The remainder of the maintenance is either routine scheduled maintenance, random failed component replacement, or various types of damage.
Does all this maintenance have any impact on overall reliability? Study and analysis on the relationships between reliability / safety and scheduled maintenance activity has been accomplished by major aircraft and component manufacturers. Their conclusion was that the reliability of only a small number of components is directly dependant on scheduled maintenance. Most components do not benefit from it.
Components can generally be divided into 4 groups:
1. Components whose reliability is independent of age
2. Components subject to wear-out beyond equipment life
3. Components whose inherent reliability levels are directly dependent upon scheduled maintenance activity
4. Components that are susceptible to age related deterioration
Each group has a unique reliability / age relationship
Group 1 - Components who's Reliability is Independent of Age
This group includes components whose failure rate is exponential, such as electronic equipment or other equipment with no moving parts. They may or may not exhibit infant mortality, and regularly scheduled maintenance has no effect on reliability.
Regularly scheduled maintenance on these components is only effective as a "failure finding" task. This group represents about 90% of aircraft equipment.
Components in Group One tend to have either a fairly high infant mortality rate or none at all, but once that initial burn-in period is over, component reliability stabilizes. If it is going to fail, it will fail early on after installation. Once it gets past this infant mortality period, it can pretty much be counted on to last.
Predictive, proactive, or preventive maintenance on these types of components will have little or no effect on their reliability.
An example on your car would be the radio. If it gets past the first few weeks trouble free, it will likely not fail for the life of the vehicle. There is no preventive maintenance task you could perform that would affect the reliability. It will work until it doesn't work anymore. Then you change it.
Group 2 - Components Subject to Wear-out Beyond Equipment Life
This group includes Hydro / Electro Mechanical Equipment that has adequate design margins to push the wear-out region beyond the life of the equipment it is installed on.
They behave a lot like Group 1 Components, and their reliability is generally unaffected by scheduled maintenance. Failures, when they occur, tend to be abrupt and random, and provide little or no pre-failure indication.
Occasionally Group 2 components experience premature failure or infant mortality, but in general, failures of components in this group are discovered through effective reliability monitoring rather than through the maintenance program.
Components in this group have an inherently stable design that under normal circumstances will have a life that significantly exceeds that of the equipment it is installed on. Some might require an occasional lubrication or cleaning task, but most do not benefit from ongoing maintenance. Not only do they not benefit from it, many do not even have provisions or requirements to perform any maintenance. With these components, there is generally little you can do to extend component life, and since the component is already designed to exceed the life of the equipment it is installed on, there is little need to bother.
An example on your car might be the windshield. Barring any accidental damage, it should last well beyond the life of the vehicle it is installed on. When you tow that old clunker to the scrap yard, chances are the windshield will still be okay provided no one has broken it along the way. There is nothing you can do from a maintenance standpoint to extend the life of the windshield, and even if there were, you would have to carefully consider whether it was worth the time and effort to do it. If there were something you could do that would extend the life of the windshield by fifty percent, would there be any benefit in doing it? Probably not, since it will still be fully functional when the car is scrapped anyway.
Group 3 - Components Whose Inherent Reliability Levels are Dependent on Scheduled Maintenance Activity
These typically have heavily loaded dynamic interfaces or high speed rotating parts.
Scheduled tasks generally involve lubrication and / or analytical inspection. Components in this group clearly benefit from routine maintenance.
Sticking with the car example, this group would include things like oil and filter changes. You can operate your vehicle without performing this task - for a while - but doing so will accelerate the failure rate.
Performing the maintenance task, though, does not absolutely eliminate the failure from occurring, but it does delay it and may lessen the severity when it does occur.
Group 4 - Components Susceptible to Age Related Deterioration
Components in this group will wear and will eventually fail regardless of the type or amount of maintenance performed.
You can sometimes delay the inevitable failure by employing operational procedures, but maintenance procedures will have little if any effect, and may yield no real value.
An example would be the brakes on your car. They will eventually wear to the point of failure. Period. There is nothing you can do from a maintenance standpoint that will prevent or even significantly delay that failure from occurring. All you can do is plan for the failure and be prepared for it when it occurs. It is not a question of "if" they are going to fail, it is merely a question of "when".
There are a few things you can do from an operational standpoint - things like not "riding" the brake pedal - that will extend brake life a small amount. From a maintenance point of view, all that can be done is to check the components on a routine basis and change them when wear patterns indicate that they need to be changed.
Reliability ≠ Scheduled Maintenance
The majority of the equipment in Groups 1 and 2 do not have a direct relationship between scheduled maintenance activity and reliability. If they fail, it will be a random, unpredictable type of failure. Routine maintenance would be ineffective and pointless.
Sort of like rearranging the deck chairs on the Titanic.
Scheduled maintenance should be limited to those Group 3 components where inherent reliability levels are dependent on scheduled maintenance activity and those Group 4 components where they are susceptible to damage or other forms of age related deterioration. Basically, if it ain't broke, don't fix it. If performing routine maintenance on it won't have any effect on component reliability, why do it?
If you have an exceptional reliability program and can predict with any degree of accuracy when a Group 3 or Group 4 component will fail, changing that component as a preemptive measure might be worthwhile, particularly if you want to be able to control when and where that component gets replaced. In an airline environment, there are many parts we change for that very reason.
For example, if an aircraft tire passes inspection during a routine maintenance check at the hangar, but the technician knows that the tire will probably be out of limits before the next time the aircraft comes in for maintenance, it is better to change the tire early than to have to change it at a remote airport a few days later and cause a flight delay. Sure, you lose a little life on the existing tire, but you have to compare that cost to the cost of a flight delay.
Sometimes maintenance can help improve reliability, but in many cases, it can't.
Doing more does not always bring about a commensurate increase in results.
An additional factor that makes aviation reliability different from other types of equipment reliability is the differences in how reliability is calculated. For most reliability calculations, the standard is developed based on MTBF - or Mean Time Between Failures. For an airline, this is no doubt the worst possible factor to use in a reliability calculation.
Simply, an airline has to be proactive - and running a component to the point of failure is certainly not proactive - and it isn't safe. Equipment and component failures on an aircraft have the potential to ruin your whole day - as well as those of the passengers.
Granted, failures do occur, but every possible precaution is taken to limit it. Mean Time Between Removals - or MTBR - is not necessarily a good metric of measurement, either, as most major components are on a time or cycle controlled replacement schedule anyway, so MTBR is known before the component is even installed.
The factor that becomes important in the airline world is Mean Time Between Unscheduled Removals - MTBUR. While MTBUR includes MTBF, as a failure is an unscheduled removal - there are also unscheduled removals that are not the result of a failure of the removed component.
This paper was originally delivered at IMC-2007 - the 22nd International Maintenance Conference.