Time-based maintenance (TBM)

Time-based maintenance (TBM) refers to scheduled maintenance activities. In some industries TBM is sometimes referred to as calendar-based maintenance. Time-to-maintenance, however, is a more flexible term whose meaning varies depending on context. In some cases, it's used interchangeably with TBM to describe the interval between scheduled maintenance tasks. In other contexts, like in reliability engineering or asset management, it may refer to the mean time between maintenance (MTBM). MTBM is a metric that indicates the average operation time between maintenance events. This usage focuses more on performance tracking and predictive planning rather than fixed schedules.

Depending on the industry or system, time-to-maintenance could imply a countdown to the next schedule maintenance, a performance metric, or a predictive estimate based on condition monitoring. Understanding the context, whether it's preventive scheduling or reliability analysis, is key to interpreting accurately.

The United Airlines study: Rethinking reliability

The most overlooked weakness of TBM is not simply wasted effort, it’s the increased chance of failure immediately after maintenance. This insight was first illuminated by groundbreaking reliability research in the aviation industry.

In the 1960s, United Airlines commissioned a landmark reliability study on commercial aircraft, later published in Reliability-Centered Maintenance by Nowlan and Heap (1978) [1]. Their findings shook the foundations of traditional maintenance thinking.

By analyzing thousands of components, they discovered six distinct failure patterns. The results were startling:

• Only ~11% of failures were clearly related to age or wear-out.
• The remaining ~89% of failures were random or unrelated to age.
• 68% of failures followed what's known as the infant mortality curve, where failures were more likely to occur right after a component was installed or replaced.

The implication of these findings: Performing fixed-interval overhauls often reset equipment into the “infant mortality” phase, making failures more likely rather than less. In other words, TBM didn’t just waste resources, it actively reduced reliability.

Supported evidence beyond the aviation industry

The UAL study was just the beginning. Independent research across industries confirmed that most equipment failures are not age-related:

• Broberg (1973, Sweden): Found similarly low percentages of age-related failures [2].
• MSDP (1982) and SUBMEPP (2001, U.S. Navy) [3]: Reinforced that random failure patterns dominate in complex systems, especially electronics and hydraulics.

Across sectors the message has been consistent, the assumption that things wear out on a schedule is in fact, often false.

Why Condition-Based Maintenance (CBM) is the modern alternative

These findings paved the way for Reliability-Centered Maintenance (RCM) and the adoption of Condition-Based Maintenance (CBM) strategies. Unlike TBM, CBM relies on real-time monitoring, sensors, and predictive analytics to determine when maintenance is required. CBM reduces intrusive maintenance, avoids premature part replacements, and focuses resources where they matter most.

In practice, many organizations adopt a hybrid approach where they use TBM only for simply, predictable wear and tear (e.g., wear on parts like filters or seals). They also use CBM and reliability-based strategies for critical assets with complex failure patterns.

Is TBM right for you? Evaluating suitability and getting started

When exploring time-based maintenance, many maintenance teams are not just looking for definitions but the ways this strategy aligns with their unique assets, budget constraints and operational goods. Knowing how to roll it out. Start small, choose a room or an asset that you could change from, and then expand from there. Crawl on your learning then expand across your business.

Evaluating suitability for your operation

Time-based maintenance works best when:

1. Asset failure is predictable: Equipment tends to wear out after a known period or usage cycle.
2. Downtime is costly: Preventing unexpected breakdowns is more economical than reactive repairs.
3. Regulatory compliance is critical: Industries like healthcare, aviation, and manufacturing often require scheduled maintenance for safety and legal reasons.
4. You have budget flexibility: While time-based maintenance can reduce emergency costs, it does require scheduled maintenance and resource allocation.

If your assets are newer, digitally monitored, or show unpredictable wear patterns, you might consider condition-based or predictive maintenance instead. But for many operations, especially those with standardized workflows, TBM offers a solution.

How to set up your preventive maintenance schedule

Getting started doesn’t have to be overwhelming. Here’s a simple framework to launch your first time-based maintenance plan:

1. Take inventory of your assets: List all equipment and systems that require regular upkeep. Include details like manufacturer recommendations, usage frequency, and criticality to operations.
2. Define maintenance intervals: Use OEM guidelines, historical data, or industry benchmarks to set time-based triggers (e.g., every 3 months, 500 hours of operation, or quarterly).
3. Prioritize risk and impact: Focus first on high-risk or high-impact assets. These are the ones that, if they fail, could halt production or pose safety risks.
4. Assign responsibilities: Clarify who performs the maintenance, internal teams or external vendors and ensure they have access to the right tools and documentation.
5. Track and optimize: Use a CMMS (Computerized Maintenance Management System) to track completed tasks, analyze trends and refine your schedule over time.
6. Communicate and train: Make sure your team understands the schedule, the importance of adherence, and how to report issues or delays.

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TBM has its place when applied thoughtfully

The biggest lesson from decades of research is clear: more maintenance does not always mean better reliability. When used indiscriminately, TBM not only wastes time and money but can also increase failure risk by reintroducing infant mortality. That’s why today’s most resilient organizations are moving beyond the calendar and toward condition-driven, reliability-centered approaches.