4 ms·

TCAS averts possible head-on collision in Austin

50 points by philip1209 2y ago

cjbprime 2y ago
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.
- unsnap_biceps 2y ago
  > 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_system#Alerts 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.
  - quink 2y ago
    RA? Climb or descend. Or any of these more accurately: https://en.wikipedia.org/wiki/Traffic_collision_avoidance_system#Types_of_traffic_and_resolution_advisories https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...
    - unsnap_biceps 2y ago
      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?
      - crthpl 2y ago
        I think there is a deterministic algorithm (based on GPS position?) for determining who climbs and who descends.
        deleted 2y ago
        [deleted]
      - weaksauce 2y ago
        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.
        unsnap_biceps 2y ago
        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.
        weaksauce 2y ago
        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_system https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...
- akira2501 2y ago
  It picks one. If this does not improve the situation it issues a reversal.
- willidiots 2y ago
  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.
  - cjbprime 2y ago
    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".
- polishdude20 2y ago
  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.
olliej 2y ago
I’m sorry, but in what world is it acceptable for an aircraft flying in an airport’s atc controlled airspace to not be coordinating with ATC? I thought that was literally the entire reason we have controlled airspace rules?
- noirbot 2y ago
  It's F1 weekend in Austin, so I'd assume that's part of the reason someone may think they don't need to care where they fly their private jet.
  - dfc 2y ago
    Are you referring to the 45 year old Cessna when you say private jet? It looks like that plane flies a lot around Louisiana and Texas. Weird to jump to a conclusion about F1 money and an entitled pilot.
    - noirbot 2y ago
      I mean, I'm just guessing, but AUS becomes one of the busiest airports for non-commercial planes this weekend every year. Even an old Cessna is literally a private jet, and they clearly weren't listing to ATC, so it's either entitlement or some sort of minor disaster that almost became a major disaster. It's a choice to fly your airplane across the land/takeoff trajectory for a major airport. They also essentially flew by the F1 track on the way.
      - ceejayoz 2y ago
        > Even an old Cessna is literally a private jet... No, it isn't. It's a small non-jet aircraft. https://en.wikipedia.org/wiki/Cessna_182_Skylane https://en.wikipedia.org/wiki/Cessna_182_Skylane > they clearly weren't listing to ATC They are not required to in that airspace at that altitude.
        noirbot 2y ago
cyberax 2y ago
I wish we had TCAS for small aircraft as well! PCAS is great, but it's passive, so it doesn't help if two small aircraft are in dangerous proximity.
- imglorp 2y ago
  There's also FLARM for small aircraft. No help in this case though. https://en.m.wikipedia.org/wiki/FLARM https://en.m.wikipedia.org/wiki/FLARM
- cynical_spar 2y ago
  We do, it’s called ADS-B. It’s required in class C, B, and A airspace. As well as in class E above 10,000 feet and within a mode C veil. Unfortunately we have thousands of airplanes that do not have electrical power so it would be very difficult to require it everywhere within the US.
  - buildsjets 2y ago
    No, we don't. ADS-B transponders do not negotiate with each other in real time to compute a mutual avoidance maneuver and provide audio guidance to the pilots of what to do to avoid a collision. ADS-B is better than nothing, but all it does is report where another airplane claimed to have been about 20-30 seconds in the past. Flying formation with people while watching their ADS-B return is very illuminating.
    - cynical_spar 2y ago
      You are down playing it way too much. It’s way better than nothing. You also don’t seem to understand how ADS-B works. There are two kinds ground to air and air to air, air to air latency is measured in milliseconds and in the case of converging airplanes the latency would be near zero since you are receiving the other airplanes broadcasts directly. Having TCAS in small GA airplanes is never going to be possible. The cost is too high and the performance differences are too great. You don’t need an RA to make a decision, TAs are completely sufficient in GA and ADS-B works quite well for that. Not sure why you would care about this while flying formation. TCAS isn’t helping you there either. https://www.researchgate.net/publication/343689163_A_study_of_uncompensated_latency_in_ADS-B_reports https://www.researchgate.net/publication/343689163_A_study_o...
philip1209 2y ago
General discussion point: TCAS works (well) for flying aircraft. But, I believe it disables below 1000f above ground. And, we’ve seen many issues recently with aircraft on or near the ground - from the collision in Japan to many near-misses on the ground. So, how might we make a system as robust as TCAS for aircraft on or near the ground?

50 points by philip1209 2y ago

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.

unsnap_biceps 2y ago
> 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_system#Alerts 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.
- quink 2y ago
  RA? Climb or descend. Or any of these more accurately: https://en.wikipedia.org/wiki/Traffic_collision_avoidance_system#Types_of_traffic_and_resolution_advisories https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...
  - unsnap_biceps 2y ago
    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?
    - crthpl 2y ago
      I think there is a deterministic algorithm (based on GPS position?) for determining who climbs and who descends.
      - deleted 2y ago
        [deleted]
    - weaksauce 2y ago
      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.
      - unsnap_biceps 2y ago
        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.
        weaksauce 2y ago
        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_system https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...
akira2501 2y ago
It picks one. If this does not improve the situation it issues a reversal.
willidiots 2y ago
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.
- cjbprime 2y ago
  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".
polishdude20 2y ago
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.

olliej 2y ago

I’m sorry, but in what world is it acceptable for an aircraft flying in an airport’s atc controlled airspace to not be coordinating with ATC? I thought that was literally the entire reason we have controlled airspace rules?

noirbot 2y ago
It's F1 weekend in Austin, so I'd assume that's part of the reason someone may think they don't need to care where they fly their private jet.
- dfc 2y ago
  Are you referring to the 45 year old Cessna when you say private jet? It looks like that plane flies a lot around Louisiana and Texas. Weird to jump to a conclusion about F1 money and an entitled pilot.
  - noirbot 2y ago
    I mean, I'm just guessing, but AUS becomes one of the busiest airports for non-commercial planes this weekend every year. Even an old Cessna is literally a private jet, and they clearly weren't listing to ATC, so it's either entitlement or some sort of minor disaster that almost became a major disaster. It's a choice to fly your airplane across the land/takeoff trajectory for a major airport. They also essentially flew by the F1 track on the way.
    - ceejayoz 2y ago
      > Even an old Cessna is literally a private jet... No, it isn't. It's a small non-jet aircraft. https://en.wikipedia.org/wiki/Cessna_182_Skylane https://en.wikipedia.org/wiki/Cessna_182_Skylane > they clearly weren't listing to ATC They are not required to in that airspace at that altitude.
      - noirbot 2y ago

cyberax 2y ago

I wish we had TCAS for small aircraft as well! PCAS is great, but it's passive, so it doesn't help if two small aircraft are in dangerous proximity.

imglorp 2y ago
There's also FLARM for small aircraft. No help in this case though. https://en.m.wikipedia.org/wiki/FLARM https://en.m.wikipedia.org/wiki/FLARM
cynical_spar 2y ago
We do, it’s called ADS-B. It’s required in class C, B, and A airspace. As well as in class E above 10,000 feet and within a mode C veil. Unfortunately we have thousands of airplanes that do not have electrical power so it would be very difficult to require it everywhere within the US.
- buildsjets 2y ago
  No, we don't. ADS-B transponders do not negotiate with each other in real time to compute a mutual avoidance maneuver and provide audio guidance to the pilots of what to do to avoid a collision. ADS-B is better than nothing, but all it does is report where another airplane claimed to have been about 20-30 seconds in the past. Flying formation with people while watching their ADS-B return is very illuminating.
  - cynical_spar 2y ago
    You are down playing it way too much. It’s way better than nothing. You also don’t seem to understand how ADS-B works. There are two kinds ground to air and air to air, air to air latency is measured in milliseconds and in the case of converging airplanes the latency would be near zero since you are receiving the other airplanes broadcasts directly. Having TCAS in small GA airplanes is never going to be possible. The cost is too high and the performance differences are too great. You don’t need an RA to make a decision, TAs are completely sufficient in GA and ADS-B works quite well for that. Not sure why you would care about this while flying formation. TCAS isn’t helping you there either. https://www.researchgate.net/publication/343689163_A_study_of_uncompensated_latency_in_ADS-B_reports https://www.researchgate.net/publication/343689163_A_study_o...

philip1209 2y ago

General discussion point: TCAS works (well) for flying aircraft. But, I believe it disables below 1000f above ground. And, we’ve seen many issues recently with aircraft on or near the ground - from the collision in Japan to many near-misses on the ground. So, how might we make a system as robust as TCAS for aircraft on or near the ground?