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Comment by cjbprime | Hacker News Reader

The Cessna probably didn't have TCAS, right? So how did the system know whether to tell the Boeing pilots to raise or lower their altitude? I thought it was a system with a negotiation to avoid instructing both planes to turn the same way.

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TCAS will use the Mode C or Mode S altitude provided by a standard SSR transponder if the intruder aircraft isn't TCAS-equipped. To determine the appropriate maneuver, the system calculates which trajectory will increase the vertical separation at the closest point of approach. TCAS generally favors minimizing disruption by requiring the smallest possible deviation from the current vertical trajectory, but if the intruder is climbing or descending, the system biases the RA based on the threat's current altitude trend, with the expectation that the intruder might level off.

You can read all about the threat resolution logic in section 3.10.8 of ICAO 9863.

Oh cool, thanks! TCAS is much more capable than I expected, I thought it was more like a decentralized coin-toss consensus algorithm.

An aside:

> 3.10.1.1 The ACAS algorithms operate in a cycle repeated nominally once per second. At the beginning of the cycle, surveillance reports are used to update the tracks of all intruders and to initiate new tracks as required. Each intruder is then represented by a current estimate of its range, range rate, altitude, altitude rate, and perhaps, its bearing. Own aircraft altitude and altitude rate estimates are also updated.

Words that should probably never appear in technical specifications: "perhaps".

I'm sure it tracked the cessna's rate of ascent descent and decided? Probably also determines that a jet can climb hella faster than a Cessna can.

> When a TA is issued, pilots are instructed to initiate a visual search for the traffic causing the TA. If the traffic is visually acquired, pilots are instructed to maintain visual separation from the traffic. Training programs also indicate that no horizontal maneuvers are to be made based solely on information shown on the traffic display. Slight adjustments in vertical speed while climbing or descending, or slight adjustments in airspeed while still complying with the ATC clearance are acceptable

https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...

I am unsure what it would have told them to do in the RA case though.

RA? Climb or descend. Or any of these more accurately: https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...

Sure, but which one? The Cessna doesn't have a TCAS to coordinate with, so how does it know which one is best? Does it just pick one and change it if the other aircraft reacts in the wrong way?

I think there is a deterministic algorithm (based on GPS position?) for determining who climbs and who descends.

don't the planes know where the other is based on the fact that the tcas is yelling at them in the first place? easy enough to tell them to pull up if it knows they are below them.

Certainly the one plane is aware of the other, however, I wonder if the algorithm takes into account the physical characteristics of the planes. For example, a 737 descending would take longer to pull up on and there would be a period of time where the engine are spooling up that it continues to descend before it gains altitude. Would it be smart enough to figure out that the 737 is going to end up below it before gaining altitude and thus should actually descend faster? Does it just go, that plane is blow me, so pull up? It's a fascinating problem to think about the different edge cases and how it could possible handle it.

yeah the system is constantly calculating velocities and positions in 3d space. there's a traffic advisory portion where it tells the pilots to be aware of traffic nearby and there's a resolution advisory which tells the pilots exactly what to do and at what rate. commercial pilots practice that in the sim extensively. the faa made having ads-b out mandatory for all aircraft some time ago which helps the planes see each other so the commercial one knew exactly where the smaller plane was. (though you can shut the device off or it could be malfunctioning but it wasn't here)

https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...

Ah, you meant which specific RA. While I don’t know, that’s become a more deterministic answer in the case of an airplane not following the instructions (or not having TCAS) because of Überlingen and the introduction of TCAS reversals… in other words exactly as you said, it’ll 1. change if the other aircraft reacts in the wrong way and 2. there’s a much greater clarity and emphasis in following the RAs in all cases except for GPWS in great part because of Überlingen.

Edit: never mind the last bit, RAs actually get completely suppressed if GPWS does its thing, TAs still go through.

It picks one. If this does not improve the situation it issues a reversal.

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