Life profile and time units

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Life profile and time units

Hello, Lambda!
Could you please help me to understand FIDES approach to Life profile setup.
I tried to apply FIDES formulae to calculate mission failure rate, but the result seems to me being misleading:
1) Lambda_Calendar = Lambda_ARW*(t_ARW/8760) + Lambda_OFF*(t_OFF/8760)

2) Lambda_Mission = Lambda_Calendar * 8760/t_ARW = Lambda_ARW + Lambda_OFF*(t_OFF/T_ARW).
The second term in this equation is confusing me. I think the result should be Lambda_Mission = Lambda_ARW.

Could you please comment this or send me to the right reference on this topic.

Thank you in advance,

Leonid Boim (RAMS engineer)

Re: Life profile and time units

Dear Leonid,

Your equations are right. Your confusion only illustrates how much it is misleadind to use "mission failure rate" or "mission MTBF"

The mission failure rate comprises all the failure of the product. For example real flight failure, storage failure, maintenance failure... In your example with only two phases, the mission failure is the flight failure rate plus a smal amount coming from the few failures of the other phase (off). Therefore the mission MTBF is always a bit worse than the pure flight MTBF.

Once again, mission failure rate are confusing. For example, much flight hours leads to a better "mission MTBF". Say that to a customer -a user- and he will understand "the more I flight the better is the reliability of my product", which is a big mistake.

Helicopters are an interesting example because FIDES shows that flight is not the harsher phase of some life profile (instead it is Ground On, because the thermal cycling may become worse than flight vibrations for short flight).

I hope it helps,