Introduction

Manufacturers often design devices to fail based on specific metrics, such as Mean Time to Repair (MTTR), Mean Time Between Failures (MTBF), and Mean Time to Failure (MTTF). These metrics are crucial for ensuring system reliability and balancing costs against potential consequences.

Understanding MTTR, MTBF, and MTTF

First, let's define the three key metrics:

MTTR - Mean Time to Repair

MTTR is the average time between malfunctions and the time taken to repair them. For instance, a consumer vehicle's braking system might have an MTTR of a few hours, equating to a single day from a consumer's perspective.

MTBF - Mean Time Between Failures

MTBF measures the average time a system operates between failures. In the context of a consumer vehicle, the braking system might last around 36,000 miles (approximately 2-3 years), depending on the type of driving.

MTTF - Mean Time to Failure

MTTF represents the average life of a system until failure. In the past, vehicle braking systems from the 1950s and 1960s often failed frequently due to single-cylinder hydraulic systems and fragile rubber hoses. These systems pose a significant risk to life and safety, prompting advancements in technology that significantly reduced failure rates.

Engineers' Role and Design Constraints

Engineers design systems with multiple constraints, including cost, reliability, and potential consequences. Fault-tolerant systems, like those used in wide-scale communications and stock exchanges, are designed to mitigate the risk of failure. The consequences of such failures often far outweigh the costs.

Examples of Fault-Tolerant Systems

For instance, in high-reliability systems, all calculations run through three separate circuits, and any circuits that start giving non-matching results are automatically shut down for replacement. This design ensures that the system can operate without interruption, prioritizing safety over cost.

Consumer Vehicle Standpoint

From a consumer vehicle perspective, the braking system is a perfect example of how MTTR, MTBF, and MTTF are applied. A braking system typically lasts 36,000 miles, with an average life of around two to three years. Improved engineering, such as dual hydraulic systems and steel-belted hoses, has drastically reduced the likelihood of brake failures, making it a rare event.

Consequences and Liability

The potential consequences of device failure, especially in cases where lives are at risk, often drive decision-making processes. Engineering solutions that mitigate these risks are sometimes rejected by management due to the potential costs, such as legal liabilities from tort cases.

Tort Cases and Legal Considerations

Companies can be sued for not taking appropriate corrective measures, leading to costly consequences. In these cases, engineers are often tasked with presenting solutions or identifying problems, but these recommendations may be disregarded by management to avoid the associated costs.

Balance and Technology Advancements

There is always a balance between engineering and costs. Technological advancements play a significant role in improving product design and reliability. If a cell phone expected to last 20 years was produced in 2000, it would likely not be well-received. Instead, expected useful life and product design are key considerations in modern manufacturing.

Conclusion

Manufacturers make deliberate design choices based on MTTR, MTBF, and MTTF to ensure system reliability and manage costs effectively. Understanding these metrics is crucial for consumers, engineers, and manufacturers alike. As technology continues to advance, the focus on reliability and safety will only become more significant.