What is the function of that small hole?

The small hole, called the breather hole or pressure balancing hole, allows air from the cabin to flow between the panes, balancing the pressure and ensuring that only the outer pane bears the brunt of the pressure difference.

As the plane ascends, the air pressure decreases. The air pressure outside the aircraft is much lower than inside the cabin, which exerts physical stress on the airplane window. The aircraft’s internal pressure control system adjusts the cabin pressure to a safe and comfortable level for passengers.

The middle and outer acrylic layers of the airplane window must withstand this pressure difference. Both layers are designed to handle the pressure, but thanks to the small hole, only the outermost pane has to absorb the full pressure load.

A critical detail

Marlowe Moncur, Chief Technology Officer at GKN Aerospace, explains: “The purpose of the small breather hole in the middle pane is to balance the pressure between the cabin and the gap between the acrylic panes, so that the cabin pressure only acts on the outermost pane.”

In the rare event that the outer acrylic pane fails under pressure (which almost never happens) and cracks, the middle pane will serve as a backup. Of course, the small hole in the middle pane will allow a little air to pass through, but the aircraft’s pressure balancing system will handle the rest.

In the unlikely scenario where one of the window panes breaks, the backup structure ensures that the pressure doesn’t harm the window. The remaining panes can still withstand the pressure, keeping passengers safe.

Bret Jensen, a senior aviation technician at Boeing, shares: “This little hole also helps prevent moisture and frost from building up on the window. That’s why your window doesn’t fog up completely when the plane flies through clouds.”

On long flights, a thin layer of frost may accumulate around the breather hole area. Many flights reach altitudes where the outside air temperature can drop to as low as -57°C.

Though small in size, these tiny holes in airplane windows play an essential role in ensuring safety and comfort for passengers in the sky.