Cable car disaster in Italy 3

[LittleInch]

Not a huge amount of detail at the moment but a dreadful tragedy and difficult to understand how this could happen. The factor of safety on a cable car cable must be huge as it is so vital.

https://www.theguardian.com/world/2021/may/23/at-least-eight-dead-in-italian-cable-car-accident

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[LittleInch]

A "fork like device" which may actually be a fork...

Although one report said it was a device they used to hold the brake pads apart during a service so maybe some sort of U bolt like device, which normally would have a red tell tale string so you don't leave it in place. A bit like the "doors to manual" operation you used to see on planes when we used to travel by plane.

This is a bit of a wild leap here but you wonder if the cars were kept one at each end for months on end, did the pull wire end up at a strange angle or bent over a pulley for a long time, not moving and stretched a bit or somehow work hardened? The report said the cable sheared just as the top car was about to enter the top station.

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[dik]

News...

"Police arrested three people Wednesday in the cable car disaster that killed 14 people in northern Italy, saying workers placed a clamp on the emergency brake to deactivate it as a patchwork repair - one that prevented the brake from engaging when the lead cable snapped."

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

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[1503-44]

LI, yeah I know. Its just a statistics thing with me. All we can do is follow the science. Sometimes it works and sometimes it doesn't. I try to remember the reason we try to make it practially impossible to happen is because it does, seemingly one way or another, as we tell ourselves its a low P event, it should never. Then on the other hand, each one that happens lowers the P of the next. Is it asymptotopic to something? I wonder what? It all reinforces an idea that at least low P events are way more common than we can possibly calculate, that's all. Maybe just calculating such things lowers the P, like measuring the position of a photon or something. Just another intriguing question in a list of many.

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[Retiredat46]

Could the tow cable have been damaged by an excessive load caused by the emergency brake malfunction?

 

[IRstuff]

The brake wouldn't be applied unless the tow cable broke; that's its one job.

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[dik]

It appears the brake was disabled...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Retiredat46]

From an article in ABC News May 26:

https://abcnews.go.com/Internationa...le-car-crash-clamp-deactivated-brake-77910900

"Carabinieri Lt. Col. Alberto Cicognani said at least one of the three people questioned overnight admitted to what happened. He said the fork-shaped clamp had been placed on the emergency brake to deactivate it because the brake was engaging spontaneously and preventing the funicular from working."

Seems like the brake would engage all at once with enough force to hold several times the weight of a fully loaded car in the event the tow cable broke. Likewise, the tow cable would be sized to pull several times the weight of a fully loaded car. There would be a short time between brake engagement and shutdown of the tow motor (because of the overload) when the forces would be very high.

It's possible the shock loading of a spontaneous brake engagement was about the same as the strength of the tow cable, damaging the cable. We don't know the safety factors used in the design or how many times the brakes malfunctioned before they were disabled, but it could have been a contributing factor.

 

[MartinLe]

But the emergency brake doesn't act on the tow cable (that broke) but on the other (carrying?) cable.

 

[1503-44]

I imagine once the brake did not engage, 5-6G dynamics did the rest.

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[LittleInch]

Retiredat46, (did you?, really??, how???) You might be onto something there.

Safety devices need to work when they're needed and not work when they're not. Spurious operation can often be only slightly worse than the event they are meant to avoid.

Yes the brake is on the main cable but the tow cable is fixed solid to the car so suffers shock loading. Probably worse if the stop is very close to the main winch wheel.

I would imagine the brake is quite powerful as the down car would be moving quite fast before the activation so needs to allow the car full of 30 people to a halt within a reasonably short distance/ time.

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[1503-44]

What is supposed to trigger the brake's engagement? 1.1 x Normal Velocity? 1.5G aceleration?
It would seem to be a poor design if the brakes engaged late enough to break the cable. After "V2" speed, they should only deploy a drag chute or something. No point deploying only to snap the cable.

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[Alistair_Heaton]

I was think about that as well..

Maybe tension on the pulling cable?

 

[LittleInch]

No idea, but probably a mixture of the two.

So max velocity x 1.2 sounds about right given that the speed is fixed by the cable. A bit of stretch or sag every now and then, but within small limits and then lose the cable / lose tension it triggers activation also.

Given that this was designed 50 years ago and the brakes are never supposed to activate, maybe the impact on the tow cable wasn't fully thought through or calculated in the event of spurious operation - who knows? I don't know what the equivalent would be on a plane to test a safety device but in pipeline terms I guess this is like test triggering an ESD whilst at full flow in a pump station / compressor station - could get a bit hairy...

So what was apparently repeated spurious activation before they locked it open won't have done it any good for sure.

The excess speed of the car that crashed caused it to hit the pylon then jump off the cable by all accounts. Truly terrifying and it could have held 30 people. You would have needed a massive drag chute to stop it doing that.

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[LionelHutz]

I'd bet on the tow cable pulling on a mechanism that holds the brake off. Release the tension on the tow cable and the brake engages.

The brake coming on during towing certainly could have been shock loading the tow cable and caused some damage to it.

 

[Retiredat46]

In normal operation there would always be a substantial load on the tow cable, so my guess is that the brake is triggered if the load falls below a specified level. It's probably something like the slack line detector/alarm/shutoff systems on cranes, which stop the hoist before the wire rope can wander out of the grooves in the drum. I could tell an interesting story about my experience with one of those, but this is probably not the place.

LittleInch, yes, I did retire in 1992 at age 46. I had several years warning that my job would go away if I didn't want to relocate, so I started saving and investing everything I could. I got a substantial bonus payment for volunteering to leave a little before I'd expected to. My mother, who grew up during the Great Depression, broke down in tears when I told her I no longer had a job.

My plan (if you can call it a plan) was to start looking for a job if my nest egg ever fell below half of what I started with. That never happened, so I spent my time sailing, building and flying model planes, learning how to operate machine tools, getting my private pilot's license, owning my own plane, and doing welding, machining, and fabrication at the local tech college. I started my latest hobby of tree climbing a couple years ago. I'm a huge fan of early retirement, and I marvel at the fact that I was able to pull it off.

 

[waross]

I agree with Lionel.
If the tow cable breaks,the brake grabs the stationary support cable.
I wonder if, for some reason, the car ran up against the stops at the terminal with the tow cable still pulling.
That may break the cable.
I know that that is not supposed to happen, but the brake was not supposed to be disabled either.

Bill
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[Retiredat46]

"I wonder if, for some reason, the car ran up against the stops at the terminal with the tow cable still pulling."

The load on the cable in that (very unlikely) scenario would be higher than having the brake engage spontaneously while the car was being towed, but the load from a spontaneous brake engagement is going to be much higher that the normal load of towing the car up the incline. Either may be enough to damage the tow cable. It just depends on what safety factor was applied in the design.

 

[MartinLe]

I'm not convinced the emergency break engaging would seriously stress the tow cable - here's why:

* the cable is long, this means there's (small) 'springiness' in the calbe itself
* more importantly: the cable sags. more tension=less sag=lifting the cable. This is another mechanisms that can absorb some energy and reduce the acceleration

That said it's still suspicious that the tow cable broke near (or at?) the car with the damaged brake.

 

[itsmoked]

I think there is a horrible failure mode here.

If a single car's brake spontaneously applies it's going to have a huge load on it since it's going to have to stop both cars at the same time. That's also going to put a huge load on the the tow cable connecting the two cars together.

I can see that all overlaid on the assumption that if the cable breaks each car is instantly isolated and each car's brake is sized to stop it's own/only car.

Keith Cress
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[LittleInch]

Depends where the car was when the brake was applied. If within say 50 m of the end point then there isn't much line stretch available.

If, as we suspect, one activation of the emergency brake is no force on the pull wire then as it approaches the end, you would expect the force in the car to reduce so more likely to activate.

That's also the worst location for high load on the pull wire.

But we really are working on very little information.

All we really know is that to get the service running they knowingly deactivated the safety system. And have no info on the cable.

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