oscillation in as350b3
today i experienced an oscillation in the eurocopter as350b3 for the first time. the b3 has just been returned to service after a 1500 hours (i believe). it had been flown a couple of times since then, until i departed the airport to ferry the helicopter out in the woods for sling load. i flew at 1500', 135 kias, 82% torque (90% on the fli, i believe), OAT 5 deg C. i had put lots (!) of friction on the collective, not touching the control, and gently keeping the cyclic to maintain the attitude. no turbulence in the area. suddenly, with no power or attitude changes, i got a oscillation up and down in the whole helicopter that corresponded to the main rotor rpm, same intensity, like the rotor system wasn't tracked properly. i gently reduced power just before the bleed valve opened (appx 91% ng) and the oscillation stopped. i returned to base with this power setting and landed normally.
i have heard of new b3's having this problem; but this was my first time. i want to make clear that this was no pilot induced oscillation, since the friction was almost max on, my left hand was on my thigh (until it had lasted for about 3-4 seconds), and i was not moving the cyclic, it was completely still. momentarily i thought it was turbulence, but it turned out to be rythmic. the technicians are checking the helicopter tomorrow morning, since i was on a night flight. my guess is that this could be the collective linkage (somwhere between the collective and the servo). is this the case? i have heard of other pilots who have had the same problem, and even worse oscillations than what i had.
the thing that worried me was that it came without any warning in smooth, calm air, with no changes in power/attitude, like lightning from clear sky, and disappeared when i reduced power.