Power of your words

To be a functional safety manager, it is very important to have a power of words. As a functional safety manager, if my asking, which is a request somebody to do something, sounds like a dog’s barking, what will happen?

I remember a Sun Tzu’s history. Sun Tzu is an author of “The art of War”. He believed that usage of military strategy is effective compared to just “fighting”. It is natural, but at that time there was no such a concept of strategy.

At first his concept is not accepted when he was appointed as a war strategist. King said that if your word is true, show me using my several maid of honor.

They felt that it is a fun. But he is very serious. He felt hurt in his pride. So he commands but they did not follow.

He considered this situation is a violation of military rule, and its punishment is beheading of two heads of maid of honor.

The king was in panic, and they were also. King tried to hesitate him, but he killed both.

So, how will you change your environment? You cannot kill people, how can you make them change their perception? How can you cultivate culture of functional safety?


This story might help you how to start.




FIT budgeting

FIT Budgeting occurs when a item consists of more than one system. It is very similar to apportion of Target level of safety(TLS) in avionics domain. FIT budgeting is related to division of random hardware failure rate for some systems. If more than one supplier has to develop, it will not be easy.

Regarding ASIL decomposition, decomposed QM means that the decomposed requirement do not have to develop according to ISO 26262, but FIT budgeting is still valid. Total FIT number is not changed even though you decompose safety requirements.

In this case, the system which is allocated QM(original ASIL) has determined to be no risks in the systematic aspect.

If this policy follows similarly in the avionics domain, a subsystem which is included in the high SIL systems but is not allocated to safety requirements, it might be okay develop as QM. Of course, FIT budgeting (I mean, apportion of TLS) needs to be considered. I’m not sure, it is my guess.


Safety Analysis – FTA and FMEA

I’m not sure my opinion is aligned to ISO standard, but I believe it’s practical.

When starting safety analysis, I’m recognizing that it can be used as eliciting additional safety requirement. For me, FMEA(Failure Modes and Effect Analysis) is more comfortable.

FMEA is an activity of finding SPF, while FTA(Fault Tree Analysis) is finding both SPF(Single Point Fault) and MPF(Multiple Point Fault). If FTA can reveals SPF only, I’m not sure why I conduct FTA. Based on FMEA, FTA can be extracted automatically. If additional information about SM are considered in FMEA then some MPF can be drawn in FTA. So I believe that the purpose of FTA is to find MPF, not SPF.

I have one more comment about FTA.

It is a kinds of logical expressions. So MPF can be extracted by analyzing identified safety requirements. Let’s assume that Safety Requirements are specified as follows;

Top Requirement = AND(Group_REQ1, Group_REQ2, Group_REQ3)

Group_REQ1 = OR(REQ11, REQ12)

Group_REQ2 = AND(OR(REQ21, REQ22), OR(REQ23, REQ24))

Group_REQ3 = OR(AND(REQ31, REQ32), AND(REQ33, REQ31), REQ35))


Violation of Safety Goal is Negation of Top Requirement.

Based on this, Violation of SG can be expressed as logical expression.

To find CF, CCF, and MPF, these logical expression should be prepared, and it is during safety requirement elicitation phase.

Safety analysis in the architecture level is a deeper level of elicitation of safety requirement.

There is a mechanical procedure of drawing FTA from architecture such as Hip-Hops method. But its demerit is they do not consider logically expressed safety requirements, so constructing logical expression is weak point. It surely reviewed by safety analyzer.