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Comment by ErroneousBosh | Hacker News Reader

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.

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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.

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.

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

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

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.

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.

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.

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.

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

it was also moving forward, not just downward.

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

341mph or 500ft/s

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