A $100m Tank Incident Is Squeezing the World's Aircraft Window Supply

GKN Aerospace is still running its aircraft window plant in Garden Grove, California, at roughly 50% of normal output after a chemical storage tank nearly exploded in late May, triggering the evacuation of more than 50,000 residents.

The tank held over 7,000 gallons of methyl methacrylate (MMA), a skin and lung-irritating compound used in plastics production. As its temperature climbed — the gauge maxed out at 100F (38C), so it may have gone higher — workers realized the vessel risked exploding and showering nearby neighborhoods with toxic material. Fire crews hosed the tank down. At one point, the vessel "actually bulged," according to the local fire chief.

No explosion occurred. In August, GKN and local authorities announced a $100m (£74m) program to compensate evacuated residents. But window production at the site, halted after the incident, remains hobbled. Melrose Industries, GKN's UK parent company, says it aims to restore full production by 28 September. GKN declined an interview with the BBC.

Few suppliers, big consequences

The disruption sent a shockwave through aircraft manufacturing because GKN is one of surprisingly few companies worldwide that make aircraft windows — highly engineered, safety-critical parts.

"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 says it is "taking steps to mitigate any potential impacts" and describes the production shortfall 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."

The timing is bad. Airlines "can't get new aircraft fast enough," says Mike Stengel of AeroDynamic Advisory, a consultancy. "They're holding on to aircraft for longer."

New competitors struggle to enter the window business because of heavy regulation, Stengel adds. "It can take time to pivot to a new supplier." The result is what he calls a "balancing act" between plane-makers and the parts firms they depend on.

What goes into a window

To understand the engineering, the BBC spoke to Jean-Eric Vermont, general manager of French company Saint-Gobain Aerospace, another of the few window makers. "We focus mostly on commercial aircraft, regional aircraft and helicopters," he says. Vermont declined to say how much market share his company holds in aircraft windows — or "transparencies," as they are known in the trade.

The oval-shaped cabin windows next to passengers' seats are typically made of plastic-based materials. "You start from a sheet of acrylic," Vermont explains. "Then you cut to size, [to] give the shape." A Saint-Gobain spokesman confirmed the company does not store or process MMA like GKN; it purchases sheets of polymerised MMA, or acrylic, instead.

Aircraft manufacturers are ordering larger cabin windows to give passengers a wider view — a selling point that requires additional testing to confirm the bigger panes hold up as well as smaller ones. Saint-Gobain tests its windows by exposing them to pressures many times what an aircraft experiences at altitude. Cabin windows usually consist of two plies, and the company also verifies the window survives even if one ply is damaged.

Cockpit windows: a different animal

Cockpit windows, Vermont says, are "totally different." They are generally made of glass chemically reinforced by adding potassium — swapping smaller sodium ions for larger potassium ions fills out the molecular structure, so the glass compresses into an extra-strong formation as it cools during production.

The latest aircraft feature curved cockpit windows that make planes more streamlined and fuel-efficient. But curvature is hard to manufacture: the slightest distortion or defect is clearly visible to the pilot, so quality control checks screen out such flaws.

Cockpit windows are especially vulnerable to bird strikes. "The issue is not the speed of the bird, the issue is the speed of the aircraft," says Vermont. Saint-Gobain uses numerical simulations and "physical tests representative of a sizeable bird impact" to test against the threat.

"Typically, if you go through a hailstorm or if you hit a large enough bird, the external ply will crack," he says. "The cockpit window is designed for that." A London to Londonderry flight was recently affected by a cracked cockpit window; a mid-air emergency was declared, but 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's interfacing with two very different environments," says Stengel. "That's why these are more highly-engineered products."

With GKN not expected back at full capacity until 28 September, and few qualified alternatives available, plane-makers will keep managing a bottleneck that one overheating tank was enough to create.