Airplane Cabin Windows – Why the Window Pane Small Hole?

Have you noticed that your airplane cabin windows have a small hole towards the bottom of the window panes? 

This tiny hole is called a “Bleed Hole”, or a “Breather Hole”.

This article was updated on June 20, 2020

Airplane Cabin Windows, more than a view

A pressurized airplane enables you to breathe and stay in a comfortable temperature setting when flying.

In order to install windows in a pressurized airplane, the windows need to be strong.

Airplane cabin windows consist of three layers of the window, designed to equalize the low outside pressure of the airplane with the high inside cabin pressure of the airplane.

There are 3 passenger cabin window layers

3 layers are necessary to make this system work. Two of the layers serve as a fail-safe structural construction.

1. The outer window pane (closest to the outside)

.47 inches (12mm) thick, this windowpane is the thickest one.

It seals the cabin completely from the outside air, and it is also the most structurally sound window pane.

If the outer layer window pane fails, the middle window pane takes over.

2. The middle windowpane

.23 inches (6mm) thick, this is the windowpane with the bleed hole.

So what’s the purpose of this airplane cabin window hole?

The hole balances the pressure which exists between the cabin and the air gap between the outer and middle window panes.

3. The inner window pane (closest to you, the passenger)

.16 inches (4mm) thick, this one is an insulating barrier.

It is also called the “dust cover” or “scratch pane,” since it protects the middle pane from scratching and dust.

This pane acts as an insulating barrier by protecting us from the cold outside air temperatures at cruising altitude.

The outside temperature at cruise is typically around minus 76 Fahrenheit (-60 degrees Celcius).

The cabin window hole serves two different purposes. 

Purpose #1

The hole allows pressure to equalize between the cabin and the air gap that exists between the window panes.

Therefore, and thanks to the hole, the in-flight cabin pressure is mostly “pushing” on the thickest outer pane. Remember, the inside cabin air pressure is much higher than the pressure of the outside air.

Purpose #2

The purpose is to release moisture from the air gap between the window panes. 

This prevents fog and frost from excessively forming on the window panes.

The frost and fog you sometimes see are from water condensation. This as a result of the cold outside air continuously hitting the cold surface of the window.

Why is the frost pattern round?

It’s from a mixture of the window surface temperature, cabin air humidity, and the rate of airflow through the bleed hole.

But doesn’t that hole affect cabin pressurization?

No, that tiny air leak going through that window hole does not affect cabin pressurization. 

Cabin air does not leave the airplane through that hole.

The air is ultimately kept inside the structure of the airplane, kept secure by the outer window pane.

What if we did not have that small hole in the cabin window?

If we did not have that tiny hole to equalize the air pressure the difference of air pressure between the window panes would become so great and unequal that the windowpane may bust!

Airplane cabin windows hole
Take-off from Newark Airport. Credit: Captain Novak/ALPA (Airline Pilots Association)

New cabin window technologies

In the past few years, new technologies have been invented, where the airplane design no longer has to require the breather-hole in the cabin window.

One such new airplane, eliminating the need for the breather-hole is the Boeing 787.

B787 cabin window dimmer
Boeing 787 cabin window. Arriving LAX Airport. Los Angeles downtown view. Photo: CaptainJetson.com.

Electro-chromatic dimmable windows

On the 787 (and on the 777X), window technology has been advanced even further with its window dimming switch that replaces the traditional window shade. The feature is based on electro-chromatic-dimmable technology.

The dimming effect is achieved by electrified gel sandwiched between two thin glass-layers:

  • When the passenger pushes the dark side of the button the electric current is increased and the gel darkens.
  • When the passenger pushes the light side of the button the electric current decreases and the gel lightens.
B787 cabin window dimmer
B787 cabin window dimmer switch. Photo: CaptainJetson.com
electro-chromatic-dimmable windows Boeing 787
An electro-chromatic dimmable window on a 787. Boeing.
Electro-chromatic dimmable window schematic
Note the electrochromic panel between the two outer structural windows and the inner dust cover panel. This is where the dimming effect takes place by electrified gel sandwiched between two thin glass-layers. Illustration: Gentek.

Airplane cabin windows, what’s next?

There is exciting progress in the further development of improved passenger cabin window visibility.

Airplane cabin window holes are long gone from new airplanes. Even the B787 larger cabin windows are rapidly becoming old technology!

Virtual cabin windows

One technology currently being greatly explored is virtual windows. These windows are not real cabin windows, but, rather, signals from cameras mounted all around the outside of the airplane.

This technology allows airplane manufacturers and airlines to present the actual view from outside the airplane with as much detail as desired. Here is an example of virtual plane window views from Boeings’ rendition of a virtual cabin ceiling.

Boeing's virtual cabin ceiling
Image: Boeing/Allure.

Or, how about a virtual window arrangement like this one from a supersonic business jet? The same concept will be applied to future hypersonic jets.

Airplane cabin windows, Spike jet
Spike-S-512-Interior. Image: Spike Aerospace.

Cargo compartment windows

cargo compartment window seat, business

Are you a passenger who prefers real cabin windows over virtual, or “fake” windows? Even real window technology has wiggle room for more cabin window possibilities. How about airplane cargo compartment seating with windows? This concept is the brainchild of Florian Barjot. You can read more about the development of the cargo compartment windows here.

Featured Image: CaptainJetson.com

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