Sikorsky S-92: From Design to Operations
Looks to me Sikorsky never did an oil out test, just closed off the oil cooler and ran with hot oil and ignored loss of oil from elsewhere, inspiring unwarrented confidence.
Sikorsky stated that they did do such a test and that it failed, hence the bolt on Bypass Valve (drop me a PM if you need a reference). The valve was never part of the plan, full compliance was and as has been stated elsewhere the Blackhawk gearbox demonstrates that it is possible.
SASless, I don't know if the original oil out test was observed by the FAA, but I would have thought it would be. If I were a manufacturer I wouldn't want to write-off a MGB just to try it out, then have to do it all over again with an inspector present
VL
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The supposition that the transmission seized up in flight is a logical conclusion kicked around on this board given the known loss of oil pressure and the impact evidence. But did it? It will be of great interest what can be extracted from the HUMS and what the transmission tear down will reveal.
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Having may in an RFM text serves no useful purpose at all - especially when allied with the notorious sense of optimism shown by most helicopters pilots.
As with all accidents, there were a series of events that led to this catastrophic outcome:
Up to this point all could have been well apart from the obfuscation and prevarication that occurred on this (and probably other) forii. Many believed (and were not disabused of the belief) that the gearbox could 'run-dry' for 30 minutes. If you don't believe that was the case look to the spat between HC and Brian Abrahams.
Was this lack of clarity instrumental in the decisions of the crew to turn back and make for land instead of 'landing immediately'? We just don't know but, from discussion on PPRUne, we could imagine others making that same decision at that stage.
Was it considered that this (new) failure mode was 'extremely remote' and that the probability of it occurring again within a year or 1,250hrs was zero (or close to it)? Well it must have been because that was the essence of the letter to operators issued by SAC in January. If the S92 had had a 30 minute run-dry capability, this could have been a reasonable (stop gap) policy to keep the fleet in the air - without it (and in hindsight) it was not.
We are not solely concerned with the apparent lack of compliance with 29.927 (but the questions will be asked "was the FMECA rigorous enough"), or the use of titanium in the filter studs.
We are concerned that there was insufficient information provided to the pilots; worse, that they were led to believe that the aircraft was in compliance.
Perhaps we ought to be most concerned that, having had a near catastrophy, a further risk assessment (FMECA) had not been carried out which considered the probability of further filter stud failures and the likely consequences for an aircraft which spends most of its life over the open seas.
As with all accidents, there were a series of events that led to this catastrophic outcome:
- The gearbox failed its unaided 29.927 'run-dry' test.
- The Subsequent FMECA postulated that the only possible loss of oil would be from the external piping leading to the oil cooler.
- The isolation switch was fitted and the gearbox run without cooling for three hours.
- The aircraft was released to service.
Up to this point all could have been well apart from the obfuscation and prevarication that occurred on this (and probably other) forii. Many believed (and were not disabused of the belief) that the gearbox could 'run-dry' for 30 minutes. If you don't believe that was the case look to the spat between HC and Brian Abrahams.
Was this lack of clarity instrumental in the decisions of the crew to turn back and make for land instead of 'landing immediately'? We just don't know but, from discussion on PPRUne, we could imagine others making that same decision at that stage.
- There was a failure of (titanium) filter studs mid-2008 resulting in a rapid loss of oil.
Was it considered that this (new) failure mode was 'extremely remote' and that the probability of it occurring again within a year or 1,250hrs was zero (or close to it)? Well it must have been because that was the essence of the letter to operators issued by SAC in January. If the S92 had had a 30 minute run-dry capability, this could have been a reasonable (stop gap) policy to keep the fleet in the air - without it (and in hindsight) it was not.
We are not solely concerned with the apparent lack of compliance with 29.927 (but the questions will be asked "was the FMECA rigorous enough"), or the use of titanium in the filter studs.
We are concerned that there was insufficient information provided to the pilots; worse, that they were led to believe that the aircraft was in compliance.
Perhaps we ought to be most concerned that, having had a near catastrophy, a further risk assessment (FMECA) had not been carried out which considered the probability of further filter stud failures and the likely consequences for an aircraft which spends most of its life over the open seas.
js0987 The supposition that the transmission seized up in flight is a logical conclusion kicked around on this board given the known loss of oil pressure and the impact evidence. But did it?...
Most of that news conference can be heard via podcast here TSB News Conference March 26 via CBC Podcast (from CBC NL website)
gwillie, be careful what assumptions you make. The investigator clearly states that there was some rotation but he cannot say how much energy there was.
So the rotor was turning, but was it turning enough to produce the necessary lift to effect a ditching?
So the rotor was turning, but was it turning enough to produce the necessary lift to effect a ditching?
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With an estimated impact force of min 20g, I doubt the available rotor rpm did much to assist the landing. The condition of the recovered rotor blades as seen in the photos would also indicate very low rpm.
carholme
carholme
Likewise.....I wonder what the ROD and Ground Speed was at the time of impact?
I noticed he emphasized "some rotational movement" and talked of some strike effect.
Certainly no controlled ditching with the severe airframe damage and minor damage to the blades.....as I would expect it to be the otherway around if it were controlled.
I noticed he emphasized "some rotational movement" and talked of some strike effect.
Certainly no controlled ditching with the severe airframe damage and minor damage to the blades.....as I would expect it to be the otherway around if it were controlled.
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I'm curious about the testing that was done on this gearbox. I would guess that it was done on a test stand and not on an actual airframe tied to the ground. This being the case does is accurately show what the gearbox is experiencing in flight? After all, on a real helicopter it not only has to transmit the power, which is done on the test stand, but also support the weight af the entire machine as it is in flight. I doubt the test stand has the capability to simulate all this.
We see it all the time with engines that show a particular symptom on the wing but will not show it on the test cell. I'm sure nobody wants to take the chance on ruining an entire aircraft to test a gearbox failure, but at what point do we consider it an apples to apples test?
Max
We see it all the time with engines that show a particular symptom on the wing but will not show it on the test cell. I'm sure nobody wants to take the chance on ruining an entire aircraft to test a gearbox failure, but at what point do we consider it an apples to apples test?
Max
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Maxtork,
I agree. Sitting under a gearbox that I know will run for 30 mins dry on a bench would not make me happy in the knowledge that it would run dry in an aircraft for 30 mins. I don't know how they do bench testing but to be valid in my opinion, they have to account for thermal stress from engines and cooling effects of airspeed, torsional and bending moments imparted by the airframe, engines, controls, rotor head, tail rotor drive and of course the effects of aging/fatigue. I don't think that that kind of detail can be replicated on a bench. Am I wrong?
So much emphasis these days are placed on Cat A performance (rightly so) and yet is it me or has the regulation of gearboxes and drive train not kept up? Or is it that now we are putting so much power through gearboxes, the regulations haven't accounted for how close we are to the viable technological limits?
It is the FAA and JAA/EASA that need to be looking at their belly buttons.
I agree. Sitting under a gearbox that I know will run for 30 mins dry on a bench would not make me happy in the knowledge that it would run dry in an aircraft for 30 mins. I don't know how they do bench testing but to be valid in my opinion, they have to account for thermal stress from engines and cooling effects of airspeed, torsional and bending moments imparted by the airframe, engines, controls, rotor head, tail rotor drive and of course the effects of aging/fatigue. I don't think that that kind of detail can be replicated on a bench. Am I wrong?
So much emphasis these days are placed on Cat A performance (rightly so) and yet is it me or has the regulation of gearboxes and drive train not kept up? Or is it that now we are putting so much power through gearboxes, the regulations haven't accounted for how close we are to the viable technological limits?
It is the FAA and JAA/EASA that need to be looking at their belly buttons.
Droopy,
What helicopter types you got in your logbook so far?
What are you flying now on a regular basis?
Are any of them tested and certified for any run dry capability?
What helicopter types you got in your logbook so far?
What are you flying now on a regular basis?
Are any of them tested and certified for any run dry capability?
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filter installation
Maybe they should redesign the transmission oil system to relocate the filter to the external section of the oil system. Then they could isolate any leak or failure of the filter with the existing system in place.
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Mars, good post.
If there was a change to the assumptions or known modality of failure that would otherwise alter the original assessment to certify, one would think that a reassessment would be in order. Do we know if such a reassessment was conducted after the Broome incident?
Cheers
AV8
If there was a change to the assumptions or known modality of failure that would otherwise alter the original assessment to certify, one would think that a reassessment would be in order. Do we know if such a reassessment was conducted after the Broome incident?
Cheers
AV8
Better yet.....why not make it an internal filter....build it into the case...cover it with a blank gasketed cover then even if the filter bowl separates for any reason the oil is trapped "inside" the Gear Box.
Sikorsky uses internal oil passages rather than using external piping to get the oil around the case as it is.
The key is to keep the oil "inside" the gear box as much as possible and limit the sources of "external" leaks as much as possible.
Sikorsky uses internal oil passages rather than using external piping to get the oil around the case as it is.
The key is to keep the oil "inside" the gear box as much as possible and limit the sources of "external" leaks as much as possible.
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Filter covers..
"...build it into the case...cover it with a blank gasketed cover then even if the filter bowl separates for any reason the oil is trapped "inside" the Gear Box."
You still have a cover and a gasket with attaching hardware to hold the cover on, that mechanics need to remove and install during inspections.
Mounting the filter in the external system would address these human factors.
Have you ever seen a gasket blow out on a Huey transmission internal oil filter? All the oil goes away....
You still have a cover and a gasket with attaching hardware to hold the cover on, that mechanics need to remove and install during inspections.
Mounting the filter in the external system would address these human factors.
Have you ever seen a gasket blow out on a Huey transmission internal oil filter? All the oil goes away....
I have posted that the Cougar accident had great similarity to the Shuttle Challenger and that the Fineman Appendix F of that report discusses how this can happen.
Now it appears Columbia suffered the same fate. Lessons never learned.
Linked article of interest. Especially the crew saying it looks bad and managers saying it did not look too bad to them.
Spaceflight Now | STS-119 Shuttle Report | Legendary commander tells story of shuttle's close call
The relevance is the mind set of decision making which does not extrapolate the potential results of an event and mitigate the chance of the worst case scenarios.
The Sultan
Now it appears Columbia suffered the same fate. Lessons never learned.
Linked article of interest. Especially the crew saying it looks bad and managers saying it did not look too bad to them.
Spaceflight Now | STS-119 Shuttle Report | Legendary commander tells story of shuttle's close call
The relevance is the mind set of decision making which does not extrapolate the potential results of an event and mitigate the chance of the worst case scenarios.
The Sultan
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Transmission Design
I have been looking for my copy of the HARP Report (Helicopter Airworthiness Review Panel) produced by the UK CAA in the 80s following too many helicopter accidents that had technical related issues - can't find it so this is from memory.
The report essentially said that many aspects of helicopter design equated to the Dakota era of fixed wing design and that transmissions that included high speed sections within the main casing like the S61 - which at that time dominated the offshore world - were definately out of order. The report implored manufacturers to find a better way with less criticality. I believe the 30 minute run-dry time was meant to indicate that the design indeed had less critical failure modes. I don't believe it was ever envisaged that we, the flight crew, would interpret that to mean that it gave us more 'get-you-home' time. It is easy to see though that when you are up against it the pressure is on to make it those last few miles to the beach.
I recommend the HARP report (maybe someone can post a link if it exists on-line) because I think that as a retrospective it would make fascinating reading in the context of recent developments.
G.
The report essentially said that many aspects of helicopter design equated to the Dakota era of fixed wing design and that transmissions that included high speed sections within the main casing like the S61 - which at that time dominated the offshore world - were definately out of order. The report implored manufacturers to find a better way with less criticality. I believe the 30 minute run-dry time was meant to indicate that the design indeed had less critical failure modes. I don't believe it was ever envisaged that we, the flight crew, would interpret that to mean that it gave us more 'get-you-home' time. It is easy to see though that when you are up against it the pressure is on to make it those last few miles to the beach.
I recommend the HARP report (maybe someone can post a link if it exists on-line) because I think that as a retrospective it would make fascinating reading in the context of recent developments.
G.
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Run-dry
Busdriver 2
The "no oil" capabilities of the 60 was in the Army -10 in 2007. Dont know what it says currently.
Remembering back to 2007 I recall that anytime a transmission oil pressure low situation was introduced in the 60 simulator, the following would happen. Oil px low light, rapidly followed by: TX Oil hot, multiple chip lights then within approx 2 minutes horrible noise and general breakdown.
I dont know if the simulator software is based on manufacturers (Sikorsky) data from tests, or if it is programmed according to the Armys own experiences with gearbox problems. Anyhow I think the message the Army is giving its aviators both in emergency drills and simulator use is: Anytime you have main transmission issues....land without delay!
It would be very interesting to know what the Navy an AF says for the same gearbox related malfunctions in the HH-60. The Army in most cases operate over Terra Firma vs Navy blue water ops (offshore) considerations.
Torcher
The "no oil" capabilities of the 60 was in the Army -10 in 2007. Dont know what it says currently.
Remembering back to 2007 I recall that anytime a transmission oil pressure low situation was introduced in the 60 simulator, the following would happen. Oil px low light, rapidly followed by: TX Oil hot, multiple chip lights then within approx 2 minutes horrible noise and general breakdown.
I dont know if the simulator software is based on manufacturers (Sikorsky) data from tests, or if it is programmed according to the Armys own experiences with gearbox problems. Anyhow I think the message the Army is giving its aviators both in emergency drills and simulator use is: Anytime you have main transmission issues....land without delay!
It would be very interesting to know what the Navy an AF says for the same gearbox related malfunctions in the HH-60. The Army in most cases operate over Terra Firma vs Navy blue water ops (offshore) considerations.
Torcher