Friday, June 24, 2016

Why do Airplane Windows have Holes?

If you have ever traveled on a window seat in an Airplane, you would have noticed a small hole near the bottom of the window pane. This hole is not accessible from the inside (it's not on the pane closest to you), and appears to be on a 'middle' pane. On cold mornings, you can even see ice crystals forming on this pane, like the image below.

Ice on Airplane Window
It's a little unsettling if you don't know why airplanes would need a hole on each window. Isn't the whole purpose of the window to keep the cabin pressurized while allowing for pleasing views to us passengers? A hole in even one of the window panes would make the window as a whole weaker - no? As it turns out, the opposite is true - the hole is a safety feature!

Of the three panes of a typical aircraft window, the first one (from inside) is merely to protect the second and third ones. If you recall your experience with the first pane, it is a little flimsy and bends a little under pressure from your finger. Surely this pane isn't going to hold out against the low outside pressure at cruising altitude. The first pane keeps the passengers and little kids with their toys and spilling hot / cold beverages from interfering with the second and third panes - both of which are independently capable of maintaining cabin pressure. Wait - if both panes are capable of holding cabin pressure at cruising altitudes, why do we need two? For safety, of course. But why the hole?

Icy Art

The hole makes sure only the outer pane takes the load of Air pressure 'usually'. In case the outer pane fails, the inner one takes up the role of maintaining cabin pressure - albeit with a little leakage. This also makes window inspection easier - if the outer pane is OK, all is good and there's no need to inspect the middle pane. Pilots are anyways trained to descend to safer altitudes in case a window pane breaks, and the cabin pressurization system can handle the additional small load (due to leakage) easily in such a case.

So next time you see ice crystals forming on your airplane window, relax and take comfort in the fact that it's been designed as a safety feature. The added visual pleasure of seeing nature make intricate art right at your window is an added bonus.

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