PPRuNe Forums - DRIVESHAFT FAILURE

PPRuNe Forums (https://www.pprune.org/)

- Rotorheads (https://www.pprune.org/rotorheads-23/)

- - DRIVESHAFT FAILURE (https://www.pprune.org/rotorheads/583598-driveshaft-failure.html)

Quote:

Originally Posted by John Eacott (Post 10063405)

I suspect that it hasn’t been pointed out to you because your interpretation is incorrect...

@megan 100% does know what he's talking about, he's just been misunderstood. If I may clarify his point. Within the S76 MGB, if the gear (that spins clockwise) between the main drive shaft gear (that spins anti-clockwise) and the gear (that spins anti-clockwise) that connects the #1 engine input gear to the TR drive shaft, should it fail, then the #1 engine will drive only the TR but not the main rotor. In that instance, should you misdiagnose the indications and shut down the #1 engine you will lose drive to the TR (the power from #2 engine only gets as far as the main drive shaft because of the failed gear). This has happened in an S76 before.

It's a scenario that gets discussed during recurrent training in the classroom, but I've never been asked to demonstrate it in the simulator. If the trainee is totally on-top of all the other malfunctions covered in the syllabus, and there is time available to look at other things, then sure, take a look at this one in the sim.

gulli, you’re quite right and my interpretation of megan’s statement was of a No 1 engine failure. Not a No 1 drive failure.

RTFQ John!!

Quote:

Originally Posted by megan (Post 10063369)

Sikorsky Archives | S-76

Thank you! Nothing beats a good schematic. Or: A picture is worth a thousand words.
With that schematic the possible failure mechanism and consequence becomes very clear!

Quote:

Originally Posted by henra (Post 10063514)

..With that schematic the possible failure mechanism and consequence becomes very clear!

What is also interesting, with this malfunction in S76 equipped with DECU, what behavior might you expect from the engines? The pilot response as per the ECL should be the same whether you have DECU or not, because this would be primarily indicated as a MGB failure (with secondary indications)...just don't get suckered into shutting down the #1 engine otherwise you'll give yourself a TR drive failure as well. As @megan correctly points out.

Isn't that scenario a bit far-fetched? Surely if that shaft failed, the engine would very rapidly spin up due to the removal of the load and shut itself down?

I don't know what overspeed protection there is but if it has a DECU, it would surely shut the engine down.

Has that shaft ever failed? If not, then it is quite understandable that it isn't covered in annual sim training.

Quote:

Originally Posted by [email protected] (Post 10063698)

Isn't that scenario a bit far-fetched? ...

The scenario is not far-fetched because it has happened before. We aren't discussing a shaft failure, we drifted to discussing a particular gear failure inside the MGB. With this malfunction the #1 engine doesn't rapidly spin up, it is still governed at 107% N2 and under load driving the TR, but not the main rotor. The #2 engine is still driving the main rotor but not the TR. If you move the collective the TQ on both engines will respond accordingly, #2 engine because of the main rotor pitch change, and the #1 engine because of collective yaw coupling and the resulting TR pitch change. A pedal input will result in a change in TQ on #1 engine but not #2 engine. When DECU are involved which talk to each other to maintain constant N1 on both engines, when there are vastly different loads placed on each engine due to the MGB gear failure, then the discussion gets interesting. But, as I mentioned before, not normally practiced in the simulator unless the trainees are a whiz at everything else. However it is discussed in the class during systems review, particularly if the trainees are sleepy.

One thing I always wondered is why on the Bell 212 tripletach the Rotor RPM is a small, short needle and the eng RPM are 2 big long needles? Isn't Rotor RPM the most important?

Reason being, there are two N2 needles and one NR needle. If the big needle was NR, there wouldn't be enough length in the 2 shorter N2 needles so the "1" and "2" flags on the respective needles didn't overlap.

Gullibell - I still don't see it - if that gear fails (not sure what you mean by gear failure - does it disintegrate or just stop spinning?) the gear that links it to the TR drive must surely also fail/stop spinning since it is attached to the same shaft (as far as I can see in the diagram).

And, if the gear fails and the load from the MRGB is suddenly removed leaving only the load of the TR on the No1 engine (especially in the cruise with relatively low TR thrust) there must be a Tq split until the N1 governor for No 1 engine catches up and backs off the fuel.

Quote:

Originally Posted by gulliBell (Post 10063763)

Well if all three are lined up it doesn't matter and if you have a non- engagement or drive shaft failure the eng needle number will appear. But perhaps I am grinding the coffee a little fine ...I just think that Rotor RPM is the most important indication in the cockpit.
I was always taught...caution light, strange noise, disturbance in the force...check Rotor RPM and control that before you do or say anything else.

Quote:

Originally Posted by [email protected] (Post 10063831)

Gullibell - I still don't see it...

gear fail = the shaft between the input and output end of that gear breaks, however so. The input part of that gear still turns freely thus continues to transmit power from #1 engine output to the TR drive gear.

yes, of course there will be a TQ split between the engines if one engine is driving the main rotor and the other is driving the TR. There will be a point in the collective range of travel where the TQ on each engine will be matched.

Quote:

Originally Posted by albatross (Post 10063889)

Well if all three are lined up it doesn't matter...

But it does matter. If N1 was the short pointer there is not enough length on that radius for the "1" and "2" flags not to overlap. Just draw it on a piece of paper and it should be clear why. Also, just because the NR pointer is shorter than the N2 pointers on the indicator doesn't assign it any lesser priority.

Lets roll back a bit here:

Nick Lappos 2nd Nov 2001, 18:09

Quote:

The procedure is based on an actual failure that occurred about 15 years ago, where the input gear attachment bolts lost torque and the separation that you describe actually occurred.
The gear was redesigned, and no repeat failure occurred. The flight crew noted the problem as noise and rumbling, a momentary upspeed of #1 engine, a swing to the left (extra tail thrust)
and then back to normal, with very low #1 torque and high #2 torque.
After a bit of discussion, the crew left well enough alone, and flew home without shutting down #1 (what a good pair of guys! If it works, leave it alone!).

When they landed, they noted that the failure, in that the tail rotor was not connected to the main rotor.
We id'd the problem and fixed it asap, of course, and no repeat has occurred. We inspect all boxes on overhaul for signs of lost torque on that gear to see if any recurrence is creeping back,
and everything is fine now.

For Nr Fairy
The above is not like a more common failure that NR fairy notes, where the engine shaft going to the transmission can fail, and N2/Np can go up while torque goes down. In that case, you have an
engine power loss, but a healthy rotor drive train otherwise. That can be confusing because the engine rpm on the triple tach goes up, but the rotor goes down. The rotor is your closest friend,
so it is wise to make it happy first, of course. Crews can get confused when those needles, always stuck together before, start to disagree.

Also, I am surprised that any A's are flogging around with disconnected electric overspeed systems. They were a pain in the early days (1979) but should be healthy now. The normal governor
will catch these failures we describe, I think, so it is not essential to the failures on this thread, but the electric overspeed is helpful for internal engine failures where the power section can unbutton
from the compressor, the internal engine overspeed can get very high and engine rupture is possible.

There are other clues to Nr - noise and vibration - pretty obvious but unless you have experienced it before it may be a little bit of a surprise as to what the strange noise is.

We used to do touchdown autos in the old days with the RRPM obscured for the student - surprising how close you can keep it to correct as you are attuned to the noise and vibration.

Quote:

When DECU are involved which talk to each other to maintain constant N1 on both engines, when there are vastly different loads placed on each engine due to the MGB gear failure, then the discussion gets interesting.

I think you might mean NG or N2.

Quote:

Originally Posted by RVDT (Post 10063971)

..I think you might mean NG or N2.

No. The DECU talk to each other to match N1, unless TQ limit on one engine is reached before the other then the DECU will stop matching N1.

Quote:

No. The DECU talk to each other to match N1, unless TQ limit on one engine is reached before the other then the DECU will stop matching N1.

So if you have a stronger (better performing) N1 or Ng than the other you must have Q splits all the time?

Quote:

Originally Posted by RVDT (Post 10064004)

So if you have a stronger (better performing) N1 or Ng than the other you must have Q splits all the time?

Correct. The DECU match N1 for load sharing. RFM Part 2 Section 1 pg 1-10 refers, together with all the other intricacies of the DECU. And just to extend on that concept, the pilot has no control over TQ matching with the engines in automatic mode. So even if the pilot wanted to, he can't match/trim the TQ output of each engine. The DECU will only allow N1 matching until TQ limits are reached. If one engine has a better TQ margin than the other, the DECU will hold it at the TQ limit and then N1 will split until the weaker engine reaches its TQ limit. The DECU gets pretty busy at that point with soft limiting logic, and eventually blow-away logic, if you keep demanding more power. When you back off from a TQ limit the DECU reverts back to N1 matching for load sharing. It's the way the system is designed.

Unlike helicopters like B212 or B412, which have a TQ split limitation, there is no such limitation in a S76 fitted with DECU. As long as the engines make power assurance, flying with DECU matched N1 and a TQ split due to engines having different margins is no big deal.

gulliBell, the 76B allowed the pilot to match either N1 or Tq. I usually matched Tq to make the dial line up in a pleasing and eye-catching manner.

76B, who for AC, in what role, and how did it compare with regard to the rest? Have lot of time in the A and C.

Quote:

Originally Posted by Ascend Charlie (Post 10064269)

gulliBell, the 76B allowed the pilot to match either N1 or Tq..

Sure. I was particularly referring to DECU in S76C++...C+ also, sort of.

Quote:

Originally Posted by megan (Post 10064298)

...Have lot of time in the A and C.

Big jump between A/C and C++, I think you'd like it. I'm old fashioned, I like the 212.