So your company has decided to follow ISA S84 / IEC 61511 and you find yourself trying to decide if your current test plans are “good enough” as-is. So this raises the question of what makes a functional test plan “good”? While some of the answers are things one would definitely want their test plans to address, some of the requirements conflict with one another. The primary purpose of a functional test plan is to “detect dangerous undetected failures”. Keeping this concept in mind, it becomes much easier to answer the question of what makes a functional test plan “good”.