Home' RTCA Documents for Review : C2 Link Systems MASPS_Draft Contents Appendix D
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Initial results from those simulations showed that the point during the encounter at which
the C2 Link System interruptions had the most impact on Loss of Well Clear (LoWC) count
and Near Mid-Air Collison (NMAC) count was at the DAA Warning Alert time.
Subsequent simulations determined the effect on the LoWC count and NMAC count of a
range of C2 Link System interruption lengths (spanning from 0 to 60 seconds). Each
interruption was timed to occur at the DAA Warning Alert time so that it would have the
most impact on the LoWC and NMAC counts.
The methodology used to determine the longest allowable C2 Link System interruption
was to assess how much of an increase in LoWC and NMAC count could be allowed, due
to C2 Link System interruptions, while remaining below the Allowable Quantitative
Probabilities associated with the MAC and NMAC severities. Based on the Safety Risk
Management Document (SRMD) for Unmanned Aircraft Systems (UAS) Detect and
Avoid (DAA) System Safety Assessment (SSA) , these probabilities are respectively
Extremely Improbable at a probability of 1 x 10-9 and Extremely Remote at a probability
Since the “Inadequate or lack of maneuver when one is required in uncontrolled airspace
conditions” hazard was closest to its Allowable Quantitative Probability limit, this
condition was used to determine the longest allowable C2 Link System interruption. A 5
second C2 Link System interruption was the longest that could be tolerated before the
MAC likelihood equaled its Allowable Qualitative Probability of 1 x 10-9 which was used
as the target level of safety in the SRMD UAS DAA SSA . The analysis also showed
that the corresponding NMAC likelihood increase was still significantly below its
Allowable Qualitative Probability value of 1 x 10-7
Consequently, a 5 second interruption-based TET was determined to be the limiting case.
This time already include consideration of the, 1 second, C2 Link System latency that was
built into the encounter model. It should be emphasized that this analysis is conservative
in that it is based on a 5 second C2 Link System interruption occurring at the worst possible
time in the encounter and that every encounter among the many millions simulated had this
maximum length of C2 Link System interruption at that worst possible time.
Availability for Long C2 Link System Interruptions
Analyses of the SRMD UAS DAA SSA  Event and Fault Trees, associated with the
allowable probability of the “C2 Link System being not available when needed for DAA”
fault, was performed to determine the minimum Availability for long interruptions.
The SRMD UAS DAA SSA  Event Tree already includes an “Event Tree” factor to
account for the fact that not all “inadequate or lack of maneuvers when one is required”
will lead to the most severe outcome of a MAC.
The SRMD UAS DAA SSA  Fault Tree already included a “Fault Tree” factor to
account for the fact that the UA is not permanently in a situation where a DAA initiated
“maneuver when one is required” is occurring. Consequently, Step 2 of the Availability
calculation described in the Introduction to this Appendix was not required.
The calculated C2 Link System Availability of 95.087% for long interruptions, with a 5
second TET, still allows the UAS to meet the Allowable Quantitative Probability of a MAC
of 1x 10-9
Although the above analysis is mathematically accurate, it was the view of the DAA and
C2 Link System Subject Matter Experts involved that this Availability may not reflect what
is required for safe operation. This view is based on the fact that the calculated Availability
is not derived from statistical C2 Link performance simulations validated with operational
experience. Consequently, until such time as more accurate analysis has be en undertaken,
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