  Identifying drawbacks or insufficiencies in terms of safety is important also
in early development stages of safety critical systems. In industry,
development artefacts such as components or units, are often reused from
existing artefacts to save time and costs. When development artefacts are
reused, their existing safety analysis models are an important input for an
early safety assessment for the new system, since they already provide a valid
model. Component fault trees support such reuse strategies by a compositional
horizontal approach. But current development strategies do not only divide
systems horizontally, e.g., By encapsulating different functionality into
separate components and hierarchies of components, but also vertically, e.g.
Into software and hardware architecture layers. Current safety analysis
methodologies, such as component fault trees, do not support such vertical
layers. Therefore, we present here a methodology that is able to divide safety
analysis models into different layers of a systems architecture. We use so
called Architecture Layer Failure Dependencies to enable component fault trees
on different layers of an architecture. These dependencies are then used to
generate safety evidence for the entire system and over all different
architecture layers. A case study applies the approach to hardware and software
layers.
