Introducing 'Critical Design Functions'

'The recent product recalls by General Motors and Takata, among others, raise difficult questions that will occupy lawyers and politicians for many years.

Anthony Valukas's Report prepared for the GM Board offers a detailed analysis of the ignition switch - and 'who-knew-what-when' as crisis slowly developed within GM. Reading the 325 page report is a sobering experience for anybody involved in product design and  manufacturing - or supply chain management. Sadly, none of the systems and processes used by GM prevented a dangerous and defective design reaching the consumer.

Perhaps the most troubling suggestion is that GM Engineers with their wealth of experience and product knowledge failed to understand how the failure of one component would affect the vehicle systems that relied upon it.

Reading the report got me thinking about 'Critical Design Functions' and how they could have saved GM - and a lot of other companies - millions of dollars.....

Failure Modes Effects Analysis (FMEA) is a well established technique used to identify and control risk during product and process development, by preventing or detecting the potential causes of failure, in the product or process design.

FMEA's can be used at different points in the product life cycle, to improve the:-

• Product specification, during the System FMEA,
• Product design, in the Design FMEA
• Production Process, using a Process FMEA, or
• Performance and reliability of plant and equipment, using a Machinery FMEA

The different types of FMEA have a common purpose, to identify the potential causes of failure and put in place controls to prevent or detect potential failure conditions.  

People who use the FMEA's may already be familiar with the concept of a 'critical characteristic' of the design or production process, that identifies particular dimensions or characteristics (for example a dimension or setting) that require 'special care' because:-

• Non-compliance may cause serious failure, resulting in potential loss and injury, or prevent the product complying with a legal requirement, and
• The production process may does not have the  capable to produce parts with zero defects

When a 'critical characteristic' is identified the engineering team are required to show that all potential of error conditions have been:-

• Eliminated by design, or
• Prevented by controlling the inputs to the process, or will be
• Detected by inspection and sampling

In a manufacturing environment, the Production Control Plan documents how process control will be achieved and maintained, based on the knowledge gained from the Process FMEA. Suppliers are routinely required to prove that they have a 'Capable Process' and that the control plan will prevent 'non-conforming' products being supplied.

We are therefore proposing that a similar logic should be used in the System FMEA to identify as Critical Design Functions - the component functions that are required to prevent a safety critical failure of the system. This information should be included in the design brief and specification for all components. Designers and component suppliers would then be alerted to the fact that particular requirements in the specification are safety critical, because of the impact any loss of function may have on system performance.

Component designers and suppliers would then be required to provide objective evidence that:-

• The proposed design performs the Critical Design Function in all operating scenarios 
• The design has been fully validated, using test conditions that are correlated with customer usage.

The explicit definition of Critical Design Functions in the requirements for components, such as the GM ignition switch, would focus attention on the need for extra care in the design process - and adequate validation of sample parts under realistic test conditions.

As product complexity has increased, the 'Recall Ratio' (Units Recalled / Units Sold) continues to rise - and the cost of recalls has become an existential threat to weaker organisations.

Although sophisticated design technology may accelerate the product development process - without effective communication of Critical Design Functions we may not understand the interaction between components in a complex system.

About the Author

Phil Stunell is a professional engineer and consultant with a special interest in product liability and quality improvement.