DRIVESHAFT FAILURE

[KPax]

As an ex military ATCO I was at a small airfield near Shawbury today when I heard ' Driveshaft failure not good'. Could someone explain what would happen if this happens on a rotary

[Mustapha Cuppa]

Depends which drive shaft is being talked about.

[Arm out the window]

Most likely the driveshaft from the engine to the main rotor gearbox, leading to loss of drive naturally and therefore an autorotation, but also perhaps confusingly to engine overspeed because it's not under load any more.

[Widewoodenwingswork]

Those two words strike as much fear into me as "I'm" and "Pregnant", there's absolutely no need to capitalise them.

[gulliBell]

Just to elaborate on #3.
The different driveshafts are ET (engine to transmission), mast, and TR (tail rotor).
Twin engine helicopters can either have a single driveshaft between a combining gearbox and the main gear box (e.g. Bell 412), or a separate drive shaft from each engine to the main gear box (e.g. S76).
Single engine helicopters have a single driveshaft between the engine and the main gear box.
If you only have a single drive shaft to the main gear box, and it fails, you have 2 issues. Loss of drive to the main rotor, requiring autorotation; and if that engine/s doesn't have an electronic overspeed system it will explode (likely uncontained engine failure). Not that the engine was helping you in any way once the driveshaft failed. So you could have a fire, severed electrics, hydraulics, other damaged components etc etc
If the mast fails (the driveshaft that the main rotor connects to), you'll arrive at the scene of the accident as a passenger.
If the TR driveshaft fails, it doesn't matter how many engines you have, without drive to the tail rotor you'll be shutting down all the engines very soon and executing an autorotation.

[Ascend Charlie]

On the B206, if the engine to txmsn driveshaft fails, you still have the drive going back to the tail rotor, which will initially go berserk with the overspeed. The N1 governor should stop the N1 at 105%. Some other aircraft have an overspeed trip and will kindly shut the engine down. Not really needed, if the overspeed is caused by a faulty sensor and it isn't overspeeding at all.

Do not shut down the 206 engine for your auto, or you will lose all tail rotor control, get to the bottom with a big spin, sideways impact, stall flick spin crash burn die.

[riff_raff]

There is also the interconnect driveshaft systems used between the front/rear rotors on tandem helos like the CH-47.

[krypton_john]

"... the tail rotor, which will initially go berserk with the overspeed"

Why would the tail rotor overspeed if the engine to transmission shaft fails? The tail rotor is driven out of the transmission not the engine, right? As long as the MR is driving the transmission (in the absence of the engine input) won't the transmission just drive the TR at the prescribed ratio to MR %?

"Do not shut down the 206 engine for your auto, or you will lose all tail rotor control, get to the bottom with a big spin, sideways impact, stall flick spin crash burn die."

I don't understand that either. What's the difference between "shut down the 206 engine for your auto" and having an actual engine failure auto? The latter needn't spin crash burn die.

All the above is contingent on the main transmission unit driving the TR shaft as well as the MR shaft.

[LRP]

on the 206/407 the T/R driveshaft is driven by the freewheeling unit. If the driveshaft fails the xmsn will not (back) drive the T/R so the engine will be the only drive available for the tail rotor.

[RVDT]

hahahaha - priceless!

You guys should stick to flying as your engineering knowledge is just dismal.

About 50% of what is posted so far is close to fact.

:ugh:

[krypton_john]

LRP: Thanks for that.

But then why, on a full down engine out auto, is there no torque spin on raising the collective?

[Sloppy Link]

Don't shut down engines with a thruster failure in a MD902.

[Sir Korsky]

Megan - the S76 tail rotor drive is geared between the two engine inputs / Free wheel units. The C++ will shut down an over speeding engine at 122.5 N1. The tail drive will not be effected.

[Ascend Charlie]

Krypton asks:
But then why, on a full down engine out auto, is there no torque spin on raising the collective?

On an engine off auto (have you ever done one? Not just having the engine at idle) you still have control of the tail rotor, unless you are in a 206 with a driveshaft failure and you have shut down the engine. Raising the lever will make the nose go left, through friction, and right pedal should keep you straight.

I seriously hope you are not a 206 pilot. If you are, you need to read the flight manual.

[212man]

http://www.tsb.gc.ca/ENG/rapports-reports/aviation/2011/a11c0152/images/A11C0152-figure-1.png

[krypton_john]

No need to get snarky, Ascend Charlie.

I am not a Jetranger pilot hence the question and it was clear from the context that I was asking about the Jetranger transmission configuration so not sure why you needed to comment "unless you are in a 206". That's the point of my whole question.

When you say "Raising the lever will make the nose go left, through friction" what do you mean by "friction"? Do you mean "main rotor torque"? Or perhaps drag on the MR creating torque? Well yes I know that.

But when you say "right pedal should keep you straight" - why would pedal do anything at all if the tail rotor is not being driven?

212 Man - thanks for that diagram.

[gulliBell]

The friction referred to is the friction in the main gearbox with all that stuff turning, the residual affect without any power from the engine going into it is for the gearbox to also turn in the same direction the main rotor is turning.

In the B206 the tail rotor might still be turning under power because it's driveshaft comes off the back of the engine reduction gearbox. Hence, right pedal would give you a counter-torque to the drag effect of the main gearbox turning. However, I'd be most surprised if the engine kept running after ET driveshaft failure if it was under load at the time of failure.

[LRP]

The friction referred to is the friction in the main gearbox with all that stuff turning, the residual affect without any power from the engine going into it is for the gearbox to also turn in the same direction the main rotor is turning.

In the B206 the tail rotor might still be turning under power because it's driveshaft comes off the back of the engine reduction gearbox. Hence, right pedal would give you a counter-torque to the drag effect of the main gearbox turning. However, I'd be most surprised if the engine kept running after ET driveshaft failure if it was under load at the time of failure.
I haven't done any sort of research but I've never personally heard of a catastrophic engine failure following a short shaft failure in any Bell. YMMV

[Fareastdriver]

Over fifty years ago on the Bristol Sycamore all practise EOLs were with the engine shut down. There was a cam on the collective that would open the engine to full when you raised the lever to cushion the touchdown; to avoid this the engine had to be shut down on finals when one was sure of arriving at the right place.
The droop stops held the blades clear of the boom on shutdown so in case one of them failed after landing the engine had to be started immediately before the blades slowed down to the critical stage otherwise a blade could hit the boom.
Starting the engine was already a delicate operation to avoid slamming the freewheel unit so you were using three hands and your knees..

[RVDT]

I haven't done any sort of research but I've never personally heard of a catastrophic engine failure following a short shaft failure in any Bell. YMMV

Hence the previous derogatory post.

A mechanical failure of a driveshaft would most of the time NOT be related to the amount of torque on the shaft.

It could just as easily fail with little or no torque so the hellfire and brimstone regarding overspeed is meaningless.

The N1 governor should stop the N1 at 105%

Only after the normal N2 governor (Pr/Pg)has failed and if we are talking about B206 and C-18 and C-20 derivatives there is also the N2 mechanical overspeed governor (Py dump) in addition.

Electrical overspeed governors are a whole different thing and for different reasons and certification requirements.

[gulliBell]

Yes, but a short shaft failure on a B206 that has an engine producing 300 shp one moment, which when suddenly unloaded, makes me wonder of the instantaneous engine acceleration that might occur, and how effective a hydromechanical GP or PT governor might be at limiting any resulting affects from that. I haven't seen a C20 fail before, but I have seen a C30 hand out free samples of expensive engine bits and pieces flung far and wide. It gets very messy very quickly.

[Fareastdriver]

how effective a hydromechanical GP or PT governor might be at limiting any resulting affects from that

The Turmo 3C fitted to the 330 Puma was quite good at it. On the two occasions of flexicoupling failure that I know about the engine was just throttled back to idle with very little, if noticeable, surge.

[mixing lever]

The investigation results are not yet posted, but the below crash video mimics well what a short shaft failure might look like.


https://www.youtube.com/watch?v=aQUV_NwNHyQ

[RVDT]

The investigation results are not yet posted, but the below crash video mimics well what a short shaft failure might look like. or NOT.

Unless of course you know more than the NTSB.

[RVDT]

but I have seen a C30 hand out free samples of expensive engine bits and pieces flung far and wide. It gets very messy very quickly.

Related to a shaft failure? Or just an early C30 issue? Which airframe?

[Flyting]

I asked the question a few years ago for the 350...
http://www.pprune.org/rotorheads/229674-350-short-shaft-failure.html
Some usefull info there

[vfr440]

GB, I think you are correct.
IIRC, several years ago there was a classic HF incident in Canada when for a chain of reasons, a B206 was wheeled out for gd runs/hover check after major maintenance. The Jesus nut had been left on the bench. Everything OK at ground idle, but, on wind up to flight idle, (and perhaps a little collective), the rotor departed the airframe. This was inconvenient,...... but the engine, relieved of its load, went to nose-bleed RPM on the N2 train. Result - welded PT wheels to the labyrinth seals on the casing. Only the Lord knows how many RPM, but certainly exceeded the plastic deformation of the wheels' constituent steel. This just for info - VFR

[RVDT]

vfr, Ever seen someone run an MD500 without blades? The procedure is in the AMM to balance just the head.

How much torque do you really think there is at FL? All you are doing is removing that torque. Easily caught by the governor. .

I'm calling BS on this one.

[Fareastdriver]

I don't know about the MD500 but any helicopter with a rotor speed governing system will quite happily run with both engines running with the rotor blades removed.

Quite difficult to believe in some cases. I remember running a 332 in storage without blades and the engineer beside me was having kittens as I went through the normal start with the first engine and then straight into the second.

It was sitting quite happily at about 270 Rrpm with both engines well above Idle.

[ericferret]

On either the Arrius or Arriel engine course (can't remember which) at Turbomeca it was said that the primary overspeed protection (certification requirement) was blade shedding with the electrical overspeed system being secondary.

In the early 80's the BO 105 had a problem with freewheel slippage which manifested itself as
100% N2 with idle N1. End result was that the freewheel would re-engage on shut down destroying the driveshaft. What you saw depended on whether one or both engines were running.

[vfr440]

OK, RVDT, I see your point .........
HOWEVER I requested and borrowed the graphics of the resultant damage (and still use them in initial HF courses to emphasise the importance of 2nd inspections). To correct my post, perhaps I should have stressed this was into a hover, and probably the machine was heavy. The resultant departure of the head was apparently 'unexpected' (!!) - but the damage was extreme, I promise you. But thank you for pointing out the lack of specifics in my post, safe flying - VFR
PS on a rethink, the reason it all went pear-shaped was simply because the Jesus nut was not fitted, RPM at 100%Nr, min-blade pitch angle and the split cones dropped out. Then in just one revolution the trunnion dropped putting o-my-God collective pitch to an energy charged M/R, Sorry forgetful - must be an ageing thing LOL

[212man]

Actually, VFR, I thought if was obvious what you described and I got it first time!

OK, RVDT, I see your point .........
HOWEVER I requested and borrowed the graphics of the resultant damage (and still use them in initial HF courses to emphasise the importance of 2nd inspections). To correct my post, perhaps I should have stressed this was into a hover, and probably the machine was heavy. The resultant departure of the head was apparently 'unexpected' (!!) - but the damage was extreme, I promise you. But thank you for pointing out the lack of specifics in my post, safe flying - VFR
PS on a rethink, the reason it all went pear-shaped was simply because the Jesus nut was not fitted, RPM at 100%Nr, min-blade pitch angle and the split cones dropped out. Then in just one revolution the trunnion dropped putting o-my-God collective pitch to an energy charged M/R, Sorry forgetful - must be an ageing thing LOL

[FH1100 Pilot]

or NOT.

Unless of course you know more than the NTSB.
The Bell 206 accident in Hawaii is far from explained by the NTSB. They only say that they pulled the thing out of the water and all of the big structural pieces are accounted for.

Luckily there is video evidence of the last few seconds of the flight. This video shows a fairly shallow approach that suddenly gets interrupted. The video contradicts the pilot's statements that he initiated an autorotation to include a left pedal turn to land parallel to the shoreline. That simply did not happen. The videographer's statement was that the helicopter was coming in low and straight. Doesn't sound autorotative to me...

What we can see AND HEAR is the end of a fairly shallow approach. All of a sudden the ship plummets out of the sky, accompanied by the very clear sound of the tail rotor rpm *increasing* as the 206B's notoriously slow N2 governor, taken quite by surprise hadn't yet caught up with the lack of load. And there was a slight left yaw which would also be expected. This can ONLY be caused by one thing: Main driveshaft failure.

No matter what the pilot *says* happened, he's wrong. We always get it wrong. Whether he's being deliberately dishonest or just remembers things wrong doesn't really matter. I'd guess that it all happened too fast for his puny brain to comprehend, and felt that he had to immediately come up with an explanation of why he crashed. CYA? Why not just shrug and say, "I dunno, I don't remember. I'll get back to you when my memory of the event is more clear."

So don't think that the NTSB has ruled on that one yet.

[riff_raff]

The worst rotorcraft driveshaft failure example I can imagine would be the CH-47's interconnect drive between the fwd/aft rotors. The 1982 Mannheim crash (http://www.chinook-helicopter.com/history/aircraft/C_Models/74-22292/74-22292.html) is a very graphic example of what happens when the interconnect drive system fails on a CH-47 tandem helo.

The most complicated rotorcraft driveshaft system I can think of is the V-22 cross-wing interconnect. Three gearboxes (2 TAGB & 1 MWGB), a dozen or so driveshaft sections, a similar number of hanger bearings, and a large number of diaphragm type flex couplings. What makes things difficult with this particular application is the driveshaft system must accommodate flexing of the wing structure. The V-22 interconnect drive system's primary function is to power both rotors during OEI operation. And failure of the drive system during certain OEI operating conditions (hover, transition, etc) can be catastrophic. The 1992 Quantico crash (https://www.flightglobal.com/FlightPDFArchive/1993/1993%20-%201208.PDF) was an example of a driveshaft failure during transition and OEI conditions.

[newfieboy]

vfr440

Re your post referring to the Canadian Jesus Bolt. You might want to pull up the TSB report and read before posting bull****.
The Pilot and Engineer sadly killed in this incident were colleagues and highly experienced operators. The A/C actually disintegrated in flight due rotor separation, as you correctly state due lack of JB. There was not a lot left for the TSB to investigate, except the JB sitting in an apprentice engineers tool box. Very sad I remember the phone call.

[mixing lever]

NTSB report is out - short shaft failure.

https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20160218X71040&AKey=1&RType=Final&IType=FA

[Viper 7]

So there was discolouration on the coupling and the temp strips were missing.


We check these on every pre-flight.


In the spirit of a lesson not being learned unless a behaviour is changed, is there anything else to look out for on a pre-flight, maybe grease being thrown around? We don't check for any lash or play in the shaft, does anyone else?


V7

[fijdor]

Last time I have flown the B212 was about 9 years ago so I am a bit rusty on the procedures. Last I flew the aircraft, I was the company training pilot on the 212 and 205 and was told and shown a new training procedure for the 212 but not the 205 (don't know why, 205 is in the same situation) and the procedure was called Main driveshaft failure training, here also called shortshaft or power shaft. The training was to recognize what happened in flight when the failure occur and what to do about it.
This came about due to 2 recent (at the time) accident in the US military where 2 212 crashed with lost of lives and it was found out that the problem was due to a powershaft failure AND the wrong response to it by the pic.
According to the report both pilots in both accident had the reaction of pulling collective up to slow down RPM to investigate the problem.
Doesn't take much to loose rotor RPM especially when the engines are not connected anymore to the trany and RPM would come down to a point of no recovery possible.
I do not have the paperwork related to it but it showed dual tach and different needles position depending if it was a powershaft failure or gov-overspeed etc.
Don't know if they are still training for that nowadays.

jD

[megan]

but I have seen a C30 hand out free samples of expensive engine bits and pieces flung far and wideA good reason for not doing ground runs to find where the problem is when the trend check has taken a massive nose dive.the S76 tail rotor drive is geared between the two engine inputsSir Korsky, most of the schematics I've seen would have you interpret that as being the case, because the orientation of the graphic is so poor. See below for a clearer view. You can see the tail rotor is only driven from the #1 engine side. As Sikorsky saysEach engine has a separate power train all the way up to the bull gear through a single spur and a single bevel mesh. The tail takeoff is from the left engine power train. In case of a left engine failure the tail takeoff still drives through the right engine to the bull gear back through the bevel set to the tail take off.So its theoretically possible for a #1 engine drive failure into the MGB where #2 is what supplies main rotor power and you you have to keep #1 running to drive the tail rotor ie shutting down #1 will deprive you of tail rotor drive. A fact I've never seen pointed out in training or simulator.

Sikorsky Archives | S-76 (http://www.sikorskyarchives.com/S-76.php)

[John Eacott]

A good reason for not doing ground runs to find where the problem is when the trend check has taken a massive nose dive.Sir Korsky, most of the schematics I've seen would have you interpret that as being the case, because the orientation of the graphic is so poor. See below for a clearer view. You can see the tail rotor is only driven from the #1 engine side. As Sikorsky saysSo its theoretically possible for a #1 engine drive failure into the MGB where #2 is what supplies main rotor power and you you have to keep #1 running to drive the tail rotor ie shutting down #1 will deprive you of tail rotor drive. A fact I've never seen pointed out in training or simulator.

Sikorsky Archives | S-76 (http://www.sikorskyarchives.com/S-76.php)

I suspect that it hasn’t been pointed out to you because your interpretation is incorrect. The tail rotor drive will continue regardless of the engine giving power, via the MGB bull gear as described in the previous quote. Should the No 1 fail then the freewheel disconnects it from a drive input.

Have you not started No 2 first and seen the main and tail rotors spin up together?

[gulliBell]

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.

[John Eacott]

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!!

[henra]

Sikorsky Archives | S-76 (http://www.sikorskyarchives.com/S-76.php)

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!

[gulliBell]

..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.

[[email protected]]

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.

[gulliBell]

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.

[albatross]

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?

[gulliBell]

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.

[[email protected]]

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.

[albatross]

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.

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.

[gulliBell]

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.

[gulliBell]

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.

[RVDT]

Lets roll back a bit here:

Nick Lappos 2nd Nov 2001, 18:09

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.

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.

[gulliBell]

..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.

[RVDT]

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?

[gulliBell]

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.

[Ascend Charlie]

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.

[megan]

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.

[gulliBell]

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.

[gulliBell]

...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.

[[email protected]]

gear fail = the shaft between the input and output end of that gear breaks, however so. so it is a shaft failure or the bolts that hold the gear to the shaft and not the gear breaking up - that makes more sense.

However, Nick's post indicates that the problem won't happen again since the shaft and gear were redesigned - therefore not much point in teaching it in the sim.

On the N1 governing by the DECU - with AC talking about matching N1 or Tq - I think that is confusing the ability to beep the engines up and down to match them in the way you prefer with what the DECU is designed to do (govern N1 and load share with the other engine)

[Ascend Charlie]

The B in the 76B stood for BALLS, which it had plenty of with the PT-6 and FADEC.

[gulliBell]

1. so it is a shaft failure or the bolts that hold the gear to the shaft and not the gear breaking up - that makes more sense.

2. However, Nick's post indicates that the problem won't happen again since the shaft and gear were redesigned - therefore not much point in teaching it in the sim.

3. On the N1 governing by the DECU - with AC talking about matching N1 or Tq - I think that is confusing the ability to beep the engines up and down to match them in the way you prefer with what the DECU is designed to do (govern N1 and load share with the other engine)

1. I think that gear is 2 separate pieces that are bolted together. As NL pointed out the failure mode in the example cited was bolts coming loose. Gears in helicopter gear boxes can certainly fail and make metal for other reasons. I know, not long after when we got C model there was an AD about the possibility of loose bolts within the MGB. Sikorsky sent a maintenance team to open all the gear boxes in the new helicopters and check the torques on the bolts. They had some pretty impressive test gear with them. It is very rare event to see inside a S76 MGB.

2. As I mentioned earlier, we do not cover this malfunction in the simulator. It is discussed in class room training, more-so to get the trainees to think about and diagnose a set of circumstances which might not be covered in the RFM. Not all malfunctions are in the ECL.

3. There is no beep available to the pilot with DECU in automatic mode. The DECU gives you 107% N2 on both engines with N1 matched. That's it. The engine trim switch on the pilot collective - of which there is only one - only functions when the engine is in manual mode (i.e. using the MFCU controlling fuel flow, DECU controlled AFCU is disabled). It will beep up/down the engine/s in manual mode. Things get a bit tricky with the single eng trim switch and both engines in manual mode, particularly if there is a TQ split between the engines. This isn't usually practiced in the simulator, but it is easily manageable if you know what you're doing.

[212man]

On the N1 governing by the DECU - with AC talking about matching N1 or Tq - I think that is confusing the ability to beep the engines up and down to match them in the way you prefer with what the DECU is designed to do (govern N1 and load share with the other engine)
DECUs, and any governing system, only govern N1/Ng when the engine(s) are at idle. With the Nr in the normal operating range they govern N2/Nf. Matching N1 is something different.

[212man]

The B in the 76B stood for BALLS, which it had plenty of with the PT-6 and FADEC.
Pretty sure it doesn’t have FADEC.

[[email protected]]

212man - yes, my mistake, I wrote N1 when I should have put N2 - it comes from years of calling them Ng and Nf instead of N1 and N2 - doh!:ok:

Gulli - thanks, the Bell DECUs and Airbus DECUs clearly work in subtly different ways.

Since many aircraft have the facility to let you beep up Nr for Cat A, either automatically or via a switch/beep, I assumed the 76 was the same.

[gulliBell]

DECUs, and any governing system, only govern N1/Ng when the engine(s) are at idle. With the Nr in the normal operating range they govern N2/Nf.

Not quite. After the engine is started, the DECU only controls N1 once the N2 exceeds 9%. Which is why, if you start the engine with the rotor brake on and then move the ECL from IDLE to FLY, the engine stays at idle.

And the other point. The DECU has no idea what the NR is...it doesn't even have an input for NR signal. The trigger for the DECU to govern N2 is when it senses the ECL in FLY position.

[212man]

Not quite. After the engine is started, the DECU only controls N1 once the N2 exceeds 9%. Which is why, if you start the engine with the rotor brake on and then move the ECL from IDLE to FLY, the engine stays at idle.

So you start with the rotor brake on and the engine sits there happily at idle N1 - what do you suggest is controlling that value?

And the other point. The DECU has no idea what the NR is...it doesn't even have an input for NR signal. The trigger for the DECU to govern N2 is when it senses the ECL in FLY position.

I didn't suggest it did sense Nr, but if you don't have Nr you won't have N2 and in normal conditions they are matched.

I was also talking in generalities about any governor, whether a FADEC, a DECU, an ECU or a hydro-pneumatic system using P3 air and bob weights. You start the engine and the N1 is governed then, as you increase the power the N2 increases and at a certain point it becomes the datum that is then governed.

The trigger for the DECU to govern N2 is when it senses the ECL in FLY position

Are you positive about that? In the S92 when you advance the throttles from IDLE to FLY the N2/Nr will rise and then reach the normal value of 105% at which point pushing the throttles forward further has no effect. This happens a couple of inches before reaching the FLY detent and clearly indicates that the FADEC has taken over N2 control. There also some flight checks that require the throttle to be retarded out of the FLY gate, but the N2 is still governed. It was the same in the 76A+ with its steam driven system.

(Note - I do know that in the S92 the Nf/N2 is referred to as Np, but was keeping it simple)

[Salusa]

I think we should all think carefully about..

N1/Ng
N2/NP
N3/Nr?

Goes to show standardisation is required..

Hated Torque matching 212 - 3B engine with TCU. 412/212 with ITT trim better but can lead you away... PTGs obviously don't match perfectly and one OHC unit against a almost timex unit can be interesting. PWAC removed field cleaning from MM so orifice clean no longer an option

Back to original question.

Most twins have overspeed protection of some sort. Singles driveshaft failure will be interesting...

Revert to the pilots...

[[email protected]]

But Ng = gas generator rpm
Nf = free power turbine rpm
and Nr = rotor rpm

That is both logical and easily understood:ok:

[megan]

Nick's post indicates that the problem won't happen again since the shaft and gear were redesigned - therefore not much point in teaching it in the simSometimes the dog of fate lifts its leg and urinates on the pillar of science. Never believe in the word never, as in, it can't happen. The only MGB problem in this regard I ever had was the rotor brake quill on a 212 snapping in half for some unknown to me reason. Bevel gear dropped and bounced around in the MGB and the brake disc left lying on the cabin roof. Any part can fail for any number of reasons, heat treating, tooling marks, corrosion, inclusions being a few. I'm sure some one once said a MGB can't fail and release the main rotor. We know better now, and still await the initiating cause.

[Ascend Charlie]

There was a case of a 412 in Oz, airborne for a check flight after service, and the drive shaft from the combining gearbox let go.

Two serviceable engines having a little overspeed, rotor having somewhat of an underspeed, but the cool captain pulled off a good auto and all was well.

[gulliBell]

..Two serviceable engines having a little overspeed, rotor having somewhat of an underspeed, but the cool captain pulled off a good auto and all was well.

The aircraft was damaged but they walked away...I think I read that the auto technique as described by the Captain was to develop the flare at the bottom until the tail stinger hit the ground, and then level off and land. And that's exactly what happened: the impact bent the tail boom but, as I said, they walked away.