The next time you ride a high-speed rollercoaster, it could easily be the case that there are tiny flaws in the rails on which you travel.
There might even be small cracks.
These are however unquestionably safe due to decades of operation and subsequent engineering understanding of these steel thrill rides.
Any weld in any structure is nearly always the weakest link in a stressed engineering component that is made from steel.
Flaws can be present at the manufacturing stage and remain when the item enters service.
They may also be absent initially, but appear as a direct result of the applied stresses seen during service.
For a rollercoaster, ‘service’ is of course the ride being in operation, where the passenger-carrying train applies significant forces to both itself and the track.
The areas of highest stress are the points in the ride where you most feel the forces of the ride: bottoms of drops, helixes, sharp turns, etc.
But it is not just the track which experiences significant levels of stress in this area, but also the train chassis.
The train being held safely onto the track results in considerable force being applied to axles, wheel assemblies and crucially, welded parts.
On a busy day, trains might complete a circuit more than 200 times. Over a season that might equate to tens of thousands of stress cycles.
It is this cyclic nature of stress being applied which can cause cracks to both form and grow in areas of high forces.
A weld in these areas that was already flawed when it entered service – either after the ride’s fabrication or repair – is highly vulnerable to the stresses of service.
That’s why most almost all flaws are identified at the fabrication or repair stage, after the weld has been made and before the ride is put into service.
This is done via a combination of visual examination and a selection of non-destructive testing methods which include ultrasonic and magnetic techniques among others.
It is rare, but sadly things can and do go wrong.
In 2016, a rollercoaster derailment at the M&Ds theme park in Scotland was ultimately proven to be a result of flawed repairs to the Tsunami ride’s train.
Poorly-executed repairs saw incorrect weld metal applied to the train’s axles.
In a crucial area of stress application, this area was massively exposed and ultimately fractured.
The train’s chassis fractured, and the train became detached from the track at the bottom of one of the ride’s drops.
The inspector involved in signing off the defective welds, was ultimately prosecuted, fined and suspended from his profession.
Such incidents are almost unheard of, but they are ultimately not the result of design or operational issues.
A rogue worker in any system can cause severe problems, but thankfully this isn’t a common occurrence.
There is however always room for improvement in reducing risk – maybe more so at some parks than others, or in certain countries more than others.
An army of safe management
But with millions of rollercoaster circuits being completed across the world every year, it is clear that there is generally a well-understood system of operation, inspection, maintenance, repair and replacement in place for such rides.
This safe system of operation can on occasion include the the monitoring of known cracks, which in isolation are proven through mechanical engineering to be safe at certain sizes, quantities and locations.
These days, software known as finite element analysis (FEA) is used to identify key areas of stress and thus ensure inspections are appropriately targeted.
Cracks can also be modelled in FEA with the stresses of ride operation factored in to predict growth over a given number of rides.
Safety factors are then applied before the area is re-inspected, repaired or even replaced.
Critical crack sizes may take months or even years to be reached, but no responsible operator knowingly waits anywhere near that long.
And when taking relevant legislation, regulations and guidance into account, operators aren’t usually allowed to wait that long even if they wanted to.
But there comes a point in every ride’s life where it is no longer economically viable to maintain the existing structure.
As appears to be the case with the Ultimate at Lightwater Valley, this can lead to retirement of the ride completely.
In other cases, a near-complete rebuild of the ride may be warranted, as we are seeing with the plans for Alton Towers’ Nemesis to be re-tracked in the next year.
So during every closed season, it is a small army of inspectors, technicians and engineers that identify and remediate flaws in their early stages to keep every rollercoaster rider safe.