A mystery for The Cat
A mystery for The Cat
Guest
Posts: n/a
To: Mriya225
The work that I do involves four phases.
1) Perform a reliability analysis to determine the rate of failure for the piece parts that make up an assembly and calculate the overall failure rate or deftct rate in the assembly based on the total failure rates of the piece parts.
2) Perform a Maintainability and maintenance analysis on the assembly to determine the way the part is maintained in the most expedient manner, and what maintenance must be performed and how often in order to maintain the reliability of the assembly.
3) Perform a Failure Mode Effects Analysis (FMEA) to determine the effects on the assembly in the event of a failure of a piece part within in the assembly and then, kick that up one level to determine the effect on the system and then one more level to determine the effects of the aircraft.
4) Perform a safety hazards analysis using the reliability failure rates for the component relative to system operation and factor in the elements of the FMEA to show the inter relationships of individual failures relative to safe system operation.
In performing the above analyses it is necessary to have sufficient failure rate data to base your calculations on. In many cases aircraft companies do not have a historical data base and if they do, they do not make it available to the companies that build appliances for the aircraft company. In a case like that, the analyst must come up with his own figures based on his past experience and any data base that he/she may have accumulated. These figures in most cases must be massaged to fit the case. This enters in a degree of inaccuarcy in the calculations. The classic explanation used by most numbers crunchers (which I am not) is that the results of the calculation are in effect performance numbers and do not necessarily reflect the item being analyzed.
The other factor is the definitions of critical failures as indicated in the safety documents of the FAA. They state that if a single point failure is identified it must be designed out. If it can't, the defined failure must not occur more frequently that one time in a billion hours of operation or, 1 10-9.
That automatically skews the figures in the analysis as in the case of the rotor shaft it cannot fail more frequently than 1 10-9 hours of accumulated flight of the S-76 and all of its' variants.
This means that Sikorsky myst perform 100% quality checks on each and every shaft. It also means that the designers must calculate the maximum loading and the accumulated effects of vibration, corrosion and anything that would have an effect on the shaft. The stress engineers must calculate the loading and provide an adequate safety margin. Usually 1.5 times the yield loading.
In the performance of the FMEA the analyst must determine the exact modes of failure and their causes to ensure that they are either designed out or taken into consideration in the design and subsequent stress calculations.
Now, another problem surfaces. There is an adversarial relationship between the product integrity engineers and the design engineers. When a person like myself identifies a potential problem the engineers in most cases wil not consider it because as they often say we are only numbers crunchers.
Now we get to the S-76 rotor shaft problem.
If I had done the analysis on the shaft I would have considered the following:
1) Inclusions in the forging
2) Faulty heat treat or, improper vacuum degassing
3) Hydrogen embrittlement
4) Machine strikes
5) Stress corrosion due to exposure to highly alkaline material used in cleaning or in the heat treating process.
To verify that there were no problems I would have recommended the following:
1)Perform 100% radiographic inspection or,
2)Perform 100% ultrasonic inspection or,
3)Perform 100% Magnaflux/Magnaglo inspection
4)When heat treat is performed include a sample that can be tested for Hydrogen embrittlement and. if practical, perform all of the tests above.
This what I would do. The question is, what did Sikorsky RMS engineers tell the design and production engineers to do to ensure a high level of reliability.
Aircraft companies don't always get it right.
FAA requirements as stated above for a single point failure that would result in loss of the aircraft and / or injury or death is 1 10-9. Figure out how many shafts were on the way to total failure prior to the discovery of the problem. Divide the number of potential failures into the total number of hours of flying for the S-76 fleet. If they had failed then statiscally, there should not be a failure in several hundred thousand years of continuous operation.
This same problem was presented in my argument about the lack of safety in the R22 and R44.
Here are three examples of the manufacturers not getting it right.
Of forty three B214s delivered to our facility in Iran we found 18 that had an improperly heat treated bull gear in the transmissions.
On those same helicopters (all 43) we detected a magnetism problem in the rotor shafts. This resulted in several problems in the navigation system and magnetic compasses. This was caused by the fact that Bell never included bonding straps on the rotor heads.
A perfect example of this catastrophic failure problem is the fan disc on the No. 2 engine of the United DC-10 in Iowa. The failure of the disc was attributed to a grain of sand imbedded in the disc and not found during the various testing of the disc during production. Based on the calculations of the engine manufacture the catastrophic failure of the disc was greater than 1 10-9 and should therfore never happen during the life of the DC-10 fleet life.
The entire safety of all commercial airliners is based on the manipulation of numbers in order to meet the allowed rates of failure specified by the FAA.
One final point. In the preparation of the safety hazards analysis the analyst will construct a boolean logic diagram using and gates and, or gates. These elements will be assembled in such a way that they accurately depict the system under analysis and the systems that provide services to the system under analysis. All of these diagrams terminate at the highest level in an And gate. At this point the analyst using Boolean algebra will calculate the frequency at which the And gate will indicate failure.
In all cases, it will be at a minimum of 1 10-9 or as high as 1 10-12 or even 1 10-17. In the eyes of the FAA that is sufficient. But,it is not.
If they took all of the And gates at the system level and ran them to an Or gate that indicated the aircraft level and then ran the same calculations they would come up with a number more close to 1 10-8 or-9 which is well below the safety level af any of the systems.
This is analagous to the Astronauts in the Apollo capsule saying that "Just think, this whole thing was built by the lowest bidders".
------------------
The Cat
[This message has been edited by Lu Zuckerman (edited 12 November 2000).]
[This message has been edited by Lu Zuckerman (edited 12 November 2000).]
[This message has been edited by Lu Zuckerman (edited 12 November 2000).]
[This message has been edited by Lu Zuckerman (edited 12 November 2000).]
[This message has been edited by Lu Zuckerman (edited 13 November 2000).]
The work that I do involves four phases.
1) Perform a reliability analysis to determine the rate of failure for the piece parts that make up an assembly and calculate the overall failure rate or deftct rate in the assembly based on the total failure rates of the piece parts.
2) Perform a Maintainability and maintenance analysis on the assembly to determine the way the part is maintained in the most expedient manner, and what maintenance must be performed and how often in order to maintain the reliability of the assembly.
3) Perform a Failure Mode Effects Analysis (FMEA) to determine the effects on the assembly in the event of a failure of a piece part within in the assembly and then, kick that up one level to determine the effect on the system and then one more level to determine the effects of the aircraft.
4) Perform a safety hazards analysis using the reliability failure rates for the component relative to system operation and factor in the elements of the FMEA to show the inter relationships of individual failures relative to safe system operation.
In performing the above analyses it is necessary to have sufficient failure rate data to base your calculations on. In many cases aircraft companies do not have a historical data base and if they do, they do not make it available to the companies that build appliances for the aircraft company. In a case like that, the analyst must come up with his own figures based on his past experience and any data base that he/she may have accumulated. These figures in most cases must be massaged to fit the case. This enters in a degree of inaccuarcy in the calculations. The classic explanation used by most numbers crunchers (which I am not) is that the results of the calculation are in effect performance numbers and do not necessarily reflect the item being analyzed.
The other factor is the definitions of critical failures as indicated in the safety documents of the FAA. They state that if a single point failure is identified it must be designed out. If it can't, the defined failure must not occur more frequently that one time in a billion hours of operation or, 1 10-9.
That automatically skews the figures in the analysis as in the case of the rotor shaft it cannot fail more frequently than 1 10-9 hours of accumulated flight of the S-76 and all of its' variants.
This means that Sikorsky myst perform 100% quality checks on each and every shaft. It also means that the designers must calculate the maximum loading and the accumulated effects of vibration, corrosion and anything that would have an effect on the shaft. The stress engineers must calculate the loading and provide an adequate safety margin. Usually 1.5 times the yield loading.
In the performance of the FMEA the analyst must determine the exact modes of failure and their causes to ensure that they are either designed out or taken into consideration in the design and subsequent stress calculations.
Now, another problem surfaces. There is an adversarial relationship between the product integrity engineers and the design engineers. When a person like myself identifies a potential problem the engineers in most cases wil not consider it because as they often say we are only numbers crunchers.
Now we get to the S-76 rotor shaft problem.
If I had done the analysis on the shaft I would have considered the following:
1) Inclusions in the forging
2) Faulty heat treat or, improper vacuum degassing
3) Hydrogen embrittlement
4) Machine strikes
5) Stress corrosion due to exposure to highly alkaline material used in cleaning or in the heat treating process.
To verify that there were no problems I would have recommended the following:
1)Perform 100% radiographic inspection or,
2)Perform 100% ultrasonic inspection or,
3)Perform 100% Magnaflux/Magnaglo inspection
4)When heat treat is performed include a sample that can be tested for Hydrogen embrittlement and. if practical, perform all of the tests above.
This what I would do. The question is, what did Sikorsky RMS engineers tell the design and production engineers to do to ensure a high level of reliability.
Aircraft companies don't always get it right.
FAA requirements as stated above for a single point failure that would result in loss of the aircraft and / or injury or death is 1 10-9. Figure out how many shafts were on the way to total failure prior to the discovery of the problem. Divide the number of potential failures into the total number of hours of flying for the S-76 fleet. If they had failed then statiscally, there should not be a failure in several hundred thousand years of continuous operation.
This same problem was presented in my argument about the lack of safety in the R22 and R44.
Here are three examples of the manufacturers not getting it right.
Of forty three B214s delivered to our facility in Iran we found 18 that had an improperly heat treated bull gear in the transmissions.
On those same helicopters (all 43) we detected a magnetism problem in the rotor shafts. This resulted in several problems in the navigation system and magnetic compasses. This was caused by the fact that Bell never included bonding straps on the rotor heads.
A perfect example of this catastrophic failure problem is the fan disc on the No. 2 engine of the United DC-10 in Iowa. The failure of the disc was attributed to a grain of sand imbedded in the disc and not found during the various testing of the disc during production. Based on the calculations of the engine manufacture the catastrophic failure of the disc was greater than 1 10-9 and should therfore never happen during the life of the DC-10 fleet life.
The entire safety of all commercial airliners is based on the manipulation of numbers in order to meet the allowed rates of failure specified by the FAA.
One final point. In the preparation of the safety hazards analysis the analyst will construct a boolean logic diagram using and gates and, or gates. These elements will be assembled in such a way that they accurately depict the system under analysis and the systems that provide services to the system under analysis. All of these diagrams terminate at the highest level in an And gate. At this point the analyst using Boolean algebra will calculate the frequency at which the And gate will indicate failure.
In all cases, it will be at a minimum of 1 10-9 or as high as 1 10-12 or even 1 10-17. In the eyes of the FAA that is sufficient. But,it is not.
If they took all of the And gates at the system level and ran them to an Or gate that indicated the aircraft level and then ran the same calculations they would come up with a number more close to 1 10-8 or-9 which is well below the safety level af any of the systems.
This is analagous to the Astronauts in the Apollo capsule saying that "Just think, this whole thing was built by the lowest bidders".
------------------
The Cat
[This message has been edited by Lu Zuckerman (edited 12 November 2000).]
[This message has been edited by Lu Zuckerman (edited 12 November 2000).]
[This message has been edited by Lu Zuckerman (edited 12 November 2000).]
[This message has been edited by Lu Zuckerman (edited 12 November 2000).]
[This message has been edited by Lu Zuckerman (edited 13 November 2000).]
Guest
Posts: n/a
I have California teaching credentials in four subject areas including Aircraft and Aerospace Technology. I started teaching at Fort Eustis, Virginia on contract managing a course on three types of Sikorsky helicopters. I had platform instruction experience on two assignments with the Air Force and NASA. I have also taught Aerospace Technology in the Los Angeles and Santa Barbara school systems.
Normally, when I am on assignment as a consultant I locate the nearest A&P school and offer my services teaching helicopter aero dynamics. The last time I was able to do this was at Frederick Community College in Frederick, MD and at Ivy Tech State College in Terre Haute, IN
I also want to use this space to add some additional information to my post in response to your first post in this thread.
Another point that effects the reliability of components is a condition known as batch sensitivity. This means that if more than one item of the same design is exposed to the same manufacturing processes at the same time. If one is found defective in service another item that was processed at the same time could have the same defect. This is especialy true for springs or any other item that requires heat treating and vacuum degassing. Another thing effecting springs is the wire draw process which can introduce inclusions which can weaken the spring. This inclusion can be in only one spring or it could be in all springs that were made from that same wire draw. Springs that are used in critical functions such as those used in servo mechanisms should all have 100% QC. Most companies don't do this as it is very expensive.
Another problem resulting in failure is maintenance error or mishandling either during manufacture or during component maintenance.
Sorry for going on so long.
------------------
The Cat
Normally, when I am on assignment as a consultant I locate the nearest A&P school and offer my services teaching helicopter aero dynamics. The last time I was able to do this was at Frederick Community College in Frederick, MD and at Ivy Tech State College in Terre Haute, IN
I also want to use this space to add some additional information to my post in response to your first post in this thread.
Another point that effects the reliability of components is a condition known as batch sensitivity. This means that if more than one item of the same design is exposed to the same manufacturing processes at the same time. If one is found defective in service another item that was processed at the same time could have the same defect. This is especialy true for springs or any other item that requires heat treating and vacuum degassing. Another thing effecting springs is the wire draw process which can introduce inclusions which can weaken the spring. This inclusion can be in only one spring or it could be in all springs that were made from that same wire draw. Springs that are used in critical functions such as those used in servo mechanisms should all have 100% QC. Most companies don't do this as it is very expensive.
Another problem resulting in failure is maintenance error or mishandling either during manufacture or during component maintenance.
Sorry for going on so long.
------------------
The Cat
Guest
Posts: n/a
Lu,
Please don't apologize for "going on", it's extremely informative! If you're ever heading west into Colorado... We have a school here, Westwood College of Aviation Technology (formerly known as Colorado Aero Tech) that desperately needs someone with your knowledge! TRUST ME!
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Un diva très doué.
Please don't apologize for "going on", it's extremely informative! If you're ever heading west into Colorado... We have a school here, Westwood College of Aviation Technology (formerly known as Colorado Aero Tech) that desperately needs someone with your knowledge! TRUST ME!
------------------
Un diva très doué.
