Priority AND-Gate

Question

Hi Together,     I want to model a FT, where I have a priority fault. Normally I would model with an AND gate, that a System A AND B has to fail, but in my case System A has to fail first, only then it is getting dangerous.     how can I model this in the FTA? In FT+ there is a "priority AND Gate" available.     Sven

medini_1059007410 · Answer

Hi Sven,  You can use the approach of a Required Order Factor (ROF - that's the term used in ARP4761 for Aero Safety Analysis):       The factor below the AND gate has a fixed probability of 0.5 (for 1 out of 2 variants). It will show up in the cut sets (could be filtered out for special calculations, as in the VTOL example), but the probability is rightly scaled.      I hope that helps.     Regards,  Nils