turn freely by hand after keying the connecting hardware. ( in the book ) If this hardware is tightened too tight it will cause feedback in the control.
I have seen on many aircraft that the head of the Pilots collective stick additional switches / buttons have been added.
This adds to the weight at the end of a long arm, making it hard to keep the collective from dropping in flight. Making it hard or impossible to correct with friction, preset or Pilot applied.
One mechanical item that can effect this is not enough built in or ground adjustable friction. ( TM 55-1520-210-23-2 pages included ) & ( TM 55-1520-242-MTF page included )
This adjustment should be done with the use of a hydraulic power supply. UH-1 requirements are 6 GPM flow and 1,000 psi.
With the Pilots collective boot removed you will see at the base of the collective a boss with two set screws that can be tightened or loosened onto a large washer. The screws are used to adjust the preset friction and should be adjusted equally. If they are not they can cause binding in the shoe application.
This usually happens because the bottom one is hard to get to so people will only adjust the upper one.
I have included a photo of tools used to do the job. If you have the collective pulled up to about half travel it is relatively easy to make the adjustments. ( see enclosed photo ) This is what is used to set the basic friction of 8 to 10 lbs. up & down. Inspect the shoes and pads for serviceability.
Notice the CAUTION a force of less than 8 lbs. could result in damage to the aircraft “ collective bounce “ Tools
I have included a photo of the tools used to set the built in or ground adjustable friction.
A length of cord. The one pictured is parachute cord with loops on both ends long enough to go around the twist grip and attach to the scale hook end.
The scale pictured is a push/pull rod on one end and a hook on the other. JONARD TOOLS P/N GPP-15 This one from GRANGER about $65.00 You use the hook to the cord to pull up. Turn the scale around and push down with the rod end.
The other a 5/32 allen wrench with a reach of 16” is best. Both of the ones pictured are homemade.