.
So far, in reading bewteen the lines, I suspect that we did accomodate some of your concerns.
To: lomapaseo
They may be mixed up but here are the basic problems:
1) Most reliability and maintainability and many safety engineers have never turned a wrench in anger. Most of them have been transferred to the RMS department to avoid layoff. Most being design engineers will deal with the item under analysis as a mathematical entity instead of an element that is subject to abuse by its’ environment, by a hamfisted mechanic or possibly it has a design flaw or a manufacturing flaw. These things are very seldom taken into consideration during the reliability analysis.
2) In order to perform a reliability analysis the analyst requires a database that contains failure rates under a stated environment and stress level. Such a document does not exist. As previously stated the documents provided by the Government for use on reliability programs for government equipment are totally useless but if the analyst can prove that his number came from that database the government will accept it. These same databases are used for commercial aircraft and many other programs requiring Reliability analysis. The databases for electronic parts are quite complete so the problem of unusable information is mooted.
3) Reliability analysis was developed by the US Air Force to determine the best way to improve electronic system architecture. The analyst would create a block diagram reflecting the system design. He would insert fictitious failure rates in each block and using Boolean Algebra run a calculation to determine the system failure rate. Then by inserting redundancy of several kinds and using the same failure rates he would run his calculation again to see if there was any improvement in the end number. This would be performed several times until the analyst arrived at the optimum design. The end numbers were meaningless but they showed that by altering the design you could improve reliability. By requirement RMS analysis must be accomplished prior to freezing the design. Now because of financial constraints or whatever excuse engineering can come up with the analysis is done very close to design freeze and in some cases after design freeze. So whatever analysis is performed it does not mean a thing.
4) Now when a block diagram is constructed for a mechanical entity the analyst will create a block diagram reflecting the system architecture and plug in failure rates that come from the meaningless government database. The analyst will run his calculation to determine if he meets the contract requirements. If he can’t he will go back to the database and manipulate the numbers in order to meet contract requirements. One year later the company must show reliability growth and in order to do that the analyst will select even better numbers in order to show growth even though the design has not changed.
5) Engineers have a not invented here attitude when it comes to reliability or maintainability input.
I could go on for much more but I will be eating up space on Danny’s’ server. Suffice it to say, if you use the government-supplied system and can show that the analyses complied with those guidelines and specs and your numbers comply with the contract then it is acceptable. The UK has a Def Stan, which reflects the US Government specs, and they have the same problems.
Now consider the FTA, which is submitted to the certification authorities, comprises the failure rates determined in the reliability analyses. Computer engineers came up with the following quote and it is applicable here. Garbage In Garbage Out (GIGO).
I believe this has been carried far enough. I haven’t been influential in changing the system in 34 years and I doubt it will ever change as too many people have a vested interest in not changing it.
