An industrial nightmare was brewing south of Los Angeles.

In late May, a chemical storage tank with more than 7,000 gallons of toxic material began heating up – fast. The tank belonged to GKN Aerospace, a company that supplies parts to the aviation industry, including aircraft windows.

As the temperature inside the shiny metal storage vessel rose, workers realised they had a problem. The tank was at risk of exploding, potentially showering nearby neighbourhoods with methyl methacrylate (MMA), a skin and lung-irritating, external compound used in plastics production.

The tank's temperature reached 100F (38C) – though possibly went higher – 100F was simply the maximum reading, external on the temperature gauge inside.

More than 50,000 residents of Garden Grove city were evacuated and fire crews began hosing down the tank in a bid to cool it. At one point, the vessel "actually bulged" according to the local fire chief.

Thankfully, no explosion occurred. Months later, in August, GKN and local authorities announced a $100m (£74m) programme, external to compensate residents evacuated during the incident. But production of aircraft windows at the site, which had been halted, is still not fully restored.

This has sent a minor shockwave through the aircraft manufacturing industry because GKN is one of surprisingly few companies, anywhere in the world, that make aircraft windows – highly engineered, safety-critical parts for planes.

Production at GKN's Garden Grove facility currently remains hobbled, at roughly 50% of normal, according to public statements, external by its UK parent company Melrose Industries. The aim is to have full production restored by 28 September. GKN declined an interview with the BBC.

"They're one of only a handful of suppliers – less than a handful, really – of critical windows," says Marisa Garcia, an aviation industry analyst. "One of the breaking points of supply chain in aviation is you have very few companies qualified to do something and, when something goes wrong, it really upsets the entire system."

Boeing tells the BBC it is "taking steps to mitigate any potential impacts", describing the shortfall in production as an "industry-wide" situation.

"We are supporting our supplier," a spokesman adds. An Airbus spokeswoman says, "We are closely monitoring and are seeing positive progress toward a return to normal."

It comes at a time when airlines "can't get new aircraft fast enough," says Mike Stengel of AeroDynamic Advisory, a consultancy. "They're holding on to aircraft for longer."

Even though there are just a few aircraft window-manufacturing firms in existence, it's hard for new competitors to emerge because of how regulated the industry is, adds Stengel. "It can take time to pivot to a new supplier."

This leads to a "balancing act" between plane-makers and the firms they rely on for various parts, Stengel says.

To find out what goes into the making of an aircraft window, I spoke to Jean-Eric Vermont, general manager of French company Saint-Gobain Aerospace.

"We focus mostly on commercial aircraft, regional aircraft and helicopters," he says. Vermont declines, though, to say how much market share his company has in aircraft windows – or transparencies, as they are known in the trade.

The oval-shaped windows in the cabin, next to passengers' seats, are typically made of plastic-based materials, says Vermont. "You start from a sheet of acrylic," he explains. "Then you cut to size, [to] give the shape."

A Saint-Gobain spokesman confirmed to the BBC that the company does not store or process MMA like GKN, but rather purchases sheets of polymerised MMA, or acrylic.

Aircraft manufacturers are increasingly ordering larger cabin windows, says Vermont, in order to offer passengers a wider view of the world outside. It's a selling point but it requires additional testing to ascertain whether the bigger windows hold up as well as the smaller ones.

Saint-Gobain tests its windows by exposing them to pressures many times what would be normal on an aircraft flying at altitude.

These acrylic-based cabin windows are usually made out of two layers or plies, and Saint-Gobain also performs tests to confirm whether the window will survive even if one of those plies gets damaged.

Cockpit windows, Vermont says, are "totally different".

They are generally made of glass, which is chemically reinforced by adding potassium to it. This involves swapping smaller sodium ions for larger potassium ions, which helps to fill out the molecular structure of the glass, meaning that when it cools during production, it compresses into an extra strong, tight formation as it cools.

Some of the latest aircraft feature cockpit windows that are also curved, to make the plane more streamlined, which improves fuel efficiency. But curved cockpit windows are challenging to make – the slightest distortion or defect is clearly visible to the pilot.

Quality control checks are used to ensure no such impediments exist in the final product.

All aircraft windows must be resistant to impacts but cockpit windows, at the front of the plane, are especially vulnerable to bird strikes, says Vermont: "The issue is not the speed of the bird, the issue is the speed of the aircraft."

Saint-Gobain uses numerical simulations and "physical tests representative of a sizeable bird impact" to test its windows against this threat.

"Typically, if you go through a hailstorm or if you hit a large enough bird, the external ply will crack," says Vermont. "The cockpit window is designed for that."

A London to Londonderry flight was recently affected by a cracked cockpit window. While a mid-air emergency was declared, the plane landed safely. "There can be [such] cases," says Vermont, though he adds that Saint-Gobain's windows have not been involved in any recently.

"You're obviously dealing with a surface that is interfacing with two very different environments," says Stengel. "That's why these are more highly-engineered products."