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What Are Airplane Windows Made of? (And What Is That Little Hole?)

Jan. 12, 2019
9 min read
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When you're in flight, the only thing separating you from the minus 60 degrees Fahrenheit and unbreathable, thin air outside is an airplane window. On one side, there's a warm, pressurized cabin where you can work, watch movies, sleep — and on the other, a world that would kill you in minutes if you were exposed to it. Between the two, just incredibly sturdy windows. A very exciting subject for AvGeeks, but maybe a bit of a mystery for the flying public. So, let's take a deep dive into aircraft cabin windows and flight deck windshields.

Cabin Windows: No, They're Not Glass

A cabin window consists of three panes: 1) an outer pane flush with the outside fuselage, 2) an inner pane — which has a little hole in it you may have spotted, and 3) a thinner, non-structural plastic pane called a scratch pane. Passengers can't touch the inner pane (the one with the hole in it) or the outer pane, for safety reasons. Instead, passengers can rest their weary heads against the scratch pane, press their iPhone against it, or simply muck it up with greasy fingers. The scratch pane isn't actually part of the window assembly itself, but installed separately.

The main thing to know is that aircraft cabin windows are not made of glass, but with something called "stretched acrylic". It's a lightweight material manufactured by a few global suppliers for the various aircraft flying today. One such supplier is UK-based GKN. The largest manufacturer of cabin windows worldwide, GKN makes cabin windows for the Boeing 737 and the Boeing 787, and most other aircraft.

"Stretched acrylic is produced by stretching the base material of as-cast acrylic. It provides better resistance to crazing [hairline cracks], reduced crack propagation and improved impact resistance," said Jason Webb, Director of Business Development and Aftermarket Services at GKN in California, in an email.

A typical commercial airplane passenger window, showing outer pane, inner pane with bleed hole, and scratch pane (which, other than the shade, is the only part you can touch). Image via Aeorlineas Argentinas Boeing 737 Maintenance Manual and modified by author to include scratch pane for clarity.
A typical commercial airplane passenger window, showing outer pane, inner (or middle) pane with bleed hole, and scratch pane. Image via Aerolineas Argentinas Boeing 737 Maintenance Manual, and modified by author to include scratch pane for clarity.

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 Webb. 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 little hole for?

The tiny breather hole in the inner pane with some frozen condensation. (Image via Getty Images).
The tiny breather hole in the inner pane with some frozen condensation. (Image by Getty Images)

As your flying tube gains altitude, the pressure acting on the outside the plane drops — the air is much less dense the higher your plane climbs. Because aircraft cabins are pressurized to about 6,000 feet for passenger comfort (and survival), there is more pressure inside the plane than acting on it from the outside. That pressure is bearing on the fuselage and the cabin windows. 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.

The scratch plane covering the inner pane doesn't block air from escaping into the hole. "It's not airtight. Air can flow into that space through multiple gaps in the interior wall assembly," Webb said.

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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.

The marvel of Boeing 787 windows

If you've flown on a Boeing 787 Dreamliner, 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, as noted above, 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 magic: 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. "Applying a small electrical voltage across the gel causes it to darken, while removing the voltage allows the gel to return to its natural transparent state. The voltage can be precisely controlled and adjusted in small increments to allow intermediate states of light transmittance to be selected," is how PPG explains it.

Below, windows are darkened in a 787 cabin, except for the two at the back.

Photo by Alberto Riva/TPG
Photo by Alberto Riva/TPG

The results are amazing: I flew the Dreamliner with JAL recently, and the cabin was dark enough for people to sleep comfortably with the windows at the darkest setting for people could sleep. Nevertheless, even with darkened shades you could tell it was bright and sunny out as we passed over the Canadian Arctic. Trippy.

The PPG Alteos window on the Boeing 787. Image via PPG.
The PPG Alteos window on the Boeing 787. Image via PPG.

The window has no moving parts, operates silently, and goes to transparent in the event of loss of power. According to PPG, the electrochromic windows block 99.997% of the visible light.

Flight Deck Windshields

Unlike cabin windows, the flight deck windshields are made with glass-faced acrylic — an outer layer of glass bonded to stretched acrylic. Then, there's a layer between them, made of urethane. Each has anti-ice and anti-fog systems. In the case of the Boeing 787, there are then layers of stretched acrylic, just like the cabin windows, albeit much thicker — between one and three inches thick depending on the aircraft.

Cockpit windshield on the Boeing 787. Image courtesy of PPG.

In some cases, such as the Boeing 737 and Boeing 747, the windshield features two plies of tempered (i.e. hardened) glass along with an interlayer. This design is likely a throwback to the originally-approved designs, with the Boeing 787 sporting a lighter version with an outside, tempered glass ply.

Cockpit windshield on the Boeing 747. It's thicker and heavier than the 787 windshield. Image courtesy PPG.

Have you ever noticed that the cockpit windshields sometimes look like they have oil on them? That oily sheen is actually a coating of indium tin oxide, which is a conductive material between the layers and transmits heat. Accordingly, this thin coating is all that is needed to keep the windows nice and clear in frosty weather. Brilliant.

Lufthansa's fleet as seen in Frankfurt Airport in Germany, the primary hub for the airline. Lufthansa is the world's 10th largest airline by passengers carried in 2016. The airline owns a fleeet of 273 aircrafts and 130 orders. Lufthansa both operates the super jumbo jets Airbus A380 and Boeing 747-8 and 747-400. Lufthansa is also the launch customer of the Airbus A320neo series and already flies the A350. The airline is one of the founder members of Star Alliance aviation affiliation. Frankfurt airport is an international airport connecting the German city to the world (Photo by Nicolas Economou/NurPhoto via Getty Images)
Is that a splotch of oil causing the rainbow? No—its a thin coating of indium tin oxide, which is electrically conducive and transmits heat to keep the windshield clear on this Lufthansa A321. (Photo by Nicolas Economou/NurPhoto via Getty Images)

According to PPG, in the past "this was accomplished with thin wires of a design similar to those in rear car windows, but the main manufacturers now use a coating of indium tin oxide. Just nanometers thick, this coating sits between glass plies and is completely transparent."

What About Bird Strikes?

Bird strikes are a cause for concern for pilots, airlines and manufacturers. Accordingly, flight deck windows are rigorously tested by the manufacturers. Federal regulations in the US require the window panes "withstand, without penetration, the impact of a four-pound bird when the velocity of the airplane" is equal to around 340 knots indicated airspeed in the case of a 737. (That is quite fast.)

Below, you'll find a video of a birdstrike on a cockpit windshield which occurred less than 100 feet off the ground, just prior to landing. As you can see, this startled the first officer. The bird makes a solid thwack against the windshield but the aircraft goes on its merry way. Not so the bird, sadly.

Mike Arnot is the founder of Boarding Pass NYC, a New York-based travel brand, and a private pilot.

Featured image by (Photo by Darren Murph / The Points Guy)