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Justification for Engine Parts Testing in Manufacturing
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Again, we are blessed with a great success story for NDT. The author tells us more on how to keep such success from being turned into a disaster! He is right, I believe, when he tells us that getting NDT accepted and then keeping it operating are very basic to the success of NDT. Frank Iddings Tutorial Projects Editor |
There is nothing more basic in nondestructive testing (NDT) than getting a test implemented and keeping it operating. Often the battle on both fronts is against those who want to eliminate NDT and substitute statistics, batch testing, auditing and "continuous improvement." This article is about the successful implementation and defense of an NDT test on a jet engine part against the objections of these "quality professionals."The company in question produces titanium turbine disks for small jet engines. A certain production run for one type of engine lasted from 1983 through 1988. Using good industry practice and state of the industry ultrasonic NDT equipment at the time, the manufacturer did ultrasonic testing on these disks as they were produced. The actual production varied from year to year and was recorded, as were the ultrasonic testing results. Out of a total of 25 965 parts produced, ultrasonic testing found 34 discontinuities and visual and optical testing detected one more after final machining. The disks with discontinuities were scrapped. The production quantities and the number of discontinuities found per year are listed in Table 1.
Table 1 Production and number of ultrasonically detected discontinuities
Year Production Number 1983 2516 12 1984 4541 9 1985 6523 4 1986 6222 3 1987 2663 1 1988 3500 5 Total 25 965 34
The remaining surfaces were free of discontinuities. The next manufacturing step is shot peening, which sets up compressive stresses in the surface. In this configuration, a crack developing inside the disk would be blocked by the compressive stresses from growing to the surface and becoming dangerous. The time for such retarded crack growth would be from six to eight years in fleet service. As a point of fact, all of the disks which passed the NDT test survived their expected lifetimes.
In summary, it can be said that testing can make a profit
A broken disk would have destroyed an engine/nacelle set or a whole airplane. The liability cost, without accounting for life and limb, would have been $500,000 or $7,000,000, respectively, in the economics of the time. If a failure had occurred, then a recall would have been pursued at a cost of $8,000,000 per occurrence to eliminate all disks "related" to the failed one. "Related" has a strict technical meaning and might include, but is not limited to, disks from the same heat of titanium, those from the same billet of titanium, those made within some period of time and so on. The aircraft engine industry has developed the strict definition of "related." In legal terms, one is attempting to assure that the failed part will be unique in the universe after the "related" ones are eliminated.
Production of jet engine disks continued after the production run reported in Table 1. As seen from Table 1, the number of discontinuities was decreasing over time. On this basis (attributed to "continuous improvement") and on the basis that there had been no destruction of aircraft or engines, in 1990 the "quality professionals" within the company advocated the cessation of ultrasonic testing to save money. This advocacy position was absurd because ultrasonic testing had caused the good safety record. Nevertheless, they continued to advocate the termination of ultrasonic testing.
The company, a member of the Center for Nondestructive Evaluation consortium at Iowa State University, called upon me (associate director there at the time) to use my recently developed economic theory of quality, productivity and profit to prove financially that ultrasonic testing should continue. I showed that the destruction of six aircraft at the minimum liability, followed by recalls each time to eliminate related disks, would have had the effect of reducing the expected profit from the manufacture of all 25 965 discs from more than $82,000,000 to a loss of more than $7,000,000. The cost of the ultrasonic testing was $465,000. So, in order to save $465,000, the "quality professionals" would blow away $82,000,000 or more.
The actual method to calculate the effect of quality on productivity, profit and corporate stability is given in Papadakis (1995; 1996; 1997). Of these, Papadakis (1996) is the clearest. The calculation on disks is given in its entirety in the references.
Another approach to justifying the investment in NDT equipment for use over several years is some variation of return on investment. In this calculation, a cost comparison is made between two scenarios. One scenario is to purchase new equipment with its associated annual costs, while the other scenario is to stay with the old system with its annual costs. The cash flows are put into the return on investment calculation and the profit due to buying the new equipment is calculated. Every factory controller has a canned program for this calculation. If the return on investment is higher than some percentage of profit agreed upon by the company (hurdle rate), then the purchase is justified.
The return on investment approach was applied to the engine disk data. The investment in 1982 in new ultrasonic NDT equipment was $400,000. The return on investment would be the profit made on that investment. The scenario with the investment has annual operating costs, maintenance costs, depreciation and corporate taxes saved as the depreciation is expensed during the production run, plus the resale value (capital regained) at the end of the production run. The scenario with no ultrasonic testing investment is the costs incurred as planes are destroyed and recalls are carried out. For the 35 faulty disks in the six year production run in question, assuming reasonable "relatedness" to eliminate many of them by recalls, the profit on the $400,000 investment after 12 years was 90.53%. The return on investment calculations are reported in Papadakis (1995; 1996; 1997), with Papadakis (1997) being the most complete.
Thus it was again proved, at a hurdle rate in effect of 12%, that it was correct to use 100% NDT. In summary, it can be said that testing can make a profit. The assertion of the "quality professionals" that you cannot test quality into a part is bunkum. They are addressing the wrong problem. The real issue is keeping nonconforming products out of the downstream pipeline and out of the final product. NDT is the ideal solution. Without NDT, many airplanes flying today would have long ago been holes in the desert.
ACKNOWLEDGMENTS
The author wishes to acknowledge the assistance of Fletcher H. Bray, Tom Howell and Vicki Panhuise of the Garrett Division of Allied Signal Aerospace for supplying the data. Bray worked closely with the author in the defense of NDT.
REFERENCES
Papadakis, E.P., "Cost of Quality: Three Financial Justifications for Nondestructive Testing," Reliability Magazine, Vol. 1, No. 5, January/February 1995, pp. 8-16.Papadakis, E.P., "Quality, Productivity, and Cash Flow," SAE Paper 960543, SAE International Congress and Exposition, Detroit, Michigan, 26-29 February 1996.
Papadakis, E.P., "Financial Justification for Investment in Nondestructive Testing Equipment," Materials Evaluation, Vol. 55, No. 10, October 1997, pp. 1155-1158.
Papadakis, E.P., "A Cost of Quality: Three Financial Methods for Making Inspection Decisions," Materials Evaluation, Vol. 55, No. 12, December 1997, pp. 1336-1345.
* Quality Systems Concepts, Inc., 379 Diem Woods Drive, New Holland, PA 17557; (717) 355-2142; fax (717) 355-2142; e-mail <papadakis@desupernet.net>.
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