Availability, reliability, and maintainability are a set of attributes that we often talk about in Maintenance. Also known as availability, reliability, and serviceability – usually for computer-related hardware and software – the three are very different concepts. But do you know the differences between them? 


What is Availability?

Availability is the probability that the system is operational, and ready to use. It can also be described as the time an asset is expected to function. To calculate availability, compare the amount of time an asset was available, and the time it should’ve been operating. Here’s the availability formula:


how to calculate availability in maintenance


In an ideal setting, the availability of assets would be 99.999%. In the real world, you should aim at 90% or higher. Availability can also be expressed as a direct proportion (i.e. 9/10). Either way, availability equals actual operation time divided by scheduled operation time. 


Rarely, availability will be conveyed in time units (i.e. weeks or months). This represents the average time an asset should work over a given period.


There are several things facility managers can do to improve asset availability. For example, scheduling programmed corrective maintenance or preventive replacements outside regular working hours. Using predictive maintenance can prevent an asset from ever being out of order.


But is that all there is to it? Let’s play with the availability formula a little bit. If you think about it, “actual working time” equals can also be defined as “uptime”. In the same vein, “scheduled working time” would then become the sum of uptime and downtime. From here we have:


availability formula in maintenance


Ready for another twist? Think about it. “Uptime” is the time a machine operates between failures, also known as MTBF.  “Downtime” is the total time it is unavailable until failures are repaired, which is the MTTR. So here’s how to calculate availability with MTBF and MTTR:


how to calculate availability in maintenance


It’s now clear that high availability hinges on low downtime. However, if you’re using real data to estimate actual working time, you’ll probably include times when the asset was operating in a limited capacity. If you want to know the probability of an asset working correctly, then we’re talking about reliability


What is Reliability?

Reliability is the probability a system will produce the correct output, which is not the same as being available. Imagine a car, an aeroplane, or a helicopter. Do you want them to be available to travel at all times, or do you want it to be reliable? Obviously, one is not possible without the other.


Usually, an increase in reliability means increased availability. MTBF is a good indicator of reliability throughout the asset’s lifetime. This is a simplified way to calculate reliability:


how to calculate reliability


However, the reliability function over time is specified as:


reliability formula


Improving reliability means avoiding failures or malfunctions with customised plans, which is also known as reliability centered maintenance. A customised approach to ensure reliability depends largely on two factors: data collection, and knowledge of failure modes.  


To achieve the latter, facility managers usually rely on reliability block diagrams, fault tree analyses, and Markov models. For critical assets, FMEA is also an option. Total productive Maintenance and lean manufacturing propose continuous improvement to increase reliability. 


What is Maintainability?

Time for a quick recap. Availability means the asset is operational. Reliability means the asset is likely to work correctly. But what happens when a failure occurs? 


Failures should cause as little disruption as possible. That’s maintainability – sometimes called serviceability – which is how easily a system can be repaired or maintained. High maintainability suggests a low time to repair. Time to repair includes evacuation, diagnosis, assembly of resources, the repair itself, inspection, and return to service. Or MTTR for short.


While the MTTR is the main indicator to calculate maintainability, it’s not the only one. We’ve already discussed the maintainability function here, and we suggest you bookmark it for later. For now, the important takeaway is that improving MTTR will improve maintainability. 


Managers can promote several measures to expedite repairs, including using a real-time CMMS or Intelligent Maintenance Management Platform. But, perhaps, the most effective measure to repair with as little disruption as possible is to detect failures early on. Early detection of failures is more likely to provide cost-effective, agile repairs and much less hassle. 


How to improve system availability and efficiency?

We’ve now established how to calculate availability with the MTBF and MTTR. We’ve explained that MTBF is a strong indicator for reliability, while MTTR hints at maintainability. Therefore, improving both reliability and maintainability will increase system availability.


Availability, as you may recall, is one of the three factors in Overall Equipment Efficiency (OEE). Increasing availability will invariably increase your OEE, and reliability plays into performance improvement as well. So let’s review a few things you should do to improve system availability and OEE:


  • use predictive maintenance to avoid breakdowns and track changes in performance; 
  • early failure detection through predictive and preventive maintenance;
  • scheduling of maintenance activities based on MTBF and MTTR;
  • effective preventive maintenance practices, based on KPIs and failure modes analysis.