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Comment by onion2k | Hacker News Reader

I find it slightly incredible that two planes ever hit each other given the size of planes compared to the volume of space they fly in. I understand that it's simple probability (small planes, big volume, but lots of flights) but it's still amazing that it happens at all.

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The probability is much higher in terminal airspace because all planes are going to or coming from the airport so they are on converging paths by definition.

And better GNSS based navigation equipment is now making it more likely than 20 years ago. The historical accuracy was such that planes where often a bit offset left or right off the route giving extra separation. Now the accuracy is so good that planes going opposite direction on the same route are passing exactly bang in the middle on top/below each other.

Yea, this is a really good point that isn't well known.

We've improved GNSS significantly, but the technology to avoid collisions hasn't been widely deployed, even though it's orders of magnitude less complicated than high accuracy GNSS on the whole

So... a random dilution of precision generator could... save lives? ha.

It's not uncommon in uncontrolled airspace to set a 1 or 2 Nm offset to the right in the FMS. Then it will follow the high accuracy path but not the same as opposite traffic that's 1000ft above or below.

Planes going opposite directions on the same route are meant to be at different altitudes. Altimeters have been quite accurate for a long time.

1000ft separation isn't much when you have an issue, if you loose an engine at cruise altitude you typically cannot maintain that altitude for example.

Oceanic procedure, without radar control, is to turn 45 degrees off track while descending, but it takes a little bit of time to set all that in motion when things go wrong while you were just seconds before happily cruising straight ahead on autopilot (autopilot is mandatory above 29,000 ft in a lot of airspace)

The ability for planes to follow these highly accurate common flight paths was a contributing factor in the Gol flight 1907 mid-air collision in Brazil [0]. (That, plus ATC clearing the two planes at the same altitude _and_ both having TCAS (Traffic Collision Avoidance System) switched off). William Langewiesche published a great long-form article about the disaster called "The Devil at 37,0000ft" that's well worth reading if that's your sort of thing [1].

[0] https://en.wikipedia.org/wiki/Gol_Linhas_Aéreas_Flight_1907

[1] https://www.vanityfair.com/culture/2009/01/air_crash200901

That vanity fair article was a good read but I felt it continually excused the American pilots ineptitude but happily pilloried the Brazilian air traffic controllers.

>I find it slightly incredible that two planes ever hit each other given the size of planes compared to the volume of space they fly in.

The space can be constrained though. There are "airways" (jet routes) for flights, and not that much of a volume/size ratio when the volume concerns areas near airports -- where many planes approach, exit, are asked to circle in a holding pattern, etc.

So it's not like the whole sky is their domain...

"Big Sky Theory"

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

>In aviation, the Big Sky Theory is that two randomly flying bodies are very unlikely to collide, as the three-dimensional space is so large relative to the bodies. Some aviation safety rules involving altimetry and navigation standards are based on this concept. It does not apply when aircraft are flying along specific narrow routes, such as an airport traffic pattern or jet airway.

>The Big Sky Theory has been mathematically modeled, using a gas law approach. This implies that collisions of aircraft in free flight should be extremely rare in en-route airspace, whereas operational errors such as violations of formal separation standards should be relatively common. Three critical parameters are the number of flying objects per unit volume, their speed, and their size. Larger, faster objects, flying in a traffic-rich environment are more collision-prone.

http://code7700.com/big_sky_theory.htm

>It seems that there are a lot of pilots out there that believe in the "Big Sky Technique." They think the amount of airspace out there is so wide and vast, and that they are so small, that the chances of hitting another aircraft is too small to worry about. And yet history begs to differ.

The Big Sky - Kate Bush

https://www.youtube.com/watch?v=sV7w5TaYjRA&ab_channel=KateB...

    They look down
    At the ground
    Missing
    But I never go in now

    I'm looking at the big sky
    I'm looking at the big sky now
    I'm looking at the big sky
    You never understood me
    You never really tried

Big Sky - Lou Reed

https://www.youtube.com/watch?v=Ug_Z-gu7u44&ab_channel=Naoyo...

    Big sky, big sky holding up the sun
    Big sky, big sky holding up the moon
    Big sky holding down the sea
    But it can't hold us down anymore

    Big sky holding up the stars
    Big sky holding Venus and Mars
    Big sky catch you in a jar
    But it can't hold us down anymore

    Big sky, big enormous place
    Big wind blow all over the place
    Big storm wrecking havoc and waste
    But it can't hold us down anymore

The problem is that the theory mostly works. "See and avoid" ends up mostly depending on the good odds a lot of the time. Except when it doesn't as in this case.

In free flight, two aircraft can be anywhere in 3D space and collisions are rare.

Around takeoff and landing - there is effectively a 1D track for the approach and the climb-out. It's not quite that simple because different aircraft will have different descent profiles and different angles to the wind, but it's a much smaller 'search space' for collisions.

Less incredible in this case since it was two planes landing on parallel runways at the same time.

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