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.
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".
> 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
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?
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.
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.
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?
The Class C airspace at that location starts at 2,100 feet. The Cessna was at 1,300 feet. The airliner was at 1,700 feet. The Cessna did not have to be coordinating with ATC because they were NOT in controlled airspace that requires radio communication with ATC, but it is optional and recommended.
The real question here is, why does Austin Approach Control have airliners driving around lower than the ceiling of the Class C protected airspace, where EVERY pilot knows that there are little putt-putt planes pottering about down there?
Looking at the charts, the Cessna brushed right up against the ground-4500 class C space. Not the smartest place to be off-frequency, in what sounded like mixed viz conditions. At the same time yeah, Austin Approach screwed up - "that traffic's through your final", when obviously it wasn't.
No, that's essentially underneath the landing path to the airport. Aircraft operate in three dimensions, and the airspace is sized and configured in three dimensions to support all the navigation needs and users in the area.
Did you know that there is a "tunnel" of uncontrolled airspace directly about Los Angeles International, between 3500 feet and 4500 feet, and you can fly through with commercial airliners whizzing about, talking to absolutely no radar services or ATC controllers? Called the LAX SFRA.
The Class C airspace is sized to contain the approach paths, including a buffer above and to the sides, as controlled airspace. But underneath the approach path and Class C airspace is generally uncontrolled airspace, untill you are very close, within a few miles, of the airport. (Technically it is called Class E or Class G airpspace depending on the area and altitude) Think of the controlled airspace as shaped like an upside-down wedding cake. Oh look, the FAA has a graphic for that:
Also, don’t forget that Class E airspace is controlled airspace, as IFR aircraft must operate with an ATC clearance. Airliners operate in Class E all the time. 1. ATC must vector them around VFR aircraft, and 2. VFR aircraft must see and avoid.
The site seems to be very focused on what happened in the air, I assume the site summarizes air traffic control or FAA reports for hobbyists, or that sort of thing, right? So we don’t know what happened when the pilot of the Cessna got on the ground. Without knowing much about the rules of this sort of stuff, I assume they got a pretty stern talking to…
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.
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.
Private plane then. The point is the same. If you're flying a couple miles away from a commercial airport and across the path of the runways, maybe you're not required to be listening to ATC, but that's definitely a choice you're making to risk it.
No, it isn't. You can get a Cessna for less than a Cybertruck. You cannot get a private jet that cheap.
> If you're flying a couple miles away from a commercial airport and across the path of the runways, maybe you're not required to be listening to ATC, but that's definitely a choice you're making to risk it.
There's an explicit system of controlled airspace around airports.
The Cessna was operating in airspace it had the legal right to be in and no need for ATC coordination. Near some of the busiest airports, etiquitte requires you try not to make a nuisance of yourself as a GA pilot to ATC.
You clearly know this more than I do, and I'm probably wrong in general, as it seems. I guess I find it hard to believe ATC is just flagrantly flying planes full of people in airspace they shouldn't be, but maybe that's normal.
You’d be surprised! For example, when I take off from Palo Alto in the Bay Area and fly northeast over, say, Fremont, staying under 4000 feet, I am in what’s called Class E airspace where I am not required to transmit on the radio. I regularly see arrivals into Oakland in front of me at my altitude and below. It is technically considered controlled airspace, as IFR aircraft must operate on an ATC clearance, but my VFR ass can legally barrel right through without talking to anyone! Obviously that’s not the smartest or safest practice but it’s legal.
Ok, I used imprecise language in using the colloquial term. I'm not a pilot. I don't work in ATC. The exact nature of the plane doesn't seem too relevant to whether they should have been flying across the path of an airport runway without listening to ATC.
It's not as if the lack of literal jet engines makes it less of a problem or less of a rich person's toy.
> The exact nature of the plane doesn't seem too relevant to whether they should have been flying across the path of an airport runway without listening to ATC.
ATC had this controlled aircraft outside the airport's standard controlled airspace.
The fact that TCAS occurred at all means the Cessna was properly (legal requirement) broadcasting its altitude, airspeed, and GPS position to all around it.
The blame here likely lies with ATC, not the Cessna pilot.
Well, imprecise language really is key here - because a pilot with a turbojet rating is, de facto, a better pilot than one who's flying a single engine piston plane.
Honest question, why would they de facto be better? Is it harder to fly a jet-engine plane? Especially when we're talking about generally flying safely in terms of "in airspace you're allowed to be in and being aware of other planes"? I could see a more complex plane being literally harder to fly, but is there a reason a Cessna isn't easier to fly safely?
In general, a jet is much more likely to be coordinating with ATC even if they aren’t required to under the specific circumstances. But still, this was a failure of ATC to provide adequate separation from an aircraft that they knew was operating under Visual Flight Rules, and was not under ATC control.
The Cessna pilot is under no obligation to maintain contact with ATC in this airspace, and it is the job of ATC to route the aircraft under its guidance around this kind of airspace, and if penetration of uncontrolled airspace is required, then to advise about and route around any known traffic.
The Cessna was probably communicating with the relevant CTAF or tower frequency for the airport it was setting up an approach for, or monitoring Unicom for traffic avoidance for other small VFR aircraft.
A single pilot can’t fly, navigate, and maintain traffic separation in a plane and communicate on all frequencies at once simultaneously, and ATC does not have the resources to control all random manoeuvres of small aircraft.
This is why there are places where small aircraft operate, mostly under “see and avoid” VFR rules, and places where all operations are done under strict ATC controls.
In this case, the heavy aircraft was directed to fly through one of these uncontrolled areas, and the controller failed to maintain adequate separation from uncontrolled operations.
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