Originally Posted by Lucy Lastic
Is there actually any real evidence that flying non-TX is inherently unsafe, or is this just an assumption?
Given that there are still incidents of lack of separation by fully-equipped aircraft, it isn't necessarily a fix to "the problem".
Serious losses of separation as a result of infringement occur much more often with Mode A/non-Transponder aircraft than with Mode C/S.
Within controlled airspace there are serious loss of separation instances associated with misunderstood clearances and busted altitudes. Almost everyone is Mode C/S equipped and this coupled with TCAS helps save the day. NATS hopes that downlinking autopilot data will help catch these problems earlier (a benefit of Mode S enhanced)
In the open FIR airproxs between equipped aircraft rarely would count as a serious loss of separation (mostly CAT stressing that IFR separation wasn't maintained - but that is a long way from a real risk of collision). Which implies (but I have not looked at the data for this particular case) that the high collision risk incidents outside controlled airspace are higher for Mode A/non-transponder aircraft than for Mode C/S. All of this paragraph is in the context of one aircraft being TCAS equipped or receiving a radar service. Obviously transponders do nothing to help reduce collisions between aircraft without TCAS or a radar service.