NATS makes use of a Safety Severity Event
(SSE) Scheme as a tool to assess incident severity.
The SSE Scheme comes in 2 varieties; the en route version and the
airports version. Both schemes are based on the number of barriers
available preventing the incident becoming an accident after the
loss of separation has occurred.
The en route scheme assumes that the barriers work in a particular
order - ATC (timely and effective), then ATC (belated or with help
of safety nets), then Pilot, then Providence. These equate to SSE4,
SSE3, SSE2 and SSE1 respectively. A letter is annotated based on
proximity, where 'a' is less than a third of the required
separation, 'b' is between one third and a half, 'c' is between a
half and 100% of separation, and 'd' is more than 100% separation
(where, due to its nature, the event is investigated as if it were a
loss of separation).
The airports scheme recognises that the barriers may work in
different orders, and also takes cognisance of the energy of the
aircraft in various zones or phases of flight around the airfield
(e.g. the runway is a 'riskier' area than the taxiway). It also
retains the concept of latent rather than actual incidents (for
example if there was no other aircraft nearby when an aircraft has a
runway incursion, this would be seen as a Latent Zone A). Where
other aircraft are involved, a score of SSE1-4 is accorded; however,
as there are often no separation standards defined in this
environment, no letters are added as in the En Route Scheme.
