  Safety-critical infrastructures must operate in a safe and reliable way.
Fault tree analysis is a widespread method used for risk assessment of these
systems: fault trees (FTs) are required by, e.g., the Federal Aviation
Administration and the Nuclear Regulatory Commission. In spite of their
popularity, little work has been done on formulating structural queries about
FT and analyzing these, e.g., when evaluating potential scenarios, and to give
practitioners instruments to formulate queries on FTs in an understandable yet
powerful way. In this paper, we aim to fill this gap by extending BFL [32], a
logic that reasons about Boolean FTs. To do so, we introduce a Probabilistic
Fault tree Logic (PFL). PFL is a simple, yet expressive logic that supports
easier formulation of complex scenarios and specification of FT properties that
comprise probabilities. Alongside PFL, we present LangPFL, a domain specific
language to further ease property specification. We showcase PFL and LangPFL by
applying them to a COVID-19 related FT and to a FT for an oil/gas pipeline.
Finally, we present theory and model checking algorithms based on binary
decision diagrams (BDDs).
