AIN has learned that FAA officials are considering the introduction of TCAS-III to meet collision-avoidance needs when large unmanned aircraft start to operate in civil airspace. TCAS-III, which would add lateral resolution advisories (RAs) to the vertical commands current-generation TCAS-II offers, was proposed during TCAS development in the 1980s but was not taken further after investigations showed that its development would be technically challenging and could delay the implementation of the system, both in North America and elsewhere.

The renewed interest in TCAS-III comes from the expectation that future unmanned aerial vehicles (UAVs) would be unlikely to have the climb capability required in a current RA vertical avoidance maneuver. Sources have told AIN that should the FAA eventually adopt the system, it would necessarily require an airworthiness directive mandate to upgrade all TCAS-II units to TCAS-III standards to maintain airspace compatibility.

Industry specialists AIN contacted agreed that the earlier technical difficulties–mainly in antenna performance and target processing–could now be overcome, but they had some doubts about how many UAVs would be likely to carry a full TCAS installation necessary to perform coordinated lateral TCAS RAs, versus those vehicles that would be simply transponder equipped.

In other words, would the number of UAVs with a full TCAS installation be sufficient to justify the wholesale conversion of the world’s current TCAS-II installations to TCAS-III for the sake of compatibility?

Also, since there is a fairly well established understanding, at ICAO and in national aviation administrations, that any limitations in UAV performance cannot be allowed to negatively affect other airspace users, it seems likely that any upgrade mandates to accommodate UAVs would meet with strong opposition from the TCAS-II community. Nevertheless, the UAV movement is gaining momentum, and maintaining a political (as well as cockpit) lookout could be prudent.