Q 615. In FMEA, Risk Priority Number helps us assess the risk in the process. While potentially RPN can range from 1 to 1000, could there be a process step, where the RPN is 1000? Provide examples to support your answer.

 

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Failure Mode and Effects Analysis (FMEA) is the most effective of all the Defect Prevention tools in my opinion, for achieving zero defects. If done correctly and with the right attention to detail it will enable the organisation to design products and / or processes that meet the customer’s requirements at optimal cost. However, it can also be the most difficult to deploy as it relies not only on technical knowledge and experience but precision to right level of detail. If done poorly it will take a lot of resource for very little benefit and runs the risk of organisations , it is better to not to do it at all than just doing it like a form filling exercise.

 

Risk Priority Number (RPN)

Risk Priority Number (RPN) can be calculated for each Potential Cause of a particular Failure Mode.

The calculation is quite simple.

 

RPN = Severity x Occurrence x Detection

Criteria’s of Severity, Occurrence & Detection varies for DFMEA & PFMEA in Aerospace Industry. For this example let us consider only PFMEA related criteria.

 

Severity

Severity defines what will the impact incase of failure of that characteristic.

Inputs for severity 

1.      From DFMEA    2.  Critical / Significant / product KC in drawing     3.  Process Key Characteristics (KC)

 

Occurrence

The occurrence rating looks to evaluate the potential for the Potential Cause of Failure to

Occur in our current existing process. This is done on a scale of 1 to 10 where 10 is ‘will happen nearly always’ and 1 is ‘will never / unlikely to happen’. Criteria is based on, How many times per million opportunities would we expect to see the defect (PPM), Likelihood of the Potential Cause occurring or how often have we seen this type of defect (time-based examples).

 

Inputs for Occurence

1.      Customer escapes           2. Internal Rejection %   3. Capability Indices (CP,CPK,PP,PPK)

4.    PPM

 

Detection

If the Potential Failure Mode did occur how confident are we that we would detect it before it was dispatched to the Customer? Answer to this question will give us the Detection score.

 

Inputs for Detection

1.      Inspection Plan

2.      Process Flow (rating changes depending on where you are inspecting)

 

Could there be a process step, where the RPN is 1000?

Yes there could be such a process step, see below stated example.

 

When the stated characteristic is a critical dimension where failure of that feature can result in harm to person without any warning. (e.g Telephone blasting or catching fire during charging without any warning , Aeroengine welded pipe suddenly blasts due to crack and causes engine failure in running condition).

Severity = 10

 

When there is high chance of getting the defect in almost every part. (e.g Porosity / internal crack during welding of tubes if its 100% probability in all the parts).

Occurrence = 10

 

You are doing inspection and if a particular defect cannot be detected at all in your inspection. (e.g Micro Porosity or micro crack after welding cannot be seen by naked eye, but you are doing visual inspection).

Detection = 10

 

In this case RPN will be = S x O x D = 10x10x10 = 1000

 

Boeing 737 Max grounded example 

e.g MCAS Characteristic

Severity :- MCAS system was a critical feature, but it had a single point of failure. it auto changes the direction of the plane without any warning, which let to loss of several lives (Severity 10).

 

Occurrence :- This could have happened to all the planes as it had a single point of failure for a critical feature, secondly it was located at the outer side where "bird strike" had a very high chance.  Thirdly the reaction time for pilot to turn of the MCAS, once if the MCAS is triggered is 15 seconds. (Occurrence = 10)

 

Detection :- Even if the Pilot tries to turn off the MCAS within 15sec and take it to manual control, because of the MCAS sensor failure the software auto corrects the plane movement downwards, Even pilot is clueless why. (Detection = 10)

 

 In this case RPN will be = S x O x D = 10x10x10 = 1000

It would mean severity of the process is 10(most risky process), occurrence is 10 ( always fails) and detection is 10(extremely late detection).

 

We know from our experience that we need to focus on controlling the occurrence than detection. But here in this case if it is most risky process and occurrences are the highest and we get to know at the end, this would be a complete failure scenario and sounds unrealistic i.e. completely raw processes. We cannot expect an army person operating a drone at the border of two enemy countries to cross the border everytime he operates it.

 

Further, let us consider an example of a pizza makeline. Once the chef puts a pizza in an oven, the desired output is a hot pizza which must meet the food and hygiene standards and customer satisfaction. Assume that everytime pizza comes out of the oven, is damaged. So from here we know occurrence is high i.e. everytime it fails. It has more severity as every damaged pizza has a mould in it and detection is also quite late. But is it 10? From a process step of baking, it is ten and yes it is a complete failure. But could it be detected earlier? Yes it could be detected earlier by naked eye or a simple examination of the oven.

 

But it seems the processes like brick making where visibility could be low or the least or projects where one sees the final product only at the end, the detection seems to be higher or highest. However, in most cases it sounds quite unrealistic for RPN of a step to be 1000 as most processes and services have a troubleshooting mechanism in place.

RPN-It stands for risk priority number.

It is a aspect of FMEA-Failure mode effect analysis.

FMEA- it is the way to analyze the failure mode of a process before implementing to make the process FTR(first time right)

What can go wrong 

Why it will go wrong 

How bad it will go wrong

It is denoted by 3 aspects.

Severity-How bad is the effect

Occurrence -Frequency

Detection-

RPN-Severity*Occurrence*Detection

RPN table formact

Process/potential failure mode/Failure effect/Severity/cause of failure/Occurrence/Current process control/Detection/RPN

 

With RPN no we can have a prior view of new process and we can put control points to make process error/failure free.

 

 

Risk Priority Number (RPN) helps to assess risk in a process. Higher the magnitude of RPN, scarier the risk.

 

RPN = severity * occurence * detection

 

Each factor ranges from 1 to 10 thereby making RPN have 1 as minimum possible value (least risk) and 1000 as maximum possible value (highest risk). RPN of 1000 happens where all the three parameters are at maximum - impact is severe, likelihood of occurence is high and chance of detection is rare - immediate attention is needed for such processes.

 

(For example)

 

Lets take the case of card counting in blackjack game in Casino (from Casino management point of view).

 

Severity = 10 - Casino will lose money since the house edge reduces during card counting
Occurence = 10 - high possibility of players to do card counting since it increases the chance of winning
Detection = 10 - very difficult for Casino to differentiate card counters from normal players

 

RPN for this example is 1000 making it a high risk process to be worked upon immediately - else it will impact the profitability of the casino.

A RPN score of 1000 means that all the three parameters of Severity, Occurrence and Detection need to be at the maximum. That means a catastrophic failure of or on something has to happen, that cannot be detected or very difficult to detect and it will happen routinely.

 

Imagine if this can happen in the current world with all the technology development and Infrastructure!!

 

Let us try how things could have been in the past for some of the below scenarios, that probably could have their RPN as 1000

1. Volcano Eruption
2. Wars due to differing ideologies...
3. Viral diseases 

 

Let's take the example of Volcanic eruption.

 

For Volcanic eruptions, people never knew when a Volcano would erupt. It was always a matter of running for dear life and staying out of sight till the Volcano subsided.  Therefore major volcanoes that had a history of frequent eruption and that had the capability of inflicting heavy sorts of damage in any means, were a source for RPN, having a score of 1000.

 

Now with the advent of modern technologies and infrastructure, we would be able to predict the volcanic eruption to a great extent. Because the eruptions can be predicted and tracked,the detection and occurrence are known and hence you may not get a score of 1000

 

Conclusion:

As we saw here, with the advent of modern technologies and sophisticated Infrastructures, every problem/issue or challenge if put under a FMEA may not get a RPN score of 1000. The Severity of the issue may still be there. But the detection of the issue might be easy.

 

or The Severity of the issue might get lowered than what it was. 

 

Thus I would conclude by saying IMHO, a RPN score of 1000 may not be possible

Very interesting answers and examples quoted. The best answer was provided by Adil Khan and hence his answer has been selected as the winner.

 

R Rajesh also mentions a very important point that if a process step has an RPN of 1000, there will definitely be some mitigation actions that will be taken to reduce the risk at the particular step.