Hacker News Reader

Comment by haunter | Hacker News Reader

It looked like a missile https://streamable.com/lg0on2

Learjet 60 is about 18 metres long. In one frame it's a little above the ground, in the next it's gone up to about halfway between the cockpit glass and the wing roots, and in the next it's gone.

At 30fps, I make that somewhere in the region of 1000mph.

The audio (don’t listen to it) has the pilot reporting the descent speed as 22,000 feet/min

I'm not planning on listening to it. I would expect that happened long before it hit the ground.

I’m not going to listen again to verify this but it felt like 20 seconds before they hit. The recording was only about a minute long.

ADS-B shows it as 32,000 fpm in the final 2 data records.

32,640 fpm is the maximum ADS-B can encode for vertical speed.

Quite likely it exceeded that, some sensible numbers I've seen suggest between 800 to 1000 km/h final speed at impact

I got called all kinds of wrong for picking 1000mph, but it's kind of the midpoint of the numbers I get from an 18-metre plane collapsing into the ground in a little over 1/30th of a second, single frame of video.

That's suspiciously close to the max value of a signed short int.

You'd think, but it's far stranger than that.

The actual VSI reading is taken either from barometric pressure or GNSS, and the source is flagged in a bit in the ADS-B packet. Then it's encoded in one of two forms in a 9-bit value with a single-bit sign.

If the 9-bit value is 0, there is no vertical speed available. Otherwise the vertical speed is encoded as +/-64 * (value - 1), giving a range from 0 to 510 for a range of bitfield values from 1 to 511.

Now this gives you a range in 64ft/min steps up to 32640ft/min. Most civilian high-performance aircraft will be in an absolutely screaming climb at around 10000ft/min and an 8000ft/min descent would be pretty aggressive - autopilots may drop into a 8000ish ft/min descent if there's a loss of cabin pressure, for example but 6000 would be considerably more comfortable. So even with your 9-bit value you've got enough resolution to show a fairly gentle climb (try staying below 1ft/sec climb or sink when flying by hand!) and something well in excess of what you'd normally want to experience.

64 * 512 = 32768

64 * 511 = 32704

64 * 510 = 32640

So that's just a bunch of zeros for the lower bits.

Interesting. Thanks for the explanation.

Yep. Picked 32640 because the last two values are, from my understanding, inconsistently handled by transmitters due to how far from norm they are.

Too many zeros or too many ones in a row can cause problems with DC balance on the transmitter and the receiver stages.

It's a bit like how the petrol gauge on your car is out to lunch below about 1/8th of a tank.

If you're doing 32000ft/min you're clearly going *muuuuch* too fast, either up or down, implausibly so.

Unless you're in a jet fighter aircraft in which case showing up correctly on flightradar24 is not your biggest worry.

there is dashcam footage from further away that gives a better indication of speed. It was fast but not supersonic.

The speed of sound in air at sea level is around 770mph, and I got 1000mph from 18 metres of plane disappearing into the ground in about a single frame of video.

Let's not argue over the approximate nature of "1000mph" given the measurement constraints.

I think it's pretty much a mercy that no-one aboard would have felt a thing in the last couple of dozen milliseconds.

Last transmission was 22k feet/second, which is 15k mph.

(I thought ATC's cool as a cucumber 'altitude your discretion' was 'interesting' – presumably a formality they need to offer to clear the aircraft, since clearly no discretion was involved.)

No it was feet per minute. 22,000 feet/second is almost orbital velocity.

Oops, thanks. Not familiar units for me, and I hadn't had a coffee.. that would be an absurd speed of course (nevermind to also have the composure to report it while experiencing it!).

> 22,000 feet/second is almost orbital velocity.

Solves one problem, gives you a different problem...

I once read a transcript of an incident where ATC advised an aircraft with multiple engine failures and no hydraulics "Emergency team in attendance, runway at your discretion".

The reply: "Oh you want me to be all fancy and make it a runway, do you? Well, we'll see..."

They got it pretty close to the runway and it all had a far happier ending than this one.

CBC article says vertical speed was 30,000 feet per minute, which is 550km/h or 150m/s.

it was also moving forward, not just downward.

Yes but that number is the vertical speed as it's measured in feet per minute.

That was probably not when it was the height of a telephone pole above the ground though.

341mph or 500ft/s

I might be going mad but 9 metres in 1/30s (half the plane in one frame) is 270m/s * 3600s = 972000m/h / 1000m/km = 972km/h or about 600mph?

I think your 1000 is the right ballpark but the units are wrong.

"No immediate reports of anyone on the ground injured or killed." If that's correct, the people in that silver car had the luckiest day of their lives.

The sound is just crazy and the images of the crash site seems somehow worse than what we've seen from e.g. Ukraine when a missile miss and slams into the ground.

A fully-fueled private jet is an inefficient missile, but it exceeds the raw chemical energy of most in-theater single-target tactical missiles (although the energy conversion is far less precise and efficient).

But yes, that's a lot of kinetic mass dead-stopping on a hard surface and 1 to 2 kiloliters of jet fuel.

Technically called a wet explosion. Fuel deflagration, not explosive detonation.

> when a missile miss and slams into the ground.

A lot of them are designed to explode before hitting the target for maximum damage.

The private jet carries up to 3.5 tons of fuel and weighs over 10 tons at take off. A Kalibr cruise missile is maximum 500kg of explosives and the heaviest variant is 2.3 tons over all.

Its not surprising at all that the jet makes a huge hole in the ground. Jet fuel is more energetic than RDX, while converting that energy slower. So both the chemical and kinetic energy is much higher with the jet then with a russian cruise missile. If anything the crater is surprisingly small.

The fuel would not have contributed to the crater. Fuel does not explode, not at the speed needed to form a crater. It requires oxygen. No matter how much fuel was present, there was not enough oxygen in the immediate vicinity of the crash to burn that much fuel.

The fact that there was a fire long after the impact shows the fuel was not burnt in the crash. Fuel hung around long enough to still be burning at least several seconds afterwards, long after the crater was formed. That is why it wasn't a missile. Missiles use high explosives (propellants and warheads) which burn up all at once to generally leave a non-burning crater.

Some back-of-napkin math yields a crater depth of a little under a meter ... which is in the ballpark of images.

aircraft (1.5m radius cylinder x 18m long) at 6700kg = 51.54 kg/m3 aircraft density... over ~1,500kg/m3 average soil density x aircraft 18m length = 0.63m

=.63m / 2-foot penetration below ground level... which is what the pictures show.

Soil is dense. Aircraft are marshmallows compared to soil.

> The fuel would not have contributed to the crater.

In the explosive sense, I agree. Depending on how much fuel they had, it could’ve been a decent percentage of the aircraft’s weight, though.

surely velocity and elasticity need to go into that calculation as well

Nope. Speed doesnt contribute to crater depth. Faster makes a wider crater, not a deeper one.

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

When the object is near supersonic spèeds (throught the various materials) even the strenght of the materials becomes very secondary. A meteor made of sand makes the same crater as one made of stone.

Just to address the elephant in the room, that was also the "vulnerability" deliberately exploited in the 9/11 attacks.

What do you mean deliberately exploited? Given the information made available in the past 25 years, they didn't seem to plan for such engineering failure, it was more likely pure luck. I mean, crashing a plane, counting that the fuel would flow through the elevator shafts, still burning, and that it would soften the iron columns enough to make them buckle... it needs a competent team to plan that with certainty.

IIRC some the guys who hijacked the planes took flying lessons and were notoriously uninterested in takeoff and landing compared to steering, according to their instructors. They weren't very sophisticate.

I'll give you the benefit of the doubt, though. In a conspirational mindset, if it was a false flag operation (and it did help USA find support to rearrange the Middle East oil control, get rid of their old bomb stock, and a few other perks), a State certainly has the proper people to plan such scheme, and the actual hijackers would only need to know which face and height of the towers to hit, and at what angle and speed.

I meant the "vulnerability" that planes, especially early in the flight, also contain significant fuel that causes cascading damage beyond the impact.

I get that my comment probably didn't reach the "insightfulness" threshold for HN, but geez, go easy on me.

Ah, my bad. The word "vulnerability" threw me off.

I'd guess that wasn't deliberate also, since logistically, it was better to hijack the plane while it was in range of their target, and it had enough fuel to make it look like kidnapping instead of terrorism to delay the response. One can't ignore that a refueled plane weighs about 20-25% more, though.

I don't know how to break this to you, but the reason the 9/11 hijackers weren't interested in learning how to take off and land was because they were going to hijack the planes mid-air and then crash them, not land them. It wasn't because they weren't "sophisticated" and lacked interest.

What, that planes are heavy and move fast?

This is because there is more mass to the plane than most missiles. And that mass going those kinds of missile-like speeds is going to create quite a shockwave and crater. Contrary to what many people are saying, the fuel contributes very little to the crater size.

Citation: I'm a former airplane and missile guy.

Shouldn't the missile warhead contribute to the crater size though?

Reply on news.ycombinator.com