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Comment by ceejayoz | Hacker News Reader

TCAS disables below 1,000 feet because there’s too much stuff at an airport.

I have ADS-B in my airplane and can see everything on the ground on a pretty map as if it were literally a video game. I can see landing aircraft in realtime while holding short or crossing a runway. The emergency responder should have had it in their fire truck.

The technology already exists. The problem has already been solved with an iPad and a $200 receiver. Almost certainly some BS regulation or rule was at least partially responsible here.

Information overload is a thing, and there are a lot of ground vehicles at a place like LGA.

Consider that if you have access to all the local ADS-B data you can project paths forward through 3D space for the next, say, 30 seconds or so. Using GPS you can determine your own position in 3D space. At that point it's trivial (and I'm not handwaving here, it is literally extremely trivial) to filter projected paths based on passing close enough to your own in 3D space (ie accounting for altitude). Stick that on a tablet and require it to be present in all vehicles that operate on the tarmac.

It wouldn't need to work 100% of the time because you'd still be required to contact ATC. The only requirement is that it have a reasonably high chance of alerting drivers to potential mistakes before they happen.

Which is to say this incident was trivially preventable had anyone with authority over these sorts of things cared to bother.

> for the next, say, 30 seconds or so

But this is a hand-wave.

This is a situation where both vehicles got explicit permission from someone who's supposed to know what they're doing. These sorts of runway crossings aren't unusual - and this one was responding to an emergency - and at a place like LGA there's always gonna be a plane on approach.

The difference between "hold short at runway 22" and being on runway 22 is much less than 30 seconds in some cases.

I think the most generous interpretation of using 'all' ADS-B data (including things on the ground) would be to have VR and have boxes for all objects, à la the F-35 helmet:

* https://www.radiantvisionsystems.com/blog/worlds-most-advanc...

Not sure if you can do something on a 'simple' HUD that many planes have, so you could see objects in your flight path.

The entire point is that there's no need for someone driving a ground vehicle to see all the ADS-B data. They only need to know if and when a plane is projected to cross the direction in which they're facing. It might also be useful to know the projected speed as well as how far in front of your vehicle it will pass (but you can presumably figure the latter out on your own because, y'know, the runway).

If ATC says you're clear to cross the runway and then you glance down and the screen shows a plane projected to cross directly in front of you in 10 seconds you'd probably think twice, right? This hypothetical cheap appliance has GPS and a compass and probably even a camera feed facing forward. It isn't a difficult technical problem to calculate the time offset at which the object traveling along the crosscutting path will pass in front of you.

> The difference between "hold short at runway 22" and being on runway 22 is much less than 30 seconds in some cases.

What is the typical minimum temporal separation? I would have expected at least 45 or 60 seconds given the cost of a plane and the imminent threat to life.

So, it does appear I was correct here - it's a difficult thing to solve.

https://www.nbcnews.com/news/us-news/investigators-search-an...

> Jennifer Homendy, chairwoman of the National Transportation Safety Board, said at a news conference Tuesday that the airport uses a safety system called ASDE-X to track surface movements of aircraft and vehicles.

> "ASDE-X did not generate an alert due to the close proximity of vehicles merging and unmerging near the runway, resulting in the inability to create a track of high confidence,” Homendy read from an analysis of the system’s performance.

A clearance from ATC means you can land not that you must land nor that it is safe to land. PIC still has the ultimate choice. It's common practice in the US to issue landing clearances even when another plane is on the runway or there are two landing planes ahead of it also with landing clearances, if that wasn't done you would be waiting far longer at the airport.

It's obviously the right choice to give the PIC the information via avionics in a graphically concise way that highlights this potential runway contention because it is real and pilots are expected to adjust their speed to maintain the right sequence.

When it isn't possible, which does happen, IE a plane ahead is slow to clear the runway or to takeoff, pilots are expected+required to execute a go-around.

Well imagine if we designed a TCAS-like system that did work below 1,000 feet!

TCAS is mostly "how close to another plane am I?"

In flight, the answer really shouldn't ever be "less than 500 feet". During landing, the answer almost certainly will be "less than 500 feet"; a plane's queued up to enter the runway after you land, a ground vehicle is working on something near but not on the end of the runway, etc.

It's a surprisingly tough challenge to solve a) reliably and b) in a way that doesn't cause a whole bunch of false go-arounds wreaking havoc on the busy airport.

It's a much easier challenge when every moving vehicle in the airport environment is issued a relatively exact clearance.

The ATC clearance gives each vehicle a movement contract.

It would be great if avionics actually took those into account. Avionics should help pilots ensure their vehicles don't break the contract and should also alert immediately if other vehicles have, or their velocity vector is such that they will violate the contract. IE braking rate is insufficient to stop before the hold short line. ATC computers should ensure no conflicting clearances are issued.

None of that happens right now.

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