Maintainability Impact on System Effectiveness
The majority of users state that they “need the equipment functionability as badly as they need safety, because they cannot tolerate having equipment out of operation”. There are several ways that designers can control that. One is to build items/systems that are extremely reliable, and consequently, costly. The second is to provide a system that, when it fails, is easy to restore. Thus, if everything is made highly reliable and everything is easy to repair, the producer has got a very efficient system that no one can afford to buy. Consequently, the question is how much a utility of the system is needed, and how much is one prepared to pay for it? For example, how important for the train operator is it to move train from the platform, when 1000 fare paying passengers expect to leave the gate at 6.25 am? Clearly the passengers are not interested what the problem is, or that it is designer's error, manufacturers, maintainers, operators or somebody else’s problem. They are only interested in leaving at 6.25 am in order to arrive at their chosen destination at 7.30 am. Thus, if any problem develops, it needs to be rectified as soon as possible. Consequently, maintainability is one of the main factors in achieving a high level of operational effectiveness, which in turn increases users or customers’ satisfaction. Example 21.3: The main objective of this example is to illustrate the impact of maintainability on operational effectiveness of motor vehicles. The inherent maintainability characteristics for several motor vehicles are given in Table 21.1. They clearly indicate the impact of the design decisions on the maintenance resources, frequency, and ultimately operational effectiveness. motor vehicle was 75,000, the total hours spent on maintaining their functionability, by the users, is given in Table 21.2 together with the operational effectiveness achieved. In Table 21.2, MTIMp represents the mean cumulative time in maintenance caused by the execution of preventive maintenance tasks (services), MTIMc stands for the corresponding time caused by the demand for the execution of corrective maintenance tasks, and MTIM represents the mean total time in maintenance obtained as a sum of the two (MTIM=MTIMp+MTIMc).
Example 21.4: Maintenance troubleshooting is another area to be considered under the maintainability heading. For the airlines this is usually only about 1 h at the gate prior to its departure to the next destination, whereas for a racing car or weapon system this is usually a few minutes. An easily manageable device is needed for the diagnostic of all different modules in order to determine their state and identify the failed one within it. Practice shows that false removals cost about the same as an actual failure when the component under investigation is removed and replaced. Reducing this would be a big cost saver. Devices with such capabilities have been developed in the aerospace industry as a result of maintainability studies and research. For example, the design of the Boeing 777 includes “On-Board Maintenance System” with the objective to assist the airlines with a more cost-effective and time-responsive device to avoid expensive gate delays and flight cancellations [Proctor, Journal “Aviation Week & Space Technology”]. For similar purposes the Flight Control Division of Wright Laboratory, USAR, has developed, fault detection/isolation system for F-16 aircraft, which allows maintainers – novice as well as expert – to find a failed component.