EC225 crash near Bergen, Norway April 2016
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You ask a couple of very good questions. I think it raises further questions too.
I do not mean this as criticism of the AAIB but if you look at their report on G-REDL, section 1.18.2 it states that:
“Data provided by the helicopter manufacturer indicated that between 2001 and 2009 there were nine recorded cases of planet gear spalling on the AS332 L2 (see Table 2)”
and then goes on to add that:
“The information provided by the manufacturer regarding the number of planet gear rejections due to spalling was incomplete. During the investigation anecdotal evidence was provided that indicated that overhaul facilities disposed of rejected gears without routing them for investigation”.
As the EC225 and AS332L2 are widely reported to share the same epicyclic gearbox module, isn’t it surprising that only data on L2 gears with spalling was considered relevant? Was the Type 2 gear introduced for the EC225 to use exclusively, therefore considered to not be relevant to the 2009 investigation? Something doesn’t add up here.
I do not mean this as criticism of the AAIB but if you look at their report on G-REDL, section 1.18.2 it states that:
“Data provided by the helicopter manufacturer indicated that between 2001 and 2009 there were nine recorded cases of planet gear spalling on the AS332 L2 (see Table 2)”
and then goes on to add that:
“The information provided by the manufacturer regarding the number of planet gear rejections due to spalling was incomplete. During the investigation anecdotal evidence was provided that indicated that overhaul facilities disposed of rejected gears without routing them for investigation”.
As the EC225 and AS332L2 are widely reported to share the same epicyclic gearbox module, isn’t it surprising that only data on L2 gears with spalling was considered relevant? Was the Type 2 gear introduced for the EC225 to use exclusively, therefore considered to not be relevant to the 2009 investigation? Something doesn’t add up here.
Components rejected, in operation or during overhaul, were inspected in accordance with the Continued Airworthiness programme. Those which were considered to show new or unusual failure modes were then routed by the manufacturer to its materials laboratory for further analysis. However, the laboratory did not have the capacity to carry out an investigation of every component rejected during gearbox overhaul. When the Continued Airworthiness programme for the AS332 L2 was initiated it was determined, based on previous operational history, design calculations and the maintenance programme requirements, that damage to the planet gear outer race would not adversely affect the continued airworthiness of the helicopter. Therefore, planet gears which had been rejected for spalling were not routinely routed to the laboratory for additional investigation.
I think that was more of a capacity problem, rather than not of interest.
It is disturbing though, that they did not take action to investigate every one of them.
But the failure risk experienced on the L2 was assessed on the 225 by means of
decreasing the TBO.
Originally Posted by n305fa
Re the prediction of remaining gear life with spalling present, I was referring to your assertion that the gear should have lasted 300 hours, not the ability of the design team to predict residual life based on certification testing.
So 1080 to 2000 hours is 900 hours. It ran 300 hours until failure from last
modification, inspection and repair, whatever that means. According to the certification testing, it could have some spalling at 2000 hours that would not lead to catastrophic failure.
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Concentric,
Yes - I hadn't considered that the "second type" might have originated in the 225 (then possibly been distributed in L2s as units get swapped for overhaul). I guess the casual treatment of the rejected gears during 2001-9 may come down to not expecting them to fail suddenly, or regarding spalling as normal wear and tear.
The AAIB report, whilst thorough, didn't conclude why these gears suffer fatigue in the way they do. This led to some fairly woolly recommendations about assessments of continued airworthiness etc. It appears to me as if AH have then stood by their design and put extra monitoring requirements in place rather than doing much research into the component suitability.
Yes - I hadn't considered that the "second type" might have originated in the 225 (then possibly been distributed in L2s as units get swapped for overhaul). I guess the casual treatment of the rejected gears during 2001-9 may come down to not expecting them to fail suddenly, or regarding spalling as normal wear and tear.
The AAIB report, whilst thorough, didn't conclude why these gears suffer fatigue in the way they do. This led to some fairly woolly recommendations about assessments of continued airworthiness etc. It appears to me as if AH have then stood by their design and put extra monitoring requirements in place rather than doing much research into the component suitability.
Concentric/AnFi
Given the complexity of the manufacturing process and the many steps necessary to ensure that a pristine product performs safely, it struck me as odd that a unit that had been damaged in a road accident was repaired and returned to service.
Now I know we have no idea whether it was a contributing factor and clearly we can't just throw away expensive kit at the drop of a hat, but safety critical components with very limited opportunities for redundancy such as an MGB? If such a thing receives a shock loading outside of its normal operational environment which causes damage which needs to be repaired, surely it would be prudent to write it off as an insurance loss. I am not familiar with the overhaul that was carried out but it would be close to impossible to ensure the safety standards inherent in the manufacturing process or a routine overhaul from normal wear and tear.
Given the complexity of the manufacturing process and the many steps necessary to ensure that a pristine product performs safely, it struck me as odd that a unit that had been damaged in a road accident was repaired and returned to service.
Now I know we have no idea whether it was a contributing factor and clearly we can't just throw away expensive kit at the drop of a hat, but safety critical components with very limited opportunities for redundancy such as an MGB? If such a thing receives a shock loading outside of its normal operational environment which causes damage which needs to be repaired, surely it would be prudent to write it off as an insurance loss. I am not familiar with the overhaul that was carried out but it would be close to impossible to ensure the safety standards inherent in the manufacturing process or a routine overhaul from normal wear and tear.
What was disassembled?
How far down was the gear box taken down?
What was inspected?
What was found damaged?
What was replaced?
What was repaired?
Not enough information to assess the return to service decision.
As to "write it off and collect the insurance" -- that's one way to go. But doesn't that decision depend on what you found out once you opened up the gear box and assessed what was or wasn't damaged after that transportation accident?
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From the report:
Components rejected, in operation or during overhaul, were inspected in accordance with the Continued Airworthiness programme. Those which were considered to show new or unusual failure modes were then routed by the manufacturer to its materials laboratory for further analysis. However, the laboratory did not have the capacity to carry out an investigation of every component rejected during gearbox overhaul. When the Continued Airworthiness programme for the AS332 L2 was initiated it was determined, based on previous operational history, design calculations and the maintenance programme requirements, that damage to the planet gear outer race would not adversely affect the continued airworthiness of the helicopter. Therefore, planet gears which had been rejected for spalling were not routinely routed to the laboratory for additional investigation.
I think that was more of a capacity problem, rather than not of interest.
It is disturbing though, that they did not take action to investigate every one of them.
But the failure risk experienced on the L2 was assessed on the 225 by means of
decreasing the TBO.
Sorry I don't follow you. I'm referring to the given running time and the TBO.
So 1080 to 2000 hours is 900 hours. It ran 300 hours until failure from last
modification, inspection and repair, whatever that means. According to the certification testing, it could have some spalling at 2000 hours that would not lead to catastrophic failure.
Components rejected, in operation or during overhaul, were inspected in accordance with the Continued Airworthiness programme. Those which were considered to show new or unusual failure modes were then routed by the manufacturer to its materials laboratory for further analysis. However, the laboratory did not have the capacity to carry out an investigation of every component rejected during gearbox overhaul. When the Continued Airworthiness programme for the AS332 L2 was initiated it was determined, based on previous operational history, design calculations and the maintenance programme requirements, that damage to the planet gear outer race would not adversely affect the continued airworthiness of the helicopter. Therefore, planet gears which had been rejected for spalling were not routinely routed to the laboratory for additional investigation.
I think that was more of a capacity problem, rather than not of interest.
It is disturbing though, that they did not take action to investigate every one of them.
But the failure risk experienced on the L2 was assessed on the 225 by means of
decreasing the TBO.
Sorry I don't follow you. I'm referring to the given running time and the TBO.
So 1080 to 2000 hours is 900 hours. It ran 300 hours until failure from last
modification, inspection and repair, whatever that means. According to the certification testing, it could have some spalling at 2000 hours that would not lead to catastrophic failure.
Sorry, I got the wrong end of the stick about your comment on the running time, please accept my apologies.
Re the two types of planet gear, the 2nd stage planet gears are produced by two manufacturers, and they have done since the L2 was certified. Obviously the outer dimensions of the gear are identical as are the method and dimensions of the location to the second stage planet carrier. I don't know what the differences are in terms of raceway profiles, bearing roller design etc. You can also "mix and match" gears from each manufacturer within the module so you could end up with different manufacturers gears in the same epicyclic stage / module
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Neither you nor I knows what "repaired" means for that gear box. It covers a lot of ground. The details of that may or may not be in the final report.
What was disassembled?
How far down was the gear box taken down?
What was inspected?
What was found damaged?
What was replaced?
What was repaired?
Not enough information to assess the return to service decision.
As to "write it off and collect the insurance" -- that's one way to go. But doesn't that decision depend on what you found out once you opened up the gear box and assessed what was or wasn't damaged after that transportation accident?
What was disassembled?
How far down was the gear box taken down?
What was inspected?
What was found damaged?
What was replaced?
What was repaired?
Not enough information to assess the return to service decision.
As to "write it off and collect the insurance" -- that's one way to go. But doesn't that decision depend on what you found out once you opened up the gear box and assessed what was or wasn't damaged after that transportation accident?
I am not suggesting it for all the components in the helicopter but a helicopter MGB I would argue is something where those risks need not be taken. Just reject those which have been exposed to such exceptional events.
If you owned a single engined light aircraft and the repair shop accidentally dropped the engine a metre or so during overhaul and damaged it would you accept it back after repair or would you feel safer if they replace it with a new one? That is what insurance is for.
Last edited by birmingham; 2nd Jul 2016 at 14:57.
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This event in Oz transporting the MGB is interesting in regards to claiming insurance and scrapping the freight. If it was a serious crash / deceleration ( wish we knew those details ) what would you gain by returning the unit into service even after inspection and refurb ?
Reminds me of the COUGAR ACE that listed off Alaska carrying 4800 Mazda cars with water damage to some decks and cars hanging on their chains at 60 deg angles for days. Ship was righted and towed to Portland . Mazda decided to have all 4800 cars crushed to avoid any getting sold or on the road. Expensive yes, but it covered their butt in the future.
Reminds me of the COUGAR ACE that listed off Alaska carrying 4800 Mazda cars with water damage to some decks and cars hanging on their chains at 60 deg angles for days. Ship was righted and towed to Portland . Mazda decided to have all 4800 cars crushed to avoid any getting sold or on the road. Expensive yes, but it covered their butt in the future.
Last edited by TylerMonkey; 2nd Jul 2016 at 17:21.
I think the reported damage to the MGB and subsequent repair at AB will be a big factor in the ongoing investigation. I assume that this avenue is being followed.
I posted previously of an engine failure in a single engine helicopter with the determination that the root cause was believed to be that an internal gear had been dropped during assembly causing damage to the gear leading to it's in flight failure. It was interesting day for me.
Much as many think they should be privy to all information I fear they will have to wait for the final report.
I posted previously of an engine failure in a single engine helicopter with the determination that the root cause was believed to be that an internal gear had been dropped during assembly causing damage to the gear leading to it's in flight failure. It was interesting day for me.
Much as many think they should be privy to all information I fear they will have to wait for the final report.
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@riff_raff: Thank you for that enlightening explanation of the carburizing and etching processes. It illustrates well the point I mentioned about complex processes required to achieve a safe finished product, and that is just one such process. Who would imagine that to control dimensions of the bearing raceway and hardened layer that so many chemical and thermal processes would also be required, some of which to counteract potentially harmful effects of a preceding process?
Glad to help. I found your question regarding potential hydrogen embrittlement of the planet gear quite interesting, so I looked into it.
It indeed seems ironic that an inspection procedure intended to detect material damage from a previous manufacturing process could itself potentially produce damage to the material. Also consider that the hydrogen embrittlement relief relies on strict adherence to a controlled process to ensure quality. This requires every person involved to maintain accurate documentation/records of their work. With the hydrogen embrittlement relief baking of these planet gears, even something as simple as how the gears are positioned in the furnace can be important.
Hydrogen embrittlement in steel parts is often associated with finishing processes such as electroplating. Copper plating (AMS 2418 type 2 or similar) is often used with carburizing to mask surfaces that don't require case hardening. The copper plate masking is sometimes removed after heat treat using a chemical stripping process. And plating specs (such as the one noted above) usually require the stripping method to be defined as part of the approved process, and state that the process shall not produce hydrogen embrittlement in the part. But as noted above, this only works when the controlled process is strictly adhered to by everyone involved.
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Klare likheter mellom to fatale Super Puma-ulykker - Tu.no
Here they claim that the cracks have propagated from the same surface deterioration/spalling.
However they say, spalling from G-REDL but no spalling from LN-OJF.
I am not sure about if the article is correctly citing AIBN.
Because the picture they refer to showing the crack propagation is a
simulation/calculation done by AH and not an actual analysis of the cracked gear
The article cites AH, are currently testing how the impact of a similar road accident could affect the gearbox and epicyclic module.
The last statement, don´t understand it. Why would they need to do such
a test, if the gearbox was completely stripped, checked and overhauled after the accident?
Here they claim that the cracks have propagated from the same surface deterioration/spalling.
However they say, spalling from G-REDL but no spalling from LN-OJF.
I am not sure about if the article is correctly citing AIBN.
Because the picture they refer to showing the crack propagation is a
simulation/calculation done by AH and not an actual analysis of the cracked gear
The article cites AH, are currently testing how the impact of a similar road accident could affect the gearbox and epicyclic module.
The last statement, don´t understand it. Why would they need to do such
a test, if the gearbox was completely stripped, checked and overhauled after the accident?
I am curious where you got your information that " gearbox was completely stripped, checked and overhauled after the accident". The only statements I've seen are that it was inspected and repaired. Not sure anyone but the company, AH, or the insurer would actually know what that means. Obviously it was not overhauled because it was installed with continued time, not zero time since overhaul.
AIBN will also know what was done. Work Order is recorded on the log card. Go to AH, pull the Work Order and there you have everything that was done to the MGB after that incident.
Easy.
Easy.
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Lots of reports to read regarding this issue. Interesting that AH says one planet gear design has better reliability than the other. Yet they have not established the root cause of the planet gear that failed. If the root cause was a manufacturing problem, all of the planet gears and bearings would likely need to be replaced again.
Gearboxes etc
This morning's Aviation Week Network, in an update on the 400M program, notes that AH is assisting with resolution of their prop gear box issues. ( same gearbox supplier, I believe ).
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original post by megan
Certainly going to be interesting to see the root origin of the failure.
Reminded of the fan disc failure in a DC-10. Was found the disc had been machined from a piece that came from the near end of a billet, which was found to contain inclusions. Fix, discard the ends of the billet.
In the early 60's worked in a shop manufacturing steam turbines for ships. For the gear cutting shop battery back up was supplied to the cutters/grinders. Once a cut/grind on a tooth was begun it was not permitted to be interrupted, for it introduced a point of future failure.
Certainly going to be interesting to see the root origin of the failure.
Reminded of the fan disc failure in a DC-10. Was found the disc had been machined from a piece that came from the near end of a billet, which was found to contain inclusions. Fix, discard the ends of the billet.
In the early 60's worked in a shop manufacturing steam turbines for ships. For the gear cutting shop battery back up was supplied to the cutters/grinders. Once a cut/grind on a tooth was begun it was not permitted to be interrupted, for it introduced a point of future failure.
original post by riff raff
Lots of reports to read regarding this issue. Interesting that AH says one planet gear design has better reliability than the other. Yet they have not established the root cause of the planet gear that failed. If the root cause was a manufacturing problem, all of the planet gears and bearings would likely need to be replaced again.
Lots of reports to read regarding this issue. Interesting that AH says one planet gear design has better reliability than the other. Yet they have not established the root cause of the planet gear that failed. If the root cause was a manufacturing problem, all of the planet gears and bearings would likely need to be replaced again.
Hopefully someone more knowledgeable on total-fleet-hours and total-gears-manufactured can improve my numbers .
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buzz: "Surely from here Oil Company's will start with the basics.
1. He who has the best Gearbox run dry times (no cheating e.g. Glycol)
2. Best Single Engine performance"
That would be a single engined aircraft but the authorities won't allow it.
1. He who has the best Gearbox run dry times (no cheating e.g. Glycol)
2. Best Single Engine performance"
That would be a single engined aircraft but the authorities won't allow it.
AnFi
Glycol is a valid means to reach the 30 minute requirements. Design mistakes which effectively neutralized this type of system (a sensor designed to report failure when the system is working and led to two ditchings) are not acceptable. This sensor screwup may point to systemic issues at AH (supported by the latest crash) which should cause operators to look elsewhere for aircraft, just not for a glycol loss of lube backup.
The Sultan
Glycol is a valid means to reach the 30 minute requirements. Design mistakes which effectively neutralized this type of system (a sensor designed to report failure when the system is working and led to two ditchings) are not acceptable. This sensor screwup may point to systemic issues at AH (supported by the latest crash) which should cause operators to look elsewhere for aircraft, just not for a glycol loss of lube backup.
The Sultan
You miss the point....using Glycol is not the same as a genuine Run Dry capability just as assuming there shall always be some trapped Oil in the Gearbox (such as in the S-92 original assumptions).
The goal would be to have a gearbox that can be run at Cruise Power for 30 Minutes with no oil, glycol, or fluid of any kind.
The question I would pose is what does the Crew do if there is no landing site within that 30 Minute Range and the Sea State exceeds the Certification Limit of the Emergency Floatation System particularly in very cold air and water temperatures and rough seas?
Why have a 30 minute run dry time if a safe landing (Ditching) can be made and SAR resources are available to retrieve the Crew and Passengers in a timely manner?
Are we perhaps chasing the wrong Horse?
The goal would be to have a gearbox that can be run at Cruise Power for 30 Minutes with no oil, glycol, or fluid of any kind.
The question I would pose is what does the Crew do if there is no landing site within that 30 Minute Range and the Sea State exceeds the Certification Limit of the Emergency Floatation System particularly in very cold air and water temperatures and rough seas?
Why have a 30 minute run dry time if a safe landing (Ditching) can be made and SAR resources are available to retrieve the Crew and Passengers in a timely manner?
Are we perhaps chasing the wrong Horse?
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I am trying to understand how the rotor mast are supported in the sun gear
and how the bearing loads would be if the rotor came out of control as a result of a crack in the conical housing leading to conical housing separation.
This question is supported by the fact that the secondary planetary gear carrier where the sun gear is attached is significantly bent, which could be a reason that the rotor mast upper mounting on the conical housing is no longer supporting the upper side forces of the rotor mast. (The rotor mast is inclined before separating from the sun gear)
this could be supported by the fact that there was no HUMS and magnetic plug readouts supporting any problems related to the MGB operation before the accident.
there are no indications that supports an actual gearbox jamming (from what I read and heard)
Would it be a possible scenario that the conical housing failed first?
And this is caused by a damage due to the rough handling that is not discovered?
there are two things that I find points towards this
the direction of the helicopter in relation to the direction of the rotor
after the separation from the aircraft.
if there was only a shearing of the gearbox, while the rotor still in a motion horisontal to the aircraft and MGB mounting plate
the rotor would also spin horisontal to that after separation due to the strong gyro forces.
If there was a breakdown of the upper bearing of the rotor mast and conical housing could this have lead to the rotor spinning out of control?
and if it was still supported by the lower bearing could this explain why the outer ring gear casing is broken, and whythe planetary gear carrier is bent?
if you look at the angle of the rotor on the videos available
it is nearly 45 degrees to the horisontal which a helicopter would normally travel.
would there be any serious argument against such a scenario?
and how the bearing loads would be if the rotor came out of control as a result of a crack in the conical housing leading to conical housing separation.
This question is supported by the fact that the secondary planetary gear carrier where the sun gear is attached is significantly bent, which could be a reason that the rotor mast upper mounting on the conical housing is no longer supporting the upper side forces of the rotor mast. (The rotor mast is inclined before separating from the sun gear)
this could be supported by the fact that there was no HUMS and magnetic plug readouts supporting any problems related to the MGB operation before the accident.
there are no indications that supports an actual gearbox jamming (from what I read and heard)
Would it be a possible scenario that the conical housing failed first?
And this is caused by a damage due to the rough handling that is not discovered?
there are two things that I find points towards this
the direction of the helicopter in relation to the direction of the rotor
after the separation from the aircraft.
if there was only a shearing of the gearbox, while the rotor still in a motion horisontal to the aircraft and MGB mounting plate
the rotor would also spin horisontal to that after separation due to the strong gyro forces.
If there was a breakdown of the upper bearing of the rotor mast and conical housing could this have lead to the rotor spinning out of control?
and if it was still supported by the lower bearing could this explain why the outer ring gear casing is broken, and whythe planetary gear carrier is bent?
if you look at the angle of the rotor on the videos available
it is nearly 45 degrees to the horisontal which a helicopter would normally travel.
would there be any serious argument against such a scenario?
Last edited by turboshafts; 7th Jul 2016 at 12:08. Reason: grammar
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The rotor only continues to spin after separation because it is driven by air from below as it falls ( as in autorotation). The tilt up 45 degs you mention is a result of this change as far as I know.
Open to suggestions ....
Open to suggestions ....
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You miss the point....using Glycol is not the same as a genuine Run Dry capability just as assuming there shall always be some trapped Oil in the Gearbox (such as in the S-92 original assumptions). The goal would be to have a gearbox that can be run at Cruise Power for 30 Minutes with no oil, glycol, or fluid of any kind. The question I would pose is what does the Crew do if there is no landing site within that 30 Minute Range and the Sea State exceeds the Certification Limit of the Emergency Floatation System particularly in very cold air and water temperatures and rough seas? Why have a 30 minute run dry time if a safe landing (Ditching) can be made and SAR resources are available to retrieve the Crew and Passengers in a timely manner? Are we perhaps chasing the wrong Horse?
These are all valid points. And there is currently a big effort underway to improve loss of lube capability of rotorcraft gearboxes.
First, let's consider what the real problem is when an MRGB operates under loss of lube conditions. The primary functions of the lube oil are to reduce friction and provide cooling of the bearings and gears. What causes failure of gears/bearings with loss of lube is the combination of increased friction heat generation and a greatly diminished cooling capability. If the gear/bearing materials are heated much beyond their tempering temperature, they begin to lose strength. The reason for a loss of lube back-up system using glycol injection rather than oil is because glycol provides better cooling. There are also new gear materials (like C64) that can operate at very high temperatures (~900degF). This will significantly improve loss of lube capability as these materials become more widely used in new designs.
Regulatory agencies such as the FAA have also recognized that their certification requirements need improvement. I think the loss of lube qual test procedure will become far more rigorous, including increasing the time requirement beyond 30 minutes.
Lastly, one change I would like to see the FAA implement is for every certified gearbox design in production to undergo a formal design review every few years using the most current analysis tools/techniques available. This analysis technology improves at a very rapid pace, and this proactive approach could likely reveal many potential problems that were missed in the original design analysis.