A few more from AC251, some bolding by moi (comments and elaboration probably superfluous) - I think they inadvertently left out the new "E+" airspace category???
15.3.6
Those portions of airspace wherein it is determined that an air traffic control service
will also be provided to VFR flights shall be designated as Classes B, C or D airspace.
15.3.7
Note that an ‘air traffic control service’ does not imply a separation service.
15.3.8
The particular criterion differentiating CTA classes is the provision of ATC service
to VFR flights. If it is NOT intended to provide ATC to VFR, then the options are:
•
Class A airspace (such as in high level CTA), which excludes VFR (the exclusion
of VFR could be due to the deliberate policy choice, or because the available VFR
aircraft lack the performance to access that airspace); or
•
Class E, which is usually used for lower level CTA (i.e. below F180 in Australia).
VFR flights are permitted, but only receive traffic information ‘as far as is
practical’, i.e. the unit providing ATC service need not and may not be resourced to
routinely provide traffic information.
15.3.9
Class E must not be used for control zones, i.e. airspace extending up from the
surface, usually associated with aerodrome traffic.
15.3.10
If an ATC service is to be provided to VFR flights, classification options range
from Class B down to D:
•
Class B - VFR flights are treated the same as IFR, and all flights are separated;
•
Class C - VFR flights are separated from IFR flights; and
•
Class D - only IFR flights are separated.
15.3.11
The defining element is the degree to which visual ‘see-and avoid’ is integrated into
the airspace traffic management plan. This dictates the degree to which traffic information
and collision avoidance advice is prescribed to support clearances in preventing collisions
and expediting an orderly flow of air traffic.
15.4.2
The class of airspace chosen will be that which best reduces the level of risk to As
Low As Reasonably Practicable (ALARP).
15.4.3
The role of risk assessment in the CTA design/classification process is such that:
•
quantitative (and qualitative) analysis must support the introduction of an ATC
service on the grounds of reducing unacceptable risks to an acceptable level; and
•
the nature of the hazards, and the resulting risks, in specific areas will largely
derive from the traffic mix, and will indicate which class of airspace will best
reduce the risks to an acceptable level.
15.4.4
The class of airspace dictates the nature of the ATC service provided.
15.4.8
Generally, a higher classification requires more resources which results in higher
costs. Cost Benefit Analysis calculates whether the expenditure is matched by a reduction in
risk, and is a first step in ensuring that the residual risks are ALARP.