DRIVESHAFT FAILURE
 

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

Depends which drive shaft is being talked about.

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.

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

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.

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.

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

"... 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.

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.

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:

LRP: Thanks for that.

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

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

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.

Krypton asks:
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.

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.

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

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..

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.

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.

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.

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.

Driveshaft Failure
 

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

or NOT.

Unless of course you know more than the NTSB.

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

I asked the question a few years ago for the 350...
http://www.pprune.org/rotorheads/229...t-failure.html
Some usefull info there

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

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.

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.

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.

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

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

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.

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 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 was an example of a driveshaft failure during transition and OEI conditions.

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.

Pearl Harbor 206 crash
 

NTSB report is out - short shaft failure.

https://app.ntsb.gov/pdfgenerator/Re...Final&IType=FA

Lessons learned?
 

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

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

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

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?