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The TCAS (Traffic Collision Avoidance System) is capable of issuing RA (Resolution Advisory) when a mid-air collision is imminent by telling the pilots to "climb", "descend", "increase climb" etc.

Why does the system only issues commands in the vertical direction, but not the horizontal direction? It seems an oversight in the design of the system to not make use of all available maneuver options to resolve a traffic conflict. Why can't the system tell the pilots to "turn left", "increase right turn" etc.?

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  • $\begingroup$ Side note: future versions probably will $\endgroup$ Mar 20, 2017 at 18:47

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TCAS gets fairly accurate altitude information from the same source as the transponder. Thus any extrapolation from this information can yield a fairly accurate prediction of whether two aircraft may collide, and provide more certainty that changes in altitude will avert any conflict.

Range information is obtained by measuring the round trip time of the interrogation and response. This will also be fairly accurate.

Direction information is obtained by using the TCAS antenna to detect the direction of responses from other aircraft.

These values are then extrapolated to provide an expected path of other aircraft. Most airliners are required to use TCAS II which only provides the vertical guidance. TCAS III was envisioned to provide horizontal guidance as well, but:

it was judged by the industry to be unfeasible due to limitations in the accuracy of the TCAS directional antennas.

Even if TCAS can be accurate to 3 degrees in direction, this translates to over 300 feet of uncertainty at 1 nmi.

TCAS IV was planned to use more accurate GPS information, but with ADS-B becoming more widely adopted, this development was stopped in favor of some future system that can use existing ADS-B technology.

Another point is that airplanes are generally larger in the horizontal plane than the vertical, and thus would need to change course more horizontally to avoid collision than they would need to vertically. Especially as planes get larger, they also tend to be more maneuverable in pitch than in turns.

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    $\begingroup$ Adding to the last paragraph -- though I'm not sure if it's been one of the factors in TCAS III -- an extreme avoidance bank also increases the plane's vertical plane, which might increase the chances of a mid-air collision. $\endgroup$
    – user14897
    Jun 12, 2018 at 23:52
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The vertical dimension is already sufficient to resolve a collision. It also makes resolution advisories (RA) much simpler to comply with. There's no need to make the software, RA and following the RA more complex than it needs to be.

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    $\begingroup$ Can you provide some statistics / research / authoritative source, about "vertical dimension is already sufficient to resolve a collision"? $\endgroup$
    – kevin
    Mar 20, 2017 at 19:25
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    $\begingroup$ Well the track record of TCAS shows that it works with just vertical resolution, and I'm sure they expected it to when they designed it. But this begs the question, if you will only use one, why choose vertical over horizontal? $\endgroup$
    – fooot
    Mar 20, 2017 at 22:30

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