Is Maintainability Only About Repair Time?

Abstract

Chris and Fred discuss one of the most popular interpretations of maintainability which is based on how long it takes to maintain something. We see so many textbooks and guides that focus on this metric as the only maintainability measure. But is this really all that matters? What about the number of technicians required? … the number of tools required? … the training required for these technicians? … the mass of these tools? Does reliability matter? Yes to all these things. But how does they relate to the time-based definition? They don’t. Listen to this podcast to learn more.

Key Points

Join Chris and Fred as they discuss what it means to focus on maintainability. This is a definition that is often quoted as put forward by Patrick O’Connor where maintainability is the probability of completing maintenance in a specific period of (calendar) time. But there is so much more to maintainability than just this. And focusing on this maintainability metric only involves substantial design risk. Why?

We focus on the following topics in this podcast:

Time-based maintainability definitions is only one part of maintainability. When the Space Shuttle was operational (and depending on the analysis referenced), it cost around $ 18 000 (2014 US dollars) per kg of payload. So having more than one tool where there was scope to only have one could be very expensive – and that is before we consider the impact it has on the scope of missions that can be completed as all the ‘payload’ budget becomes allocated to tools. Commercial companies are interested in making sure that they don’t need a lot of training for their technicians to be able to maintain new equipment. It is all about what matters to the organization – and this requires a first-principles approach.
MTTR versus MCMT. Technically, they mean the same thing. However, MTTR (mean time to repair) is often used to describe the mean time of all maintenance actions – repair or otherwise. MCMT (mean corrective maintenance time) is usually understood to be the mean repair time. However … are we ever interested in the mean maintenance time of anything? To find the answer to this – you must understand how it links to your business.
Focusing on one in specifications can put ‘blinkers’ on. If you only focus on duration based maintainability definitions, then this can limit the focus of your design engineers. Because they might then forget about the number and weight of tools required, the level of training required, access hatches and so on.
Isn’t something that takes less time to repair ‘more maintainable?’ As a rule yes. But don’t let this confuse you – there is more to maintainability.
Prognostics, diagnostics and built-in testing … what about those? They are becoming more important than ever. But if you don’t ask for them, then there is a really good chance you won’t get them.
But what about reliability? You can’t talk about maintainability without focusing on reliability. It may be better for you to have a product that is ‘less maintainable’ but significantly more reliable than its competitors. You need to be able to look at both maintainability and reliability in conjunction when measuring performance.
… and never forget operations. It is all about the business, revenue and so on. How much does one hour of maintenance cost? Consider a public pool that is open from 10 am to 6 pm every day. Does the operator prefer 10 hours of maintenance conducted overnight (outside of operating hours) instead of 30 minutes of maintenance that requires the pool to be unexpectedly closed at 3 pm in the afternoon? If both maintenance activities have the same direct costs – then perhaps the long, overnight maintenance period is preferred (… but not if you focus on duration based maintainability metrics).

Enjoy an episode of Speaking of Reliability. Where you can join friends as they discuss reliability topics. Join us as we discuss topics ranging from design for reliability techniques to field data analysis approaches.