Bell 206 Main Drive Shaft failure

[PPRuNeUser0211]

Team,

I'm not familiar with the tech for a 206 (never had the opportunity to fly one). However, I'm reading a manual for a TH67 and it states that in the event of a main drive Shaft failure you'll get a sudden increase in engine rpm (as the load is shed) decrease in rotor rpm and a yaw as the torque disappears. All of this is pretty vanilla

However, it also says the engine must remain in operation to provide power to the tail rotor and that loss of control may occur if the engine is shut down.

My understanding is that conventional wisdom drives the TR from the MRGB - is this manual talking about a drive shaft failure between mrgb and the rotor system itself? Or is there something unusual about the 206 setup?
<Edit number typo>

[Ascend Charlie]

Engine drives forward to the main TXMSN via that drive shaft, and rearwards to the tail rotor. Not driven by Txmsn, but by engine. With no load from the main rotor, who knows what the engine RPM will be, even at idle, but leave it running and you will at least have some control during the auto.

[PPRuNeUser0211]

Engine drives forward to the main TXMSN via that drive shaft, and rearwards to the tail rotor. Not driven by Txmsn, but by engine. With no load from the main rotor, who knows what the engine RPM will be, even at idle, but leave it running and you will at least have some control during the auto.
Thanks. So in the event of an engine failure, is loss of tail rotor drive always going to happen? Or is there a freewheel to allow backwards drive from the MRGB?

[hillberg]

free wheel has a shaft aft to tail rotor and the same shaft drives the main rotor,
think of a life saver on a straw the life saver is the driven free wheel and the straw the shaft that drives everything.

the candy stops moving as the straw still spins

break the front end of the strawstraw and the main gear box stops and the aft straw still has power:ok:

[LRP]

Thanks. So in the event of an engine failure, is loss of tail rotor drive always going to happen? Or is there a freewheel to allow backwards drive from the MRGB?

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1366x1760/206_freewheeling_unit_9cd6f97f464ad89ad631c76d99e99dc5cd74ac e9.jpg

[paco]

There is a continuous connection from the MGB to the tail rotor but they are driven by the engine from a point some way down, so if the engine fails the TR will still be driven. If the connection from the accessory gearbox to the MGB is lost, however, the situation is as described. The arrangement is a common one for modern helicopters as it saves a drive shaft.

I once asked a Bell engineer how you can tell when the main shaft fails and he said "Don't worry, you will know....."

[Ascend Charlie]

A friend was in a B212. Both engines running happily, but the shaft from the combining gearbox to the txmsn snapped. Massive double overspeed, rotor low warning, and a very nice auto saved the day.

The bit about "Don't worry, you will know" took a couple of seconds to process the data.

[wrench1]

Or is there something unusual about the 206 setup?
While how the T/R is driven has been explained above, attached a rough diagram: The red line shows a normal condition where the M/R and T/R are driven through the free-wheel unit by the engine. The blue line shows a loss of the IDS where the T/R is still driven through the FWU provided the engine is still operating. The green line shows an engine drive loss where the autorotating M/R drives the T/R via the FWU.

[DOASO]

Somewhat recently at factory training, I was told they actually want you to roll the throttle down to idle in this situation. You’ll still have enough controllability and potentially save the engine from a giant over speed. Seems reasonable.

[megan]

I was told they actually want you to roll the throttle down to idle in this situation. You’ll still have enough controllability and potentially save the engine from a giant over speedYou won't save the engine from an overspeed, it will have done whatever overspeeding it was going to do at the instant the shaft snapped. From the military Kiowa manual, the civil manual, at least the copy I have, is rather perfunctory in detail.

Main Drive Shaft FailureThe following symptoms occur

a. N2 RPM rapidly increases while rotor RPM decreases

b. Yaw left

c. Engine noise increases

d. Low RPM audio activates

e. N2 stabilises 103% or above

Recovery

1. Collective – Adjust to maintain rotor RPM

2. Confirm failure

3. Throttle – Maintain fully open to provide tail rotor thrust

4. Select landing area

5. Call mayday

6. Transponder – EMER

7. Brief passengers

8. Harness – Locked

9. Visor – Down

10. Land – Power on autorotation

Note: Because tail rotor will maintain constant RPM throughout the autorotative landing, tail rotor effectiveness will be greater than usual. Care should be exercised in compensating for yaw on touchdown.

[Ascend Charlie]

10. Land – Power on autorotation

Shouldn't that be "Power OFF auto", even though the engine is running?
People might think "Oh, power on auto, I can power terminate if I stuff it up..."

[PPRuNeUser0211]

Thanks for the above team, good job correcting my ignorance of types I've no experience in! Diagrams in particular paint a thousand words to me.

[megan]

Shouldn't that be "Power OFF auto", even though the engine is running?It's hinting to the throttle position is what I take away AC. Should they think,People might think "Oh, power on auto, I can power terminate if I stuff it up..."they don't have a clue as to what has just occurred (drive shaft snapped).

[GrayHorizonsHeli]

theres no doubt the engine speed will climb when the shaft fails.
my question is how fast and how high?
Will the governor at least put up a good fight and keep the engine from grenading itself?

[megan]

In the accident reports I've not seen any mention of how high the overspeed, but no mention has been made of any engine damage. I'd venture that during such an event the overspeed would be over before the pilot had a clue as to what just happened. An engineer may be able to comment on checks required on the engine, if any, following a drive shaft failure.

[PEASACAKE]

In the accident reports I've not seen any mention of how high the overspeed, but no mention has been made of any engine damage. I'd venture that during such an event the overspeed would be over before the pilot had a clue as to what just happened. An engineer may be able to comment on checks required on the engine, if any, following a drive shaft failure.

In basic terms the Engine manual states after driveshaft failure (engine to main transmission) to put all 3 engine modules (complete engine) in a box and send it for overhaul, post overspeed. Contact your bank manager for further assistance..........

[FH1100 Pilot]

Remember, in a 206 with an IDS failure, you're not going from, say, 80% torque to *zero* torque. The engine is still driving the oil cooler and tail rotor. So it's not completely unloaded. However, it will be "some" time for the governor to react. I'm sure the left yaw snap of the nose will be, as the Brits say, lively. Pilot better have quick feet, quick hands and, oh yeah, a quick brain.

The best example of this was the 206 tour helicopter that crashed in Hawaii back in February of 2016. As the ship is on short-final, you can clearly hear the tail rotor increase RPM when the IDS fails. The 206 makes a very distinctive tail rotor growl. There is some left yaw as you'd expect, and it looks like the pilot arrests it, maybe, but the ship hits the water too quickly to tell. Doesn't look like he had much time to do anything but utter an expletive and try to pull the collective up out of its mount.

It should be noted that the 206/407 is unique in its goofy tail rotor drive configuration. Other aircraft that employ the RR-250 series engines do not have the same issue. I mean, think about it: If the d/s fails at the forward (transmission) coupling, the engine is still going to be driving what's left of that shaft. Can you imagine the god-awful beating and banging going on back there? If the failure happened at, say, 500 feet agl, everything will be over pretty dang quickly, no matter what you do. If it happens above 500 feet agl, you're going to desperately wish it would be over quickly - but it won't. (This is why most helicopter pilots don't like to fly high: Subconsciously they worry about one-in-a-million failures like this.) From a higher altitude, I might be tempted to stop-cock the engine just to stop the noise, and deal with the resulting tail rotor failure at the bottom. I pray to God I never have to find out just what I'd do in that situation.

[wrench1]

my question is how fast and how high?
The only failure I'm personally aware of was a K-Flex failure and the pilot felt a klunk which put his eyes on the panel and he saw the needle splits. He caught the overspeed which he said was quick but slower than a high side gov failure.
checks required on the engine, if any,...
send it for overhaul...
FWIW: I don't have a RR manual handy, but I don't recall any specific overspeed inspection for the 250 series engines. Usually if the engine exceeds any RPM limits it doesn't stop and "self-inspects" by throwing a wheel loose. However, I believe the loss of a main driveshaft falls under the "sudden stoppage" special inspection which has several levels based on component damage with the 1st levels being inspections only, ending with an overhaul requirement if the engine or xsmn mounts are deformed/broken.

[RVDT]

It should be noted that the 206/407 is unique in its goofy tail rotor drive configuration.

Really?

AS 350 is pretty much the same. Forward shaft or couplings let go and no different.

[megan]

It should be noted that the 206/407 is unique in its goofy tail rotor drive configuration. Other aircraft that employ the RR-250 series engines do not have the same issueThe S-76, irrespective of type of engine, whether it be RR250, Turbomeca or PT6 can have the same problem as the Bell 206 family if the input drive shaft from the number one engine to main gear box fails. Tail rotor drive comes from number one engine. In my day it was not a failure mode addressed in the emergency checklist, to make matters worse, the drivetrain schematic in most of the books could make you believe that the tail rotor had drive available from both engines in the event of input shaft failures.

[PEASACAKE]

The only failure I'm personally aware of was a K-Flex failure and the pilot felt a klunk which put his eyes on the panel and he saw the needle splits. He caught the overspeed which he said was quick but slower than a high side gov failure.


FWIW: I don't have a RR manual handy, but I don't recall any specific overspeed inspection for the 250 series engines. Usually if the engine exceeds any RPM limits it doesn't stop and "self-inspects" by throwing a wheel loose. However, I believe the loss of a main driveshaft falls under the "sudden stoppage" special inspection which has several levels based on component damage with the 1st levels being inspections only, ending with an overhaul requirement if the engine or xsmn mounts are deformed/broken.

Your maintenance technician will soon find it for you in a matter of minutes in the maintenance 250 series manual at 72-00-00 after you report the overspeed, done it for over 35 years and I find it every time. However getting a pilot (especially if its being hired out) to admit an overspeed, overtorque or overtemp is the hardest part. When they report the overspeed / overtemp / overtorque and tell the how expensive its going to be, they suddenly remember it might not have been such and overspeed / overtemp / overtorque. I really appreciate the new monitoring systems that record all operational incidents.

This is an old manual unamended and not up to date (on my computer) and shows what must be sent for overhaul if you exceed limitations.

https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/2000x1124/14c08d08_1eac_44d2_a0ea_91cc125c2138_1_201_a_bd37b07978f6cb6 c0394f586f7a053eefb81a043.jpeg

[wrench1]

Your maintenance technician will soon find it for you in a matter of minutes in the maintenance 250 series manual at 72-00-00
Ha. And here I logged in to edit my previous post. Appreciate the correction. That's what I get for thinking without verifying. But after looking in an old C30 M&O, I still noted there is no Overspeed Inspection as I stated. Just the speed limit go/no-go table similar to what you posted. However, considering the T/R drive system is still intact and "loaded" on the engine after a IDS fails, I would not consider this a "Complete loss of output shaft load" as stated in our mutual speed limit tables. Perhaps performing the Sudden Stoppage/Engagement inspection (drive system component malfunction) and a quick call to RR Tech support would be in order before simply "sending it off for overhaul and contacting your bank manager." At least that's what my mechanic told me in the mirror this morning.:ok:

[megan]

I'm not aware of ovrspeed limits but i had a high side governor failure on a RR250 on a S-76 and the aircraft was not pulled off line, other than fixing the problem, governor drive shaft sheared.

[PEASACAKE]

Ha. And here I logged in to edit my previous post. Appreciate the correction. That's what I get for thinking without verifying. But after looking in an old C30 M&O, I still noted there is no Overspeed Inspection as I stated. Just the speed limit go/no-go table similar to what you posted. However, considering the T/R drive system is still intact and "loaded" on the engine after a IDS fails, I would not consider this a "Complete loss of output shaft load" as stated in our mutual speed limit tables. Perhaps performing the Sudden Stoppage/Engagement inspection (drive system component malfunction) and a quick call to RR Tech support would be in order before simply "sending it off for overhaul and contacting your bank manager." At least that's what my mechanic told me in the mirror this morning.:ok:

I think owners or operators call their bank manager before the maintenance organisation before reporting an overspeed, overtorque, overtemp incident, is that not what is states in the most Ops manuals (in my limited experience), I think there is a flow diagram showing how to do it................. you wish.😂

[wrench1]

I think owners or operators call their bank manager before the maintenance organisation before reporting an overspeed, overtorque, overtemp incident,...
In my experience, it depends on the person or entity. Fortunately, the 135 operator I worked at for the day job was not of that manner and actually quite the opposite when it came to mx issues. Unfortunately, some of the owners (helicopters/airplanes) I ran across through my side job were exactly of that manner where money overshadowed mx. Needless to say those type owners had to find another mx provider as I walked away.

[RVDT]

megan sez,

The S-76, irrespective of type of engine, whether it be RR250, Turbomeca or PT6 can have the same problem as the Bell 206 family if the input drive shaft from the number one engine to main gear box fails.

Er don't think so - the "shaft" you are thinking of is actually a bolted connection within the gearbox between 2 gears. And yes not only could it happen - it did!!

Not letting BS get in the way of a good story?

[megan]

Here is the moment for you to shine RVDT and inform we mere plebs what the particular item is called and what it's function is besides transferring power from the engine to the main rotor shaft. I acknowledge I did err in saying "engine to main gear box", but that was to avoid having to explain the drivetrain layout, call it semantics.


https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/711x461/s76_14_20_20copy_07439477f81327290f974f2e23f08738521d7ff6.jp g

[RVDT]

megan,

The convo here was about the drive shaft external to the main gearbox failing. If one of these failed on the 76 of which there are 2 the TR drive would remain intact.

The failure was inside the main gearbox and was a bolted coupling between 2 gears.

To quote from 2016 which you must have read because you commented on it.

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.

All the gears and shafts and freewheels in your diagram are inside and part of the MGB except for the TRDS in the diagram.

Semantics? Maybe, but for me the devil is always in the details!

[megan]

RVDT, you need to reread the OP''s post, the devil is in the detail as you say.it also says the engine must remain in operation to provide power to the tail rotor and that loss of control may occur if the engine is shut down.

My understanding is that conventional wisdom drives the TR from the MRGB - is this manual talking about a drive shaft failure between mrgb and the rotor system itself? Or is there something unusual about the 206 setup?The devil is in the detail, we're talking about a failure mode where power to the main rotor is reduced to zero but still leaves the tail rotor powered. The fact that the 76 failure mode occurs in the gearbox rather than external is moot. The OP asks "is there something unusual about the 206 setup", the answer is no, the 76, for one, has an identical failure mode.