Bell 412HP Yaw Instability

[Chopperlover]

Dear all,

There is an issue I've been having with a Bell 412HP on its yaw axis. During take off or approach for landing, there is a sudden left yaw that needs compensation from the pilots somewhere around the 60-65 knots area and whenever a power change is applied on either maneuver. I've already made several attempts interchanging Helipilots, CMTs and Tarsyn's but the issue still goes on. It is not the usual yaw reaction expected from a power setting change but a more inexpected and sudden motion of the helicopter.

Any thoughts?

[helix47]

Had this problem once. I am sure you know that above the 60-65 kts the yaw scas is inoperative and below that speed the yaw scas is working. The sudden yaw is the system switching on and off. Make sure you are in balance (ball in the middle) at this speed range. If it still happens our avionic man says maybe the CMTs need setting up. (?) how you do this i dunno.

[jetA1pilot]

think Helix is on the right track.

Have you monitored the API's just prior to and during the occurence? Might give some indication if the actuators are doing something 'untoward' on the relevant system AP1/AP2. Also, is HP in SAS or ATT mode and does the same happen with F/T switch off or F/T release button pressed in?

[Shawn Coyle]

Are you turning when passing through this speed range?
If my rapidly failing memory is right, below 60-65 knots the yaw channel is trying to maintain heading, and above 60-65 knots it's trying to maintain balanced flight.
Can't remember what the sensor is that tells the AFCS that the pilot is trying to over-ride the yaw channel, but that might also be a problem.
Does it do it on all 412s or just one?

[Torquelimited]

Are you sure you are operating a 412HP?
If the reaction is due to the heading control changing from YAW to ROLL commands at 60 knots........you are flying a 412EP 4axis ship.

[BJC]

The Bell 412CF used to have a similar problem on every aircraft that was corrected by a software upgrade to the TARSYN if I recall. The problem went away even if only one of the Tarsyns had the updated software. The yaw kick was only a couple of degrees though when decelerating through about 60 kts, you make it sound like this is a new thing in your aircraft and more significant then a few degs of yaw. Might be worth checking if you've recently swapped one out and perhaps have an older software version now.

[Chopperlover]

Thanks for your inputs ya'll!

I've been traveling and very busy on some other things and still hadn't had the chance to come back here to pprune to check out for possibles replies to my still ongoing issue.

BJC, the Tarsyns have not been replaced for a while in this helicopter and they have been interchanged in the helo just to troubleshoot the issue but it keeps doing it. I will check it again just to make sure, though. Thank you.

Torquelimited, yes it indeed is a BH412HP. We also do have a 4-axis 412EP which, by the way, has never had this problem. BTW, this is a recent issue as it hadn't happened before to the a/c. Thanks, though.

Mr. Shawn, No there is no turn while passing this speed. It's been doing it only on this one helicopter for a couple of months now.

JetA1Pilot, No we have not monitored the API's nor I know if they are flying SAS or ATT. ATT mode I believe it is but I'm not sure. Will check and let you guys know.

Helix, the CMT's were already checked and the one for the yaw channel was faulty (shorted) leading us to think the issue would be solved but it wasn't. It's keeps happening.

Again thank you very much for your inputs. Will try your advises and let ya'll know.

Regards,

CL.