by Thomas Carroll
- Operational Reliability
- Asset Management Programs
- Maintenance Effectiveness
- Preventive Maintenance Programs
- Maintenance Support
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Maintenance Training Programs
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Component Repair Facility
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Components Themselves
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Mechanical System Hardware
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Operator / OEM Engineering
Rogue Component Definition
A rogue component is defined as an individual repairable component, which repeatedly experiences short in-service periods, manifesting the same mechanical system fault each time it is installed, and when it is removed from the mechanical system, the fault is corrected.
The primary reason a component becomes rogue is because shop repair bench tests do not address 100% of the component's operating functions, characteristics or environment. Interviews with various component Original Equipment Manufacturers (OEM) revealed the bench test coverage is typically about 85% of the component's complete functionality. Even if all the functions were covered, the operating environment of the component when it is installed in the mechanical system is usually quite different than the repair shop, so if a failure is dependent upon a particular in-service environmental condition, it is unlikely that it will be duplicated during testing.
Additionally, the bench test is crafted to identify anticipated failures - focused on things that are expected to fail. For instance, it would not make sense to check all the screws or electrical ground straps each time the component comes into the shop, since the chance of failure for those pieces is practically zero and the cost of performing such extensive testing during each shop visit would be exorbitant.
When a component experiences a failure that was unaddressed or unanticipated by the shop testing procedures, a rogue is born. Since every test that is performed misses that specific aspect of the component's functionality, the fault will never be identified and resolved.
"Natural Selection" Phenomenon
There is a Darwinian-like "natural selection" process that ensures the rogue components are placed in the most disadvantageous position in the asset management program. The following depictions demonstrate the mechanics of this phenomenon.
Figure 1 shows a pristine condition where the component spare inventory and the in-service population are comprised of serviceable (Good) components that function as designed and expected (the In-Service Population shown in the diagram is a small representation of the general population). There are no rogue components yet.
In this situation, the asset management process will follow all the applied models. As a part fails in service, it is removed and replaced with a good part from the spare inventory. The component repair facility duplicates the problem with the failed unit, repairs it and returns it to the spare inventory. The "natural selection" phenomenon begins when a rogue component develops as shown in Figure 2.
When one of the in-service components develops a rogue failure, it is removed and sent to the repair facility. Since the failure is not addressed by the standard bench test or overhaul procedure, it is not duplicated and resolved. It checks normally, scoring a "No Fault Found" (NFF) and returns to the spare inventory as depicted by Figure 3.
This new rogue component is also removed from service and sent to the repair facility, and, like the first rogue component, it is also NFF and returned to the spare inventory, as shown in Figure 5.
Though the rogue components make up a very small part of the total component has ensured that they are sorted out to the most disadvantageous place in the asset management process - the spare inventory. In Figure 7, they have comprised 75% of the spare inventory. There are documented cases where the entire spare inventory had been replaced by rogue components.
Rogue Component Effect
As mentioned earlier, there are a number of facets of the organization that are affected by the development of rogue components. To illustrate the impact on Maintenance Effectiveness, the following scenario describes a real-life event:
"Real Life" Case in Point:
There is a system that allows air to be ventedto the atmosphere, comprised of an electromechanical control unit, sensing units A through C, a control feedback sensor and the vent valve. A system malfunction occurred that caused the vent valve to intermittently lock up in mid-position during high operational demands. The maintenance technicians could not duplicate the fault, so they replaced the control unit as it was the most likely part that could cause this problem.
The problem repeated. Since the control unit did not resolve the problem, the vent valve was replaced, which required considerable system down time and maintenance resources.
Now when the system operated during high demand periods, the valve intermittently modulated open and closed, when it should remain in a fixed position. Again, the problem could not be duplicated and since this new issue surfaced immediately after the installation of the valve, it was replaced again in the assumption that it was defective from stock. The system was down again for a considerable amount of time during this second replacement. The modulation problem repeated.
Next, the control feedback sensor was replaced and again the problem repeated. It was suspected that there could be an intermittent fault in the interconnecting wiring, which might have been caused when the valve was replaced. Several maintenance technicians spent many hours checking the wiring for faults, finding no problems.
Root Cause Analysis
The root cause of the initial system malfunction (when the valve would stop during operation) was a faulty vent valve. The control unit that was first installed was a rogue component, which caused the valve to intermittently modulate during high operational demands. However, this rogue failure would not manifest itself until a serviceable valve was installed, since the faulty valve would lock up during operation, preventing the modulation from occurring.
This type of compound problem does not happen all the time. Usually a rogue component causes the original problem to continue until it is resolved after multiple replacements of the same part, when a "good" part is finally pulled from the spare inventory.
To illustrate an effect on Asset Management, the following describes a real-life event:
"Real Life" Case in Point:
An aircraft operator had a fleet of 40 aircraft, each having an autopilot system comprised of a control panel, pitch computer, roll computer, air data sensor and a number of servo motors and sensors. In order to minimize aircraft down time, it was determined that 6 pitch computers were needed as on-hand spare inventory.
Over time, the fleet began to experience an increasing number of pitch related complaints and the spare pitch computer inventory was reduced to zero on a number of occasions. More computers were procured to accommodate the increasing demand. This chain of events repeated over a number of years until there were ultimately 28 spare computers to support the 40 aircraft that were in service.
Root Cause Analysis
The root cause of the inordinate amount of spare inventory that had accumulated was that a substantial amount of rogue components had developed. After a thorough analysis of the pitch computer population, it was discovered that 20 of the 28 spare computers were rogue. This caused the high replacement activity that would quickly decimate the inventory when an aircraft experienced a pitch related autopilot complaint.
Once the rogue components were identified and resolved, it was possible to surplus 20 of the spares. Each computer cost approximately $12,000 (US), so the cost of acquiring the excess inventory totaled around $240,000 (US). Of course, when components are sold on the surplus market, only a small fraction of the initial outlay is recovered.
Rogue Component Control
Given the fact that rogue components will develop, what can be done? The first step is to develop a good record keeping process, capturing maintenance events and tracking components by part and serial number.
Then a rogue component surveillance program needs to be developed, that will flag individual parts by serial number that experience repeated consecutive short in-service periods.
Once the program identifies potential rogue components, the next step is to separate them from those that appear to be fully functional. A review of the system maintenance records will show a bona fide rogue component has manifested the same system fault each time it is installed and the problem is resolved when that specific serial number is removed from service.
The final step is to provide the OEM with the in-service data pertaining to the rogue component, so the shop test can be amplified to identify that particular failure when components are returned for repair in the future.
Conclusion
If the asset management program utilizes repaired, reconditioned or overhauled components, it is inevitable that rogue components will develop. Their negative impact will ripple across many facets of the organization. If that unique component failure is not identified and resolved, the rogue population will continue to grow, compounding the negative effect.
Detailed record keeping will provide the foundation for building a comprehensive rogue component identification and control program. In addition to minimizing their effect across the organization, this program will also be the catalyst for improving the effectiveness of various optimization initiatives, such as Six Sigma, Lean, Lean Six Sigma, and the like. Rogue components have also proven to be a significant contributing factor in causing those initiatives to fall short of expectations - but since they are so well hidden, everything else takes the beating.
Thomas Carroll has been involved with aviation maintenance since 1972, where he served seven years in the USAF as an Avionics Instrument Systems Specialist. After that he joined US Airways, working in the Instrument Shop and the Avionics Maintenance Control department. He was the first Component Reliability Engineer in the company, and then was promoted to Manager of Reliability Engineering, where he overhauled all the existing reliability programs and processes. He established the reliability program at NetJets, Inc. and is currently serving as Director of Maintenance Technical Services. He has spoken on the subject of component reliability and performance measurement at industry meetings and conferences, written articles for several publications, and conducted training classes at OEM component repair facilities and aircraft manufacturing sites.
MaintMgmt_Feb_Mar_2009.pdf
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