Thread by @thelorryist

So how does it happen?

Maybe I can be of some use explaining, people seem to enjoy these threads, so...

Heavy Goods Vehicles Braking Systems 101.

A thread.

What you see there is two vehicles, each with a separate braking system.

Drivers need to understand how these brakes work so they don't kill themselves (or anyone else).

Heavy trucks brakes are pneumatic (worked by air) rather than hydraulic (worked by fluid) like in your car

The air lines have a long way to travel and need to operate reliably in this way and also exert huge forces, so air is a better medium but it makes for complex systems.

They also 'fail safe' so if air is removed from the system the brakes come on....

Which also means to take the brakes off, you need to apply air to the brake system.

With air supplied at sufficient pressure it will push back a huge spring inside the brake chambers allowing the parking brakes to be released.

Ok, boring, who cares?

When the driver is connecting a vehicle to a trailer, they will need to make sure the parking brakes are applied to both the trailer and the vehicle.

If the driver forgets one or the other, it removes a layer of safety that can mean the combination will roll away. Easily done.

If for example, a driver attempts to connect to a trailer and doesn't check if the park brake is applied. The brakes on the trailer will be ON.

Until the driver connects air to the trailer, at which point the brakes will be released. So the trailer is free to roll.

If the vehicle is not physically connected to the trailer at that point, via the kingpin and coupling, then there is nothing to stop it.

Perhaps the driver may also forgot to apply the parking brake in the vehicle... as well as failing to check the trailer... it happens

In which case the whole combination may roll away, and in this condition, the driver may even find themselves stood on the catwalk between vehicle and trailer as this begins to happen.

Don't ask me how I learned this one.

The hard way, it always is.

If this ever happens to you and you are stood here.

Immediately disconnect the 'emergency line', the red airline, this will dump the air from the chambers and the brakes will come on immediately.

Might save your life.

It's why I never recommend twisting the collar too far.

Coupling & uncoupling is one of the most effective ways to kill yourself if you get it wrong as a driver and it's something we are expected to carry out safely multiple times per shift.

But please tell me again how this is unskilled.

Dropped trailers like this happen every day

For clarity, I don't know the cause at Bristol Harbour.

This is not intended as direct analysis. It will be fully investigated and results will come out in time.

This is purely to illustrate something else you may not know about this most visible of invisible industries