Category Archives: RTCA

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.


If I were in the OEM’s side, I’ll manage functional safety project as follows

Management of functional safety process is so important not only in the supplier’s side but also in the OEM’s side.

In the standard, there is two type of faults. One is systematic fault and the other is random hardware fault. While random hardware fault is engineering aspect and can be treated by engineering decision, but systematic fault is not in just engineering aspect.

Let’s assume that a OEM auditor finds that supplier has random hardware faults and systematic faults. After audit, what corrective actions needs to be done?

To eliminate random hardware fault, supplier has to design and implement again. But sometimes it need not take too much time compared to the systematic fault.

To treat systematic fault, what are required?

What do you think?

Ideally speaking, all work products cannot be trusted and all activities are required to do again from beginning. But I’m not sure such an extreme approach can be happened in the automotive industry. It surely impact to lag project timing.

Then, supplier’s risk is moved to OEM’s side.

In order not to happen in this undesirable situations, what OEM have to do?

Let assume that there are two options to conduct audit.

  1. Audit once at the end of the project.
  2. Audit multiple times at the important timings.


It would not be difficult problem. You already know the answer, and may understand what my point is.

Let me simplify my point; Bothering supplier multiple times will help not only OEM but also supplier. To do this, OEM has to be diligent.