If you've ever looked out of an aircraft window, you might have wondered about the engineering that went into creating this vital part of an airplane. The answer is that aircraft cabin windows are not made of glass, but with something called "stretched acrylic". It's a lightweight material that provides better resistance to hairline cracks, reduced crack propagation and improved impact resistance, and is manufactured by a few global suppliers for various aircraft flying today. One such supplier is UK-based GKN, which makes cabin windows for many aircraft, including the Boeing 737 and the Boeing 787.
A cabin window consists of three panes: 1) an outer pane flush with the outside fuselage, 2) an inner pane with a tiny hole, and 3) a scratch pane. Passengers can't touch the inner pane or the outer pane, for safety reasons. Instead, passengers can rest their heads against the scratch pane, press their iPhone against it, or simply make it dirty with greasy fingers. The scratch pane isn't actually part of the window assembly itself, but installed separately.
The inner and outer pane thickness is specific to each type of aircraft. "Inner panes are generally thinner at approximately 0.2” thick and are only present as a fail-safe if the outer pane fails," said Jason Webb, Director of Business Development and Aftermarket Services at GKN in California, in an email. This is a very rare occurrence, most notably with Southwest Flight 1380, which was actually a failure of the engine, not the window. "The outer panes are thicker at approximately 0.4” thick and carry the pressure loads for the life of the window," Webb said. The increased thickness is meant to "to allow for engagement with the airframe structure while maintaining the required strength. The air gap is approximately 0.25” and also varies for each aircraft," Webb said.
What is the tiny hole on aircraft windows for?
The little hole on the inner panel allows some of the cabin air to escape into the pocket between the inner and outer panes and equalize. This forces the outer pane to take all of the load, albeit slowly. It's a tiny hole so that as the plane ascends, the pressure slowly equalizes. Frost can form on the inner pane because moist air from the cabin seeps through the hole as the aircraft gains altitude. It freezes on contact with the very cold windows.
If you've flown on a Boeing 787 Dreamliner or many A330 aircraft, you know the windows are a cut above. First, they are quite a bit larger than most airline windows, at 19 inches high. The structural cabin window is made by GKN, with the familiar inner and outer panes made of stretched acrylic. However, unlike other planes, Dreamliners do not have a physical window shade. Instead, the shade is controlled by electrochromic technology which uses electricity to change the color and amount of light that passes through the window. It does this by means of a medium, a gel, that darkens or lightens depending on the charge. This system, branded as Alteos, is manufactured by US-based PPG Aerospace using technology developed by Gentex, based in Michigan.
Aircraft cabin windows are just as important as the engines or wings in keeping an airplane aloft. Without them, passengers would not be able to see out of the plane, and the pressure differential between the cabin and outside air would be too great, making flight impossible. So, the next time you're on a plane, take a moment to appreciate the engineering that goes into creating these vital components.
Images in this postGraphic of passenger windows via wp.com
A tiny hole on a plane window via wp.com
Graph of electronically controlled windows via wp.com