Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle diagnosis or maintenance indication
Reexamination Certificate
2000-06-19
2001-08-21
Nguyen, Tan (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle diagnosis or maintenance indication
C702S108000, C702S127000
Reexamination Certificate
active
06278920
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to methods of quality control and reliability employed by maintenance supervisors for ensuring safety of components used in a fleet of equipment, and more particularly, to a computerized method for determining a confidence level in a schedule for removing and replacing such component within the fleet.
2. Description of the Related Art
In the field of aircraft maintenance, safety and reliability dictate that maintenance be scheduled at periodic intervals to replace critical components before they fail. Apart from safety concerns, it is generally less expensive and more convenient to take equipment out of service at regular scheduled maintenance intervals to replace components before they fail than to expedite the replacement of a component that has already failed. Particularly in the aircraft industry, unexpected component failures tend to cause flight delays and cancellations that airline companies can ill afford. Safety and reliability concerns are likewise critical for other industries, including commercial trucking, communications, and military weapons and monitoring systems. If component replacement is put off for too long, safety and reliability are compromised, and the cost of post-failure replacement in terms of the potential loss of human life and property damage can be significant. On the other hand, if a component replacement schedule is overly conservative, then maintenance costs are unnecessarily increased by excessive labor and inventory costs; likewise, frequent removal of the equipment from the active fleet also tends to unnecessarily increase the costs of operating the fleet.
Present methods for analyzing the reliability of components in a fleet of vehicles include the simple use of the anticipated life expectancy recommended by the manufacturer of the component. Using this approach, a component is replaced just before the component has been in use for its anticipated life expectancy. However, this simplistic approach typically results in a significant number of components failing. More rigorous approaches have been used in the past, but their complexity, and the difficulty of interpreting their results, has probably discouraged maintenance supervisors from using such methods on any regular basis thus far.
Accordingly, it is an object of the present invention to provide a method for forecasting and analyzing failures of a component installed in a “fleet” of aircraft, trucks, or any other equipment.
Another object of the present invention is to provide such a method that is easy to implement and operate, and which can be repeated and updated at frequent intervals to analyze and adjust component replacement schedules as changed circumstances dictate.
Yet another object of the present invention is to provide such a method that can utilize actual historical data for the reliability of a component under study within a particular fleet in lieu of, and/or in supplementation of, reliability data provided by a manufacturer of such component.
Still another object of the present invention is to provide such a method which permits a manufacturer to reduce the size of its inventory of spare repair/replacement components.
A further object of the present invention is to provide such a method which reliably guides a maintenance supervisor in scheduling component maintenance for convenient, less-expensive, down times but which does not increase maintenance costs by unnecessarily replacing components before it is really necessary to do so.
A still further object of the present invention is to quickly correct reliance upon overly-optimistic life expectancy specifications supplied by the component vendor.
Another object of the present invention is to provide such a method that can be used in conjunction with a component that is incorporated within virtually any type of equipment for which reliability is impacted by the number of operating hours and/or cycles to which the component is subjected.
Still another object of the present invention is to provide such a method which can be performed with the assistance of a conventional desktop or laptop personal computer.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.
SUMMARY OF THE INVENTION
Briefly described, and in accordance with a preferred embodiment thereof, the present invention is a method for establishing a confidence level in a replacement schedule for a component used within a group of machines, such as aircraft or the like, wherein the method includes the step of selecting an anticipated life expectancy T for the component. The method also includes the step of establishing an indicated reliability R
1
for the component, which in some cases may be computed based upon the anticipated life expectancy T. The method also includes the steps of establishing a time increment t over which one or more units of the component had to be removed due to a failure, as well as establishing the number of failed units F of the component that had to be replaced over such time increment t. The method also includes the step of determining the population of the component within a particular group of such machines under study.
The method of the present invention computes a minimum-mean-time-between-removals &thgr;
1
based upon the anticipated life expectancy T and the indicated reliability R
1
; the present method also computes an upper limit on unreliability P
u
based upon time increment t and the minimum-mean-time-between-removals &thgr;
1
. Applicant's method then searches two or more Gamma Confidence Level look-up tables based upon three factors, namely, the number of failed units F, the population of the component within a particular group of machines, and the upper limit on unreliability P
u
to determine whether such three factors simultaneously appear within one of such Gamma Confidence look-up tables.
If the three factors do not simultaneously appear on any of the searched Gamma Confidence lookup tables, then the present invention includes the additional step of raising the indicated reliability R
1
, and repeating the steps of a) computing minimum-mean-time-between-removals &thgr;
1
; b) computing the upper limit on unreliability P
u
; and c) searching the Gamma Confidence Level look-up tables until the three aforementioned factors simultaneously appear within one of such Gamma Confidence look-up tables to provide a Gamma Confidence Level for the population of the component within the group of machines under study.
If the Gamma Confidence Level determined in the above-described fashion is at or below a predetermined percentage, for example, 75%, then the original anticipated life expectancy T is reduced, and the above-described process is repeated, until the Gamma Confidence Level is above the predetermined percentage. Finally, components within the particular group of machines under study are replaced when the usage of the component reaches the minimum-mean-time-between-removals &thgr;
1
computed during the last iteration of the above described process.
Preferably, the computational steps of the method set forth above are performed with the aid of a computer, such as a conventional desktop or laptop personal computer. The values for anticipated life expectancy T, indicated reliability R
1
, time increment t, number of failed units F, and the component population, are supplied as input data to a computer program, as by keyboard entries. The computer operates a computer software program to compute minimum-mean-time-between-removals &thgr;
1
, the upper limit on unreliability P
u
, and then searches the Gamma Confidence Level look-up tables which are stored in a memory of the computer. The computer then displays on a conventional computer monitor either the Gamma Confidence Level, or prompts the user to modify the values for anticipated life expectancy T and/or indicated reliability R
1
, until the aforementioned three factors simul
Cahill Sutton & Thomas P.L.C.
Marc-Coleman Marthe
Nguyen Tan
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