Sikorsky S-76: Ask Nick Lappos
Join Date: Dec 2004
Location: USA
Age: 54
Posts: 305
Likes: 0
Received 0 Likes
on
0 Posts
Hi Flint,
Thanks for your quick response. I know what you're saying: there may be just a hint of feedback when flown sporty perhaps but with this aircraft it's very pronounced even with minor control inputs; it's certainly much more noticable than the other S76 we have here. And although it's a C++ this old bird is neither new nor tight
My initial suspicion was the same as yours, some kind of cable or bellcrank hangup. I asked the engineers to check the continuity of the T/R controls aft of the mixing unit and we all had a good look at the pulleys etc. but could not find any binding.
I did find out that the T/R control cables were recently changed. Perhaps that's the cause of it all but that raises the question; is this just a matter of letting the cables wear in or does the rigging still need to be adjusted?
Thanks for your quick response. I know what you're saying: there may be just a hint of feedback when flown sporty perhaps but with this aircraft it's very pronounced even with minor control inputs; it's certainly much more noticable than the other S76 we have here. And although it's a C++ this old bird is neither new nor tight
My initial suspicion was the same as yours, some kind of cable or bellcrank hangup. I asked the engineers to check the continuity of the T/R controls aft of the mixing unit and we all had a good look at the pulleys etc. but could not find any binding.
I did find out that the T/R control cables were recently changed. Perhaps that's the cause of it all but that raises the question; is this just a matter of letting the cables wear in or does the rigging still need to be adjusted?
Join Date: Jan 2007
Location: here and there
Age: 67
Posts: 139
Likes: 0
Received 0 Likes
on
0 Posts
Revolutionary:
You are quite right you should not have collective to yaw coupling happening in normal flight. As you probably all ready know if the collective is fully down and you push peddle will want to increase the collective up, same if you have full collective pulled up will move the pedals. ( on the ground with hydraulic test rig hooked up to see this)
If cables recently changed I would suspect your tension might not have been set right, possibly to high. If the cables were replaced as per the manual with the two rigging pins installed forward and aft quadrants should not have affected the rigging.
As you mentioned binding pulley bearings you all ready checked, other area´s I have seen in the past cause this is forward or aft quadrant pivot bearings binding, possibly bearing on the top deck with either collective or tail rotor ( I would suspect the tail rotor bell cranks more likely to cause this issue), make sure even the cotter pins have been installed correctly, I have seen in the past on the S76 a cotter pin making contact with the upper deck and the pilot could feel it in the controls.
You are quite right you should not have collective to yaw coupling happening in normal flight. As you probably all ready know if the collective is fully down and you push peddle will want to increase the collective up, same if you have full collective pulled up will move the pedals. ( on the ground with hydraulic test rig hooked up to see this)
If cables recently changed I would suspect your tension might not have been set right, possibly to high. If the cables were replaced as per the manual with the two rigging pins installed forward and aft quadrants should not have affected the rigging.
As you mentioned binding pulley bearings you all ready checked, other area´s I have seen in the past cause this is forward or aft quadrant pivot bearings binding, possibly bearing on the top deck with either collective or tail rotor ( I would suspect the tail rotor bell cranks more likely to cause this issue), make sure even the cotter pins have been installed correctly, I have seen in the past on the S76 a cotter pin making contact with the upper deck and the pilot could feel it in the controls.
Join Date: Dec 2004
Location: USA
Age: 54
Posts: 305
Likes: 0
Received 0 Likes
on
0 Posts
Thanks Flint and Twisted for your advice. Our engineers have gone through the T/R linkage over the past couple of days, checking the cable tension and any potential hangup spots but have come up empty. Their focus is now shifting to the mixing unit itself. We'll see where we end up...
Join Date: Jan 2007
Location: here and there
Age: 67
Posts: 139
Likes: 0
Received 0 Likes
on
0 Posts
If your engineers are positive is not between the fwd and aft quadrants, I assume they disconnected both ends and with the cables tensioned they ran them by hand to make sure they were smooth?
If so you can rule that out as the source of the problem, question was any work done on the tail rotor retension pilot, new liner installed? If very tight could make the tail rotor controls a bit more difficult to move and sends this back to the mixing system.
Failing that I would say your problem has to be on the upper deck.
We had a similar issue where the pilot in the hover moved the pedals and the collective would go up. Basically the same troubleshooting as your guys are doing. The mechanics told me they ended up changing the collective mag brake. I thought at the time strange the pilot would not know his mag brake was not working properly when the trigger was pulled. Changed that and the problem has not re occurred. Also on the C++
If so you can rule that out as the source of the problem, question was any work done on the tail rotor retension pilot, new liner installed? If very tight could make the tail rotor controls a bit more difficult to move and sends this back to the mixing system.
Failing that I would say your problem has to be on the upper deck.
We had a similar issue where the pilot in the hover moved the pedals and the collective would go up. Basically the same troubleshooting as your guys are doing. The mechanics told me they ended up changing the collective mag brake. I thought at the time strange the pilot would not know his mag brake was not working properly when the trigger was pulled. Changed that and the problem has not re occurred. Also on the C++
Hello all,
Does anyone in the "Sikorsky community" has ever heard about the recent possibilty to install on S76 C+/C++, larger windows (with a pop out or jettisoning system) as well as an enhanced emergency floats gear, certified for force 6 sea ?
Does anyone in the "Sikorsky community" has ever heard about the recent possibilty to install on S76 C+/C++, larger windows (with a pop out or jettisoning system) as well as an enhanced emergency floats gear, certified for force 6 sea ?
Join Date: Sep 2002
Location: Pennsylvania, USA
Age: 57
Posts: 321
Likes: 0
Received 0 Likes
on
0 Posts
New windows
They are optional for the 76D. Some companies are getting them, they a great new design. I am not sure when they will be available as a retro fit, just yet. If I hear I'll let you know..
Steve76 asks:
Hello Nick and fellow 76 drivers,
With regard to a internal gearbox failure of the #1 engine, N2 input driveshaft and the subsequent channelling of 100% of the turbines power into the tail rotor. Is it not unreasonable to expect that the tail rotor will overspeed to the point that it suffers a catastrophic failure and disintegrates?.
Nick sez:
You are postulating an internal main rotor gearbox failure, I think, where the engine is still connected to the tail rotor, but disconnected from the main rotor bull gear.
Such a failure would leave the engine driving a much smaller load than the big main rotor, so an upspeed might occur, depending on how big the power reduction is (the reduction from normal drive to just the tail rotor.)
In all cases, the engine overspeed protection will catch the drive up speed if it should get out of hand, since the overspeed protection system is designed to shutdown the engine if the engine shaft breaks, which is a very big power reduction, the biggest possible. The tail rotor is quite healthy at speeds up to somewhere over 130% (I have been to 128 in flight during tests - this is done by professionals, do not try this at home!)
Most likely, the failure you describe would create a situation where the engine would speed up for a second or two, then settle down to about 109% or so, and it would be at about 2% torque, spinning happily with almost no load (just driving the tail rotor). The 109% is because unloaded, but with the cruise collective pitch setting, the engine would be trimmed up a bit by the collective bias. The rotor and the #2 engine would be driving the helicopter, and you would be in single engined flight.
The only problem would be if you decided to then shut down the #1 engine, because that would cause the loss of tail thrust.
Hello Nick and fellow 76 drivers,
With regard to a internal gearbox failure of the #1 engine, N2 input driveshaft and the subsequent channelling of 100% of the turbines power into the tail rotor. Is it not unreasonable to expect that the tail rotor will overspeed to the point that it suffers a catastrophic failure and disintegrates?.
Nick sez:
You are postulating an internal main rotor gearbox failure, I think, where the engine is still connected to the tail rotor, but disconnected from the main rotor bull gear.
Such a failure would leave the engine driving a much smaller load than the big main rotor, so an upspeed might occur, depending on how big the power reduction is (the reduction from normal drive to just the tail rotor.)
In all cases, the engine overspeed protection will catch the drive up speed if it should get out of hand, since the overspeed protection system is designed to shutdown the engine if the engine shaft breaks, which is a very big power reduction, the biggest possible. The tail rotor is quite healthy at speeds up to somewhere over 130% (I have been to 128 in flight during tests - this is done by professionals, do not try this at home!)
Most likely, the failure you describe would create a situation where the engine would speed up for a second or two, then settle down to about 109% or so, and it would be at about 2% torque, spinning happily with almost no load (just driving the tail rotor). The 109% is because unloaded, but with the cruise collective pitch setting, the engine would be trimmed up a bit by the collective bias. The rotor and the #2 engine would be driving the helicopter, and you would be in single engined flight.
The only problem would be if you decided to then shut down the #1 engine, because that would cause the loss of tail thrust.
What am I missing? Trying to figure out why you would lose TR thrust if you shut down #1 engine.
Join Date: Jun 2006
Location: East of the border
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
Gordy I suppose this is a hypothetical situation where a specific combination of failures of gears inside the Main Gearbox meant that the #1 Engine was the only thing driving the Tail Rotor output shaft. The #2 Engine would still be driving the Main Rotor through its surviving gears but not the Tail Rotor, and the #1 Engine would still be driving the Tail Rotor but not the Main Rotor. So shutdown of the #1 Engine would cause loss of tail rotor thrust.
I'm not knowledgeable enough of the internals of the MGB to say whether this is actually possible or not the way the gears are arranged inside.
I do vaguely remember hearing a story years ago at a FlightSafety recurrent that during the initial design of the S76 MGB, an engineer messed up the design for the fastener for one of the internal gears. It was supposed to be reverse threaded to prevent it backing off when the gear was spinning under load. It was designed conventionally threaded and the fastener backed out during operation, and subsequently the gear came off in flight. I believe the story was that the engine continued to drive the MGB lubrication pump and hydraulic pump but provided no power to the main rotor or tail rotor. I'm probably wrong on the details - it was a long time ago I heard that story.
Join Date: Jun 2006
Location: East of the border
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
Main Gear Box Seizure
A semi related question, food for thought...
On the S76A/A++/C with mechanically governed engines, the collective is mechanically linked to the FCU and therefore if I understand the system correctly, actually limits the amount of power the engine can put out (in conjunction with the engine lever position).
On the S76C++, there is no longer a mechanical linkage to the FCU. Once the levers are in FLY, the DECU governs the engine to 107% N2 and the collective position transducer sends a signal to the DECU to trim the engines to prevent transient droop or spikes during collective movement. So, the collective on a C++ does not *limit* engine power output the way the mechanical linkages do on the A/A++/C. With me so far?
So can anyone explain to me why the wisdom imparted in FlightSafety checklists, recurrent training, etc is that during a Main Gearbox failure on a C++, with a slowly seizing gearbox, the pilot should "maintain low/moderate torque setting to ensure that the engines continue to drive the MGB." It is implied that the pilot has control of the minimum torque and should not lower the collective fully to 0% torque, which I think is false advice. Lowering the collective even fully could not reduce the torque below that required to overcome the friction generated by the seizing MGB as the DECU attempts to maintain 107% N2.
In other words, if say 57% torque was required to overcome the additional friction in the MGB as it began to seize, then the indicated torque would read 57% at collective flat pitch, and the pilot could not "reduce torque to 0% with the collective", so to speak.
Shouldn't the correct advice be that the pilot "Maintain the engine levers at FLY so that the DECU continues to throw fuel into the engine to govern the N2 at 107%, thereby putting torque into the MGB to overcome the friction"?
I'm reasonably happy that the prevailing wisdom is applicable to the S76A/A+/A++/C but I'm not so sure that it is applicable to the S76B/C+/C++/D or actually any modern heli that uses electronic engine controls.
On the S76A/A++/C with mechanically governed engines, the collective is mechanically linked to the FCU and therefore if I understand the system correctly, actually limits the amount of power the engine can put out (in conjunction with the engine lever position).
On the S76C++, there is no longer a mechanical linkage to the FCU. Once the levers are in FLY, the DECU governs the engine to 107% N2 and the collective position transducer sends a signal to the DECU to trim the engines to prevent transient droop or spikes during collective movement. So, the collective on a C++ does not *limit* engine power output the way the mechanical linkages do on the A/A++/C. With me so far?
So can anyone explain to me why the wisdom imparted in FlightSafety checklists, recurrent training, etc is that during a Main Gearbox failure on a C++, with a slowly seizing gearbox, the pilot should "maintain low/moderate torque setting to ensure that the engines continue to drive the MGB." It is implied that the pilot has control of the minimum torque and should not lower the collective fully to 0% torque, which I think is false advice. Lowering the collective even fully could not reduce the torque below that required to overcome the friction generated by the seizing MGB as the DECU attempts to maintain 107% N2.
In other words, if say 57% torque was required to overcome the additional friction in the MGB as it began to seize, then the indicated torque would read 57% at collective flat pitch, and the pilot could not "reduce torque to 0% with the collective", so to speak.
Shouldn't the correct advice be that the pilot "Maintain the engine levers at FLY so that the DECU continues to throw fuel into the engine to govern the N2 at 107%, thereby putting torque into the MGB to overcome the friction"?
I'm reasonably happy that the prevailing wisdom is applicable to the S76A/A+/A++/C but I'm not so sure that it is applicable to the S76B/C+/C++/D or actually any modern heli that uses electronic engine controls.
From what I can figure out, they are talking about the shaft circled here, so still not sure why the TR cannot be driven by engine #2:
Flintexpert: You raise an interesting scenario---I am flying the B model, and no mention of the things you are saying.
Flintexpert: You raise an interesting scenario---I am flying the B model, and no mention of the things you are saying.
Join Date: Jun 2006
Location: East of the border
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
Gordy, I think the tail rotor gear is driven off the #1 side only. I'm looking at a photo I have here of the back of the MGB opened up, and I'm almost certain the tail rotor gear engages the smaller gear just to the front of the red circle you have in the picture.
See pictures below. The diagram from the PTM that you posted seems to imply that the tail rotor gear engages both the #1 and #2 sides but from my photos that isn't possible. The tail rotor gear is straight cut and the ones from the #1 and #2 engine are helical cut.
The pictures show the back of a S76 gearbox. The cover is removed and flipped downwards (i.e. upside down). The tail rotor gear is the big gear on the MGB cover on the lower middle half of the 2nd picture. It engages the smaller, inset gear on the #1 side on the top left. I hope this makes sense!
So yes, if the shaft you circled were to break, the tail rotor would only be driven off the #1 engine!
See pictures below. The diagram from the PTM that you posted seems to imply that the tail rotor gear engages both the #1 and #2 sides but from my photos that isn't possible. The tail rotor gear is straight cut and the ones from the #1 and #2 engine are helical cut.
The pictures show the back of a S76 gearbox. The cover is removed and flipped downwards (i.e. upside down). The tail rotor gear is the big gear on the MGB cover on the lower middle half of the 2nd picture. It engages the smaller, inset gear on the #1 side on the top left. I hope this makes sense!
So yes, if the shaft you circled were to break, the tail rotor would only be driven off the #1 engine!
Join Date: Apr 2003
Location: USA
Age: 75
Posts: 3,012
Likes: 0
Received 0 Likes
on
0 Posts
Aircraft lore is made of these kind of questions!
For all S-76 models:
1) The tail rotor is driven by the main bull gear, and thus by either engine.
2) It is not possible to isolate the TR drive without postulating a failure somewhere.
3) For convenience, the TR drive on the S-76 family MGB is twinned from the #1 side, of the input drive assemble, but once again, the #2 engine drives the TR through that gear, as well.
Regarding the TR overspeeding, the engine overspeed should catch a runaway before the TR gets to 120%, and the TR has been tested many times to well above that, I have personally seen 125% Nr/NrTR in flight.
For all S-76 models:
1) The tail rotor is driven by the main bull gear, and thus by either engine.
2) It is not possible to isolate the TR drive without postulating a failure somewhere.
3) For convenience, the TR drive on the S-76 family MGB is twinned from the #1 side, of the input drive assemble, but once again, the #2 engine drives the TR through that gear, as well.
Regarding the TR overspeeding, the engine overspeed should catch a runaway before the TR gets to 120%, and the TR has been tested many times to well above that, I have personally seen 125% Nr/NrTR in flight.
Hello Gordy,
I remember having seen a different main gear box internal design from this one (wich is the "current one") where it was easier to understand this case of MGB failure;
1/ eng1 gearing connects MGB and T/R
2/ eng2 gears MGB only.
So MGB is geared to T/R via ENG1 and its free wheel. In case of eng1 power shaft failure to the MGB, NR will drop, eng 2 will take over the loss of power and in precipitation,you may believe you have an engine 1failure (if you don't look the N2) ... you may be tempted to shut eng1 off, and if you do so, you will loose your T/R ...
But I also know that at least 2 models of gear boxes have been manufactured; the 09500 and the 09600, to cope with the increased power of the PT 6 of the B model (and later for 2S Arriel series)
Could it be that the MGB internal drawings that we see now on training documents is this one of the 09600 series that would include this gearing modification ? Difficult to tell, but it can be compared with the drawings found in older training manuals of A versions
In this case this unlikelytricky scenario would apply to A, A+ and A++ only.
Note: No worry if you don't understand what I'm trying to mean, it is normal, it's not you ! English is not my mother tongue )))
I remember having seen a different main gear box internal design from this one (wich is the "current one") where it was easier to understand this case of MGB failure;
1/ eng1 gearing connects MGB and T/R
2/ eng2 gears MGB only.
So MGB is geared to T/R via ENG1 and its free wheel. In case of eng1 power shaft failure to the MGB, NR will drop, eng 2 will take over the loss of power and in precipitation,you may believe you have an engine 1failure (if you don't look the N2) ... you may be tempted to shut eng1 off, and if you do so, you will loose your T/R ...
But I also know that at least 2 models of gear boxes have been manufactured; the 09500 and the 09600, to cope with the increased power of the PT 6 of the B model (and later for 2S Arriel series)
Could it be that the MGB internal drawings that we see now on training documents is this one of the 09600 series that would include this gearing modification ? Difficult to tell, but it can be compared with the drawings found in older training manuals of A versions
In this case this unlikelytricky scenario would apply to A, A+ and A++ only.
Note: No worry if you don't understand what I'm trying to mean, it is normal, it's not you ! English is not my mother tongue )))
Thanks for all the replies.
flintexpert, your picture does show the TR gear connected to # 1 only, however cpt points out there are two different models and I think I was referring to the one Nick has just described. Nick just described the layout I posted.
I am sure I will have more questions as I get deeper into it. I will only be flying it occasionally---2 B models and 1 A model.
flintexpert, your picture does show the TR gear connected to # 1 only, however cpt points out there are two different models and I think I was referring to the one Nick has just described. Nick just described the layout I posted.
I am sure I will have more questions as I get deeper into it. I will only be flying it occasionally---2 B models and 1 A model.
Join Date: Jun 2006
Location: East of the border
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
Hi Gordy
Having read Nick's reply, it is consistent with what I intended when I described the photos. Both engines normally drive the Main Gearbox and therefore the Tail Rotor. The Tail Rotor drive gear is *meshed* only to the left hand, #1 side. However the left hand (#1) side is also indirectly driven by the #2 engine through all the other gears in the MGB.
The diagram you posted from the FlightSafety PTM with the red circle is drawn ambiguously, and seems to suggest that the Tail Rotor gear is intermeshed simultaneously with the #1 and #2 engine input gears, which is not how it actually is built if you look at the photos.
So yes, if the shaft you circled were to fail, I would agree that the Tail Rotor would be disconnected from the Main Gearbox and #2 Engine, and the #1 Engine would be the only thing powering the Tail Rotor.
I hope I haven't added to the confusion!
Having read Nick's reply, it is consistent with what I intended when I described the photos. Both engines normally drive the Main Gearbox and therefore the Tail Rotor. The Tail Rotor drive gear is *meshed* only to the left hand, #1 side. However the left hand (#1) side is also indirectly driven by the #2 engine through all the other gears in the MGB.
The diagram you posted from the FlightSafety PTM with the red circle is drawn ambiguously, and seems to suggest that the Tail Rotor gear is intermeshed simultaneously with the #1 and #2 engine input gears, which is not how it actually is built if you look at the photos.
So yes, if the shaft you circled were to fail, I would agree that the Tail Rotor would be disconnected from the Main Gearbox and #2 Engine, and the #1 Engine would be the only thing powering the Tail Rotor.
I hope I haven't added to the confusion!
Now I understand, the problem is with the FS diagram I posted, which clearly shows both engines driving the TR drive gear directly. Maybe someone from FS is reading and can make note......
The attached comes from the Flight Safety S-76 training manual. No differentiation is made between the different models, (A, B, C, or different + versions) so presumably all drive trains are the same.
This graphic is a little clearer than the one provided by Gordy, but you can see the gear indicated (red) does not drive the tail rotor. Tail rotor drive is only provided via the gear indicated in green.
The schematic attached also backs up flintexpert input that the tail rotor drive is via straight cut gears, rather than the helical to the MGB.
It is a possible failure mode that I feel should be emphasised a little more in training than it seems to receive. It is one I used to point out to people I flew with.
Agree the diagrams provided by training organisations cause confusion on this point, and could be improved by an overhead view.
This graphic is a little clearer than the one provided by Gordy, but you can see the gear indicated (red) does not drive the tail rotor. Tail rotor drive is only provided via the gear indicated in green.
The schematic attached also backs up flintexpert input that the tail rotor drive is via straight cut gears, rather than the helical to the MGB.
It is a possible failure mode that I feel should be emphasised a little more in training than it seems to receive. It is one I used to point out to people I flew with.
Agree the diagrams provided by training organisations cause confusion on this point, and could be improved by an overhead view.
Last edited by megan; 29th May 2017 at 02:21.
Gordy,
To quote Nick Lappos from post #81 & #87 in this thread 2nd Nov 2001 !!!
It has happened -
To quote Nick Lappos from post #81 & #87 in this thread 2nd Nov 2001 !!!
It has happened -
Steve76 asks:
Hello Nick and fellow 76 drivers,
With regard to a internal gearbox failure of the #1 engine, N2 input driveshaft and the subsequent channelling of 100% of the turbines power into the tail rotor. Is it not unreasonable to expect that the tail rotor will overspeed to the point that it suffers a catastrophic failure and disintegrates?.
Nick sez:
You are postulating an internal main rotor gearbox failure, I think, where the engine is still connected to the tail rotor, but disconnected from the main rotor bull gear.
Such a failure would leave the engine driving a much smaller load than the big main rotor, so an upspeed might occur, depending on how big the power reduction is (the reduction from normal drive to just the tail rotor.)
In all cases, the engine overspeed protection will catch the drive up speed if it should get out of hand, since the overspeed protection system is designed to shutdown the engine if the engine shaft breaks, which is a very big power reduction, the biggest possible. The tail rotor is quite healthy at speeds up to somewhere over 130% (I have been to 128 in flight during tests - this is done by professionals, do not try this at home!)
Most likely, the failure you describe would create a situation where the engine would speed up for a second or two, then settle down to about 109% or so, and it would be at about 2% torque, spinning happily with almost no load (just driving the tail rotor). The 109% is because unloaded, but with the cruise collective pitch setting, the engine would be trimmed up a bit by the collective bias. The rotor and the #2 engine would be driving the helicopter, and you would be in single engined flight.
The only problem would be if you decided to then shut down the #1 engine, because that would cause the loss of tail thrust.
Hello Nick and fellow 76 drivers,
With regard to a internal gearbox failure of the #1 engine, N2 input driveshaft and the subsequent channelling of 100% of the turbines power into the tail rotor. Is it not unreasonable to expect that the tail rotor will overspeed to the point that it suffers a catastrophic failure and disintegrates?.
Nick sez:
You are postulating an internal main rotor gearbox failure, I think, where the engine is still connected to the tail rotor, but disconnected from the main rotor bull gear.
Such a failure would leave the engine driving a much smaller load than the big main rotor, so an upspeed might occur, depending on how big the power reduction is (the reduction from normal drive to just the tail rotor.)
In all cases, the engine overspeed protection will catch the drive up speed if it should get out of hand, since the overspeed protection system is designed to shutdown the engine if the engine shaft breaks, which is a very big power reduction, the biggest possible. The tail rotor is quite healthy at speeds up to somewhere over 130% (I have been to 128 in flight during tests - this is done by professionals, do not try this at home!)
Most likely, the failure you describe would create a situation where the engine would speed up for a second or two, then settle down to about 109% or so, and it would be at about 2% torque, spinning happily with almost no load (just driving the tail rotor). The 109% is because unloaded, but with the cruise collective pitch setting, the engine would be trimmed up a bit by the collective bias. The rotor and the #2 engine would be driving the helicopter, and you would be in single engined flight.
The only problem would be if you decided to then shut down the #1 engine, because that would cause the loss of tail thrust.
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.
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.