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Component Reliability - Predictable Life Cycle
One thing you can count on: Mechanical parts eventually wear out. Think about the tires and brakes on your vehicle; periodically, you must replace them. If you don’t, they will eventually fail – causing anything from a minor inconvenience to an accident (or worse). Depending on tire and brake quality, as well as how you drive the vehicle, you expect these parts to last for a certain amount of time (usually measured in miles and/or months). In fact, manufacturers spend a lot in resources to test their products and predict how long the part will last before it wears out. This testing also helps them determine the scope and length of a warranty program.
Aircraft components contain many mechanical and electromechanical parts. Most of these parts have predictable wear/failure patterns. As an operator, you probably have a huge database of component and aircraft maintenance history. Buried deep inside this data is some very valuable information that you can use to your advantage.
As an example, consider a hypothetical pneumatic valve:
A typical pneumatic valve consists of butterfly valves, bushings, bearings, torque motors and diaphragm actuators. Each of these individual piece-parts may have a predictable wear pattern. Unfortunately, most of these parts have different wear cycles. For example: Bushings may wear out after 6000 flight hours. Diaphragms may break after 8000 hours and torque motors may fail after 10000 hours.
Consider the impact these life cycles will have on your component repair budgets. Assume the following life cycles: Bushings (6000 flight hours), Diaphragms (8000 flight hours), Torque Motors (10000 flight hours). A repair history for our hypothetical valve would look like this:
Repair # Time Since New Cause of Failure Repairs
0 0 hours N/A: New Part N/A
1 6000 hours Worn Bushings Replace Bushings
2 8000 hours Worn Diaphragm Replace Diaphragm
3 10000 hours Worn Torque Motor Replace Torque Motor
4 12000 hours Worn Bushings Replace Bushings
5 16000 hours Worn Diaphragm Replace Diaphragm
6 18000 hours Worn Bushings Replace Bushings
7 20000 hours Worn Torque Motor Replace Torque Motor
As you can see, when the valve is 20000 hours old, it will have been removed from 7 aircraft. Since we know that a diaphragm usually fails 2000 hours after bushings fail, we may want to instruct our repair shop to replace the diaphragm when the bushings are replaced (as a preventive maintenance action). Using this logic, the repair history of our valve will look like this:
Repair # Time Since New Cause of Failure Repairs
0 0 hours N/A: New Part N/A
1 6000 hours Worn Bushings Bushings, Diaphragm
2 10000 hours Worn Torque Motor Torque Motor
3 12000 hours Worn Bushings Bushings, Diaphragm
4 18000 hours Worn Bushings Bushings, Diaphragm
5 20000 hours Worn Torque Motor Torque Motor
With a little preventive maintenance, we have eliminated the need for 2 repairs during the same 20000-hour interval. If we were a little more creative, we could do even better:
Repair # Time Since New Cause of Failure Repairs
0 0 hours N/A: New Part N/A
1 6000 hours Worn Bushings Bushings, Diaphragm
2 10000 hours Worn Diaphragm Bushings,Diaphragm,Motor
3 16000 hours Worn Bushings Bushings, Diaphragm
4 20000 hours Worn Diaphragm Bushings,Diaphragm,Motor
By performing some preventive maintenance, we have eliminated the need for 3 repairs – and improved Mean-Time-Between-Failures (MTBF) 75%. The original MTBF was 2857 (20000 hours/7 repairs). The new MTBF is 5000 (20000 hours /4 failures). Of course, each repair will be a little more expensive because you are replacing more parts. However, the total repair cost over the life of the valve will be less (because you will have 75% fewer repairs).
A few words of caution: The life cycles in our example are illustrations only. The actual values may be higher or lower – depending on how your fleet operates. Therefore, you will have to evaluate your component life cycles to determine an optimal level of preventive maintenance. This is no simple task. It takes a lot of data and a disciplined analytical approach to define realistic life cycles. Plus, you will have to convince the “bean counters in high places” that your preventive maintenance process makes economic sense.
Aviation Reliability Services has developed many preventive maintenance programs (we call them Component Work scopes) based on predictable life cycles. We have the experience needed to identify life cycles, develop logical repair intervals and build business cases to support the implementation of component work scopes.
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