Home' RTCA Documents for Review : DO-343B Contents © RTCA, 2018
20 = ACSP
21 = 0.999 (safety) / 0.9999 (efficiency)
The aircraft availability target of 0.999 corresponds to today’s aircraft-level
safety analysis, where the complete loss of LRCS to a single aircraft (HAZ-10)
is considered a minor safety hazard and thus a single (non-redundant) LRCS
avionics could be sufficient.
The service availability target of 0.999 suggests, that also the complete loss of
LRCS to multiple aircraft (HAZ-20) is equivalent to a minor safety hazard. This
target is achievable with a single existing satellite system.
To summarize, the ICAO PBCS as well as today’s aircraft safety analysis
suggest, that LRCS redundancy is not needed from safety perspective.
On the other hand it needs to be recognized, that ICAO PBCS isn’t a regulatory
document, in a sense that it does not prescribe concrete RCP/RSP for concrete
airspace. It rather defines the methodology for defining and prescribing the
RCP/RSP and it defines several RCP/RSP types (including the above
mentioned RCP 240/400 and RSP 180/400). Deciding if and how this
methodology will be applied and what additional airspace-specific provisions
are required (such as HF which is not covered by the ICAO PBCS) is up to the
states and other authorities controlling given airspace.
REGULATIONS (SECTION 2) VS. SAFETY DRIVEN DESIGN (SECTION 3.1)
The applicable regulations analyzed in section 2 of this appendix do not
explicitly refer to any safety analysis or quantified safety objectives. Historically
the SATCOM technology was added as complementary to HF, because it
offered better performance and more functionality. But SATCOM was not
allowed to fully replace HF, primarily due to the existence of the legacy HF
infrastructure (section 2.4.2 Observation HL4), lack of SATCOM coverage in
some areas (section 2.4.2 Observation HL6) and to some degree also because
the combination of HF and SATCOM is perceived as more reliable and thus
more ”safe” than SATCOM alone (section 2.4.2 Observation HL5).
Even without referring to HF voice, the regulations analyzed in section 2
generally require two independent communication systems (REQ-LRCS-20),
including independent aircraft antennas (REQ-LRCS-30). This is however not
driven by quantified safety objectives, because as summarized in section 3.1,
the aircraft level as well as the airspace level availability targets for the
procedural airspace can be satisfied without redundancy. (The situation is
different for continental airspace, where loss of VHF voice is recognized as
MAJOR hazard and the associated availability targets can’t be met without VHF
avionics redundancy.) The need for two independent LRCS avionics
installations is in practice driven rather by an aircraft architectural
recommendation for ETOPS certification. More concretely it is a solution for the
20 ICAO PBCS definition: An RCP allocation that specifies the required probability that the ATS unit’s
system and the CSP systems are available to provide the required level of communication service.
21 ICAO PBCS definition: An RCP allocation that specifies the required probability that the CSP
systems are available to provide the required level of communication service, given the ATS unit’s
system is available.
In the RCP/RSP allocations in ICAO PBCS it is assumed that ATS is always available and thus ACSP =
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