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Comment by blueflow | Hacker News Reader

I'll recite an avherald comment:

  If you look at "Video of aircraft after touchdown sliding along the runway and impacting the fence:", you will find out that it took them ~1.7 sec from leaving the tarmac until they hit the construction. If you measure the distance on Google earth you come up with ~140m. That means they hit the construction with roughly 296km/h or 160 knots. If it wasn't the construction it would have been the treeline or something else. That plane was doomed, concrete construction or not.

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> it took them ~1.7 sec from leaving the tarmac until they hit the construction. If you measure the distance on Google earth you come up with ~140m. That means they hit the construction with roughly 296km/h or 160 knots.

(Assuming the math is correct:) That's the average speed over that distance. The plane would have been slowing down the whole time.

Physics hack: The average velocity at constant deceleration is halfway between the initial and terminal velocities.

So if we know the landing speed (which should come out of the flight data recorder), we'll know the terminal velocity given the average speed (distance/time) which is determinable from the video.

No doubt Jeju 2216 was moving hot, but a longer run could have bled off far more speed, and kinetic energy is based on velocity squared, so every bit helps a lot.

The plane was sliding on its belly, wheels up. It wasn't slowing down very much. Ever play on a slip-n-slide as a kid?

Can't speak about the OP but I didn't weigh a few thousand metric tons as a kid.

Or as an adult. I am on a diet though.

Set yourself a goal!

And good news! A fully-loaded 737-800 has a maximum takeoff weight of less than 80 tonnes, not thousands, so your challenge is much more attainable!

<https://en.wikipedia.org/wiki/Boeing_737_Next_Generation#Spe...>

Yeah I always overestimate those things by far :)

It might not have been slowing down much in that time due to a thing called Ground Effect. Since the wheels weren't down, the flat body of the bottom of the aircraft + wings would have actually reduced drag and cushioned the plane for a bit, causing it to not slow down as much as you would assume.

I'm not a pilot.

In the video it looked like the plane was only running on the rear landing gears, I assume with no brakes applied, since that would've caused it to violently pitch down I assume. Only in the last bit did it pitch down and started scraping along the runway. It certainly doesn't look like it was efficiently shedding speed (but looks can be deceiving).

> In the video it looked like the plane was only running on the rear landing gears,

Are we talking about the same crash? In the video I have seen[1] the plane appears to be on its belly dragging on the runway.

1: https://www.youtube.com/watch?v=qJY7oaZpxDU

True! I misremembered, in the longer video it's hard to see that the nacelles are dragging on the ground all the way. Still doesn't seem to slow down much.

It was in gound effect floating till very near the end, these two video show the story very well: https://x.com/vinfly4/status/1873285591900836307 https://www.reddit.com/r/aviation/comments/1hopl7d/longer_vi...

As you can see, it actually isn't touching the ground for quite some time, it looks like it because one of the engines is smoking and the plane is throwing up dust etc from the ground as it floats above. Pure guessing on my part as someone who isn't involved in aviation but spent a good 6 hours looking into this crash: Pilots tried to go around, put the plane in go around config, couldn't, didn't know what to do, and watched the berm come at them. Extremely sad.

How both engines failed? We won't know til blackbox I guess, either pilot error or the bird strike was nuts and took out both engines, also some speculation the go around thrust caused a compressor stall. It looks to my uneducated eyes, from the first video, the left engine is not in great shape. Either way, very awful situation.

The reason he touched down halfway down the runway could have been because the gear was up. If the gear was down, he would have touched down much earlier. He may not have known the gear was up, or did not account for the gear being up in his approach.

They executed a pretty difficult turn, apparently it's called the impossible tear drop and you're trained specifically NOT to ever do it. I looked at the flight tracking, I suspect the maneuver they pulled just put them at that point in the runway[1]. If you read my reply to the comment above you, it has some additional context you might find interesting. They pilots I watched all mentioned really, it doesn't make sense they cut off the approach, they should have taken the bird strike and continued the landing. The probable reasons given was: took evasive maneuvers to avoid the brids so came off glide slope, not enough engine power for a full go around, then started to get way behind the plane.

[1]https://s.france24.com/media/display/579312a0-c8cb-11ef-81bd...

> it's called the impossible tear drop

Minor nit, I believe it’s called the teardrop go around for such cases. You also have the impossible turn which is meant typically for engine/power failure during takeoff, and it actually is possible to be safely done - as demonstrated by the former ALPA Air Safety Institute Senior VP Richard McSpadden in one of his YouTube videos.

However, it can be deceptively difficult to have the right conditions to pull it off - as demonstrated by the ironically fatal crash that killed Commander (Ret) McSpadden (though iirc it was not clear if he was flying the craft at the time).

(Edits made for clarity/content.)

Thank you for the clarification, I hoped I couched everything enough people knew I'm just reading internet and not at all an expert, I appreciate the reply!

Assuming that's true, would earlier ground contact have saved lives by slowing the plane more before colliding with the barrier?

I don't know if we could expect the pilot to know about or expect the barrier there.

Obviously hard to answer your question but if you're curious, some other bits of info I've gleaned from my autistic research mode. Some context: Bird strike was believed to be on engine 2, hydraulics are powered by engine 1. Few things seem fine as ideas to me - gear down at the speed they were going with the flaps stuck and thrust reversers sketchy, would probably cause under carriage separation, they're apparently trained to do a belly landing if they think this would happen as it's safer. The plane is in a 10/10 perfect config for a belly landing. (note: the config for belly landing and go around and extremely similar) Separately: The manual release requires you to unscrew something that takes about 60 seconds, and then violently swing the plane back and forth, pilot would not want to come off glide slope and even if they did, rocking the plane around at that low low altitude, not good. Separately: apparently in the time they turned, and crashed, there is no way they had time to run checklists, so they where according to the pilots I watched, probably just flying.

If the birds either took out both engines or engine 1 stalled under the load of engine 2 surging and dying (apparently common) - it seems to me they had no good options but to execute that tear drop turn that is apparently VERY MUCH not recommended as it's very very hard (but they did it) and get the plane down asap asap. Provided it's not pilot error and they shut down engine 1 in a panic by mistake (has happened before, fatally) - it seems they could very well have just gotten a very very very very bad, unlikely but possible, series of events. Makes me sad.

I have been under the distinct impression for a long time that a plane slowing on the runway has very little to do with landing gear brakes.

That's under normal operation. All planes are certified to stop without any air reverse thrust[0] given they land at the right sized runway, right conditions, right position, etc etc.

But it's definitely part of the program.

They must also sit on the tarmac post heavy braking and the brakes must not burst into flames.

[0] https://www.youtube.com/watch?v=evLpE8Us-j0

True, though I don't have a breakdown of net effects.

Commercial jet aircraft utilise thrust reversers, speed (or air) brakes (usually control surfaces which can extend out from the aircraft fuselage or wings), and landing gear brakes.

The latter are not insignificant, but thrust reversers and speed brakes are major contributors, especially immediately after touchdown.

There's also the effect of spoilers which increase the load over the gear and hence the braking capabilities of landing gear brakes.

Jeju 2216 failed to utilise nearly all of these mechanisms. It landed without flaps, spoilers, or gear, and possibly w/o thrust reversers.

If one divides weight of the airplane by the number of wheels it has, one would find one wheel carries around 10 times more weight than that of a truck. You even get a slightly better deal on landing when you don't have as much fuel.

That's a lot of weight but nothing crazy, so on a dry runway wheel brakes alone are more than enough to stop normally. They would also wear out a lot, overheat and occasionally ignite if used like that, so that's what thrust reversers are for.

That mass has much more velocity than a truck.

Maybe. But maybe another 1000m of dirt would have been enough to slow them before the treeline. The area south of the runway is mostly an easement for the ILS approach equipment, then a parking lot and finally some trees.

It's also definitely the case that the cement-reinforced dirt mound is not best practice for a locator array.

Yes, but in basically every single circumstance if you have room for 1000m of dirt, you have room for another 1000m of runway, which is even safer.

This is the reality - most airports run their runways “to the fence” for some variation of “to the fence”. If they could reasonably have a thousand extra feet of runway they’ll add that, as it supports more operations and doesn’t really hurt.

Some of them even move around the recommended touchdown point depending on other factors, if the runway is extra long.

So you'd rather them have a certain 100% chance of death instead of more leeway and a chance against much less robust trees? Honestly, if I crash land, I think I prefer 150 more meters and a tree as the obstacle over the concrete block.

What is going on here and what's with this crazy ass logic?

Do you believe the plane would have stayed in one piece if it wasn't for obstacles?

> What is going on here and what's with this crazy ass logic?

Nothing is crazy about it. Many people in this thread (like you) are in a tizzy over a concrete wall for a plane landing with no gear at high speeds. Your argument is basically "having no wall would make me feel better" which has no logic and very obtuse.

The ground is also a hard obstacle and this plane would've hit uneven ground shortly after the runway regardless. It's going to disintegrate either way.

160 knots?

Google tells me: "Modern jets land between 120–150 kt. This depends on weight, weather conditions and several other factors."

So even after scratching asphalt for 2/3 of the runway it was still faster than the normal landing speed.

My uneducated gut feeling says pilot was trying to abort the landing.

Normally they land with flaps down, which reduces landing speed.

I've heard sometimes landing gear is also involved..

We really need a placebo controlled double blind study to learn if landing gear is actually effective of just a cargo cult like parachutes.. [0] https://www.bmj.com/content/363/bmj.k5094

Gears also aerobrake.

There were no flaps deployed. Without flaps it's going to be a lot faster than a normal landing.

Yes, it was a very fast landing.

just because that aircraft was doomed (maybe?), that doesn't make it a good idea to have the concrete wall there

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