FIT and requirement interpretation

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FIT and requirement interpretation

Hello everyone, I'm feeling a little bit confused with the following problem:

I'm analyzing very small duration phases: their sum is little less than 3 minutes and I obtain a FIT near to 45000. When I add another phase (stocking) with a larger duration(ex. 1000 hours), the resulting FIT can be approximated to the FIT of the phase of 1000 hours (i.e. near to 10 FIT) and I understand this because its weight is bigger than the others.

But I start getting confused when I have to compare my results with the requirement. Because it looks like is sufficient adding a "non-stressing phase", even not so longer, to obtain a very small FIT.

Is there someone that could help me to understand this?

I give you some other information: the requirement is intended in the "Mil-hdbk way" but I think that the results cannot be compared due to the different methodology (calendar hours, thermal cycles and so on), at the same time it seems to me that 2 calculation obtained with the MIL-HDBK can be compared easily because of the absence of duration's weigth. 

I would convert the requirement failure rate in a "FIT" using the equation: lambda_calendar=lambda_mission*(Mission_duration/Calendar_duration) but i think this would be valid if in the FIDES calculation there are "OFF" phases. If all the phases are "ON" is still necessary to convert the requirement ? 


Thank you all.  

Edited by: Elic8ero on 09/02/2024 - 17:34