Bell 412
Join Date: Feb 2007
Location: Canada
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Salusa,
Thanks, we do have an access but all I can find, under Tools Weight and Balance, are fillable pdf documents that would not be user friendly for viewing C of G envelope after ploting the actual a/c configuration and load before a flight. Are there other ressources from the mybell.com that I may not have seen?
Regards,
Thanks, we do have an access but all I can find, under Tools Weight and Balance, are fillable pdf documents that would not be user friendly for viewing C of G envelope after ploting the actual a/c configuration and load before a flight. Are there other ressources from the mybell.com that I may not have seen?
Regards,
DAFCS, you don't have enough posts (10 required) to use the messaging/email function here. If you like to put up your email I could contact you and may be able to help you out.
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
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
EP's are notorious for fluctuating tq's at low power settings in cooler weather/cool nights when they are rigged to generally fly in warm/hot weather , fly during the day (average 25-30 oC day) no problem , fly the same machine at night do a low power decent to an airport or low speed/power search for example and you will have massive tq fluctuations of up to 10 % , nothing to do with Auto Pilot , it just makes it worse, if you engines are rigged for the general climate they operate in , being Cyprus average day 25oC , machines run fat dumb and happy in the warm climate all the time but because the 3D has the bleed valve at P2.8 air unlike the old 3B at P2.5 air it has a greater impact to the P3 air to the engine controls, as less power is needed and the air is cooler the bleed valve flutters as it is right on the open/close threshold and causes spikes in the P3 air as the engines start hunting one chases the other, easiest just split the tq about 4% , one engine bleed valve is definitely closed and the other definitely open , when the pilot pulls power again to normal levels match the engines again,
I have worked on EMS/SAR EP's for years in hot climates and the big Tq fluctuations always happen when the weather gets cooler or at night , I just got my pilots to get used to splitting tq's as soon as big fluctuations start and problem goes away.
I have worked on EMS/SAR EP's for years in hot climates and the big Tq fluctuations always happen when the weather gets cooler or at night , I just got my pilots to get used to splitting tq's as soon as big fluctuations start and problem goes away.
Our 412s in Cyprus normally only need an adjustment to torque matching as we transition from Summer to Winter and back.
When we do get a set of engines / governors / AFCUs / control frictions / DAFCS set up and behaving, they are normally fairly stable day or night but we dread having to change a governor / AFCU as that one change can lead to weeks of pain. We can go through three or four governors until we get one that is well suited to that particular aircraft. Get a bit tired of loading and removing ballast weights!!
When we do get a set of engines / governors / AFCUs / control frictions / DAFCS set up and behaving, they are normally fairly stable day or night but we dread having to change a governor / AFCU as that one change can lead to weeks of pain. We can go through three or four governors until we get one that is well suited to that particular aircraft. Get a bit tired of loading and removing ballast weights!!
We have done endless test flights over the years with all combinations of SAS ATT FD, on various aircraft at various airspeeds and flight regimes. This exact problem has shown itself on all four of our aircraft so I still think there is an inherent problem with the AHRS / DAFCS and the way it doesn't suppress the oscillations, probably related to the yaw channel as highlighted by TB180 and then the option to change back to pre TB-180 if it improved things. The option to revert to pre TB-180 was after other customers were complaining of cases of Mast Torque fluctuations but Bell gave no details of who they were and what configuration these aircraft were in.
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"Torque-oscillation" on the 412 can have many reasons....SAS/ATT FD are only one....
Tail rotor balance issues, Hydraulic issues, FT issues, pilot input, Throttle feathering, Mast mount issues and many more are just a few things to start looking at.
I know this is not an issue with most companies-but thats one advantage of a company having a properly trained maintenance test pilot working closely with very experienced engineers-those crews are most times able to detect and solve smaller issues before they result in something bigger (or-let alone-catastrophic...)...
Tail rotor balance issues, Hydraulic issues, FT issues, pilot input, Throttle feathering, Mast mount issues and many more are just a few things to start looking at.
I know this is not an issue with most companies-but thats one advantage of a company having a properly trained maintenance test pilot working closely with very experienced engineers-those crews are most times able to detect and solve smaller issues before they result in something bigger (or-let alone-catastrophic...)...
I think you guys have covered all the bases, but would still like to offer two areas to look into:
1) Governor to AFCU matching: Pratt came out with a bulletin about 15 years ago (sorry, do not have the number) which specifies governor P/N that is tuned to the latest AFCUs and helps reduce oscillations on the -3D. You may have done this already.
2) M/R servo input lever friction: as Bell went to "D" seals (D shaped O rings to help reduce oil leaks) in lieu of O Rings in the hydraulic servos, this drove to input lever friction above the desired value. This value is very low, around 1.5 Lbs if I remember correctly. This causes overshoots as the AFCS system pushes the lever until it finally moves and then moves too much. This was especially apparent during coupled ILS approaches, in very still air. Going back to plain O rings has helped reduce the friction.
Hope this helps.
Fly Safe, Always
1) Governor to AFCU matching: Pratt came out with a bulletin about 15 years ago (sorry, do not have the number) which specifies governor P/N that is tuned to the latest AFCUs and helps reduce oscillations on the -3D. You may have done this already.
2) M/R servo input lever friction: as Bell went to "D" seals (D shaped O rings to help reduce oil leaks) in lieu of O Rings in the hydraulic servos, this drove to input lever friction above the desired value. This value is very low, around 1.5 Lbs if I remember correctly. This causes overshoots as the AFCS system pushes the lever until it finally moves and then moves too much. This was especially apparent during coupled ILS approaches, in very still air. Going back to plain O rings has helped reduce the friction.
Hope this helps.
Fly Safe, Always
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Mmmmm ......
One of the many issues the years have thrown at the 412EP .... have you investigated the matching of the M/R Elastomerics ... and at what Nr you balance the M/R in cruise ... in the past we (?) have noted that depending on the age that they can cause issues especially when lifting HEAVY loads (especially heavy sling loads) ... if the RPMs are not beeped to say 101-102 the coning angle can allow for a sort of 'Hooks joint effect' oscillation which the engines then try to catch ... the engines the have a fight with the Yaw SAS and you can end up with an O'torque which spoils all your fun .... turning OFF the Yaw Ch. reduces it .... also putting the T/R into clear air when lifting also helps (but brings you into conflict with the RFM re: wind from the right etc etc) ..... compromise maybe the key (as Bell did with the design of that M/R head) ... *sigh
Cheers
Spinny
One of the many issues the years have thrown at the 412EP .... have you investigated the matching of the M/R Elastomerics ... and at what Nr you balance the M/R in cruise ... in the past we (?) have noted that depending on the age that they can cause issues especially when lifting HEAVY loads (especially heavy sling loads) ... if the RPMs are not beeped to say 101-102 the coning angle can allow for a sort of 'Hooks joint effect' oscillation which the engines then try to catch ... the engines the have a fight with the Yaw SAS and you can end up with an O'torque which spoils all your fun .... turning OFF the Yaw Ch. reduces it .... also putting the T/R into clear air when lifting also helps (but brings you into conflict with the RFM re: wind from the right etc etc) ..... compromise maybe the key (as Bell did with the design of that M/R head) ... *sigh
Cheers
Spinny
Thanks all.....keep them coming...………...with so many variables it's no wonder the DAFCS struggles to keep things under control!
If you could specify whether your machines are AHRS or Tarsyn equipped that would be appreciated.
Anyone out there with specific experience of that original AHARs yaw rate output change in TB180 that we were then given the option to reverse depending on Tail Rotor bits friction.
We have been measuring servo input lever frictions again only last week plus tail rotor pedal breakout forces with the aircraft running!! at the request of Bell.
If you could specify whether your machines are AHRS or Tarsyn equipped that would be appreciated.
Anyone out there with specific experience of that original AHARs yaw rate output change in TB180 that we were then given the option to reverse depending on Tail Rotor bits friction.
We have been measuring servo input lever frictions again only last week plus tail rotor pedal breakout forces with the aircraft running!! at the request of Bell.
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Thanks all.....keep them coming...………...with so many variables it's no wonder the DAFCS struggles to keep things under control!
If you could specify whether your machines are AHRS or Tarsyn equipped that would be appreciated.
Anyone out there with specific experience of that original AHARs yaw rate output change in TB180 that we were then given the option to reverse depending on Tail Rotor bits friction.
We have been measuring servo input lever frictions again only last week plus tail rotor pedal breakout forces with the aircraft running!! at the request of Bell.
If you could specify whether your machines are AHRS or Tarsyn equipped that would be appreciated.
Anyone out there with specific experience of that original AHARs yaw rate output change in TB180 that we were then given the option to reverse depending on Tail Rotor bits friction.
We have been measuring servo input lever frictions again only last week plus tail rotor pedal breakout forces with the aircraft running!! at the request of Bell.
Where are you located?
Dave Ed,
My comments/suggestions were for an AHARS machine. Old day Tarsyns did not seem to get into these oscillation issues.
Good fix would be going to the EPi
. The T-9 engine is much better at keeping things steady (has been since the RSAF & TCG days!).
Fly Safe, Always
My comments/suggestions were for an AHARS machine. Old day Tarsyns did not seem to get into these oscillation issues.
Good fix would be going to the EPi
. The T-9 engine is much better at keeping things steady (has been since the RSAF & TCG days!).Fly Safe, Always
My terms and conditions would not allow me to specify location.
Worked on Tarsyn equippped 212 / 412s on and off since 1979 with SCAS / SFENA / Sperry Helipilot with no memorable issues that weren't permanently mitigated. All old technology working well together.
Worked on Tarsyn equippped 212 / 412s on and off since 1979 with SCAS / SFENA / Sperry Helipilot with no memorable issues that weren't permanently mitigated. All old technology working well together.