1.1 Absolute safety cannot be achieved in any
human activity. Naturally, this fact has to be taken into account
in developing safety requirements, which means that requirements should
not imply that safety is absolute. In the case of traditional craft,
it has frequently been possible to specify certain aspects of design
or construction in some detail, in a way which was consistent with
some level of risk which had over the years been intuitively accepted
without having to be defined.
1.2 For dynamically supported craft, however,
it would often be too restrictive to include engineering specifications
into the Code. Requirements therefore need to be written (where this
question arises) in the sense of ". . .the Administration should be
satisfied on the basis of tests, investigations and past experience
that the probability of... is (acceptably low)". Since different undesirable
events may be regarded as having different general orders of acceptable
probability (e.g. temporary impairment of propulsion as compared with
an uncontrollable fire), it is convenient to agree on a series of
standardized expressions which can be used to convey the relative
acceptable probabilities of various incidents, i.e. to perform a qualitative
ranking process. A vocabulary is given below which is intended to
ensure consistency between various requirements, where it is necessary
to describe the level of risk which must not be exceeded.