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Comment by shrumm | Hacker News Reader

This reminds me of an undergrad assignment I had when taking real time C programming. We were each given a programmable lift controller that looked very lego-ish. We were meant to write a multi threaded C program to react in real time to button presses etc to control the lift. I finished and tested every scenario i could think of - it worked great. Was feeling pretty proud of myself.

The professor gets to my table, presses a few buttons and everything works fine. He nods to himself. Then he starts yanking wires out and the lift kept going. He looks at me, shakes his head and says “safety”. I forgot to make the lift stop if anything unexpected happened :)

Thank goodness there was a safety system on this lift, even if it had to fall 84 floors for it to kick in.

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Multi-threaded program for controlling safety-critical systems? What could go wrong...

This is were time-triggered architectures [1] can kick in! You divide your program into tasks and messages passed between tasks, and can then assign each task to a separate processor with a precise execution time. Same goes for messages.

[1] https://pdfs.semanticscholar.org/8e26/a95a24569b284271a54397...

You just need to acquire the global elevator lock.

They were really lucky. If they were 12 floors lower then it would have ended horribly. I wonder why didn't the emergency brakes work immediately. Shouldn't it clamp down as soon as any elevator cable snaps?

If they were 12 floors lower then it would have ended horribly

It's not clear how fast they were falling; it may be that the brakes didn't kick in because the fall was slow.

It's also very possible they were falling slowly because the brakes kicked in. As far as I know having the lift slowly hit the buffer at the bottom shouldn't be a problem other than having to reset the buffer.

This happened to me as a kid in Eastern Europe about 30 years ago. I was with my parents and my sister, going from a high story to the ground level. We were pretty much at the ground level when the cable broke. There was a sudden jolt, I don't know if it was the brakes engaging, or the cabin hitting the buffer. Then a bit of silence, and after about one or two seconds the cable came crashing on the top of the cabin. I guess the cabin was quite sturdy, that nothing happened. My Dad just opened the doors (they were and still are mechanical, rather than the motorized gliding doors you see in the US), and we left. None of us was harmed in any way.

For future reference: do not try to escape from a malfunctioning elevator by yourself. The controller won't be locked out and the elevator can start moving without any notice. Use the emergency phone or button in the elevator and wait for a technician or the fire brigade to rescueue you.

Most likely the jolt was simply the rest of the cables catching the load of the cabin. If the brakes got engaged or the elevator ran into the buffer the elevator will not be aligned with the ground floor.

Glad to hear you got off with nothing more than a slight fright though.

Surely if it hit the bottom and the cable fell on them it wouldn’t be able to go anywhere? Not sure that I could diagnose that from within the lifts I use though.

You have to keep in mind that elevators have multiple cables. All you know is something, maybe a cable, maybe multiple cables, hit the top of the elevator. You don't know how many cables may remain.

> For future reference: do not try to escape from a malfunctioning elevator by yourself. The controller won't be locked out and the elevator can start moving without any notice. Use the emergency phone or button in the elevator and wait for a technician or the fire brigade to rescueue you.

I think this is very good advice for the US and western countries, where you can expect some technician/fire brigade to come and help. In the communist Eastern Europe of 3 decades ago, there were daily blackouts, and probably every single days a few hundred people got stuck in elevators because of the sudden loss of electricity. We all knew how to deal with the situation, it was part of life. A cable breaking was definitely unusual though. I never heard of it happening to anyone else.

You're wrong. Sorry, but The advice is not good anywhere. Any good system will be designed for flight or flight because, despite all else, humans are humans and this was not a situation anyone is trained to handle. Anyone.

A lift that is stuck misaligned with the floor is by definition in an error state. It's something that should never happen. Lift programmers are -- much like ordinary programmers -- not omniscient. The chance of an elevator running into a bug in the software during normal operation is basically zero. The chance of running into a bug in an error state is much much higher.

If you're lucky the lift will have detected something wrong and gone into lockdown itself. If you're unlucky it'll start moving with your body halfway through the doorway. It's not incredibly likely to go wrong but the price when it does is too high to risk it.

They probably weren't going fast enough for the brakes to kick in.

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