sk76C+ FFCL&mixed mode

[cpt]

Does somebody can help me to understand why or how an FFCL can become jammed at the "fly gate" position in the mixed mode ?
In this particular case, in order to discriminate wich engine was causing some fluctuations around 95% N1, we barely could move the FFCL in the mixed mode and had to freeze the DECU instead.
With engine stopped,or even with N1 below 70% the FFCL could be moved freely in the mixed mode.
Could it come for instance, from a stiff stepper motor or rather from a rigging fault ? any ideas ?

[Doc Cameron]

Hi, interesting point, and one we encountered in different circumstances: When we changed to the 'new software' DECU's last year, the same thing manifested itself (but engines not running, battery on - engineers check). Engineers went back to SK to try and resolve, but I left shortly after, before feedback). Good job you didn't try to continue the lever movement as the cable would have 'de-clutched' (as I think the term goes).

I don't think you can be encountering a 'sticky stepper', as even with the stepper frozen (ala manual reversion), lever movement would still occour, just moving the FMV.

I did have a passing fancy that maybe the resolver was freezing when trying to move the lever in mixed - this is the only thing I could think of that had some connection between the DECU/manual cable/FMV in mixed mode (though it's only supposed to monitor the FMV movement)

If the problem was with the cable/manual cable adapter then this would also be apparant in 'true manual' mode.

Out of interest, I think that FSI have changed their checklist for engine oscillations, to reflect the ease of diagnosis when you go straight to manual - ie freeze the stepper immediately - if you hit the wrong engine, simply freeze the other one instead! (this topic had some discussion earlier this year on PPRUNE).

Regards,

Doc

[NickLappos]

Great questions and Doc, I think you are on the right track. If you both PM me I will pass on the email of the DECU Guru Test Pilot, Charlie Evans, who has offered to step you through some diagnostics (pun intended).

[soggyboxers]

cpt and Doc,

The problem Doc refers to is for DECUs with the TU109 software. I think the solution for that is to remove those DECUs and wait for a new software mod.

I encountered a similar problem with a C+ during training. On one particular aircraft on one engine, If I froze a DECU at a relatively high torque setting (70-90%) it was not possible to move the FFCL in the manual track. At a lower torque setting, or even at high torque on the ground it could be moved ok. If I remember our engineers checked for any cable rigging problems or snagging. I think it was only finally resolved by changing the FCU.

[212man]

"If you both PM me I will pass on the email of the DECU Guru Test Pilot, Charlie Evans, who has offered to step you through some diagnostics (pun intended)."

If you PM me, I'll go and ask him: he's sitting in the office next to me!:ok:

[NickLappos]

212man,
Ask Charlie if he needs a string of Christmas lights, I have some extras. Then post his response here, OK?

[212man]

He chuckled;)

In answer to the question (and apologies to Charlie if I misquote):

Three scenarios could be possible.

1. The problem could be due to a mechanical linkage jam within the HMU/FMU. This is a known problem that can occur with rapid movement of the FFCL, with a certain mod state HMU, most of which should have beeen replaced. However this could only happen in manual mode. The jam could be overcome, but it would be a sudden release followed by a floppy FFCL.

2. More likely, and possible in mixed mode, would be the FFCL cable having some sort of crimp or bunching. It is a long cable and it is not normally exercised, so a latent problem may go un-noticed for some time. It would give the symptons described and would be a more progressive release.

3. A further possibility is a problem in the FFCL quadrant itself. There are two microswitches at the top end of the gate to allow entry from the outer track to the inner track. If the inner track switch was damaged it could leave the FFCL midway between the tracks, and again this would only manifest itself on the occasion the FFCL was attempted to be retarded with the pilot thinking it was in the inner track. Again, normally the problem would not manifest itself as the FADEC would behave normally in FLY mode at all other times.

Hope this helps; it certainly led to an interesting discussion with Charlie.

[Doc Cameron]

I humbly submit that I don't like any of Charlies suggestions, as the crux of this is: The engine lever will not move in mixed mode, but will move freely in full manual (DECU commanded stepper frozen in Manual reversion). This does not seem to suggest any problem with the Engine lever, Throttle quadrant or indeed manual cable or cable adapter.

212, I think that maybe Charlie didn't get all the relavent info?

I did like the sound (or lack of it) from the new C++ XMSN, can we get them on the 92 please?

Cheers,

Doc

[cpt]

Thanks for your answers, I haven't had the occasion to fly on this particular helicopter, engineers tell me that the "fluctuations" situation has been cured in changing the transducer ( common cause of NR small fluctuations) and the problem of this jammed FFCL didn't come back since that.
I don't think it's an old software problem since our helicopters have been modified some time ago.
I don't think neither it has something to do with the "transducer"
When we have a look on schematic diagramms, the resolver (metering valve) seems indeed rigged to a linkage common to manual and automatic modes.
Well, I am going to PM then, if I can get an explanation has I had on the "mixing unit" ;) ....I like this feeling of beeing suddenly enlighted !