Is this another area of the vertical stabilizer or is the repair and beefup previously incorporated in this area cracking now?. Having been involved in helicopter vibration for more years than I care to admit the tail rotor balance was easy to carry out and did not alter significantly when checked on a 50 or 100 hour routine basis . With the advent of the IHUMS we were able to look at the balance through all the phases of flight but more importantly, because an axially orientated accelerometer was fitted as well as the vertical or inplane accelerometer, we could see the levels in that plane not just the "inplane balance".
Looking across the fleet of S76A and then when modified to S76A+ there were varying vibration levels, some very high and this was thought to be a possible contributory factor to the vertical stabilizer problems. If a high axial level was observed then a check of the tail rotor rigging and a tail rotor track could be carried out.
On the S61 we did actually have a system where we could make adjustment to the pitch links, based on the axial vibration phase and amplitude, to reduce the axial vibration but due to the size of the adjustment it was difficult to keep the barrel in position while tightening the lock nuts. This sort of approach can perhaps highlight if one of the blades or paddles is trying to find it's own flight path.
I would be interested to hear if anyone else has used this approach to monitoring the tail rotor vibration.