Bell 412 with AHRS Mast Torque fluctuations
Good morning all,
I'm a B2 (avionic engineer) working on four Bell 412EPs, for the past seventeen years, fitted with AHRS gyros and 4 axis SPZ7600 DAFCS.
The aircraft have always had an underlying tendency towards a two second oscillation similar to Dutch Roll which the autopilot seems to enhance rather than dampen out especially in Rad Alt hold probably as it's a demanding mode driving through the collective actuator. FDR traces of Mast Torque, Nf, Ng, all show this two second fluctuation which I assume to be the natural governing cycle of the old mechanical governors and AFCUs. It can affect the aircraft in SAS or ATTD. In Rad Alt hold we can see 20% Mast Torque fluctuations on the FDR with controls, N1s, Nfs, Hdg all showing FDR traces with a two second fluctuation. BUT rolling back either engine reduces the oscillation sometimes to zero.
Soon after the laser gyros (AHRS) replaced the old Tarsyn (spinning wheel) gyros, Bell came out with Tech Bulletin TB180 which changed the yaw rate output to the DAFCS from the AHRS due to problems with it's sensitivity particularly in relation to differing frictions within the Tail Rotor hub!
Some customers who carried out this TB then complained about Mast Torque fluctuations so Bell gave permission to revert back to pre-TB180 to see if it would help with affected aircraft.
We did, and it helped, BUT whenever we have to change Nf governors / AFCUs this problem can reappear as we are experiencing this past month.
The engine governing system seems to be suited to the old Tarsyn / analogue AFCS but since the AHRS / DAFCS were introduced the engine governing system has to be operating ultra-perfectly which can be a challenge at times. We seem to changing too many governors and AFCUs on these aircraft.
I could go on but my main question is.....has anybody else out there with Bell 412 / AHRS configs experienced anything similar?
Dave Ed